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nvrsrinivas
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Join Date: Sep 2013
Posts: 59

CHECKING THE QUALITY OF INSULATION IN THE AGED HT MOTORS

02/21/2023 8:34 AM

Dear sirs, We are having 9 Nos. of HT motors,6.6KV with the rating of 1350 KW,750KW,600KW,560KW . These motors are very old. Since ours is a cement plant, they are exposed to more dust and contamination. Our power supply is having lot of variations from 35KV to 29 KV instead of 33KV at the plant incoming power. Since ours is a 33KV/6.9 KV power transformer, we can not reduce the voltage with tap changer, whenever voltage goes higher, hence, motors are subjected to voltage variations. Since the motors are already very much aged, we want to assess the quality/degradation/status of insulation of the motors. This information may be beneficial to us to take up the overhauling and strengthening of the motors which are having weak insulation. I want your guidance what are the tests I can do at the plant site and how much this information will be useful to me. Some of my colleagues told me that these tests procedures are such that they will apply very higher voltages to the motors and since motors are already aged, they can not withstand the test voltages and they may fail. Since these motors are running on line in the plant, I can not shift these motors for checking. I want to do these tests at the site only. Can I get any assistance from the market, with people coming to the plant with test equipment and test our motors. I request your suggestions in this regard. You may kindly suggest me the test details and also test equipment required.

Thanks.

NVRSrinivas
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PWSlack
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#1

Re: CHECKING THE QUALITY OF INSULATION IN THE AGED HT MOTORS

02/21/2023 9:40 AM

If the <...motors are already aged...can not withstand the test voltages and they may fail...> then that is sufficient to justify their replacement on the basis of staying in business. To do otherwise is merely postponing the inevitable. And hope is not a strategy.

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redfred
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#2
In reply to #1

Re: CHECKING THE QUALITY OF INSULATION IN THE AGED HT MOTORS

02/21/2023 10:44 AM

But that's not what the OP wants to hear. They want us to approve their established negligent maintenance program as the most cost effective way to run an operation. Running equipment to failure does have some advantages. Broken equipment forces down time that cannot be refuted or adjusted. Troubleshooting the problem after such a failure is often easy even though the repair may be more difficult. Often catastrophic failures will also force bean counters to open the purse strings for more than minimum expenditures. Frequently the down time from a catastrophic failure at one point in a plant will reveal other points in the plant that are soon to fail, too.

I just hope nobody gets hurt in the process.

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PWSlack
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#5
In reply to #2

Re: CHECKING THE QUALITY OF INSULATION IN THE AGED HT MOTORS

02/22/2023 7:22 AM

Quite.

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SolarEagle
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#3

Re: CHECKING THE QUALITY OF INSULATION IN THE AGED HT MOTORS

02/21/2023 3:53 PM

I would just go ahead and schedule the motor windings to be re-dipped to head off a catastrophic failure....or replaced if it seems more cost efficient...

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rwilliams
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#4

Re: CHECKING THE QUALITY OF INSULATION IN THE AGED HT MOTORS

02/21/2023 10:47 PM

I have done dielectric absorption testing on salient pole 4160V machines, slow moving hydro generators. This is done with a dc hipot machine, up to the peak voltage of your ac machine, so no risk to machine insulation. Read up on this test and what it tells you.

Much more sophisticated testing is now available with motor analyzers. These tests can also be run at reduced voltage, posing no risk to the winding insulation. Shorted windings, rotor bar problems can be found.

Dielectric absorption is best used to check for changes in insulation capacitance, which is what you are measuring, but really bad readings, especially compared with similar machines, are possible. Your polarization index number will help you decide which, if any, machines should be scheduled first.
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gideon
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#6
In reply to #4

Re: CHECKING THE QUALITY OF INSULATION IN THE AGED HT MOTORS

02/27/2023 9:43 AM

I recall having variable frequency tan-delta tests done on HV transformers, but I'm not sure if that will apply to HV motors as well. Maybe see if your country has an Electrical Engineer Society/Institution or a Non-Destructive testing Society of some sort - they should be able to assist.

An alternative is to ask reputable HV motor manufacturers in your country for their suggestions. But I would not let them be involved in doing the actual tests - they may have a vested interest in selling new motors, after all!
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raghun
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#7

Re: CHECKING THE QUALITY OF INSULATION IN THE AGED HT MOTORS

03/06/2023 5:04 AM

Very Low Frequency (VLF) AC test is considered safer than traditional AC or DC Hipot test.

"VLF hipot and VLF testing is as simple as just an AC hipot on lower frequency output (at 0.1 Hz). The only way to field test high capacitance loads with AC voltage, like cables and motors/generators, is to use a VLF AC hipot. Lower the frequency, Lower will be the current and power needed to test high capacitive loads.

It is widely used for cables (IEEE 400.2). But, can be used for motors as well.

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gideon (1); PWSlack (2); raghun (1); redfred (1); rwilliams (1); SolarEagle (1)

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