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Rotor Quality- Rotor Influence Check

Rotor Quality- Rotor Influence Check

Thursday, April 27, 2023

It is possible with the MotorGenie but it requires some patience

A Rotor Influence Check (RIC Test) is a test of motor impedance in Ohms as the rotor is indexed through typically one revolution. In this example that indexing was conducted at 15 degree intervals.

All TestPro Motor Analyser

While 3Phi Reliability recommend the All TestPro 7 Professional for Electric Motor Rotor Testing this article shows how it can be achieved with the MotorGenie.

A Rotor Influence Check (RIC) can detect many different defects in a motor. The principle of measuring impedance and producing a smooth interlaced sine wave often doesn't  result in what is expected, that can be caused from Eccentricity (Possible softfoot, or Machining defects), Broken rotor bars, Casting voids, End Rind resistances.

Magnetic flux produced by a Motor Winding cuts through the rotor and an opposing flux is produced in the rotor. The two magnetic fluxes lock together and create torque on the rotorbars ultimately turning the motor.

If these two magnetic flux fields aren't symmetrical due to a defect, measured by Impedance then the torque isn't efficient.

These losses are converted to heat and this shortens the Motor Insulation life.

So Rotor defects are important for both Motor Efficiency  & Reliability.

RIC Rotor Test

The RIC test should produce an interlaced sine wave with even peaks, same amplitude, and without inflections.

The measurement must be taken at precise indexed positions and a protractor which I made has a magnetic mount that attaches to the shaft center.

RIC Rotor Test MotorGenie

The All TestPro  MotorGenie while in standard Motor test mode measures the live Impedance. This Impedance is taken for each phase T1-T2, T1-T3 & T2-T3 in sequence before indexing to the next position.

MotorGenie Motor Tester

The position of the rotor affects the Impedance reading and is updated on the instrument a few times per second.

Two tests with MotorGenie are Phase Balance and Rotor Compensation Test.  The Phase Balance test chooses the highest position of impedance which corresponds to a peak in the sine wave on one phase, the instrument is zeroed and the leads changed to another phase.

The rotor is then rotated until the maximum negative percentage is obtained. This is then repeated on the third phase.

This Phase Balance Test is measuring the maximum imbalance in impedance  that can be contributed by the Stator  or the Rotor.

The second test is the Rotor Compensation test where the Impedance on each phase is measured while turning the shaft to the maximum value.

When each phase is measured in this way the same rotor position (Highest Impedance) ensures a comparison of only the stator imbalance, hence the name Rotor Compensation test.

When comparing these two measurements effectively (Total Imbalance Stator plus Rotor) being Phase Balance against Rotor Compensation (Stator imbalance) the difference is rotor influence.

When a RIC test is plotted the Sine wave peaks being balanced indicate a good Stator and should be within 3% of each other.

RIC test on Mill Motor

The results are plotted by shaft position (15 degrees), and you can see the peaks of the Sine Wave are relatively good.

This can be confirmed with a Phase Balance Test of 30% versus a Rotor Compensation Test of less than 2%. These tests suggests a rotor defect.

The RIC test certainly confirms the problem with at least two positions having major excursions. I've have yet to dismantle this motor for a rotor inspection but I suspect an issue.

Casting Void in Rotor, Very common Defect

This pattern in the RIC test is likely to be caused by a rotor bar with excessive Impedance, hence the magnetic flux doesn't want to flow through the air pockets. 

This is a very common issue in Electric Motors and confirms why you should conduct Acceptance Testing on Purchased & Overhauled Motors.

If you don't have the ability or time of conducting a RIC Test then certainly the quicker Phase Balance & Rotor Compensated Test is recommended.

Note: If you Don't conduct Acceptance Testing then the Problem is Yours for the life of the Motor, which is likely shorter.

3Phi Reliability coach technicians in Electrical Preventative Maintenance which starts from Purchasing, then Install and Asset Management.

Best Practice Motor Management

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