Glossary · Motion control and drives
Synchronous motor
Also known as: AC synchronous motor
German: Synchronmotor
In electric drives, a synchronous motor is an AC motor whose rotor turns at exactly the speed of the rotating stator field, because the rotor field is produced by permanent magnets, DC excitation or magnetic reluctance.
- Motion control
In one sentence
A synchronous motor is an AC motor whose rotor turns exactly with the stator's rotating field, without slip.
Example
Large compressors in a refinery use DC-excited synchronous motors, while the plant's servo axes use permanent-magnet synchronous motors.
How it applies
- Engineering: Synchronous motors offer high efficiency and exact speed. Most are operated with a frequency inverter or servo drive; line-start variants exist for some applications.
- Commissioning: The drive needs rotor position or a suitable sensorless method; motor data identification is important.
- Operation: If the load exceeds the pull-out torque, the motor falls out of step; drives detect and trip on this.
- Documentation: Specify the motor type in drawings and parameter lists precisely (PMSM, reluctance, DC-excited), because drive settings and hazards differ, for example voltage generated at the terminals when a PM rotor is turned.
Synchronous vs. induction motor
The synchronous motor has no slip and runs at field speed. The Induction motor needs slip to induce rotor currents and runs slightly slower than the field.