In 1980, several of us got the idea to develop a series of seminars for distributors and Original Equipment Manufacturers to discuss servicing, installing, and troubleshooting electric motors in the field. It was a way to educate service technicians and others about the "tricks of the trade," how to safely and properly get the most out of electric motors. We needed a name for the seminar, and some was suggested: "why don't you call yourself the Motor Doctor?" The name stuck. In the years since, we've put on hundreds of seminars all over the country, appeared at countless tradeshows, and have written a regular series for a number of trade publications.
An important part of determining the correct replacement motor for a particular application is understanding motor speed. In this article, I’ll discuss some of the basic concepts behind AC induction motor speed. Next article, we’ll take a look at using multispeed motors in the field.
Every motor has magnetic poles, just like a permanent magnet. These poles are created by bundles of magnet wire wound together in the slots of the stationary part of the motor (the stator core). Look inside an electric motor, and you can count the number of poles or windings. The number and alignment of these bundles of wires creates magnetic poles, and the number of poles in the motor determine the motor’s speed, stated in revolutions per minute. Keep in mind, no-load RPM is a factor of motor poles and power frequency, not voltage, horsepower, or motor diameter.
With that knowledge in mind, you may surmise that every four-pole AC induction motor runs at the same speed. This is in fact the case, and you can determine this speed by using the following formula:
For 60 hertz electrical systems: 7,200 divided by the number of poles in the motor gives you the no load RPM.
(For 50 hertz systems: 6,000 divided by the number of poles gives you the no load RPM.) For the purposes of this article, I’ll use the North American (60 hertz) system.
Another important element to remember from this formula is that I’ll be giving you the no load RPM in the following examples. You should note that under load, the rotating parts of the motor fall behind or “slip” the magnetic speed.
Using the standard formula, you can determine that a four-pole motor operating under no-load conditions will run at 1,800 RPM (7,200 divided by four poles). Loaded, the motor will slip to between 1,600 and 1,750 RPM. Four pole designs are the most common pole configuration for AC induction motors and are typically found in belted applications such as blowers, fans, air-handling equipment, compressors, commercial garage door openers, and conveyors.
A two-pole motor will operate at 3,600 RPM unloaded and between 3,000 and 3,450 RPM under load. Such speeds are commonly found in pump applications, such as submersible pumps, sump pumps, pool or water recirculating equipment. Another typical application is small ventilating fans. To the untrained ear, two-pole motors appear to need servicing because they sound somewhat noisier when running. This is principally due to the higher RPM, and as a service technician, you should be aware of the different “normal” sounds a motor makes that are related to speed.
Six-pole motors run at 1,200 RPM unloaded and between 1,050 and 1,175 RPM loaded. They are often used for air-handling equipment, direct-drive applications, window fans, furnace blowers, room air conditioners, heat pumps, residential garage door openers, and other equipment.
As you can imagine, lower mechanical speeds often result in quieter designs, which makes an eight-pole motor well-suited for many residential applications where noise is a factor. These motors operate at 900 RPM unloaded (between 800 and 875 RPM loaded) and are being used more extensively today in room air conditioning, outdoor heat pump, and residential garage door openers.
A less common design is the 12-pole motor. This motor, which runs at 600 RPM unloaded, is used in washing machines and other equipment that require a slow cycle.
Here’s the important point to remember: when replacing a motor, you must select a replacement with the same number of poles as the original. Changing, say, from a four-pole to a six-pole design, the speed mismatch is likely to create significant problems.
Now that you understand the principles behind motor speed, you’re ready to learn about multispeed motors and their uses. We’ll tackle that next issue.
A recent Motor Doctor® article on the subject of capacitors generated correspondence that raised some interesting issues related to servicing capacitors in the field.
The original article gave a basic description of capacitor design and function, then moved on to some suggestions for diagnosing a defective capacitor in the field and how to deal with the problem.
One point I made in the article had to do with emergency field replacement of a capacitor when you don’t have the correct size available. I stated that it is an acceptable temporary fix to go up one rating size (for example, from a 7.5 microfarad original up to a 10 microfarad replacement) so long as the voltage of the replacement capacitor is equal to or greater than the original and the current draw under load does not exceed the nameplate amps. I stressed the importance of installing a capacitor of the correct size as soon as possible.
Fred Baron of Motors & Armatures, Inc. offers the following refinement and caution to the original article:
“The general rule of being able to replace a capacitor of equal microfarad rating but higher voltage rating is certainly true as far as the capacitor is concerned. There is a circumstance in which a run capacitor failure may indicate an application problem, in which case replacing the capacitor with one of higher voltage may worsen that problem. Here it is:
“When either the discharge or inlet of a squirrel cage blower is blocked, the blower motor’s RPM’s increase. (M.D.--Remember, the less air moved, the less work is done. This leads to a decreased load on the motor and higher RPMs.) As the RPMs increase, the regenerated voltage increases and tends to stress the run capacitor. The run capacitor, in air-moving applications, acts almost as a fuse, in that its failure could be an indication of a more significant problem. If the replacement capacitor is of higher voltage, the ‘fuse effect’ is diminished.”
Fred goes on to add that he agrees about our prescription for emergency fixes, but stresses that nothing is as good as finding a replacement that exactly matches the specifications of the original.
Larry Johnston, a knowledgeable service technician from Madison, MO, offers some additional insights into suggested capacitor testing procedures. Larry suggests that when using an ohmmeter to troubleshoot capacitors, you should have a capacitor of similar value to compare how far the needle swings as it shows conformity on that particular test equipment. He points out that many defective capacitors will show some degree of continuity.
Larry relates a story in which he was asked to check on a “stuck compressor” diagnosis from a service technician who also had experience as a TV technician. The customer was pleasantly surprised to learn his problem was a weak capacitor and that he didn’t need a new compressor. Larry, in turn, was puzzled that a TV technician did not have more expertise in capacitor diagnosis. In his experience, motor start capacitors and certain round type run capacitors seem most prone to show continuity but still exhibit diminished capacitance.
Larry offers the following insights into testing capacitors: “I always discharge with a 500-ohm, 11-watt resistor. The needle swing test depends on quick connection of the probe. Note, I have repaired multiple motors in air conditioning equipment where the technician has discharged the capacitor with a screwdriver and the screwdriver brushed against the return leads. I have also seen situations where the capacitor has fallen down onto an evaporator, burning a hole in it.”
We welcome your letters and always appreciate your thoughts and comments. They help everyone expand their knowledge of issues in the field and the best way to service motors and keep them in operation.
When it comes to assuring that your motor provides consistent service for the longest time possible, it’s important to pay attention to the bearing system. Bearing system failures are one of the most common mechanical breakdowns in the field.
A sleeve bearing’s entire purpose in life is to maintain a continuous layer of lubricating oil on the surface of the motor shaft. As you can see from the illustration, the the oil flows from a reservoir to the feeder wick which rides right on the shaft. As the shaft rotates, the feeder wick pumps a microscopically thin layer of oil from the reservoir down onto the shaft. The oil flows down the shaft until it hits the "flinger" on the end which returns the oil to the reservoir.
At the same time, the sleeve bearing circulates that oil to remove any contaminants from the lubricant. Even under the best of circumstances, there are moments when the shaft is rotating that metal-to-metal contact occurs. Infinitesimal fragments of metal break off, and the oil helps move those contaminants into the bearing’s reservoir where they are filtered out by the wicking material.
So what causes mechanical breakdowns in what is essentially a closed lubricating system? The most common enemies of bearings are water (that can interrupt the flow of oil to the shaft), solvents, or the wrong lubricant.
As a rule of thumb, any type of oil that is labeled "motor oil" is okay to use as a lubricant. Detergent oils are acceptable as well, the detergents will not harm the motor’s windings. Avoid other lubricants—and tell your customers not to attempt to lubricate their motor with anything but motor oil.
