THREE PHASE SYNCHRONOUS MOTOR

A synchronous motor is used to correct system voltage, If you are distributing electricity over a long distance, the transmission lines have a capacitive affect on the system requiring a synchronous motor to correct the power factor, and therefore, the voltage.

Regards JD.

Synchronous motors are preferred in certain situations.

- As jdretired said, they can be used to correct power factor by over-exciting the rotor field. This is a common practice in large industrial facilities, and is especially true when the supply utility levies a rate penalty on customers with low power factor. Many utilities in the US have arrangements with larger customers such that the synchronous motors even supply some correction to the grid, so the utility does not have to install capacitors on their distribution feeder.

- If you need very precise speed control, a synchronous motor is desirable because the rotor is magnetically locked to the magnetic field of the stator. Induction motors require a speed differential (called "slip") to exist between the stator field and the rotor. The slip varies with load on the motor shaft, so precise speed control is not possible. A synchronous motor will maintain synchronous speed regardless of load. Add a variable frequency drive, and you can easily control motor speed to 0.01 RPM or smaller increments.

- A synchronous motor can also be used as a generator. Several navies have motor-generator sets consisting of a cumulatively compounded DC motor on one end of a common shaft, and a synchronous AC motor on the other end. During normal operation, the AC end is a motor, while the DC end becomes a differentially compounded DC generator to charge emergency batteries. If the AC power were to fail, the MG set would slow down, causing the DC generator output voltage to drop. Once the DC output drops to less than the battery voltage, the DC end becomes a motor. The AC end becomes a generator, supplying emergency power to vital equipment. The transition occurs automatically with no operator or control equipment intervention.

Synchronous motors also have some drawbacks:

- A pure synchronous motor cannot start by itself. It needs either a VFD starter or squirrel cage windings to bring it up to near synchronous speed.

- Synchronous motors are more expensive than induction motors, and require significantly more maintenance.

You choose this where synchronized operation is desired and where loads are difficult to start. Like the last post, it improves power factor. You would get the same beneifits from a DC excited synchronized motor without the added cost.

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