Scoping voltage and current on 3-phase induction motor

[Circlotron]

So I have a 3-phase motor and I want to put some power factor caps across the windings and observe just how much the current lags the voltage at part load, just as an experiment. Put a scope current clamp around one of the feed wires, no probs. What about voltage? Is that scoped from line to neutral (despite motor having a delta winding, i.e. no neutral connection), or differentially across one of the windings, or what?

[tautech]

If the scope is properly earthed (and it should be !) the only place you can safely put the probe's reference lead is mains ground therefore each phase will be measured at a single phase potential; ~230VAC.

If 4 channels are available you can probe each phase and just use a single probe reference connection or none at all for 50 Hz mains work.
Wise to use only 100x probes for high energy work like mains....230 or 415VAC. 

Be careful, mains takes no prisoners. 

[Circlotron]

I'm actually running the motor on about 150VAC max phase to phase from 3 step up transformers driven by a pair of 50 watt audio amplifiers. Delta primary and secondary. So the power is limited but could still probably give quite a bite. Yep I have 100:1 probes, rated at 2.5kV I think. The question has not been answered though.

[Ian.M]

In a real line powered situation, ordinary x100 probes can only be used for phase to ground measurements, then you have to setup  waveform maths on the scope to get phase to phase voltages or use an extra channel with the probe on neutral so the waveform maths can calculate the phase to neutral voltages.  If you want to directly measure the voltages you need HV isolated differential probes with an appropriate CAT rating, which don't come cheap.

However your experimental circuit allows you to freely ground any point on the secondary side, which would allow you to set it up as corner grounded delta, so you could measure two phase to phase voltages directly and use waveform maths for the third.  Similarly, if you have a four channel scope you could simultaneously measure two phase currents and calculate the third.

+1 for being careful.  I like the Cliff Quicktest series of mains test connectors - when the lid is open and flipped back, you can be certain the load is isolated and its safe to move test leads or make circuit adjustments.

[duak]

If the motor is connected as delta and you're measuring a winding's current, then you need to measure the delta voltage ie., phase to phase.

If everything is well balanced, measuring line to neutral voltage and line current will also work.  This usually implies a three phase wye connected secondary transformer.  If you're using two audio amps, are the transformers connected as Scott-T or as corner grounded delta?

There is phase shift between delta and wye voltages, but I can't remember exactly what it is.

[Benta]

If it's connected as delta, just make a virtual neutral with three equal resistors in star from the three phases to measure phase-neutral voltage. A current clamp on the same phase you're measuring voltage on will give you PF immediately.