section_path: Types
section_name: Types
paragraph_text: Types
  
  Electric motors operate on one of three physical principles: magnetism, electrostatics and piezoelectricity.
  
  In magnetic motors, magnetic fields are formed in both the rotor and the stator. The product between these two fields gives rise to a force and thus a torque on the motor shaft. One or both of these fields changes as the rotor turns. This is done by switching the poles on and off at the right time, or varying the strength of the pole.
  
  Motors operate on either DC or AC current (or either).
  
  AC motors can be either asynchronous or synchronous. Synchronous motors require the rotor to turn at the same speed as the stator's rotating field. Asynchronous rotors relax this constraint.
  
  A fractional-horsepower motor either has a rating below about 1 horsepower (0.746 kW), or is manufactured with a frame size smaller than a standard 1 HP motor. Many household and industrial motors are in the fractional-horsepower class.
  
  Notes:
  
  1. Rotation is independent of the frequency of the AC voltage.
  
  2. Rotation is equal to synchronous speed (motor-stator-field speed).
  
  3. In SCIM, fixed-speed operation rotation is equal to synchronous speed, less slip speed.
  
  4. In non-slip energy-recovery systems, WRIM is usually used for motor-starting but can be used to vary load speed.
  
  5. Variable-speed operation.
  
  6. Whereas induction- and synchronous-motor drives are typically with either six-step or sinusoidal-waveform output, BLDC-motor drives are usually with trapezoidal-current waveform; the behavior of both sinusoidal and trapezoidal PM machines is, however, identical in terms of their fundamental aspects.
  
  7. In variable-speed operation, WRIM is used in slip-energy recovery and double-fed induction-machine applications.
  
  8. A cage winding is a short-circuited squirrel-cage rotor, a wound winding is connected externally through slip rings.
  
  9. Mostly single-phase with some three-phase.
  
  Abbreviations: