Electric Motor Overview
What is an Electric Motor?The electric motor in its simplest terms is a converter of electrical energy to useful mechanical energy. The electric motor has played a leading role in the high productivity of modern industry, and it is therefore directly responsible for the high standard of living being enjoyed throughout the industrialized world.
When Were Electric Motors Created?The beginnings of the electric motor are shrouded in mystery, but this much seems clear: The basic principles of electromagnetic induction were discovered in the early 1800’s by Oersted, Gauss and Faraday, and this combination of Scandinavian, German and English thought gave us the fundamentals for the electric motor. In the late 1800’s the actual invention of the alternating current motor was made by Nikola Tesla, a Serb who had migrated to the United States. One measure of Tesla’s genius is that he was granted more than 900 patents in the electrical field. Before Tesla’s time, direct current motors had been produced in small quantities, but it was his development of the versatile and rugged alternating current motor that opened a new age of automation and industrial productivity.
How Does an Electric Motor Work?An electric motor’s principle of operation is based on the fact that a current-carrying conductor, when placed in a magnetic field, will have a force exerted on the conductor proportional to the current flowing in the conductor and to the strength of the magnetic field. In alternating current motors, the windings placed in the laminated stator core produce the magnetic field. The aluminum bars in the laminated rotor core are the current-carrying conductors upon which the force acts. The resultant action is the rotary motion of the rotor and shaft, which can then be coupled to various devices to be driven and produce the output.
Alternating current (AC) induction motors are divided into two electrical categories based on their power source – single phase and three phase.
AC Single Phase Motors
Types of single-phase motors are distinguished mostly by the way they are started and the torque they develop.
Shaded Pole motors have low starting torque, low cost, low efficiency, and no capacitors. There is no start switch. These motors are used on small direct-drive fans and blowers found in homes. Shaded pole motors should not be used to replace other types of single-phase motors.
PSC (Permanent Split Capacitor) motors have applications similar to shaded pole, except much higher efficiency, lower current (50% - 60% less), and higher horsepower capability. PSC motors have a run capacitor in the circuit at all times. They can be used to replace shaded pole motors for more efficient operation and can be used for fan-on-shaft fan applications, but not for belted fans due to the low starting torque.
Split Phase motors have low-to-moderate starting torque (100% - 125% of full load), high starting current, no capacitor, and a starting switch to drop out the start winding when the motor reaches approximately 75% of its operating speed. They are used on easy-to-start belt-drive fans and blowers, as well as light-start pump applications.
Capacitor Start motors are designed in both moderate and high starting torque types with both having moderate starting current and high breakdown torques.
Moderate-torque motors are used on applications in which starting requires torques of 175% or less of full load and on light loads such as fans, blowers, and light-start pumps. High-torque motors have starting torques in excess of 300% of full load and are used on compressors, industrial, commercial and farm equipment. Capacitor start motors use a start capacitor and a start switch, which takes the capacitor and start winding out of the circuit when the motor reaches approximately 75% of its operating speed.
Capacitor Start/Capacitor Run motors have applications and performance similar to capacitor start except for the addition of a run capacitor (which stays in circuit) for higher efficiency and reduced running amperage. Generally, start/capacitor run motors are used for 3 HP and larger single-phase applications.
AC Three Phase Motors
Three Phase induction motors have a high starting torque, power factor, high efficiency, and low current. They do not use a switch, capacitor, relays, etc., and are suitable for larger commercial and industrial applications.
Three phase induction motors are specified by their electrical design type: A, B, C, D or E, as defined by the National Electrical Manufacturers Association (NEMA). These designs are suited to particular classes of applications based upon the load requirements typical of each class.
Because of their widespread use throughout the industry and because their characteristics lend themselves to high efficiencies, many types of general purpose three-phase motors are required to meet mandated efficiency levels under the U.S. Energy Policy Act. Included in the mandates are NEMA Design B, T frame, foot-mounted motors from 1-200 HP.
Another commonly used motor in commercial and industrial applications is the direct current motor. It is often used in applications where adjustable speed control is required. Permanent magnet DC designs are generally used for motors that produce less than 5 HP. Larger horsepower applications use shunt-wound direct current motors.
Both designs have linear speed/torque characteristics over the entire speed range. SCR rated motors – those designed for use with common solid-state speed controls – feature high starting torque for heavy load applications and reversing capabilities, and complementary active material to compensate for the additional heating caused by the rectified AC input. Designs are also available for use on generated low-voltage DC power or remote applications requiring battery power.
A gearmotor is made up of an electric motor, either DC or AC, combined with a geared speed reducer. Spur, helical or worm gears may be used in single or multiple stages. The configuration may be either that of a parallel shaft, emerging from the front of the motor, or a right-angle shaft. Gearmotors are often rated in input horsepower; however, output torque, commonly measured in inch-pounds, and output speed are the critical values.
Gearmotors may be either integral, meaning the gear reducer and motor share a common shaft, or they may be created from a separate gear reducer and motor, coupled together. Integral gearmotors are common in sub-fractional horsepower sizes; separate reducers and motors are more often the case in fractional and integral horsepowers.