You would be amazed at the types of lubrication well-meaning but untrained people try to use in electric motors. One of our quality managers once received a failed motor from a health spa that smelled strongly of coconut. No one could figure out the source of failure until they began testing the lubrication. Turns out the customer had attempted to use analgesic cream (coconut scented of course) to lube the bearings. Smelled great, didn’t work. Other equally unsuccessful lubricants I’ve seen have included petroleum jelly, transmission fluid, and cooking oil.
Another thing to remember to tell your customers is to avoid using chlorinated solvents or industrial degreasers such as WD-40® as lubricants. The solvents can attack and destroy the insulation in the windings leading to motor failure.
Water or moisture will often get into the bearing system if the flinger is damaged or left out of the bearing. The flinger, in case you’re unfamiliar with the term, is the little plastic piece that is always left over when you take apart a motor and put it back together again. Seriously, this little piece of plastic is essential in maintaining the controlled trap system and keeping oil in circulation in the bearing system.
It may seem totally illogical, but a common form of field failure is from applying too much lubrication to a motor. Factory-fresh motors with sleeve bearing systems are always properly lubricated. Since it is essentially a closed system, there is no chance the oil will leak out during shipping or installation, so avoid the temptation to oil the motor when you put it in. When you apply too much lubrication (especially to the sleeve bearing systems in smaller electric motors), the oil by-passes the bearing flinger and dissipates throughout the motor. Too much lubrication causes oil circulation to exit the "closed loop" within the bearing system, and the bearing "freezes up."
Another potential problem you may encounter in the field is bearing stress caused by excess tension on the belts used in belt-driven applications. The excessive tension tends to pull the shaft in the direction of the bearing window. If the belt happens to cover the bearing window, it decreases the bearing surface area (and its ability to circulate oil) by up to two-thirds. The result is increased risk of overload. In addition to decreasing the belt tension, you can also rotate the motor to keep the bearing window away from the belt.
Remember that heat kills motors, and sleeve bearings help to reduce the amount of heat the motor generates. Proper care and maintenance of sleeve bearings can help to prolong the life and service of your customers’ motors.
One of the things about motors that I find interesting is the wide range of work that they do. Depending on the application, a motor may be required to run continuously or start and stop frequently. They may drive mechanical loads or move air. The challenge is to design a motor that is well-suited to the specific requirements of the application. This often involves spending a lot of time learning about related devices, such as capacitors, connectors, and the topic of this particular article, switches.
Many single phase motors used in hard-to-start applications, such as conveyor belts, oil burner pumps, carbonated beverage pumps, belted fans and blowers, and commercial garage door openers, use a set of parts called a rotating governor and stationary switch assembly. Motors in these applications must have a starting circuit that produces high starting torque while at the same time limiting a high starting current. The rotating governor and stationary switch assembly enable the starting circuit to energize for a brief period of time (typically a fraction of a second) to quickly get the motor up to running speed, limiting the starting circuit to a short "burst."
Rotating governors and stationary switch assemblies have been around for decades. Manufacturers continue to make improvements to these components, developing easier-to-assemble designs with better functionality. Some of this evolution has come as the result of testing the devices under conditions peculiar to certain applications. Depending upon the end use of the product, manufacturers evaluate component performance under very low temperatures or rapid fluctuations in temperatures. In other instances, they may create a test where the switch reverses after each operating cycle. Typically, the engineers will run the switch to failure under these extreme conditions while monitoring every step in the process using high-speed photography combined with other recording instruments. Their objective is to capture the exact moment and (hopefully) the cause of failure.
From there, the research team will analyze the problem going through a cycle of failure analysis, design improvement, and process improvement. New components or design changes are tested once again, often by restarting the failure analysis process. The results are numerous improvements not always perceptible to the casual observer (or even the trained service technician). Companies take these improvements seriously, however, and often they are kept as trade secrets of the manufacturers.
Here are some examples of how this exhaustive process yields improvements in switch design. One discovery made about cold-temperature operation is that most switch designs are not affected by just the cold. Repeated exposure to humidity while the part is cold can build up damaging layers of frost and ice on components. One such application would be an overhead garage door opener, where the repeated opening and closing subjects the opener (and the switch) to a wide range of temperature variations. Manufacturers have created designs that clear offending ice from the switch without causing malfunction.
Another factor, discovered through the use of high-speed photography, is that the switch needs to make and break contact "cleanly." If the contacts are not broken cleanly, the result may be "bounce" that causes the switch contacts to cycle on and off several times before breaking a starting circuit. This bouncing could cause the start circuit to arc unnecessarily and fail prematurely in the application.
You wouldn’t think that so much work would be needed to create a relatively simple device. But consistency and reliability—especially in the face of unusual operating conditions—call for a lot of engineering know-how. It’s that kind of effort that allows switches—and motors—to be used in so many different applications.
A critical element of motor servicing technique is being able to determine whether or not a replacement motor that is not an exact duplicate of the original is suitable for the application. As a technician, you must consider a number of factors, but for this article, I'd like to focus on one of the more important issues: nameplate amps.
One typical way that technicians determine whether the replacement motor has sufficient power output is to compare the nameplate amps of the original motor with the replacement model. If the replacement motor's amp rating is at least as high as the original, you can consider the replacement suitable. In many cases, this comparison simply confirms what other factors, such as nameplate horsepower and rated voltage, tell us. This practice is most satisfactory when there is little or no variation in the efficiency from the original motor to the replacement. This method works well, for example, with most three-phase motors.
In other cases, however, comparing amps may be misleading. The comparison process tends to break down when the motors in question are single-phase models where there is a wide range of efficiencies common for a single design. This category includes permanent-split-capacitor motors, shaded-pole, and some types of split-phase and capacitor-start motors.
Since nameplate amps reflect the total current consumption of the motor (which includes both the current converted to output power and the current lost to heat due to design inefficiency), higher nameplate amps can just as likely mean poor efficiency as higher power output.
As motor manufacturers become increasingly sensitive to the energy efficiency issue, they work hard to develop motors that deliver higher power while consuming the same or fewer watts. That efficiency may or may not be reflected in the amp rating of the motor. For the service technician, this generally means placing more importance on comparing the horsepower of certain motors rather than comparing amps of the replacement to the original.
Since there are no efficiency standards for most single-phase motors, there is one good way for the technician to verify that a replacement motor of the same horsepower but higher or lower amps is a satisfactory replacement. The method is to measure the actual amps delivered to the replacement motor in its normal operating state (under normal load) and compare that measurement only to the nameplate amps for that particular replacement motor.
In summary, using just an amp comparison is not sufficient with certain types of motors. You need to be aware of variations in efficiency and take this factor into account when determining a successful replacement.
How often do you encounter this problem in the field? The motor in question continually nuisance trips. You look at the motor and the application: the motor appears to be running properly; the driven load is working properly; and yet the motor seems to keep overheating and tripping.
Many fractional horsepower motors come equipped with an internal overload device that is sensitive to both current and temperature. This thermal-overload protector (called a “thermo,” or “overload,” or simply “OL”) may either require manual resetting or can reset automatically. It is designed to protect the motor’s windings from the damaging effects of too much heat or too much current.
In situations like this, where there appears to be no mechanical or electrical problem, you may be tempted to blame the thermal device itself. Don’t give into the temptation, however, or you may find yourself treating the symptom, not the problem itself. Remember, thermos are extremely reliable, and their job is to alert you to an unseen, but potentially catastrophic problem with the application. As a good MD (Motor Doctor® column), you should take a few moments to probe for answers.
For example, you may have inadvertently substituted the wrong motor in the application. Have you matched the motor to the operating condition? Do the voltages of the replacement motor and the original correspond? One way to monitor all of these variables is to use a clamp-on ammeter to determine that you did not exceed the motor’s nameplate amps.
A related consideration is determining that you have the right load for the motor. Has the load changed from the original equipment load? You may find that a larger diameter fan has been applied to the motor, or a larger blower wheel. Or the pulley ratio is different from the original specification. Or, in some cases, the original motor’s horsepower was marginal for the driven load—and the replacement motor’s horsepower could be even more marginal. Once again, a clamp-on ammeter can help you reach a correct diagnosis.
