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Hi,
I work in an industrial setting were we have a couple hundred electrical motors. Most are 3 phase 460 volt 2 to 25 hp. Most are relatively new, about 2 to 5 years old. We have a lot of motor failures but they are generally mechanical failures not electrical.

The procedure is to basically wait until a motor fails and quickly replace it. I am trying to get beyond that mentality and gather some data on a few troublesome 20-25 h/p 3 phase 460 volt motors that have had electrical issues in the past.
I bought a Megohm meter. Actually I bought 3 of them but the first two were very cheap Chinese Biddle knock offs. I finally found an excellent condition older Biddle meter. It is a 500 volt/ 200 meg hand cranked.
My question is, everything I meg reads infinity or at least over 200 megs. I guess that is good if the readings are correct but I expected some variance. When I take my readings I
Disconnect all three lines from the motor, Connect the earth line to the motor grounding lug or frame. Attach the line the one of the motor line connections. Cranky furiously for 30 seconds to one minute and check the readings while I am turning the crank. Nothing reads below infinity. When I was using the Chinese meters I was getting the same results but these read to 500 megs. I was still reading infinity. Are these results normal or am I doing something wrong?
Also I would like to check some hard to get to motors from the motor control cabinet. These motors are connected to vfd’s. I would kill the power going to the cabinet and disconnect the lines coming from the vfd to the motor and test these lines from line to ground. I suspect the readings might be artificially low but would this be a decent indication of the motor’s health if the readings were normal?
Last question, this meter does not have a discharge switch. Would it be acceptable to connect the motor leads to ground for a few minutes after the test to make sure there is no built up voltage.
Thanks for any advice.
 

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Discussion Starter · #3 ·
Thanks

Thanks for the link. I've already read that one. Also the instruction manual from 1954 that came with the meter. I am fairly certain I am using the meter correctly It's just that the results were not what I had expected and I want to make sure I'm not missing something simple.


I wouldn't call myself an electrician even though I work with electricity everyday and am fairly knowlegable for a non-electrician. I've worked with good electricians and to call myself one would be an insult to someone who spend years learning the information I am trying to get from a question on an internet forum. It's like someone who went through Marine Corp ROTC and calling themselves a Marine.

Thanks for your response.
 

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My question is, everything I meg reads infinity or at least over 200 megs. I guess that is good if the readings are correct but I expected some variance. When I take my readings I
Disconnect all three lines from the motor, Connect the earth line to the motor grounding lug or frame. Attach the line the one of the motor line connections. Cranky furiously for 30 seconds to one minute and check the readings while I am turning the crank. Nothing reads below infinity. When I was using the Chinese meters I was getting the same results but these read to 500 megs. I was still reading infinity. Are these results normal or am I doing something wrong?
Also I would like to check some hard to get to motors from the motor control cabinet. These motors are connected to vfd’s. I would kill the power going to the cabinet and disconnect the lines coming from the vfd to the motor and test these lines from line to ground. I suspect the readings might be artificially low but would this be a decent indication of the motor’s health if the readings were normal?
Last question, this meter does not have a discharge switch. Would it be acceptable to connect the motor leads to ground for a few minutes after the test to make sure there is no built up voltage.
Thanks for any advice.
- Disconnecting at the motor is an invitation for a mistake and an accidental rotation reversal.. I would do it from the MCC cubicle, which would also prove the cable/conduit and if I got a wierd reading, then move to the motor and investigate from there.. There is the same chance of an error, but an easier way to label/document the order... If I know the type of system and am sure of how things are connected, I won't unhook any leads at all..
- To ground if you don't get infinity on a 3 phase motor, depending on your reading, you will be looking for the problem.. Or you missed some thing else connected to the motor leads...
- Careful when disconnecting at the VFD, as there may be a load reactor between the drive the and motor...
- Discharge for a few minutes?? Not unless you are into higher voltage systems.. 500 volt megger a momentary discharge should work (has for me anyway)...

Thanks for the link. I've already read that one. Also the instruction manual from 1954 that came with the meter. I am fairly certain I am using the meter correctly It's just that the results were not what I had expected and I want to make sure I'm not missing something simple.


I wouldn't call myself an electrician even though I work with electricity everyday and am fairly knowlegable for a non-electrician. I've worked with good electricians and to call myself one would be an insult to someone who spend years learning the information I am trying to get from a question on an internet forum. It's like someone who went through Marine Corp ROTC and calling themselves a Marine.

Thanks for your response.
Are you an electtrician or a maintenance handyman?
 

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Posted previously.

