Starting Procedure for Synchronous Motors

Author: E.E. Kimberly

As explained on page 235, synchronous motors are usually started by the induction-motor action of amortisseur windings. The rotating fields sweeping past the rotor poles induce an alternating voltage of possibly 1500 volts or more in the exciting windings on the poles. If not controlled in some manner, this high voltage may be a hazard to life or it may cause failure of the field-coil insulation. If the required starting torque of the motor is small, the exciting winding may be short-circuited at start and thus relieved of the high-potential stress. A more common practice, however, is to connect a resistor, called a field discharge resistor, across the open field terminals. This resistor is usually a part of the control equipment provided for the motor. After the motor has attained nearly synchronous speed, the dangerous field voltage no longer exists and the discharge resistor is disconnected. The field circuit is then connected through a control rheostat to its source of direct-current excitation.

In order that the amortisseur winding may react vigorously with the rotating field produced by the stator, that field must find a ready passage through the rotor pole structure. If the field windings are short-circuited, they become highly reactive. As a result, the rotating-field fluxes lose much of their strength by spilling over into the leakage paths in the air-gap and between the poles, instead of passing normally through the poles where they would react with the amortisseur winding and produce starting torque. This loss of flux in the amortisseur winding reduces the starting torque to some value considerably less than that which would be obtainable with the field winding open. The starting torque may therefore be adjusted between narrow limits by changing the value of the resistance in the discharge resistor.