I am using an all test pro 31 for motor diagnostics. Few days ago, in a 3 phase induction motor (Siemens), I took a wave form that looked like a random collection of pixels. In this case, what would be the rotor diagnostic?

Core77

I am assuming that you are using the automated rotor test in which you set the upper and lower limits by turning the shaft a few times then the waveform shows. There is also a condition number that shows on the right hand side that will give a value <5 for good and >5 for poor.

The pixilating will come in one of three conditions. If the rotor is being turned too fast, it will pixilate; If the upper and lower limits are too close, it may do it; and, if the rotor is in really poor condition.

The AT31 has several methods to perform the rotor test, real time. The other two include using the numeric method with the impedance readings and the other using the bar graph. I had insisted that the design guys maintain these abilities while I was the GM at ATPro through 2004.

Using the numeric method, you adjust the test frequency so that you are viewing an integer with at least one decimal place (ie: 1.1, 1.11 or 1.111 Ohms Z) this will allow fro the best accuracy. Very small motors may actually read high Z readings such that you are looking at a three-digit integer of impedance. This is just as fine. Turn the shaft slowly, and you should see the integers increase and decrease once for each pole of the motor (also a great way to identify those 'unknown soldiers'). The upper and lower values should be the same 180 degrees apart from each other. So, for a four pole machine, if you start 1.11Ohms lower, 1.22 Ohms peak, 1.11 lower, 1.30 peak, 1.11 lower, 1.22 peak, 1.11 lower and 1.30 peak, you have a good rotor.

The bar is set up in such a way, before using the waveform graph, to get a visual of the numeric test. You rotate the shaft at a constant rate until the instrument shows the bar graph. The bar graph should extend all the way from the right to the left for all poles of the machine. If it does not, then there is a rotor problem. I also use this feature to verify whether a motor (ie: inline duct fans) is rotating before performing other tests.

I had made up my own manual a long time ago that explains much of this in detail. Please feel free to contact me via email for a copy.

howard@motordoc.net

Sincerely,
Howard