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Motor Insulation Resistance in cement factory
I have on ABB 6.6 motor of cement factory, when I use PDMA MCEmax tester we have RTG is 108Mohm, but PI = 1.3. After that we stop this motor for 1 month, then we retest again the RTG just 0.5Mohm. In the past our customers meet the same problem with this motor but don't fix it and continuous running and after one week later they retest again the RTG increase again (>100Mohm). I don't why? Should we be let this motor continuous running?
The cement companies are a major part of our business and Low Insulation Resistance is something that most of them deal with at some point.
First of all, if the motor is at a cement company, it is probably operating outside the conditions change. Personally, I would not check Insulation Resistance right after the motor has been shut down. Directly after shut down the insulation system will still be holding a charge. I always allow the windings to get down to ambient temperature. It is the same after washing a motor and drying it in the oven, not electrical testing until it is cool.
If you do a Polarization Index right after the motor is shut down, your insulation system will be polarized before you even start, so your P.I. will be low.
You said it is a 6600-volt machine. What voltage are you using, it should be 2500-volts.
Reading between the lines, is this scenario accurate.
(1) The motor is running. (2) You shut it down and check the Insulation Resistance and it is 100meg-ohms. (3) you leave it for a month and check it again before energizing, now it is .5Meg-ohms.
If I am close with this scenario and the motor is out in the open and being subjected to high humidity that would account for the low Insulation Resistance after the month of non-use. Start the motor successfully and build up some heat in the winding eliminating the moisture and the Insulation Resistance goes back to 100meg-ohms.
If, in fact the PI is 1.3 that would be a concern. If your Insulation Resistance readings were in the high Gig-ohms maybe not but in the low meg-ohms it is a concern.
In a cement company, I would suggest that the motor is heavily contaminated with raw materials or finished product. If that product is laden with moisture or the company is adding metallic particles into their concrete for strength, instead of polarizing, the insulation system is continually discharging into the contaminant.
A polarization test is very like a capacitor. You have the conductor that the coil is made with and you have the insulation around the coil. Throughout the 10 minutes of the PI test you are charging the insulation. If you have a contaminant, that is in any way conductive, as the charge in the insulation builds, it will discharge into the contaminant thereby keeping your PI under 2. If this is the case the motor needs a good wash and dry.
The cement companies are a major part of our business and Low Insulation Resistance is something that most of them deal with at some point.
First of all, if the motor is at a cement company, it is probably operating outside the conditions change. Personally, I would not check Insulation Resistance right after the motor has been shut down. Directly after shut down the insulation system will still be holding a charge. I always allow the windings to get down to ambient temperature. It is the same after washing a motor and drying it in the oven, not electrical testing until it is cool.
If you do a Polarization Index right after the motor is shut down, your insulation system will be polarized before you even start, so your P.I. will be low.
You said it is a 6600-volt machine. What voltage are you using, it should be 2500-volts.
Reading between the lines, is this scenario accurate.
(1) The motor is running. (2) You shut it down and check the Insulation Resistance and it is 100meg-ohms. (3) you leave it for a month and check it again before energizing, now it is .5Meg-ohms.
If I am close with this scenario and the motor is out in the open and being subjected to high humidity that would account for the low Insulation Resistance after the month of non-use. Start the motor successfully and build up some heat in the winding eliminating the moisture and the Insulation Resistance goes back to 100meg-ohms.
If, in fact the PI is 1.3 that would be a concern. If your Insulation Resistance readings were in the high Gig-ohms maybe not but in the low meg-ohms it is a concern.
In a cement company, I would suggest that the motor is heavily contaminated with raw materials or finished product. If that product is laden with moisture or the company is adding metallic particles into their concrete for strength, instead of polarizing, the insulation system is continually discharging into the contaminant.
A polarization test is very like a capacitor. You have the conductor that the coil is made with and you have the insulation around the coil. Throughout the 10 minutes of the PI test you are charging the insulation. If you have a contaminant, that is in any way conductive, as the charge in the insulation builds, it will discharge into the contaminant thereby keeping your PI under 2. If this is the case the motor needs a good wash and dry.
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