Methods and circuits for line synchronization during transition of an electric motor between variable frequency inverter power and line frequency power

A controller of an electric motor configured to:

• determine to operate the electric motor using a line frequency power instead of a variable frequency power from an inverter of the electric motor;
• determine a time to generate a signal to close a contactor of the electric motor by energizing a coil of the contactor;
• initiate transition of the electric motor to operate using the line frequency power instead of the variable frequency power from the inverter by applying the signal to close the contactor at the determined time;
• and
• increase an amplitude of a voltage or a current fed to at least one of a main winding or a start winding of the electric motor during a predetermined number of last cycles of operation using the variable frequency power from the inverter.

A method comprising:

• determining to operate an electric motor using a line frequency power instead of a variable frequency power from an inverter of the electric motor;
• determining a time to generate a signal to close a contactor of the electric motor by energizing a coil of the contactor at least in part by analyzing previous data corresponding to a delay in closing of the contactor after application of the signal under a corresponding set of operating conditions of the electric motor;
• and
• causing transition of the electric motor to operate using the line frequency power instead of the variable frequency power from the inverter by applying the signal to close the contactor at the determined time.