Rotor Testing.

[mbailey]

Has anyone had experience performing Rotor Integrity checks in a no-load condition? We currently do a 1.0SF test, but would like to do no-loading testing by measuring inductance and rotor influence checks but have not been able to correlate no-load to load tests. Any information you can provide is appreciated.

 

[DougMSOE]

What are you trying to accomplish by this testing. Please be more specific.

 

[electricpete]

If you are looking for pole-pass sidebands around 60hz current at no-load as reliable indicator of rotor problems I think you are out of luck. You have to add load for that test to be effective.

R.I.C. as you know is separate test done off-line. I'm not that familiar but have heard residual magnetic can cause a major headache.

Single-phase test is perhaps a more reliable off-line test.

 

[mbailey]

Doug,

We are interested in determining if there is porosity or broken rotor bars in the cage. Currently we run the motor on a dyno to be able to load test to 1.0 SF or better. This is all part of troubleshooting if a bad rotor is suspected. On larger motors, the cost to run the motor under load is expensive. We would like to be able to test the rotor cage integrity without loading the motor, thus reducing our testing cost. Current analysis is performed at the present time and we look for sidebands around 60 hz at +/- slip. Evaluate based on the delta dB of 45 db or less to determine the presence of bad rotor bars. So far, correlation has been verified with visual inspection after teardown. We've also seen some correlations between single phase testing and current analysis. If it fails single phase test, it fails current analysis. However, if it passes single phase, it still may not pass current analysis. So single phase testing is not a catch all. Any information you can lend is appreciated. Thanks.

 

[electricpete]

My opinion: the single phase is the more reliable test. The ones that failed current analysis and passed single phase probably didn't have a problem (unless it was temperature dependent and therefore only showed up at load).

Back to the main question, you will find some good info at:

http://www.compsys.com/drknow/aplpa...45f2f1731b3a66c7852565a20066ef3a?OpenDocument

The above link has brief discussion of results based on large study of current signature analysis. The recommendation for most reliable results is monitor at high a load as possible. If you monitor at lower load the error will be in a direction to underestimate the severity of the problem. Figure 4 and discussion preceeding it relate to this aspect.

 

[GusD]

Hello mbailey

I have performed the RI test (RIC) using a Motor Circuit Evaluator.This is strictly a static (no load) test and
its purpose is to find possible problem with rotor bars.
The resultant Graph Pattern should show the condition of your rotor.
As for dynamic tests performed to evaluate rotor condition
they should be performed at least 75% of full steady load.
If load is surging and less than 75% of full load, the
results are questionable.If you are simply trying to evaluate the rotor's integrity (core & cage) or Windings for a WRM there are a numerous tests that will help.
Megger for WRM,Watt core loss,single phase test and etc,etc.
The single phase test has been around forever and I believe it is still just as reliable as any other test, or more so

Seasons Greetings


GusD

 

[testtech]

I have done many RIC tests. In my opinion, the RIC test will not permit you to distinguish porosity from a rotor bar fault. However, I believe the test can find porosity. I am not convinced you can depend on the RIC test to reliably find broken rotor bars. I had a motor showing clear evidence of broken rotor bars in a vibration test. This was confirmed by a current signature analysis. The RIC test shows very uniform sine waves, no evidence of a problem. The motor shop confirmed the broken rotor bars.

I suspect you are better off with a single phase test than a RIC test.

By the way, an excellent new resource on this topic is Electrical Insulation for Rotating Machines by Greg Stone, et. al., from IEEE press. Great resource for any motor insulation questions.

 

[jbartos]

Suggestion: More recent Reference:
1. St. J. Manolas, J. A. Tegopoulos "Analysis of Squirrel Cage Motors with Broken Bars and Rings," IEEE Transactions on Energy Conversion, Vol. 14, No. 4, pp 1300-1305, Dec. 1999.
A method is developed for PC to compute transient and steady state motor analysis results under asymmetrical conditions. An example is provided for the induction motor broken bars and rings.