Electric Motor Types: The Definitive Guide
Electric motors can be found in many different applications, from common household items to various types of transportation and even advanced aerospace applications. Here, we share a guide to provide you with a better understanding of the options available.
Electric Motors vs. Generators
Both electric motors and generators are electromagnetic devices with an armature winding or rotor, which rotates within a field winding or stator; however, they have opposite functions. Generators convert mechanical energy into electrical energy while motors convert electrical energy into mechanical energy.
Two Types of Electric Motors
The field winding in electric motors provides an electric current to produce a fixed magnetic field, which the armature winding uses to produce turning torque on the motor shaft. Distinctions between different types of electric motors relates to their unique operation, voltage, and application requirements. There are at least a dozen different types of electric motors, but there are two main classifications: alternating current (AC) or direct current (DC). How the windings within AC and DC motors interact with each other to produce mechanical force creates further distinctions within each of these classifications.
Brushed motors comprise of four main components:
- Rotor or Armature
There are four main brushed motor types, including:
- Series Motors. The stator is in series or identical with the rotor, causing their field currents to be identical. Characteristics: used in cranes and winches, great low speed torque, limited high speed torque.
- Shunt Motors. The field coil is parallel (shunt) with the rotor, making the motor current equal to the sum of the two currents. Characteristics: used in industrial and automotive applications, excellent speed control, high/consistent torque at low speeds.
- Cumulative Compound Motors. This type combines aspects of both series and shut types, making the motor current equal to the sum of both the series field and shunt field currents. Characteristics: used in industrial and automotive applications, has combined benefits of both series and shunt motors.
- PMDC Motors (Permanent Magnet). The most common type of brushed electric motor, PMDC motors use permanent magnets to produce the stator field. Characteristics: used in commercial production of toys and appliances, cheaper to manufacture, good low end torque, limited high end torque.
Motors in the brushless category do not have a commutator and brushes. Instead, the rotor is a permanent magnet and the coils are on the stator. Rather than controlling the magnetic fields on the rotor, brushless motors control the magnetic fields from the stator by adjusting magnitude and direction of the current in the coils. One of the main advantages of brushless motors is its efficiency, which allows for greater control and production of torque in a more compact assembly.
Motors within the AC motor classification are either synchronous or asynchronous, primarily distinguished by the speed of the rotor relative to the speed of the stator. The speed of the rotor relative to the stator is equal in a synchronous motor, but the rotor speed is less than its synchronous speed in an asynchronous motor. In addition, synchronous motors have a zero slip and require an additional power source, while asynchronous or induction motors do have slip and do not require a secondary power source.
A synchronous motor is a doubly excited machine, meaning it includes two electrical inputs. In a common three-phase synchronous motor, one input, usually three-phase AC, supplies the stator winding to produce three-phase, rotating magnetic flux. The rotor supply is usually DC, which excites or starts the rotor. Once the rotor field locks with the stator field, the motor becomes synchronous.
In contrast to synchronous motors, induction allows asynchronous to start by supplying power to the stator without providing a supply to the rotor. Induction motors have either a wound or a squirrel-cage design. Some examples of asynchronous induction motors include:
- Capacitor Start Induction Run Motors. This is a single-phase wound motor with a cage rotor and two stator windings, started by a capacitor. Their use includes compressors and pumps in refrigerators and AC systems with frequent starting and stopping.
- Squirrel Cage Induction Motors. A three-phase supply creates a magnetic field in the stator winding in this motor, which includes a squirrel-cage rotor made of highly conductive steel laminations. They are low cost, low maintenance, and high efficiency motors used in centrifugal pumps, industrial drives, large blowers and fans, machine tools, lathes, and other turning equipment.
- Double Squirrel Cage Motors. These motors overcome poor startup torque issues in squirrel cage motors. Their design balances out the reactance to resistance ratio between an outer and inner cage, increasing startup torque while maintaining overall efficiency.
Electric Motor Identification
Selecting the motor best suited to a specific application depends upon meeting the needs of four characteristics:
- Horsepower and Speed
- Motor Frame
- Voltage Requirements
- Enclosures and Mounting Positions
A metal nameplate attached to the motor contains critical information related to these characteristics with the exception of enclosure information.
Electric Motor Horsepower & Speed Rating
Both the horsepower rating and rotational speed rating (RPM) should match the load requirements for the installed application. Motors come in different horsepower categories, including: fractional motors (1/20th HP to 1 HP), integral horsepower motors (1 HP to 400 HP), and large motors (100 HP to 50,000 HP). RPM ratings include 3600 RPM (2 pole), 1800 RPM (4 pole), and 1200 RPM (6 pole).
Electric Motor Frame
Motor frame size does not indicate its performance values, especially its horsepower rating. National Electrical Manufacturers Association (NEMA) designed frame numbers to correspond to mounting sizes with their digits relating to their “D” dimension or the distance from center of shaft to center bottom of mount. In general, two-digit labels are for fractional motors, but larger horsepower motors can be built in them.
Voltage, frequency, and phase are all a part of voltage requirements. In most North American and European cases, three-phase motors include dual voltage displays like 230/460. The standard operating frequency for most electric motors is 60 Hz, though 50 Hz motors are common in Europe. This variation in hertz indicates that the motor will operate at 5/6 of its normal RPM speed. Phase is the final bit of information included with a motor’s voltage requirements, indicating the type of supply required, such as three-phase, single-phase and DC.
Enclosures and Mounting Positions
Enclosure information depends on the motor’s installation environment. There are two main categories of enclosures—open motors and enclosed motors.
Applications for open motors include indoor locations that are relatively clean and dry, which is important since open motor enclosures allow air circulation through the windings.
These types do not permit free air exchange between the exterior and interior of the motor. Variations in enclosure air-tightness and cooling features further distinguish enclosed motor types, including:
- Totally Enclosed Fan Cooled (TEFC)
- Totally Enclosed Non-Ventilated (TENV)
- Totally Enclosed Air Over (TEAO)
- Totally Enclosed Wash Down (TEWD)
- Explosion-Proof Enclosures (EXPL)
- Hazardous Location (HAZ)
Find the Electric Motor Best for Your Application
Thomson Lamination Company is a leading manufacturer of stamped motor lamination components with the capacity to produce high volume runs of rotor and stator laminations from high-conductivity metals.