Glossary · Motion control and drives
Induction motor
Also known as: Asynchronous motor, Squirrel-cage motor
German: Asynchronmotor
In electric drives, an induction motor is an AC motor in which the rotor current is induced by the rotating stator field, so the rotor turns slightly slower than the field; this speed difference is called slip.
- Motion control
In one sentence
An induction motor is an AC motor whose rotor current is induced by the stator field; the rotor lags the field by the slip.
Example
Most conveyor, fan and pump drives in a plant use squirrel-cage induction motors, either direct-on-line or fed by a frequency inverter.
How it applies
- Engineering: Induction motors are robust and inexpensive. Direct-on-line starting draws a high inrush current; soft starters or frequency inverters reduce it and allow speed control.
- Operation: Speed depends on supply frequency and load (slip). Efficiency classes (IE codes) according to IEC 60034-30-1 apply to many motors.
- Maintenance: Bearings, cooling fins and fans, and insulation resistance are typical check points.
- Documentation: Make sure nameplate data, connection (star or delta) and the permitted supply mode are consistent in the wiring diagram, the parts list and the manual. Warn that an induction motor on a frequency inverter may need thermal protection by a sensor, since cooling drops at low speed.
Induction motor vs. synchronous motor
A Synchronous motor turns exactly with the rotating field, usually because of permanent magnets or a DC-excited rotor. An induction motor always has slip under load.