System and method for detecting loss of isolation while an ac motor is operating

A method of identifying loss of isolation in an electric motor system including a motor having a motor chassis connected to electrical ground, the motor operating at a fundamental frequency;

• and a power supply operating at a power supply voltage;
• the method comprising:
• receiving a first voltage signal related to a voltage between a terminal of the power supply and the motor chassis, the first voltage signal including a first component related to the fundamental frequency and a second component related to the power supply voltage;
• filtering the first voltage signal to attenuate the first component related to the power supply voltage;
• measuring an amplitude of the second component related to the fundamental frequency;
• and
• determining if a phase to ground loss of isolation condition exists by comparing the amplitude of the first component to a fault value.

A system comprising:

• a power supply that is substantially isolated from electrical ground having a power supply voltage;
• a power inverter electrically coupled to the power supply, the power inverter configured to provide AC current from the power supply in an AC phase at an AC terminal, the AC phase having current at a fundamental frequency;
• an electric motor electrically coupled to the AC terminal of the power inverter, the electric motor having a chassis that is substantially electrically isolated from the AC terminal of the power inverter under normal operating conditions;
• a processor configured to control the AC current provided by the power inverter, the processor configured to receive a first voltage signal related to current flowing through a motor chassis, the first voltage signal including a first component related to the fundamental frequency of the AC phase, and a second component related to the power supply voltage, the processor further configured to filter the first voltage signal to attenuate the second component, to measure an amplitude of the first component, and to determine if an AC loss of isolation condition exists by comparing the amplitude of the first component to a fault value.