3 phase AC, 30 degrees to 120degrees

[danny davis]

When Measure a 3 phase AC waveform

The oscilloscopes probe can only have one ground lead, because when you use multiple probes and ground leads it will fry the probes and oscilloscope, why is that?

When measure 3 phase AC waveform , the phase shift is only 30 degrees apart from each other before the OUTPUT transformer, how and why does an AC output transformer convert 3 phase AC waveform that are 30 degrees apart into 120 degrees apart from each other?

I'm not sure how can a transformer turn 30 degree phase shift into 120 degrees

 

[chuckey]

For safety reasons you MUST have the CRO chassis earthed. If you now trigger the beam from one phase, by using a 10:1 probe from the reference phase to the "Ext. trigger" in socket with its earth to any piece of earthed metal work. Now youselect "Ext trigger on the CRO and the CRO sweep is synchronised to the reference phase. Now select the "A-B" option on the Y amp, put both the A and B amps to 50V/Cm, use a 10:1 probe on each amplifier. Stick the A probe on the reference phase (no earth)and the B on another (no earth), mark on the graticule the positive, zero and negative transitions of the waveform. move the B probe to a different phase (no earth), measure the time difference to the reference - this is the phase difference. Move the A probe to where the B probe started (no earth), measure the time offset, this is the negative phase shift.
Frank

 

[FvM]

The phase voltages (referred to star point, either an existing or artificial one), as well as the interphase voltages and currents of a symmetrical three-phase system will always show 3x120 degree phase shift related to each other. I prefer vizualizing the phase relations in a phasor diagram.

On the other hand, you can generate arbitrary phase shifts by superimposing the three phases. In so far, a transformer is able to "turn" phase shifts, depending on it's circuit.

You didn't tell what's the circuit "before the output transformer" and how you connected the measurement. I guess, the 30 degree point is a misunderstanding or misinterpretation of results.

 

[danny davis]

so u don't use any probe ground? to measure the ac waveforms? any reason why u can't use the probe grounds?

- - - Updated - - -

The circuit is a 3 phase high freq fru usp unit

 

[FvM]

The circuit is a 3 phase high freq fru usp unit

I fear, you don't refer to commonly known terms.

When using ground referenced standard oscilloscope probes, the probe ground would be usually connected to a ground of the device under test. This ground must not expose a potential difference related to the oscilloscope ground.

If the DUT circuit is floating, you can possibly ground one (and only one) circuit node through the probe ground. But you'll possibly invoke equalizing currents that affect the measurement. A better way is using true differential probes.

 

[danny davis]

A better way is using true differential probes

How would u do it as true differential probes? how do i set up my oscilloscope that way?

So u don't use the ground on the probes at all? the waveform would be all noisy and messy right?

 

[FvM]

Industry standard differential probes have relative good common mode rejection and often achieve better noise rejection than grounded probes for low and medium frequency measurement problems (up to several MHz). There are special differential probes for high MHz and GHz measurements. They need some grounding due to limited common mode range.

But you didn't yet clarify your measurement setup, thus the discussion is very general.