Why We Need Soft Starting for Synchronous Motors?
Let us take a case where the synchronous motor is started initially as an induction motor and brought to synchronous condition later. In this, when a synchronous motor is connected directly to a three phase AC supply without any starter, a very large stator current flows through the stator winding. The stator current drawn by the motor during startup will be up to five to eight times the full load current. This is due to the fact that the rotor cuts a maximum rate of flux, creating a large induced current in the rotor of the motor due to large induced EMF. Because of this increased stator and rotor current, the supply power factor will be very low typically about 0.2 lagging. This rises to 0.5 lagging on no load then to about 0.85 lagging on full load. This starting surge current reduces as the motor accelerates up to its running speed.
The motor operating at low power factor (0.5 lagging) at the light load condition reduces the efficiency as the supply current is relatively high causing significant I2R resistive losses (copper losses). In order to attain good efficiency from the motor, the power factor has to be improved.
The only way to improve the power factor of the motor at light load is to reduce the supply voltage given to the motor. This can be achieved with the applicable voltage controller. A type of voltage controller working on electronic circuits employed to control the supply voltage of the motor is called a soft starter.
The soft starter, which is an electronic voltage controller, is also known as an energy manager. It can match the supply voltage to the startup and load conditions. Such a controller aims to maintain the operating power factor as high as possible to minimize supply current and power losses. An important thing to note is that this type of voltage controller does not control the speed of the motor; that can be controlled by frequency of the supply. However this speed control is only possible in induction motor whereas the synchronous motor is a constant speed motor that runs at its synchronous speed.