Mainly there are three methods to Start Synchronous motor that are mostly used. If you want to reduce the speed of the rotating magnetic field of the stator to a low enough value. The rotor can accelerate and lock in with it during one half-cycle of the rotating magnetic field’s rotation. This is achieved by reducing the frequency of the applied electric power supply. This method is mainly used in the case such as of the inverter-fed synchronous motor operating under variable speed drive applications.
The external prime mover is used to accelerate the rotor of the synchronous motor near to its
synchronous speed and then supply the rotor as well as stator. Take care should be taken to confirm that the direction of rotation of the rotor and the rotating magnetic field of the stator is the same.
This method is mostly followed in the laboratory of the synchronous machine is started as a generator and is then connected to the supply mains by following the synchronization or paralleling procedure. To continue to run of the synchronous machine as a motor the power supply to the prime mover is disconnected.
In most of the large synchronous motors damper windings or amortisseur windings are provided in order to nullify the oscillations of the rotor whenever the synchronous machine is subjected to a periodically varying load.
1. Synchronous Motor Started by Reducing the supply Frequency
If the speed of the rotating magnetic field of the stator in a synchronous motor rotates at a low speed then there will be no problem for the rotor to accelerate and to lock in with the stator’s magnetic field. The speed of the stator magnetic field can be increased to its rated operating speed by gradually increasing the supply frequency f up to its normal 50Hz or 60Hz value.
Where can we get the variable frequency supply?
The power supply systems regulate the frequency to be 50Hz or 60 Hz. However, a variable-frequency voltage source can be obtained from a dedicated generator only in the olden days and such a situation was obviously impractical except for very unusual or special drive applications.
We now have the rectifier- inverter and cyclo converter, which can be used to convert a constant frequency AC supply to a variable frequency AC supply. With the development of such modern solid-state variable-frequency drive packages, it is thus possible to continuously control the frequency of the supply connected to the synchronous motor all the way from a fraction of a hertz up to and even above the normal rated frequency.
When the synchronous motor is operated at a speed slower than the rated speed, its internal generated voltage (usually called the counter EMF) EA = K \phi \omega will be smaller than normal. As such the terminal voltage applied to the motor must be reduced proportionally with the frequency in order to keep the stator current within the rated value. Generally, the voltage in any variable-frequency power supply varies roughly linearly with the output frequency.
2.Synchronous Motor Starting with an External Motor
The second method to starting synchronous motor is to attach an external starting motor (pony motor) to it and bring the synchronous machine to near about its rated speed (but not exactly equal to it, as the synchronization process may fail to indicate the point of closure of
the main switch connecting the synchronous machine to the supply system) with the pony
Then the output of the synchronous machine can be synchronized or paralleled with
its power supply system as a generator, and the pony motor can be detached from the shaft
of the machine or the supply to the pony motor can be disconnected. Once the pony motor is turned OFF, the shaft of the machine slows down, the speed of the rotor magnetic field BR falls behind Bnet, momentarily and the synchronous machine continues to operate as a motor.
As soon as it begins to operates as a motor the synchronous motor can be loaded in the usual manner just like any motor. The starting motor is required to overcome only the mechanical inertia of the synchronous machine without any mechanical load. Since only the motor’s inertia must be overcome, the starting motor can have a much smaller rating than the synchronous motor it is going to start. Generally, most of the large synchronous motors have brushless excitation systems mounted on their shafts. It is then possible to use these exciters as the starting motors.
3.Synchronous Motor Starting by Using damper (Amortisseur) Winding
Most of the large synchronous motors are provided with damper windings, in order to nullify the oscillations of the rotor whenever the synchronous machine is subjected to a periodically varying load. Damper windings are special bars laid into slots cut in the pole face of a synchronous machine and then shorted out on each end by a large shorting ring, similar to the squirrel cage rotor bars.
When the stator of such a synchronous machine is connected to the 3-Phase AC supply, the machine starts as a 3-Phase induction machine due to the presence of the damper bars, just like a squirrel cage induction motor. Just as in the case of a 3-Phase squirrel cage induction motor, the applied voltage must be suitably reduced so as to limit the starting current to the safe rated value. Once the motor picks up to a speed near about its synchronous speed, the DC supply to its field winding is connected and the synchronous motor pulls into step i.e. it continues to operate as a Synchronous motor running at its synchronous speed.