If a motor in an older application suddenly begins to nuisance trip, you may want to look for blockages in air flow caused by airborne debris. Other air flow problems include applying a motor with excessive horsepower for the load in an air-over application. Too much horsepower often provides inadequate air flow to dissipate heat—even if the load is light. You may find that added or re-positioned baffles or filters have re-directed air flow or decreased the amount of air flow.
Another condition that may cause unexpected thermal tripping is excessive ambient temperature. Almost all motors are designed to produce their nameplate-rated output up to a specific temperature. If the environmental temperature is higher than the nameplate rating, the motor is at risk, even if everything else about the load and power supply fall within normal ranges. If a motor begins to nuisance trip, consider the environmental temperature issue. With motor-driven devices located in mechanical rooms, check to see if additional heat-generating equipment has been installed in the room. Or perhaps room ventilation has changed, either due to construction or ventilating equipment failure.
Consider the effects of foreign material or contaminants. Material can build up on the surfaces of motors, even enclosed designs that prevent foreign materials from entering the motor. Oily vapors caused by cooking oil or other chemicals can condense on the motor. Dust and lint adhere easily to these oily films, creating a very effective insulation. This may affect both the air movement over the motor as well as the motor’s ability to dissipate heat. The results are often higher internal temperatures and thermal tripping.
One last item to check is the power supply leading into the motor. Overvoltage and undervoltage can cause the motor to overheat, resulting in nuisance tripping. To check for this condition, use a volt meter to measure the power supply while the motor and the other equipment on the same circuit are running.
It’s very easy to blame nuisance tripping on the thermal device. Don’t be fooled! Listen to what the thermo is trying to tell you and always take the time to search for the root cause of the problem. You’ll avoid bigger problems down the road.
In other articles, I have discussed thermal protection, cycling, and preventing mineral build-up in electric motors. In this column, I would like to focus on a related topic: the importance of air flow as a characteristic of certain motor applications.
Many motor applications that involve moving air from one place to another, either by fan or by blower, rely all or in part on application air flow to dissipate motor heat. In other words, the motor and fan or blower must operate as a system. Given the importance of application air in these instances, let's look at some of the impediments to air flow that could make the difference between a short motor life and a long-lasting application.
The first thing a service technician needs to consider is whether and where the original motor is located in the air stream. The replacement motor should be positioned as closely as possible to the location of the original motor. This positioning can be critical depending upon whether the motor is belly-band mounted or cradle-mounted, since these mounting methods will affect freedom of movement. If the motor is driving a fan located in a venturi, pay particular attention to positioning the fan blade with respect to the venturi.
Single-phase, split-phase, and capacitor-start motors usually incorporate some system of self-cooling, such as an internal or external cooling fan. This makes these motors somewhat less sensitive to positioning when compared with permanent-split-capacitor or shaded-pole motors. Do be aware, however, that some enclosed motors that are split phase or capacitor start, particularly those designated as totally enclosed non-ventilating (TENV), also rely on application air for cooling.
A second point to remember is that dirt and chemical deposits-over time-can deteriorate the motor's ability to radiate heat and thus compromise its temperature limits. In open motors, such materials can clog not just the visible vent openings of the motor, but also internal air passages that are often part of the rotor core. Short of burnout, nuisance tripping of the motor's thermal protection may indicate clogging of this nature. If you notice this type of material build-up, take care to examine and clean the internal air passages as well as clearing any clogged vents.
Even totally enclosed motors are subject to deteriorating heat dissipation capacity if dirt and chemical residues accumulate on the exterior of the motor. These materials may act as an insulating blanket, preventing the necessary heat radiation that is part of the motor's cooling design. Simply cleaning this material off the exterior will help restore cooling capacity in these applications.
One often-overlooked consideration is application underload. Underload can cause excessive heat in two ways:
First, underloading a motor designed to move air may not produce enough cooling air to dissipate motor heat;
Second, a motor not operating at its designed load may be operating at less than peak efficiency. This will cause a larger percentage of input power to be turned into heat rather than moving force. We've talked before about the importance of considering nameplate amps when selecting a replacement motor. This is another instance where it may not be good practice to oversize a motor, since the larger motor may be underloaded. Bigger is not necessarily better in air-moving applications-in fact it could lead to premature failure of the motor. Always be cautious when dealing with replacing motors in air-moving applications.
Remember, application air flow is one of those factors that can reduce a motor's life expectancy. Understanding the characteristics of air flow and how they affect a motor's performance will enable you to select the right design and provide proper motor maintenance in the field.
In a perfect world, each electric motor would be 100 percent efficient. In other words, 100 percent of the power input into the motor (watts) would be converted into work (horsepower).
Alas, the world we live in is far from perfect, and that imperfection extends to the motor as well. Advances in technology have brought today’s motor closer to the ideal of 100 percent efficiency, but the best the manufacturers have been able to produce so far reaches the low 90s. As a result, whenever you energize a motor, you will get two outputs: a desirable one (work) and one that is not so desirable (heat). That can be a real issue in many cases. For example, many motors used in single-phase applications (such as shaded pole motors), barely rise above the 50 percent mark in efficiency. So you know these types of motors will use almost as many input watts to produce heat as produce work.
Equipped with this knowledge, you can understand why one of the criteria you must consider when selecting a motor for an application is the effect of operating temperatures on that motor.
A number of universal factors come into play when you deal with operating temperatures, no matter what the application. These include:
- The electrical efficiency of the motor in question;
- The ambient temperature for which the motor is rated;
- The ambient temperature in which it will operate;
- The temperature rise the motor will undergo when it is working as well as its nameplate-rated temperature rise;
- The class of electrical insulation with which the motor is made;
- The motor’s service factor.
One of the most fundamental design criteria relating to motor lifespan is the selection of materials used to insulate the electrical parts of the motor and the capacity of those materials to withstand heat. Insulation is critical to the safe and consistent operation of the motor. If the insulation system fails, the electrical parts become short circuited which causes the winding to break down. The result is motor failure.
To help you identify which system is right for a given application, insulating materials are grouped into classes designated with letters that identify the maximum temperature capability of the materials in that class. These identifying letters are virtually universal among motor manufacturers because they are specified by the trade organization, NEMA. For example, Class A insulation materials are designed to withstand a maximum temperature of 95 degrees Centigrade (approximately 205 degrees Fahrenheit) in most motor applications. Class B insulating materials must be capable of withstanding maximum temperatures of 110 degrees C (or about 230 degrees F). These are the two most common classes of insulation for general-purpose motors. Other classes (for example, Class F, Class H) exist for unusual, high-temperature applications.
Consider a motor that is operating normally. The temperature of its insulation will be the sum of two components. The first is the ambient temperature (in other words, the temperature of the environment surrounding the motor when it is at rest). If that motor is operating in a room, the ambient temperature would be room temperature. The second component is the temperature rise that motor experiences when it converts some of its input power to heat rather than work.
It is common practice for a manufacturer to rate the maximum ambient temperature in which a motor is designed to operate. Thanks to our friend, NEMA, this maximum ambient temperature is commonly specified at 40 degrees C (or 80 degrees F) unless the motor is designed for a specific duty.
Using this bit of information, you can now begin to figure out the limits of temperature rise on a motor. Take, for example, a motor with a Class B insulation system. You know that its maximum rated temperature is 110 degrees, and you know its maximum ambient temperature is 40 degrees. This tells us the temperature rise is limited to 70 degrees C if it is operating at its maximum ambient temperature.
You can use this knowledge in a number of ways when installing or replacing motors. For example, if the motor in question is capable of reaching its nameplate horsepower without its temperature rise reaching the maximum for its insulation class, you can think of that motor as having "spare" temperature capacity. That excess capacity can be translated into the capability of delivering more horsepower than the nameplate specifies without exceeding the maximum insulation temperature.