Brake MotorsA brake motor is a pre-connected package of industrial-duty motor and fail-safe, stop-and-hold spring-set brake. In case of power failure, the brake sets, holding the load in position. Brake motors are commonly used on hoists or other lifting devices. Brake features can also be added to standard motors through conversion kits that attach to the shaft end of either fan-cooled or open motor.
Permanent Magnet PMAC Motors
The PMAC (Permanent Magnet AC) motor is traditionally of a more complex construction than the standard induction motor. With the new motor type, the design has been simplified by using powerful permanent magnets to create a constant flux in the air gap, thereby eliminating the need for the rotor windings and brushes normally used for excitation in synchronous motors. This results in the accurate performance of a synchronous motor, combined with the robust design of a standard induction motor. The motor is energized directly on the stator by the variable speed drive.
Standard induction motors are not particularly well suited for low-speed operation as their efficiency drops with the reduction in speed. They may also be unable to deliver sufficiently smooth torque across the lower speed range. This is normally overcome by using a gearbox. The new solution provides a high torque drive coupled directly to the load. By eliminating the gearbox, the user saves space and installation costs, as he only needs to prepare the foundations for one piece of machinery. This also gives more freedom in the layout design.
The PMAC motor can deliver more power from a smaller unit. For instance, powering the in-drives of a paper machine directly at 220 to 600 r/min with a conventional induction motor would require a motor frame substantially larger than that of a 1500 r/min motor. Using permanent magnet motors also means higher overall efficiency and less maintenance.
Enclosures and Environments
- Open Drip Proof (ODP) motors have venting in the end frame and/or main frame, situated to prevent drops of liquid from falling into the motor within a 15° angle from vertical. These motors are designed for use in areas that are reasonably dry, clean, well-ventilated, and usually indoors. If installed outdoors, ODP motors should be protected with a cover that does not restrict air flow.
- Totally Enclosed Non-Ventilated (TENV) motors have no vent openings. They are tightly enclosed to prevent the free exchange of air, but are not air tight. TENV motors have no cooling fan and rely on convection for cooling. They are suitable for use where exposed to dirt or dampness, but not for hazardous locations or applications with frequent washdown.
- Totally Enclosed Fan Cooled (TEFC) motors are the same as TENV except they have an external fan as an integral part of the motor to provide cooling by blowing air over the outside frame.
- Totally Enclosed Air Over (TEAO) motors are specifically designed to be used within the airflow of the fan or blower they are driving. This provides an important part of the motor’s cooling.
- Totally Enclosed Hostile and Severe Environment motors are designed for use in extremely moist or chemical environments, but not for hazardous locations.
- Explosion Proof motors meet Underwriters Laboratories or CSA standards for use in the hazardous (explosive) locations shown by the UL/CSA label on the motor. The motor user must specify the explosion proof motor required. Locations are considered hazardous because the atmosphere contains or may contain gas, vapor, or dust in explosive quantities. The National Electrical Code (NEC) divides these locations into classes and groups according to the type of explosive agent. The following list has some of the agents in each classification. For a complete list, see Article 500 of the National Electrical Code.
- Class I (Gases, Vapors)
Group A Acetylene
Group B Butadiene, ethylene oxide, hydrogen, propylene oxide
Group C Acetaldehyde, cyclopropane, diethlether, ethylene, isoprene
Group D Acetone, acrylonitrile, ammonia, benzene, butane, ethylene dichloride, gasoline, hexane, methane, methanol, naphtha, propane, propylene, styrene, toluene, vinyl acetate, vinyl chloride, xylene
- Class II (Combustible Dusts)
Group E Aluminum, magnesium and other metal dusts with similar characteristics
Group F Carbon black, coke or coal dust
Group G Flour, starch or grain dust
The motor ambient temperature is not to exceed +40°C or -25°C unless the motor nameplate specifically permits another value. LEESON explosion proof motors are approved for all classes noted except Class I, Groups A & B.
- Rigid base is bolted, welded, or cast on main frame and allows motor to be rigidly mounted on equipment.
- Resilient base has isolation or resilient rings between motor mounting hubs and base to absorb vibrations and noise. A conductor is imbedded in the ring to complete the circuit for grounding purposes.
- NEMA C face mount is a machined face with a pilot on the shaft end which allows direct mounting with the pump or other direct coupled equipment. Bolts pass through mounted part to threaded hole in the motor face.
- NEMA D flange mount is a machined flange with rabbet for mountings. Bolts pass through motor flange to a threaded hole in the mounted part. NEMA C face motors are by far the most popular and most readily available. NEMA D flange kits are stocked by some manufacturers.
- Type M or N mount has special flange for direct attachment to fuel atomizing pump on an oil burner. In recent years, this type of mounting has become widely used on auger drives in poultry feeders.
- Extended through-bolt motors have bolts protruding from the front or rear of the motor by which it is mounted. This is usually used on small direct drive fans or blowers.