Your diagram doesn't show any part of the circuit connected to Earth.  So any connection would be stray capacitive or inductive coupling, and leakage through insulators that aren't perfect.

A typical multimeter has an input resistance of 10M ohms on volts ranges.  So every time you connect the meter, you're adding a new resistance to it, and changing the circuit you're measuring.  Each measurement is inserting that extra resistor in a different place, so the results are totally inconsistent with each other.

Your diagram doesn't show any part of the circuit connected to Earth.  So any connection would be stray capacitive or inductive coupling, and leakage through insulators that aren't perfect.

A typical multimeter has an input resistance of 10M ohms on volts ranges.  So every time you connect the meter, you're adding a new resistance to it, and changing the circuit you're measuring.  Each measurement is inserting that extra resistor in a different place, so the results are totally inconsistent with each other.

Hi Simon. Thank you for the reply.

Cay you please advise which part should I connect to the earth? The converter is supplied from a single phase TN-S system. I also read it is not recommended to ground the DC bus link.

Please DO NOT measure to earth with a meter - and earth nothing. !!  The usual arrangement for a 3 phase rectifier is a six diode bridge, and the DC bus negative is connected to each phase that is most negative in turn spending 1/3 of the time connecting to each. Similarly the DC positive is connecting by its diodes to whichever of the 3 phases is most positive, again connecting to each phase in turn for 1/3 of a cycle, but with the timing offset by 1/6 of a cycle from the negative side commutation.

So If you ground one side of the DC bus, you will blow 3 diodes and probably some hefty fuses as well.

If you really must measure the DC bus relative to ground, you need a scope. 

What you would see is that   the DC bus is a mixture of a DC offset, on average half the Dc bus voltage superimposed on a  wibbling up and down at 3 times the mains frequency of all the waveform crests by ~ sin 30 times the peak voltage of the cycle.

(actually the PFC >PAM confuses this somewhat but the 150Hz p-p wibble is more or less the same, the waveform is more insanitary being chopped by that circuit and the average DC is pulled over to one side - the side without the series inductor...)

If there was any sensible  'earth' reference to measure your 3 phases with respect to, it would be relative to  the bouncing centre of the DC bus. Now the voltages applied to the ends of the motor windings are not  sinewaves, rather each winding end is toggled between one side  of the DC bus and the other at a supersonic rate, with a rectangular switching pattern whose mark to space ratio ('duty cycle' ) is varied to give a similar motor  torque to a sine wave.

Maybe if you low pass filtered it you would see a component. at 50Hz or whatever the inverter rate is .

So why on earth do you expect a digital meter that has a peak measuring diode circuit, with a division factor that only gives the right RMS  when expecting sinewaves  to make any sense of this at all ?

If you must measure, you need a scope, and you need to know what to expect ;-)

On a meter you may measure the DC bus voltage between the two sides of the reservoir capacitor - that should be a DC with only a few tens of volts of ripple.

This explanation is as concise as I can make it, to fit in a lunch break, so if that means some parts are not clear, come back and myself and or others  can re-do those bits.

Mike

mapj1 said:

If there was any sensible  'earth' reference to measure your 3 phases with respect to, it would be relative to  the bouncing centre of the DC bus.

In the examples, I found it is usually a three-phase rectifier or inverter producing a DC voltage, so grounding the mid-point is done as shown in the Figure 2.b from Ref. 1. Fig. 9 in the same reference shows DC grid grounding systems, where DC bus midpoint point solid grounding gives the lowest common-mode voltage. 

In a three-phase supply system, none of the input phases are grounded, but in my application, I am using a single-phase TN-S system so isn't the neutral line already grounded at the substation? Is it safe to ground the DC mid-point in that case?