Motor driving device and measuring method

A measuring method by which a motor driving device for driving a plurality of motors measures an insulation resistance of the motor, wherein,

• the motor driving device includes:
• a rectifier circuit configured to rectify an AC voltage supplied from an AC power supply via a first switch to thereby obtain a DC voltage;
• a capacitor configured to smooth the DC voltage obtained by the rectifier circuit;
• a plurality of inverter units each including upper arm semiconductor switching elements configured to connect a positive-side terminal of the capacitor with motor coils of a corresponding one of the motors, and lower arm semiconductor switching elements configured to connect a negative-side terminal of the capacitor with the motor coils, the inverter units being configured to drive the motors by converting a capacitor voltage across the capacitor into an AC voltage by switching operation of the upper arm semiconductor switching elements and the lower arm semiconductor switching elements;
• a second switch configured to connect one terminal of the positive-side terminal and the negative-side terminal of the capacitor to ground;
• a first detector configured to detect a ground-referenced current flowing between the one terminal of the capacitor and the ground, or a ground-referenced voltage between the one terminal of the capacitor and the ground;
• and
• a second detector configured to detect the capacitor voltage across the capacitor,
• the measuring method comprising:
• a switch controlling step of establishing a measurement state for enabling measurement of the insulation resistance of a measurement target motor of the motors by turning off the upper arm semiconductor switching elements and the lower arm semiconductor switching elements of all the inverter units to thereby stop operation of every motor while turning off the first switch and turning on the second switch, then turning on a semiconductor switching element that is connected to another terminal of the capacitor, among the upper arm and lower arm semiconductor switching elements to which the motor coils of the measurement target motor are connected, and turning on a semiconductor switching element that is connected to the one terminal of the capacitor, among the upper arm and lower arm semiconductor switching elements to which the motor coils of each of the motors other than the measurement target motor are connected;
• a convergence determining step of, in the measurement state, calculating a change amount in a waveform of the ground-referenced current or the ground-referenced voltage detected by the first detector, and determining that the ground-referenced current or the ground-referenced voltage has converged if the change amount becomes equal to or lower than a threshold;
• and
• an insulation resistance calculating step of, when the convergence determining step determines that the ground-referenced current or the ground-referenced voltage has converged, calculating the insulation resistance of the measurement target motor, based on the ground-referenced current or the ground-referenced voltage detected by the first detector and the capacitor voltage detected by the second detector.