This spare horsepower is sometimes expressed as service factor. The service factor number found on the nameplate (for example, 1.25) can be used to multiply the motor’s nameplate horsepower to give you a maximum horsepower that exceeds the nameplate rating without exceeding the temperature capability of the motor.
But what if the motor must operate in an environment that is warmer than its rated ambient temperature? In that case, the temperature rise must be reduced if the motor is to stay within the temperature capacity of the insulation. In these cases, you may use a motor in an environment that is warmer than its rated ambient temperature provided you reduce the load horsepower.
It’s also important to realize that the operating conditions of the motor may also affect ambient temperature. If the motor is enclosed (in a furnace, for example, or within a protective housing such as a pump housing), the ambient temperature that motor experiences is actually the temperature of the air immediately surrounding the enclosure. This suggests you will have to consider dissipating the temperature within the enclosure by passive or positive ventilation. If you are comparing an enclosed motor with a similarly rated open and ventilated motor, you will need to consider the difficulty involved in dissipating the heat involved in the operation of the enclosed motor.
Temperature considerations rank right up there with mechanical parts failures in shortening the life expectancy of a motor. Understanding all of the factors involved in temperature can help you make intelligent choices when installing or replacing motors in the field.
There are few more frustrating issues to deal with than the problem of noise in electrical and mechanical equipment. There are really two issues involved here: the subjective issue of noise perception and the physics of noise itself.
On the subjective side, the challenge for anyone dealing with the problem of noise is that the tolerance of noise varies (often substantially) from person to person. This explains (at least in part) how teenagers can tolerate rock music at ear-splitting decibel ranges while their parents cannot.
Noise physics includes how to define, measure, and control noise. This can be a much more complex issue than you might think, with variables such as frequency and intensity of harmonics. Complicating matters further is the challenge of dealing with the interaction of the noise-producing device with the physical environment.
All noise is mechanical in origin. For noise to occur, some source must create waves of pressure that are transmitted through either air, liquids, or solid materials and have components within the frequency range discernible to the human ear (generally between 30 cycles and 20,000 cycles for a young person). For the purposes of this article, I’ll concentrate on motor noise (after all, this is the Motor Doctor® column).
There are numerous sources of noise in an electric motor. A motor produces so-called "electrical noise," which is generally at line frequency or a multiple of line frequency, when the magnetized parts of the motor have room to physically move. This movement occurs as these parts are alternately attracted and repelled from one another and is often referred to as "60-cycle hum." This tends to be a manufacturing problem, and there is little you can do to alleviate this problem on the job site.
Other noise sources in the motor result from air disturbances caused by moving parts and particularly by vibrations derived from imbalances in spinning parts. Once again, this is a manufacturing issue, and as an installer, you don’t have much control over this problem.
You can have an impact on the third area of noise—the interaction of the motor with the equipment in which it is mounted. Not only is this a potential source of noise generation, but also of noise amplification. Typically, when you are called to the job site, the customer will simply complain of a noisy motor. He or she won’t necessarily describe the 60-cycle hum or excess vibration. In situations like these, the first thing you’ll need to do is determine if the motor is performing sonically to its designed specifications. Most of the time, the best instrument to test for this doesn’t come in your toolbox, it’s your well-trained ear. The more time you spend in the field, the better you will become at determining if the problem lies with the motor or the application. Here are some creative remedies to the problem of noise on the job.
Mechanical isolation is usually the most straightforward action you can take. This is because any noise inherent in the motor transfers very efficiently through metal mounting assemblies. You can break this sound path in one of three ways:
Separate the motor base from the surface on which it is mounted with a resilient pad;
Couple the motor output shaft to the driven equipment with a "soft coupling" (one that incorporates a rubber bushing);
Make a substitution from a rigid-based fractional horsepower motor to an equivalently rated one with a resilient base.
But what if these three measures fail to produce the desired outcome? Don’t despair, resolution is still possible, but it will be somewhat more complicated. Your corrective action is based on the concept of harmonics. First a quick definition of the concept. All mechanical objects tend to vibrate at a number of specific frequencies. (For example, when you rub a wet finger over the rim of a glass, you tend to get a specific sound each time from that glass). These favored frequencies are called harmonic frequencies.
Now, if by circumstance, design, location, weight, mounting configuration, or any of an endless list of mechanical parameters, a piece of equipment (door operator, pump, fan) has a harmonic frequency that matches the output frequency of a motor, the result will be excessive noise. The good news is that you can attack this problem—and make a dramatic improvement in noise levels--by changing any one or several of that long list of parameters I just gave you.
While it may be difficult to calculate the exact effect, virtually any change to mass, speed, or separation distance will have some impact on harmonic noise. The trick is to find the most effective course of action. Here are some suggestions:
For a belt-driven device, try making a slight change in drive speed by varying the pulley sizes;
If the application uses rubber isolation (like a soft coupling) but appears ineffective, try changing the density (hardness) of the isolation devices. This may be enough to move the assembly off harmonic frequency;
Where space and the application permit, you can try changing the length and configuration of the drive train by adding or subtracting a belt length or chain link;
Once you understand the role of harmonics in creating noise, you can direct your efforts in the field to solving the problem by tackling those harmonics. I just wish it were that easy to deal with the teenagers and their loud music.
Aluminum is an excellent electrical conductor, and consequently, most electric motor manufacturers offer it as an option in their motor designs. So why isn’t it used more commonly in motor applications? There are economic reasons and performance reasons that limit aluminum’s usefulness in electric motors.
There is more to the economics argument than just the cost of the wire itself. Beyond relative commodity prices between aluminum and copper, the electrical properties of aluminum itself creates other economic issues. Since aluminum is not as efficient a conductor as copper, it takes more aluminum magnetic wire to equal the performance of copper. So any temporary per-pound cost advantage can be quickly eliminated.
Aluminum magnet wire is generally found in smaller motors, where the ratio of wire to iron is relatively low and physical size is not an especially important factor. It is also found in applications where the motor runs intermittently and in applications where motor efficiency is not a major factor. Examples of these types of applications include hobby compressors, garage door openers, stationary power tools, and garbage disposals.
Motor performance is another factor that tends to limit the use of aluminum magnetic wire. In applications where the highest possible efficiency and the physical size of the motor are the main criteria (in equipment that runs continuously, such as air handlers) aluminum tends to be harder to apply than copper. Since aluminum’s conductivity is lower than copper, aluminum magnetic wire must be made in larger diameters (gages) to achieve the same linear conductivity as an equivalent piece of copper wire. If you factor the larger gage wire, multiplied by the number of coil turns in the winding, you quickly see that if you want to accommodate sufficient quantity of aluminum wire to equal the conductivity of a copper version, you’ll need a larger diameter motor. In high-efficiency designs, such as room air conditioners, furnace fans, or central air conditioners, this increased size requirement tends to rule out aluminum in favor of copper.
In some circles, aluminum suffers from a bad reputation as an electrical conductor—which it does not deserve. When aluminum was originally approved by the National Electric Code (NEC) for house wiring in the 1960s, problems occurred in instances where the wire was attached to devices such as switches and receptacles. Improper connections tended to develop high resistance, which led to thermal failures. In some cases, these thermal failures created enough heat to cause smoke and occasionally fires. A small number of these fires were catastrophic—in some instances, burning down entire houses. The trade press (and others) denounced aluminum as an electrical conductor. To this day, many people remember the "bad press" and still view aluminum as an unsuitable material for electrical use.
None of these problems ever affected motor design, where connections are made with equipment suitable for the purpose. Manufacturers use oxide-piercing connectors and specialized, high-pressure connector crimpers to create a uniform, high-conductivity connection which virtually eliminates the possibility of oxide affecting the integrity of the connection. They have found that it is critical to motor reliability to have the right connection design, materials, and manufacturing processes.
