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Inverter voltage measurements don't make sense: phase voltage to earth is higher than the line and DC + to earth not equal to DC - to earth

mike buba

I am trying to get the inverter running and for 163 Vdc in an open-loop, I am measuring line voltage (Vuv, Vvw and Vwu) 76 Vrms which is ok.

But when I measure Vu-earth I get 273 Vrms, Vv-earth = 243 Vrms and Vw-earth = 318 Vrms.

Even stranger is when I measure DC bus + to earth which is 133 Vdc is DC - to earth is -30 Vdc.

Voltage from the load neutral to earth is 276 Vrms.

Schematics and values are shown below:

Why am I measuring this?

I would expect to measure DC + and DC - to earth to be equal and phase voltages to be line/sqrt(3) and neutral to earth to be zero.

Kind regards.

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    I think that you are suffering from the delusion that Earth is something useful once you move into a circuit like this. It is not, simply a meaningless concept of no value. You ask yourself "why are you measuring this?". A better question would be "why am I measuring anything"? Because of the way that inverter drives work, using PWM and not AC voltages as such at all, you will measure a lot of very strange numbers, many of them not related to very much. As Mike says, if you want to investigate the circuit you will need to use an oscilloscope, and doing so safely really requires a pair of high voltage probes used differentialy. If you want to understand what happens if there is an Earth fault somewhere on the motor, the result will be a high current through quite a lot of parts of the circuit, and these will either be electronically limited by controlled self preservation of the electronics or trip the circuit protection.

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    I think that you are suffering from the delusion that Earth is something useful once you move into a circuit like this. It is not, simply a meaningless concept of no value. You ask yourself "why are you measuring this?". A better question would be "why am I measuring anything"? Because of the way that inverter drives work, using PWM and not AC voltages as such at all, you will measure a lot of very strange numbers, many of them not related to very much. As Mike says, if you want to investigate the circuit you will need to use an oscilloscope, and doing so safely really requires a pair of high voltage probes used differentialy. If you want to understand what happens if there is an Earth fault somewhere on the motor, the result will be a high current through quite a lot of parts of the circuit, and these will either be electronically limited by controlled self preservation of the electronics or trip the circuit protection.

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