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Identifying Rotor Anomalies with Electric Motor Testing

Don Donofrio
Don Donofrio | Instructor & Consultant, The Snell Group
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Webinar Description:

Finding and confirming rotor problems can be challenging, frustrating and sometimes easy to misdiagnose. Electric motor testing is the optimum technology for identifying rotor problems as well as providing many methods to confirm some of these serious faults. This presentation will provide examples of many methods of utilizing electric motor testing to identify rotor anomalies and provide confirmation and correlation.

Learning Takeaways:

  1. Learn how to differentiate between a good motor, a stator fault, a rotor fault and defective circuitry (PF Caps), utilizing energized and de-energized electric motor testing.
  2. Learn how to confirm rotor bar faults with multiple motor test data sets.
  3. Learn how to identify rotor issues for static and dynamic eccentricity.
  4. Learn how to identify rotor unbalance or misalignment using current signature.
  5. Learn how to identify rotor axial movement using current signature.

About the Presenter

Don came to work for Snell in 2002 to train our thermographers to conduct electric motor testing. He has been all over the world, testing, evaluating and troubleshooting problems with electric motors and motor circuits.  He has worked on motors rated in ounce inches of torque all the way up to a NASA wind tunnel synchronous motor that is 80,000 horsepower.

Don’s diverse electrical, electrical/mechanical, electronic, and sound motor knowledge is founded in twenty years of service in the Navy Nuclear Submarine Service. As the Snell Group’s Technology Lead for EMT and Power Quality Analysis, he brings 44 years of knowledge in the operation, maintenance and testing of electric motors and motor circuits. His 20-year exposure to the maintenance environment on US Navy nuclear submarines has provided invaluable insights for our customers’ reliability needs.

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Oscar

How do you test the rotor bar to bar test and which instruments you use.

mohamed ali hmida

Nice presentation Mr Donofrio. Is the axial rotor movement always exhibits a pic at 300 Hz ?

Donald Donofrio

It will cause a split peak at 300 Hz. This is a result of the magnet fields pushing and pulling on the rotor to center it magnetically. The distortion cased results in the split peak indication.

saeed naghdizadegan

Hello,
Thank for the webinar. Is it possible to identify the rotor bar problem with this method?
Inject a low voltage to two of the winding and measure current with analog Amper meter. Then start to rotate the shaft very slowly and look at the amperemeter. If we have an unusual current reduce (base on the number of stator pole) it is a sign of rotor problem?

Donald Donofrio

That is sometimes referred to as a quarter voltage test. One fourth of 480V is 120V. Apply 120V across 2 phase leads of a three phase motor. Place a amprobe on one of the powered leads and slowly turn the rotor. Observe the meter for a jump, indicating a possible rotor bar fault.

David Diez corral

Hello Don

Why is it that te Db are going donw when the posible failure posibilities increase?

best

Donald Donofrio

The Spectral presentation most often used for displaying Field Pole Pass Frequencies is dB spectrum. The noise floor is below zero and magnitudes are measured down from zero. The lower the number, the greater the magnitude of the frequency. For example, I start paying attention to possible rotor bar issues when dB is around 60, as rotor fault progress the dB level number gets smaller, closer to zero. When dB levels reach 35dB, ( with data correlation) the motor should be repaired or replaced as soon as possible.

patrick raymond

Hello Don,
I am having a RTG value of 0Mhm, on the rotor of 2nos MV motors, one is 4200KW while the other is a 4000KW motor, from my PDMA MCE test results. The 4000KW motor is currently in use while the 4200KW is a refurbished motor that has been stored for 3 years. Do you recommend opening to check the rotor of the 4200KW motor or to go ahead and install it? They is an urgently need for it.

Donald Donofrio

You need to split the circuits to isolate the source of the ground, then find the cause and repair or replace as necessary. It would be much easier to find and fix the ground on a motor that is not installed.

patrick raymond

Thanks for the feedback Don. However, do note that the motor stator has a very good RTG value, thus eliminating the need to slit the circuit. The excessive leakage happens at the rotor. My major concerns are as follows: (1) Should i go ahead with installation of the 4200KW motor, based on the fact that i have another MV motor currently in use, with same kind of fault (good stator RTG with excessive leakage at the rotor). (2) Will the motor run after installation based on the assumption that a motor with similar fault is currently in use. (3) What… Read more »

About the Author

Don Donofrio
Don Donofrio Instructor & Consultant, The Snell Group

Don came to work for Snell in 2002, to train our Thermographers how to conduct electric motor testing. He has been all over the World testing, evaluating and troubleshooting problems with electric motors and motor circuits.  He has worked on motors rated in ounce inches of torque all the way up to a NASA wind tunnel synchronous motor that is 80,000 horsepower.

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