The field is closed through a resistance R at the time of starting which is connected to the DC source only when the motor has come up to speed as an Induction motor. The application of field current can be manual or automatic. The above figure shows an arrangement with DOL starting and automatic application of field by a frequency sensitive relay FR. when motor is started by pressing start button, Ma and Mb close and current at line frequency is induced through the motor field. This current flows through relay FR as reactor X offers high impedance and through discharge resistance R. the relay FR picks up rapidly, keeping the control circuit of contactor FS open. As the motor speed increases frequency of induced current through the field winding decreases, with the result that increasing portion of current flows through reactor X till near synchronous speed, there is not enough current through FR to hold it. Hence FR opens, NC contacts of FR close, closing contact FS and opening NC of FS. The field disconnected from discharge resistance R (as NC of FS is open) andis connected across the DC bus. The motor now pulls into synchronism.