Motor makers have conducted extensive studies on the impact of adverse environments on aluminum magnet wire. These studies have centered on some of the more challenging applications, such as domestic water pumps (where the motor windings are subjected to mineral salts) and pool pumps (where the windings must withstand chlorinated water). Aluminum is well-suited to adverse environments and is used, along with copper for magnet wire, in many motors.
One last note on aluminum magnetic wire. The polyester insulating varnish commonly used on a motor’s magnetic wire has a copper hue. This makes it almost impossible to distinguish the material used in the motor windings. Other than understanding the application and the limitations of aluminum, you typically have no way of knowing if you have a motor with aluminum or copper windings.
It’s 10:30 on a Sunday night, and you get a call from a nearby hospital. The door operator has failed at the hospital’s emergency entrance. Or it’s minutes before the dinner hour at a popular restaurant, and their air conditioning system isn’t working. The nearest wholesaler is 75 miles away, and they close at 5:00 p.m. Or there’s a flood, and your telephone is ringing off the hook with customers needing replacement motors for their jet pumps.
The question you always face in emergency situations is: what’s okay or, more importantly, what’s not okay when it comes to replacing failed electric motors. You do have one element in your favor when replacing motors in an emergency--NEMA provides you with a set of minimum standards for performance and mechanical interchangeability of motors. Even with these standards, however, emergency replacements represent a particular challenge for the service technician.
The advice I’m about to give applies to squirrel cage induction motors of general purpose construction without special mechanical or electrical features that would make an exact replacement mandatory. For that reason, I’m excluding special motors, such as residential garage door opener motors, but including motors used in general-purpose applications such as commercial garage door openers.
Before the Motor Doctor® column gives you any replacement advice, however, let me state two important cautionary notes:
- Safety is your first—and foremost—consideration. Never do anything that your experience, common sense, or good practice would tell you is unsafe. If the motor you are using as a replacement is not an exact match with the original, the minimum check you must do is to measure the input amps to the substitute motor. Never exceed the input amps specified by the manufacturer for that motor.
- Second, consider any emergency replacement a temporary solution. This is particularly true when you are using a motor that does not exactly match the original in terms of manufacturer specifications. In an emergency situation, you must weigh the desirability of making the substitution against the customer’s immediate health, property, and economic needs.
Keeping those precautions in mind, let me give you six possibilities you can consider:
- Capacitors. In general, you may make substitutions for oil-filled run capacitors on permanent split capacitor (PSC) motors by going to the next standard or stock microfarad rating. For example, a 10 microfarad capacitor may be substituted for a 7 ½ microfarad capacitor. It is always safe to use a capacitor with a higher voltage rating—but it is never safe to attempt to use a capacitor with a lower voltage rating. Another possibility when substituting capacitors is to wire them together in parallel to add microfarad ratings. For example, you may wire a 5 microfarad capacitor in parallel with a 7 ½ microfarad capacitor to achieve the equivalent capacitance of a 12 ½ microfarad capacitor. You also may use the same procedures when making substitutions for the start capacitor on a motor.
- Fractional horsepower direct drive motors that are designed for air handling applications generally are either shaded pole or PSC type motors. You may always safely replace a shaded pole motor with the equivalent rated PSC motor. Keep in mind however, that it is never safe to replace a PSC motor with the equivalent shaded pole design. The reason is that the relatively lower efficiency of the shaded pole motor will create problems with heat dissipation.
- Motor speeds. This advice applies to all squirrel cage induction motors, including single phase (shaded pole, PSC, split phase, and capacitor-start split phase) as well as most three-phase motors. When making replacements here, the important factor to keep in mind is to match the number of poles of the replacement motor to that of the original. This is not a tricky process. Remember that the relationship of the number of poles to nameplate speed causes the speed of that squirrel cage motor to fall into discrete bands. For example, two-pole motor speeds cluster around 3,400 revolutions per minute (typically ranging from 3,000 to 3,600 RPM). Four-pole motors cluster around 1,750 RPM (in a range from 1,500 to 1,800 RPM). The point to remember is that nameplate speeds need not match exactly—but the number of poles must.
- Multi-speed motors. This advice applies primarily to single-phase, direct-drive motors. It is acceptable emergency practice to replace a multi-speed motor with the equivalent-rated single speed motor, and vice versa. Remember to properly treat the unused speed taps of any multi-speed motor used to substitute for a single-speed design. Each unused tap connection must be electrically insulated individually at the electrical connection. Leaving unused taps uninsulated or connected together likely will result in motor failure.
- Voltage. Any motor built to NEMA standards must be capable of delivering its nameplate horsepower without overheating over a voltage range of plus or minus 10 percent of its nameplate voltage. This means that a 115-volt motor could replace a motor rated from 110 volts to 120 volts. It also means you could replace a 208-volt motor with a 230-volt motor (230 volts minus 10 percent equals 207 volts). Be careful when making this substitution. Most local power companies specify that line voltage will be plus or minus 5 percent of nominal voltage. This means a 208-volt system might have actual line voltage as low as 197 volts. If the line voltage is at the low end of the power company’s range, it may be out of range for the substitute motor you are considering.
- Here’s an important tip: motors with multiple voltages separated by a dash on the nameplate (208-230, for example) are called "wide-voltage band" motors. Properly designed, they are capable of operating in a range that extends from 10 percent below the lower of the two voltages to 10 percent above the higher voltage. Take it from the Motor Doctor—you need some of these motors on your repair truck.
- Enclosures. Fully enclosed motors carry the nameplate designations TENV or TEFC. These are generally designed to operate under a wider range of environmental conditions than an equivalently rated open motor. For that reason, an enclosed motor can usually replace the equivalent-rated open motor in an emergency substitution—but not vice versa. Remember, however, that an enclosed motor may have a harder time dissipating heat generated during operation. That would make it less than an ideal candidate in any enclosure that does not allow for the free exchange of air. If the motor comes with thermal protection, it may be prone to nuisance tripping.
There are some motors built specifically for replacement purposes, and you should be familiar with these products and have them on your repair truck. You can use them with the confidence that they will do the job for an extended period of time in an emergency situation. In many cases, however, the advice I have provided is to get you through the emergency at hand. Good practice dictates that you return to the job site with an exact replacement as quickly as possible.
Among the many mysteries of life is the question of why the North American and Middle Eastern standard for line frequency is 60 hertz and why Europe uses 50 hertz. While the Motor Doctor® column can’t clear up that mystery (and later in this article I’ll add to the complexity), I can explain some of the physics behind line frequency. While you may not need this information every day, in our increasingly global economy, it may come in handy.
Actually, frequency is one of two potential issues that electric motor manufacturers face when selling to international customers. The second concern is voltage, but this is a relatively simple problem because most electrical equipment is designed to operate between plus and minus 10 percent of its rated voltage. To determine compatibility, you just need to know if the voltage source falls within the voltage range of the equipment in question.
The issue of line frequency (expressed in a unit called hertz) can be a bit more perplexing, especially when magnetic devices such as motors, equipment with transformers, or equipment with magnetic ballasts (fluorescent or vapor-type lamps) come into play.
One critical relationship between line frequency and magnetic devices is efficiency. The physics of electric circuits tells us that AC magnetic devices increase in efficiency as line frequency increases. Sounds simple, doesn’t it? Just build your power systems and devices to the highest frequency possible. Not so fast. Another physical characteristic to keep in mind is that current flow in a conductor tends to be closer to the surface of the conductor as frequency increases. So as the frequency goes up, solid conductors begin to resemble hollow pipes as the electrons making up the current flow migrate to the outer surfaces of the conductors.
At these higher frequencies, the energy of the electrons has a tendency to actually leave the surface of the conductor. A common example of this principle in action is radio transmission. As the frequencies get higher, all of the energy can be made to leave the conductor in a form of energy called radio waves. This also helps explain why overhead power lines tend to interfere with radio reception (the annoying 60-cycle hum). What you are hearing is energy loss from the power lines becoming a radio wave that is intercepted by the radio receiver.
