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A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the alternating current and resulting magnetic field which drives it. Another way of saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor, which must slip in order to produce torque. A synchronous motor is like an induction motor except the rotor is excited by a DC field. Slip rings and brushes are used to conduct current to the rotor. The rotor poles connect to each other and move at the same speed - hence the name synchronous motor. The speed at which synchronous motors rotate is dependent on the frequency of the AC power line, commonly in the United States, at 60 cycles per second (60Hz). They are used in analog electric clocks, timers and other devices where correct time is required.
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Uses
Sometimes a synchronous motor is used, not to drive a load, but to improve the power factor on the local grid it is connected to. It does this by providing reactive power to, or consuming reactive power from the grid. In this case the synchronous motor is called a Synchronous condenser.
Electrical power plants almost always use synchronous generators because it is important to keep the frequency constant at which the generator is connected.
Low power applications include positioning machines, where high precision is required, and robot actuators.
Mains synchronous motors are used for electric clocks.
Advantages
Synchronous motors have the following advantages over non-synchronous motors:
- Speed is independent of the load, provided an adequate field current is applied.
- Accurate control in speed and position using open loop controls, eg. stepper motors.
- They will hold their position when a DC current is applied to both the stator and the rotor windings.
- Their power factor can be adjusted to unity by using a proper field current relative to the load. Also, a "capacitive" power factor, (current phase leads voltage phase), can be obtained by increasing this current slightly, which can help achieve a better power factor correction for the whole installation.
- Their construction allows for increased electrical efficiency when a low speed is required (as in ball mills and similar apparatus).
Examples
- brushless permanent magnet DC motor.
- stepper motor.
- Slow speed AC synchronous motor.
- Switched reluctance motor.
See also
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