Each phase of the motor is connected internally on the controller in a part of the power stage that is consisted by a top and bottom MOSFET.
When a MOSFET fails, usually it gets shorted and this can be easily detected my measuring the MOSFETs body diode. The diode can be measured by using a multimeter in diode mode. By connecting the negative lead of the multimeter to the negative part of the diode and the positive lead of the multimeter to the positive part, one can measure the voltage drop across the diode. If the voltage drop is close to 0 V, then we can conclude that the MOSFET is failed. The expected voltage across A MOSFETs diode is around 0.4 V.
Warning: When measuring the controller MOSFETs the motor and power supply should be disconnected!
So in total the following MOSFET measurements can be taken for each channel. If any of the readings is below 0.1 V then the MOSFET can be considered shorted.
| CHANNEL 1 | CHANNEL 2 | ||||
| Multimeter | Measured voltage | Multimeter | Measured voltage | ||
| + | - | + | - | ||
| U1 | Vmot | U2 | Vmot | ||
| V1 | Vmot | V2 | Vmot | ||
| W1 | Vmot | W2 | Vmot | ||
| GND | U1 | GND | U2 | ||
| GND | V1 | GND | V2 | ||
| GND | W1 | GND | W2 | ||
In drives featuring STO, the STT command can be used to assess the MOSFETs' health. First, the following command must be sent to run the MOSFET test:
!STT
Then, the test results can be queried with the following command:
?STT 1
A reply of 0 indicates that there is no MOSFET damage, while a 5 indicates that one or more motor drive MOSFETs are damaged.