Consequently, the designers of electrical devices must strike a balance: the desire to use higher frequencies to improve the efficiency of converting electrical power to mechanical work and the need for lower frequencies to keep power from escaping the conductor as radio waves.
So the evolution of 50 and 60 hertz systems developed as a result of this need for balance, with additional influences coming from politics and geographic considerations. North America and other regions struck the balance at 60 hertz, while Europe settled on 50 hertz. Who’s right?
You probably realize by now that frequency is a parameter for motor selection and application. What you may not realize is that a motor designed to operate at the lower 50 hertz frequency may operate in a satisfactory manner at 60 hertz, at least from an efficiency and heat loss standpoint. This is because a 50 hertz motor contains added material to make up for the modestly decreased efficiency found at the lower line frequency. But keep in mind that when going from 50 to 60 hertz, the motor’s speed will increase proportionately to the increase in frequency (60/50=6/5=120 percent).
On the other hand, applying a 60 hertz motor to a 50 hertz line frequency application is more problematic. A motor designed to operate efficiently at 60 hertz may not have enough active material (copper and iron) to sustain efficiency at 50 hertz. Heat loss and dissipation become issues. Another consideration is that some motors designed to operate on single-phase power may have an internal switch that is speed sensitive. At the lower frequency, the motor may never reach the normal switched operating speed. The internal start switch will not open, and this will lead to burnout of the starting circuit.
I promised to add a little more complexity to the 50 hertz/60 hertz debate, and you don’t have to look too far from home to find it:
- Until very recently, portions of Philadelphia and New York City’s subway systems used 25 hertz line frequency.
- Portions of Canada used 50 hertz until the 1950s and the completion of the Continent-wide power network.
- Power frequency on most aircraft is 400 hertz. This is due to the very short power transmission distances combined with the need for very high efficiency (lightweight) motors.
In our increasingly interrelated world, don’t be surprised if you encounter the 50 hertz/60 hertz debate in your work. Now, you’ll have some understanding of how to deal with the issue.
Anyone whose job involves servicing electric motors has encountered the problem of a missing nameplate. Other articles in this series have covered ways of determining the specifications of a motor lacking the nameplate, but what if you are trying to figure out how to wire that motor?
For some kinds of motors, principally motors with terminal-based connections, basic wiring is self evident. The terminal board itself usually has markings that indicate where line one and line two are to be connected. But what if you need to reverse that motor, use a different (but available) voltage setting, or have a motor that has nothing more than a bunch of color-coded or numbered leads coming out of it?
The colors or numbers themselves are often a clue, but they alone may not provide sufficient information. There is always the trial and error method, but I don't recommend that because of the potential for destructive results. Instead, the Motor Doctor's suggestion is to equip yourself with an ohmmeter (don't settle for just a continuity tester) and learn to perform a few simple tests with it.
The first thing you'll need to discover is whether you're dealing with a three-phase motor. You may already know this from the application, but another giveaway is that the lead wires of most three-phase motors are single colors, not multiple colors, and usually identified with numbers. If, on the other hand, the motor diameter is less than seven inches and has a terminal board, it is most likely a single-phase motor.
For wiring a single-phase motor, the most important objective is to distinguish the starting circuit from the main winding. These two circuits are isolated from one another electrically if the lead wires are separated and not in contact with each other. Initially, the ohmmeter can be used to determine which wire belongs to which circuit as well as checking continuity between leads. You should be able to isolate into two groups any leads which have continuity with one another. The starting circuit is likely to isolate to two leads, the running circuit may have two or more leads that show continuity. If the running circuit has more than two leads, you will need to determine how those leads are to be used for voltage or speed changes.
You'll need to use the ohmmeter as an ohmmeter and not as a continuity checker for the next step in the procedure. You'll want to use the lowest ohm scale your meter offers, as the typical winding resistance in motors such as these is less than 100 ohms. If the motor is a permanent split-capacitor motor, you're going to be looking for common and speed taps of the winding. Using the ohmmeter, find the pair of wires that has the highest resistence as measured in ohms. This will give you your common and lowest speed tap. Using each of these two leads in turn, find the pair that gives you the the second-highest resistance. This should provide you the common and second-lowest speed tap and should also allow you to isolate which of the two leads from the first test is the common.
In addition, note that the common lead in this type of motor is usually white or purple. If there are additional leads in the run widing group, continue to use the ohmmeter to test the now-identified common and additional leads. Descending resistance will give you ascending speeds.
All is not lost if you don't have a diagram for a particular motor, at least not if you understand how to use and ohmmeter. As with any problem-solving exercise, the more tools you have at your disposal, the more effective you become in the field.
Motor efficiency remains one of the top issues in our industry, but when you talk about efficiency, often you're talking about trade-offs. In other words, it is relatively easy to make a motor efficient, if money is no object. But since cost is a factor, motor manufacturers keep seeking the right balance of increasing motor output without driving up the price of the product.
Occasionally, a technician or service person will ask me, "why not just increase the output by increasing the voltage (the current flow) to the motor?" While that may seem logical, increasing the voltage (in effect, creating an overvoltage situation) will not necessarily boost the output of the device. To understand why, you need to become familiar with a physical characteristic called "hysteresis loss."
Think of the atoms of magnetic material as an unruly herd of cattle. Running electric current through the material will polarize these atoms, creating the magnetic field. But as I mentioned, this is an unruly herd, so it takes time for the current to bring all those atoms into formation.
As you might suspect, when you reverse the current in an alternating current motor, it takes time for those atoms to get going in the opposite direction. And the amount of time is not necessarily the same as the time it took to get the herd moving properly in the first place.
Without getting into a lengthy physics lecture, this process of reversing polarity produces heat (or wasted energy). This is known as hysteresis loss. And that helps explain why increasing the voltage into the motor will not necessarily increase the output. Instead, it can fight the resistance of magnetic materials to reverse polarity--and simply heat iron.
For service technicians, this is also an explanation why a motor heats unexpectedly when the voltage supplied is higher than the device's nameplate voltage.
One way to overcome this situation is by using "magnetically soft" material. Magnetically soft material has atoms that readily reverse polarity (a docile herd?) when exposed to alternating current. Naturally, since the reversing process happens more quickly, there is less wasted energy.
Here's where metallurgy comes into play. A motor rich in magnetically soft material will be more efficient, producing more work with less heat. And since the magnetic capacity of a motor also is influenced by the amount of active material (more core, more laminations), the tendency might be to try to add as much magnetically soft material to your design as possible.
Magnetically soft materials, however, tend to be more expensive. The motor manufacturer must find that proper blend of just enough magnetically soft material to do the work required without putting too big a dent in the customer's wallet.
It's important to keep this struggle between performance and cost in mind when you talk to customers about energy-efficient motor-driven equipment. Yes, efficiency is probably more important to homeowners now than ever, but that efficient operation comes at a price. And motor manufacturers will keep working to strike that balance between motor performance, efficiency, and cost.
Electric motors, in essence, are conversion devices. They convert one form of energy (electrical energy) into another form (mechanical energy). In the process, they consume power, and they do work.
It is easy to be imprecise about these terms as well as the units of measurement we use in connection with the terms, such as horsepower, watts, and amps. Obviously, it’s beneficial to have a precise understanding of the relationship between these units of measurement, especially since they can help you make sense of the related issues of efficiency and power system sizing. So, here are some precise definitions of terms.
“Work” is performed when something is moved. It may be the work involved when several stagehands move a piano or when a gas engine moves an automobile. Appropriate examples for this article include water being moved through a pump by an electric motor or a garage door being lifted by a motor-driven opener.
“Power” is the measurement of how much work is accomplished in a specific amount of time. A bulldozer is capable of moving a hill of earth much faster than a garden tractor, therefore we say the bulldozer is more powerful than the tractor. Before bulldozers and garden tractors, horses performed much of the heavy work needed by humans. Thus, when the scientific concept of power was developed hundreds of years ago, they described it in terms of “horsepower,” a term still in use today.
