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

Question

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.

mapj1 · Answer

Ah well, in the lab when initial testing the inverter part in isolation the DC bus comes from a regulated DC supply with all sorts of safety interlocks and fast shutdown, and is much more complex than the off-line version in the final product. Similarly when testing the off-line PSU part, initially the inverter may be replaced by another adjustable load that is easier to manage. 
 Once it gets off the bench and is being integrated then pairs of probes and differential measurements are your friend. 
 
 By CVM I assume you mean the movement of the motor star-point - assuming a star load and not a delta, relative to some notional DC bus centre. Ideally it will be well centered over a full cycle but will precess at the cycle frequency and probably in reality be a few volts off-centre. 
 In your shoes I'd create that load centre from 3 equal loads on the phases, and then AC couple it to one side of a diff-amp, and put the bus voltage, top, centre or bottom, to another probe, and then subtract. 
 Please take care, experimentally this is not easy stuff to do safely - hence the first reference you gave - the IEEE paper - being about modelling it, not measuring it. 
 LT spice or something more powerful is your friend for initial design verification, it saves a lot of red faces. 
 Mike.

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

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