Rotation check of motor using 2-channel O-scope û for comment/info 2

Your probes as shown in Figure 1 need to be differential (and isolated) so they aren't shorting each other out. You might want to add a note about that so someone doesn't try this with standard single ended probes and get a big surprise!
(maybe that's implied by the Fluke 199B? I'm not familiar with that device)

John D

Wouldn't which phase is leading be determined by which channel the trace is triggered off of?

Thanks for the comments.

The probes are differential – splits into positive and negative probe clips for each channel. I sketched it to show the positive and negative input to each channel which I think implies differential. The channels are separate from each other and from ground.

The lead/lag releationship is not sensitive to the trigger setup. Whatever trigger is used triggers both channels on this analyser. While the two channels can be moved independently in the vertical direction on the screen, there is only one common horizontal movement control. In short, there is no way you can have display of two waveforms that don’t reflect their true time relationship on this analyser (as long as polarity is right). I believe this is common for many multichannel analysers but I’m not sure.

My preference is set it up for “auto-trigger” which gives a continuous sweep whenever any signal is present on the input. Setting timescale to 1 sec per division allows very easy view of the slowly-scrolling waveform (waveform can also be frozen with the save key).

Your comments made me realize I should add a few general comments about setting up the scopemeter since this procedure will be used by our electricians who don’t have as much experience with this equipment as our instrument/control guys. I will add suggested instructions to do a reset to the factory configuration (to eliminate variability due to previous settings), and then about 10 keystrokes to get it in the required configuration for the measurement.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

Pete:

Thanks for the procedure. Personally, I would not have known you are utilizing differential probes based on your diagram. I assumed you were taking advantage of the isolated, floating inputs on the Fluke Scopemeter. Obviously a line powered scope without diff probes would be problematic.

Good to see a 'trick turned to a method'.

The concern about isolated, non-isolated or differential probes is unnecessary. Remember that motor is disconnected (breaker open, contactor off) when the test is made. So, any scope or recorder can be used. Also those where PE is directly connected to the GND side of the probes.

But - don't forget to remove probes before starting motor. Or...

Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

I'm not following you Skogs; whenever I hook the tip of a scope probe to ground I see 0V. That's what I see in Figure 1, Channel B measuring the ground of Channel A.

John D

Yes, Zapped, right.

I SHOULD have said: "It is not a problem to measure with any scope - isolated, differential or grounded - you just have to know what you are doing".

In the case shown, it would be better to put both probe 'minuses' on, say, terminal B and then adjust the resulting scope pattern accordingly.

Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...