“Energy” is the ability to do work. Energy is stored in such things as coal, gasoline, and the food we eat. For energy to be released, some chemical or mechanical action must be performed on whatever stores that energy. Coal is burned, gasoline is compressed and heated to make it explode in an internal combustion engine, and our bodies oxidize the food we eat.
Electrical energy is produced mechanically by a generator or chemically by a battery. Electrical energy performs work once it’s applied to an electro/mechanical conversion device, such as a motor.
One of the measures of work is a unit called “foot/pounds.” A foot/pound is simply the work done when a one-pound weight is lifted vertically the distance of one foot. So, if a 55-pound weight is lifted vertically 10 feet, 550 foot/pounds of work has been accomplished.
As I said before, power includes a time factor. It takes more power to move the 55-pound weight in our example 10 feet in one second than it would to move the same weight the same distance in two seconds. The power it takes to move that 55-pound weight 10 feet in one second would be measured as 550 foot/pounds per second. This power value is equivalent to one horsepower. Therefore, a four-horsepower electric motor would be able to move a 2,200-pound load (4 x 550) a vertical distance of one foot in one second, or an 1,100-pound load two feet in one second.
Just as horsepower has an equivalence of foot/pounds per second, a “watt,” the unit of electrical power, has a relationship to horsepower. One horsepower equals 746 watts. Therefore, a 10-horsepower motor also can be said to produce 7,460 watts of power. The input watts to this motor, however, will be higher because not all the electric power can be converted to mechanical power. Some of that input power is wasted in the form of heat. The relationship of the input power and output power represents the motor’s efficiency.
An electric motor does its work by turning a shaft. Whenever work is accomplished by rotating something, it is referred to as rotational power or “torque.” A common measurement of torque is the pound/foot. When a force of one pound acts on a radius of one foot, the result is one pound/foot of torque. For example, if a motor drives a pulley with a two-foot radius, and the belt on the pulley has a force on it of eight pounds, the torque supplied by the motor is 8 x 2 or 16 pound/feet of torque.
Having the precise, scientific definitions of terms not only enhances your understanding, it also helps you to better see the relationship of concepts such as efficiency and torque when you look at electric motors and their applications in the field.
Every service technician should have at least one multi-speed motor in his or her truck to help in making acceptable substitutions in the field.
Multi-speed motors come in two basic varieties. The first variety has an extra set of windings called a booster winding that behaves like a transformer. The second variety comes with two distinct separate sets of windings.
So how do these motors work? Remember from last issue how you determine motor speed by the number of poles (poles divided by the constant 7,200 gives you revolutions per minute). When the motor is under load, however, the rotating part of the motor slows down or “slips.”
If the load is constant, you can increase the slip by weakening the strength of the spinning magnetic field. One way is to decrease the voltage to the magnet wire that makes up the poles.
You can decrease the voltage externally by using a speed control or internally through the use of the booster winding in a multi-speed motor. In other words, the booster winding acts like a transformer, changing incoming line voltage to a lower voltage at the windings.
The booster winding may come with taps that allow you to apply different voltages to the poles, creating different speeds in the motor. Remember that “speed” taps just affect the strength of the spinning field—not the actual speed—meaning you can only affect motor speed with a load on the shaft. This is because slip occurs when the load works against the weakened magnetic field. Consequently, if you bench test a multi-speed tapped motor using a tachometer or strobe, you will detect little variation between speed taps since there’s no load.
Since the booster winding method of creating multiple speeds involves using a motor with just a single set of pole windings, you’ll find that the horsepower is always lower as the speed (voltage) is reduced. Consequently, this design is generally unsuitable for loads other than fans.
The second type of multi-speed motor with two completely separate sets of windings allows you to use one or the other speed at a given time. Having two pole sets wound independently offers you more flexibility to produce constant horsepower in mechanical applications since you are energizing just one set of poles at a time.
We also refer to a multi-speed motor “weakened” by speed taps as a multi-horsepower motor. For example, if you have a 1/3 horsepower three-speed motor, it generally would deliver 1/3 horsepower when connected to its high-speed tap, ¼ horsepower at its middle-speed tap, and 1/6 horsepower on low speed.
Knowing this, you can begin to appreciate the versatility of multi-speed motors in the field. You could use the above multi-speed motor to replace single-speed 1/3 horsepower, ¼ horsepower, or 1/6 horsepower motors with the same number of poles. To achieve the correct results, simply select the correct tap and carefully insulate the two unused taps. The result would be a motor that produces the same performance, similar fan noise characteristics, and the same static pressure as the original single-speed model.
Multi-speed motors give the service technician another versatile tool in the field. That's why is always good to have some in stock for emergency substitutions.
One way to determine that you are making good replacements in the field is to understand the concept of motor speed so that you can match the speed of one AC induction motor to another. At the same time, you also need to become familiar with the concept of poles, since poles represent one key to a successful replacement.
Let’s begin the discussion by describing what causes an AC induction motor to run at a particular speed. Every AC induction motor has poles, just like a magnet. Unlike a simple magnet, these poles are formed by bundles of magnet wire (called windings) wound together in slots of the stator core. In most cases, you can look inside the motor and count the number of poles in the winding: they are distinct bundles of wire, evenly spaced around the stator core.
The number of poles, combined with the alternating current line frequency (HZ), are all that determine the no-load revolutions per minute (RPM) of the motor. So all four-pole motors will run at the same speed under no-load conditions, all six-pole motors will run at the same speed, and so on.
The mathematical formula to remember in helping make these calculation is the number of cycles (HZ) times 60 (for seconds in a minute) times two (for the positive and negative pulses in the cycle) divided by the number of poles.
Therefore, for a 60 hertz (cycles) system, the formula would be:
60 x 60 x 2 = 7,200 no-load RPM divided by the number of poles.
For a 50 hertz system, the formula would be:
50 x 60 x 2 = 6000 no-load RPM divided by the number of poles.
Using this formula, you can see that a four-pole motor operating on the bench under no-load conditions runs at 1,800 RPM (7,200 divided by four poles). Note that when an AC motor is loaded, the spinning magnetic field in the stator does not change speed. Instead, the rotor or moving part of the motor is restrained by the load from “catching up” to the field speed. The difference between the field speed of 1,800 RPM in this example and the rotor speed of approximately 1,725 RPM is called the “slip.” The slip varies with the load over a narrow operating range for each motor design.
So, going back to our spinning four-pole motor, it operates at 1,800 RPM under no-load conditions and approximately 1,725 RPM under load. Motors of this speed are commonly found in belted applications such as blowers, fans, air-handling equipment, compressors, and some conveyors.
A two pole motor operates at 3,600 RPM (7,200 divided by two) unloaded and approximately 3,450 RPM under load. Two-pole motors often are found in pump applications, such as sump pumps, swimming pool pumps, or water recirculating equipment. One thing to keep in mind in the field is that the higher the RPM, the “noisier” a motor may sound to the untrained ear. It is beneficial to become aware of the different speed-related sounds motors make.
Six-pole motors run at 1,200 RPM unloaded (7,200 divided by six) and between 1,050 and 1,175 RPM loaded. They are often used for air-handling equipment, direct-drive applications, window fans, furnace blowers, room air conditioners, heat pumps, and other equipment where the relatively slower motor speed makes for quieter operation. All can come in either totally open, totally enclosed, or combination models, adding to their versatility.
To satisfy consumers’ desires for quieter motors, manufacturers have developed eight-pole motors. These operate at 900 RPM (unloaded) and approximately 800 RPM under load. They are being used in applications where customers expect quieter operation, such as room air conditioners and outdoor heat pump applications.
Less-common pole configurations include 12-pole motors (600 RPM) that are used in applications requiring slow speeds, such as washing machines, and 16-pole motors (450 RPM unloaded), often found in ceiling fans.
