buzzlectric
Member
- Location
- Iowa
how many meg ohm should a motor have. i condem them at 500M or less to ground or 10% diff in lead to ground. i also like 4-20 ohm between leads how wrong am i?
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motor meg.
- Thread starter buzzlectric
- Start date
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friebel
Senior Member
- Location
- Pennsville, N.J.
Re: motor meg.
To: buzzlectric,
buzz, I have a manual that the AVO company has sent to me. This company was the former Biddle Megger company.
I have one of their manuals, called " A Stitch in Time". It is a complete guide to Electrical Insulation Testing.
On page 45, they discuss the One-Megohm Rule, and what that means is this.
Example: A 2400 volt motor should have a minimum insulation resistance of 2.4 megohms.
You are not wrong when you condemn a motor at 500 megohms, you are just going on the safe side.
buzz, as I stated in my last post, I would allow that motor to run at that low reading, but I would prepare to have it taken out of service for a overhaul.
On the subject of 4 to 20 ohms difference between phases of a motor winding. I do feel that the 20 ohm reading is a little bit high, telling me that I may have problems with the windings and would be best to take the motor out of service for a cleaning, overhaul, and possibly a dip in the insulating varnish tank.
To: buzzlectric,
buzz, I have a manual that the AVO company has sent to me. This company was the former Biddle Megger company.
I have one of their manuals, called " A Stitch in Time". It is a complete guide to Electrical Insulation Testing.
On page 45, they discuss the One-Megohm Rule, and what that means is this.
Example: A 2400 volt motor should have a minimum insulation resistance of 2.4 megohms.
You are not wrong when you condemn a motor at 500 megohms, you are just going on the safe side.
buzz, as I stated in my last post, I would allow that motor to run at that low reading, but I would prepare to have it taken out of service for a overhaul.
On the subject of 4 to 20 ohms difference between phases of a motor winding. I do feel that the 20 ohm reading is a little bit high, telling me that I may have problems with the windings and would be best to take the motor out of service for a cleaning, overhaul, and possibly a dip in the insulating varnish tank.
Re: motor meg.
As friebel stated you do need to look at the voltage of the motor. What's even more important is you need to have something to compare it to. Has the megger reading been coming down to 500meg or is that what it has been for some time? It is very possible that the reading has been 500meg and there is nothing wrong with your motor. If you are concerned check it again in a week and keep checking for a couple months. If you find it being steady change to taking readings every month and then to 6 months. Trend tells you everything. I would not panic at 500meg. The standard I have come to know is to let things go until they reach about 1meg, then it's definitely time to do something. But again all depends on the trend. As for as phase to phase, you need to look for a balance and I thing 4 ohms is high. Many motors you have to use a low resistance meter on because you'll be lucky to see 1 ohm. All you are looking at is a big coil of wire and a DC meter won't even give you indication of inductance. Again a balance of all three phases A-B, B-C, and A-C, usually within at least 10% of each other. Where are you taking your readings from. Motor connection box, or starter?
As friebel stated you do need to look at the voltage of the motor. What's even more important is you need to have something to compare it to. Has the megger reading been coming down to 500meg or is that what it has been for some time? It is very possible that the reading has been 500meg and there is nothing wrong with your motor. If you are concerned check it again in a week and keep checking for a couple months. If you find it being steady change to taking readings every month and then to 6 months. Trend tells you everything. I would not panic at 500meg. The standard I have come to know is to let things go until they reach about 1meg, then it's definitely time to do something. But again all depends on the trend. As for as phase to phase, you need to look for a balance and I thing 4 ohms is high. Many motors you have to use a low resistance meter on because you'll be lucky to see 1 ohm. All you are looking at is a big coil of wire and a DC meter won't even give you indication of inductance. Again a balance of all three phases A-B, B-C, and A-C, usually within at least 10% of each other. Where are you taking your readings from. Motor connection box, or starter?
O
oliver100
Guest
Re: motor meg.
1 mg per 1000 volt is a proven rule. DC resistance is dificult to mesure. The Z value gives you better indication. Mesuring the current will give you the real picture. Should be within 4-5% group.
1 mg per 1000 volt is a proven rule. DC resistance is dificult to mesure. The Z value gives you better indication. Mesuring the current will give you the real picture. Should be within 4-5% group.
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