ALTERNATING CURRENT MOTOR STARTING APPARATUS AND METHOD

A starting set including an induction machine and a synchronous machine is brought up to speed before connection to the main motor that is to be started. The induction machine may have a number of poles that is a fraction, such as one-half, of the poles of the synchronous machine of the starting set so that the secondary of the wound rotor motor produces a voltage at that fraction of the line frequency at full speed. Alternatively, the induction machine and the synchronous machine of the starting set may have the same or nearly the same number of poles but the machines are concatenated to provide the desired frequency change. After the starting set is up to full speed, the main motor is connected to the secondary of the wound rotor motor and accelerates to a speed near synchronism with the secondary supply. If the main motor is a synchronous machine it is then synchronized with the secondary supply by applying field excitation. Means are then provided to increase the load on the synchronous machine of the starting set. The starting set is then disconnected from the power line so that it will begin to decelerate and, when brought to as low a speed as possible, the main motor will be running at nearly line synchronous speed and can be synchronized with the line.