When making replacements, there is one key thing to remember: always select a replacement motor with the same number of poles as the original. If you change from, say, a four-pole motor to a six-pole model, the RPM will be different, affecting the motor’s performance. Understand that nameplate speed is an approximation of the rotor speed under load. You can tolerate some variation here, since motors are designed to accommodate a range of loaded speeds. If the other characteristics match (nameplate amps, etc.), and the pole count in the same, you have a suitable replacement.
Learning to understand the relationship between motor speed and poles will help you become a more knowledgeable, effective service technician in the field.
The great copper windings versus aluminum windings debate goes on. It’s a topic that I first wrote about in 1988. It remains a topic of discussion today as engineers in a variety of industries question whether the quality and performance of aluminum windings can possibly compare with copper.
What’s interesting to me is one source of the debate. Some of us remember the 1960s, when aluminum house wiring was the subject of much attention because of the apparent fire hazards it created. It turned out that the cause of the house fires was not the wire itself, but rather connection problems. The junctions would become so hot that the heat would transfer to the wire itself, eventually deteriorating the wire insulation.
Consequently, all forms of aluminum wire received a bad rap. This is unjustified: aluminum can be a perfectly good material for motor windings in many applications. Aluminum is a good conductor and, when applied properly, performs quite well. There are, however, two factors to keep in mind.
First, aluminum’s conductivity is lower than copper. To compensate, aluminum magnet wire must have larger cross-sections than the equivalent copper wire to offer the same conductance. This means windings wound with aluminum wire will likely have greater volume compared with an equivalent copper wire motor.
The second consideration is properly connecting the ends of the aluminum magnet wire. The reason for this goes back to your high school chemistry course. You may remember that aluminum oxidizes much faster than other metals—in fact, if exposed to air, powdered aluminum will completely oxidize in a few days, forming a fine white powder. But, when exposed to air, fabricated aluminum (sheets, wire, etc.) tends to form a hard insulating oxide layer that stops the oxidizing process.
To make a proper connection that ensures conductivity, the oxide layer of the aluminum magnet wire must be completely pierced and yet pierced in such a way to prevent air from coming in any further contact with the aluminum. Motor manufacturers have developed high-pressure, piercing crimp connectors to do the job. These improved connection methods have helped make motors with aluminum windings every bit as reliable as motors with copper windings.
It must be pointed out that motor efficiency is a much trickier issue in the great copper versus aluminum debate. It is possible to match the power performance of a motor wound with aluminum to a motor wound with copper wire. But since aluminum requires more turns and/or a larger diameter wire, this may not always be economically feasible in some applications. In situations where efficiency and volume are not issues (such as where the motor only has to work occasionally or for very short periods of time), aluminum magnet wires make an acceptable motor.
The bottom line is that, in terms of motor quality, reliability, and life span, aluminum windings can be every bit as good as copper-wound motors. Comparisons are fair as long as you keep efficiency issues apart when looking at copper versus aluminum.
One of the most frequent challenges you'll face as a service technician is determining if a replacement motor that is not an exact duplicate of the original is suitable for an application. Often, you can use the motor's nameplate to help you in making the selection, but a nameplate doesn't always provide you with everything you need to know.
Take nameplate amps, for example. It is common practice to determine the power input of two motors by comparing nameplate amps of the original motor with those of the replacement. In other words, if the replacement motor's rated amps are at least as high as the original, you are reasonably safe in using it in the application. This practice works best when you're working with motor types whose efficiency varies little from one design to another. A good example would be three-phase motors.
But, the nameplate may not tell the entire story, and as a superior service technician, you need to be aware of the chapters that are missing. Motor nameplates typically do not include input watts as a power measurement, nor do they identify the motor's efficiency. In certain applications, these criteria may be critical factors in deciding if a replacement motor can do the job.
One case where the nameplate-amp comparison often breaks down is when you're working with single-phase motors. These motors which include permanent split capacitor motors, shaded pole motors, as well as split phase and capacitor-start versions, may vary widely in terms of efficiency within a single motor design. You have no way of knowing that, however, simply by looking at the nameplate.
So what happens to the nameplate amp comparison in this instance. Since nameplate amps reflect total current consumption of the motor (including current converted to output power and current lost to heat), higher nameplate amps on a single-phase motor can just as likely indicate poorer conversion efficiency as increased power output.
Faced with this challenge, the service technician dealing with single-phase motors needs to look beyond amps and compare the horsepower of the replacement motor to the original. If nameplate amps and horsepower compare favorably, you likely will have a suitable replacement.
To be certain, however, it's important that you test the replacement motor in the application itself. This is especially true when you remember that there is not a single industry standard for motor efficiency. Best practice is to measure the amps through the motor terminal or power leads of the replacement and compare that reading with the amp rating shown on the replacement motor's nameplate. When you perform this test, make absolutely sure that the motor is in its normal operating state with all belts, blowers, baffles, and enclosures in place. If you do not duplicate the motor's normal working environment, you will not get a true reading of the motor's total current consumption. You very possibly will end up with an overloaded condition and eventually an unhappy customer.
Motor manufacturers are becoming increasingly sensitive to motor efficiency and are working to design motors that deliver higher power while consuming the same-or fewer-watts. All of this product development means that you, as a service technician, will probably see motors in the field of similar power output but with significantly different ampacity. By being aware of these differences and understanding that you won't find this important information just by looking at the nameplate, you will be able to provide correct replacements-and better service-to your customers.
Dealing with a user’s complaint of a noisy motor can be a frustrating experience. After all, the perception of noise is extremely subjective (just ask the parents of teenagers). Not only does the range of human hearing differ considerably among people, but it also varies by specific frequency.
Another frustration is that noise is not an easy condition to measure. Part of that difficulty goes back to the subjective perception of noise. A more technical reason is that noise is the perceived result of a complex interaction of sound waves. Measuring noise can be like measuring chaos.
All noise has a mechanical origin, which is to say it is the result of waves of pressure transmitted through air as the result of the mechanical movement of some object. In a motor, the sources of mechanical noise are numerous:
So-called "electrical noise" is the result of mechanical pressure produced when the parts of a motor that can be magnetized are attracted and repelled from one another. This happens when the magnetic field that drives the motor alternates.
Another source of noise is inherent in a motor’s relationship to both the mechanical and electrical effects of spinning parts moving through the air gap.
Additionally, since the motor has a spinning internal part (namely, the rotor) imbalances are transferred to the frame of the motor as noise.
Noise inherent in a motor generally cannot be "cured" by the motor installer. But short of specifying a low-noise motor for the application, there are several things the savvy installer can do to minimize the effects of inherent motor noise.
The first course of action is fairly straightforward—isolation. Inherent noise is very efficiently transferred to the motor’s frame through its mechanical parts. Isolation breaks that efficient path to the motor-driven device. You can isolate the motor in several ways, such as using rubber motor pads, soft couplings and/or resilient cradles.
There’s a second course of action that you need to consider when the first step fails to produce the desired result. This is based on the concept of harmonics. Harmonics are a set of specific frequencies that noisy mechanical equipment tends to favor as vibration frequencies. Unfortunately, harmonic frequencies are not easy to calculate, as they are the result of complex interactions of speed, mass, and separation (or the distance between moving assemblies).
Though difficult to calculate, you can deal with the effect of harmonic frequencies effectively by changing the speed, mass, or separation distance of the motor-driven apparatus. For example, in a belt-driven application, pulley diameters could be changed to vary the speed of the driven load. Slight increases or decreases in speed from the unit’s designed point could move the motor out of the harmonic frequency. Sometimes changing the density (hardness) of isolation devices, such as the rubber pads or resilient rings is enough to move a mechanical assembly off a harmonic frequency. Where space and application permit, changing the length of the train of driven equipment can also move that equipment off a harmonic frequency.
Although subjective, motor noise is often the cause for call-backs by unhappy customers. Many times, you can solve the problem easily with isolation. But when the noise persists, a working knowledge of harmonic frequencies may mean the difference between a happy customer and an unhappy one.