In this article we will discuss about the methods used for starting synchronous motors.
Methods for Starting of Synchronous Motors:
The synchronous motor has got no self-starting torque and the motor must first be brought up to synchronous speed by some external means before it is capable of operating with load angle δ.
Various methods of starting of synchronous motors are described as follows:
1. From DC Source:
ADVERTISEMENTS:
If the dc supply and dc compound motor is available, the synchronous motor is coupled and started by means of a dc compound motor. The speed of dc motor is adjusted by the speed regulator. The synchronous motor is then excited and synchronised with ac supply mains. At the moment of synchronising the synchronous motor is switched on with the ac mains, and either the dc motor is disconnected from the dc supply mains or the field of the dc machine is strengthened until it begins to function as a generator. Now the synchronous machine is operating, as a motor, from ac supply mains and dc machine acts as load on it.
The synchronous motor can also be started by the exciter mounted on an overhung synchronous motor bracket and shaft extension. Here again, an available dc source operates the exciter as a motor during the starting period; then, after the synchronous machine is brought up to speed and synchronised, the exciter assumes its normal function.
2. By Means of AC Motor:
A small direct-coupled induction motor, called the pony motor, may be used for starting the synchronous motor unless the motor is required to start against full-load torque. The induction motor frequently has two poles less than the synchronous motor and so is capable of raising the speed of the latter to synchronous speed.
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Before switching the ac supply to the synchronous motor, it must be synchronised with the bus-bars. After normal operation is established, the pony motor is sometimes uncoupled from the synchronous motor. This method is not very satisfactory and not suited to industrial needs. Modern machines are usually of the self-starting type and are arranged to start as induction motors.
3. By Means of Damper Grids in the Pole Faces:
The synchronous motor is made self-starting by providing a special winding on the rotor poles, known as damper winding or squirrel cage winding. The damper winding consists of short- circuited copper bars embedded in the face of the field poles. AC supply given to the stator produces a rotating magnetic field which causes the rotor to rotate, therefore, in the beginning synchronous motor provided with damper winding starts as a squirrel cage induction motor.
The exciter moves along the rotor. When the motor attains about 95% of synchronous speed, the rotor winding is connected to exciter terminals and the rotor is magnetically locked by the rotating field of the stator and the motor runs as a synchronous motor.
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Advantages:
When the motor is overloaded it does not stop, continues running as a squirrel cage induction motor. When the speed falls slightly below the synchronous speed due to overloading an emf is induced in the rotor short-circuited winding, called the damper winding. Due to this emf a torque is produced which the motor keeps running even on no load.
Disadvantages:
Since damper winding resistance is low so at start it takes large current from supply mains.
ADVERTISEMENTS:
To avoid large starting current drawn from the supply mains the synchronous motor is started usually as slip-ring induction motor. The rotor winding is connected in series with triplex rheostat at the time of starting by throw-over switch. Rotor circuit is completed and an emf is induced in the rotor circuit as in the rotor of a slip-ring induction motor.
As the motor attains synchronous speed the throw-over switch is put on the side of the exciter terminals and the dc supply passes through the rotor winding. While running, the rotor acts like rotor of a synchronous motor. Hence the motor runs as a synchronous motor. If the starting and excitation is done automatically the motor is known as auto-synchronous motor.
Precautions for Starting of Synchronous Motors:
In starting a synchronous motor the field winding must be sorted through a suitable resistance so that the induced voltage is distributed throughout the whole winding and no part can be subjected to the high voltage induced in the entire winding.
Another method of eliminating the hazard of breakdown of insulation of the field winding due to induced high voltage in it is by means of dividing the field winding into several sections during the starting period. While emf is still induced in the field coils, the number of coils in series is reduced, so that the magnitude of emf induced in any section is kept within safe limits. Sectionalizing of field winding may be accomplished by a centrifugally operated switch that keeps the field circuit sectionalized until the rotor revolves at near synchronous speed.