Synchronous Motor - Starting Methods

Starting Methods

Small synchronous motors are able to start without assistance due to the low mass of the rotor, and the low starting current needed to rapidly accelerate the rotor to full speed from a dead stop. Synchronous motors are commonly used in line-powered electric mechanical clocks that use the powerline frequency to run the gear mechanism at the correct speed.

Three phase and two phase (uncommon) synchronous motors are naturally driven to rotate following the direction of the stepped-overlap phases. Single-phase synchronous motors such as in electric wall clocks can freely rotate in either direction and keep in phase with the rising and falling field strength. Wall clock motors typically use an anti-reversing mechanism so that the synchronous motor won't start turning the mechanism backwards.

Above a certain size, synchronous motors are not self-starting motors. This property is due to the inertia of the rotor. When the power supply is switched on, the armature winding and field windings are excited. Instantaneously, the armature winding creates a rotating magnetic field, which revolves at the designated motor speed. The rotor, due to inertia, will not follow the revolving magnetic field, but instead acts as a synchronous generator (alternator) and creates a very large counter-current that opposes the rotation. More supply current is needed to overcome this resistance to motion, which results in a power draw so large that it may trip the overcurrent protection. For very large industrial synchronous motor/generators with large power supply capabilities, it may be possible to accelerate the rotor to full speed from a dead stop, but both the motor and the driven equipment may be damaged by the near-instantaneous acceleration.

When synchronous generators are brought online and synchronized, it is important they be closely matched before switched to full current, as the generator can be capable of tearing itself off the foundation or damaging its internal power windings, acting as a motor to accelerate to the correct synchronization.

In practice, the rotor should be rotated by some other means near to the motor's synchronous speed to overcome the inertia. Once the rotor nears the synchronous speed, the field winding is excited, and the motor pulls into synchronization.

The following techniques are employed to start a synchronous motor:

• A separate motor (called pony motor) is used to drive the rotor before it locks in into synchronization.
• The field winding is shunted or induction motor like arrangements are made so that the synchronous motor starts as an induction motor and locks in to synchronization once it reaches speeds near its synchronous speed.
• Reducing the input electrical frequency to get the motor starting slowly, variable-frequency drives can be used here which have rectifier-inverter circuits or cycloconverter circuits.