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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.

Parents
  • 0

    there is no direct part between the input phase and the ground.

    errm, sort of but not quite. the lines are referenced to Earth (by the opposite end of the source windings) - so definitely not floating, but if you like Earthed but with an offset - i.e. Earth+240V (ac) - so Mike's comment to be careful what you Earth still stands I think.

        - Andy,

Reply
  • 0

    there is no direct part between the input phase and the ground.

    errm, sort of but not quite. the lines are referenced to Earth (by the opposite end of the source windings) - so definitely not floating, but if you like Earthed but with an offset - i.e. Earth+240V (ac) - so Mike's comment to be careful what you Earth still stands I think.

        - Andy,

Children
  • 0 in reply to AJJewsbury

    In a three-phase system between the phase and ground, there is a transformer winding impedance.

    In a single-phase system the winding impedance is only between phase (L) and ground, while "phase" N is grounded. If I ground the mid-point it looks like I am shorting the input to the mid-point:

    I would like to ground the DC link mid-point, and as I understand, for a three-phase input this is usually done. But should I do the same in a single-phase system?

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