A quicky

For 600 VAC and lower systems

I perform these test at 1000 VDC after verifying no loads are connected. Time frames very with type of test, go-no go short conductor runs 30 seconds to a minute (when the meter reaches a stabilized reading) and or long runs of cable or busway, which can require 5-10 minutes. Minimum readings are published in NETA specifications. The often-quoted one (1) megohm is, in my opinion a mistake, 5 megohm would be a minimum under certain conditions (buried cable, emergency situations on 208/120 VAC systems). 50 megohms is a minimum we prefer with 100 megohms being the norm. BUT on new systems one would expect and hope to see full scale of the meter. At one time this full scale was typically 2000-4000 megohms, today that varies with the manufacture some meggers now read into gigohms and teraohms. One item to consider is comparison readings, such as a 3-wire circuit “L1” reads 2000 megohms; “L2” reads 200 megohms and “L3” reads 2000 megohms. While not a major issue I would want to know why “L2” was low. This would hold true for any type of circuit with similar conductors.
When Meggering it is important to verify all loads are turned off epically where there is sensitive electronics. You can damage this equipment or at a minimum get false readings.

Typically start with the lowest voltage your megger has, starting at 50, 100 or 250 VDC will minimize any chance of damaging equipment.

For trouble shooting single phase, I would megger Line to ground, line too neutral, neutral to ground. Typically when trouble shooting this is a go no go test allow megger reading to stabilize, no reason to wait one minute.

For new houses with no loads on (and smokes and similar equipment disconnected]
And the neutral bond lifted, I megger the panel with the meter lifted and all CB’s off for one minute, “L1” to neutral and ground, “L2” to neutral and ground, Megger neutral to ground.

As for exposed copper at terminals, this does nothing to the readings unless there is a problem with the terminal, such as some conducting medium between the terminal and ground or another polarity.

We megger everything and document all readings:
All new installations at a standard time frame (minimum one minute). We do gear separate from feeders prior to termination.
All equipment serviced once again at one-minute minimum, we like to megger when de-energized and prior to re-energizing to verify we did not screw up. Plus if you get low readings with the first reading the customer MUST be aware of this, because human nature would be to blame you if you just megger at the end of service.
Houses after a lightning strike, with separate readings for the panel, then individual branch circuits, then the completed system.
Neutrals for all services worked on to determine if there are down stream shorts.
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Donuts > Fried Eggs
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While 200M is a safe value for 480V equipment, it's a "low" value in that I would expect the vast majority of healthy equipment to read well over this range (often in the 50,000M+ range) so the fact that every test is maxing out is nothing to worry about.

Unfortunately, because your megger has a low range, and is hand-crank, it makes it not very useful for PM trending: Ideally you want one that will record the really good values so when things start declining to a "not-as-good-but-not-bad" values you can know to pay attention to it. Besides which, even doing a basic absorption test with a crank megger will teach you to hate life, and a PI is basically impossible.

So what you have is basically a Go/No-Go tester that's good for troubleshooting.

The only piece of advice I will give for that megger is if you're writing down values, never write "infinity" because it's a meaningless number. Infinity on one megger might be 10 billion megs, but on yours it would be less than 1000 megs, so if everyone is writing down "Infinity" no one has any idea what the highest measurable value was. Write it as >200M (or whatever the maximum scale is on the tester you're using).
 

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You should be recording PI values as well, you test duration should be 10 minutes to allow the capacitive charging current and polariation currents to receed.

And dont forget to tempature correct your readings to 40C, if you fail to do that your readings are worthless for a PM/trending persective.
 

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THE "BIG RED MACHINE"
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so when you have a low reading on a motor, what do you maintenance gurus do about it?
1. buy a new motor

2.get that motor rewound

3.or have the motor shop steam clean it, bake it out, dip and bake the motor installation

guess it depends on the budget of the factory
 

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Donuts > Fried Eggs
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so when you have a low reading on a motor, what do you maintenance gurus do about it?
1. buy a new motor

2.get that motor rewound

3.or have the motor shop steam clean it, bake it out, dip and bake the motor installation

guess it depends on the budget of the factory
Our motor department will do fractional-horsepower if that's what the customer wants. And for really old or screwball OEM motors, it might even be cost-effective. But frankly for a lot of small motors, they customer often chooses to replace.

Otherwise we do a PI, conductor resistance, and surge test to look for faults or impending turn failures. If it appears that there's just cumulative leakage due to dirt, we go through step 3 and clean and bake, then re-test. Sometimes that alone will restore a motor. Then it will get dipped or VPI'd.

If there's an actual winding fault, then it gets completely re-wound.
 

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so when you have a low reading on a motor, what do you maintenance gurus do about it?
1. buy a new motor

2.get that motor rewound

3.or have the motor shop steam clean it, bake it out, dip and bake the motor installation

guess it depends on the budget of the factory

We almost always replace small AC motors, up to around15-20HP, unless they are something special (I have had a couple of old U frame motors re-wound). After 20HP, we kind of look at having it done, but if the refurbish quote comes back to 65-75% of new, new usually gets the nod.

I have most all of my DC crane motor armatures, main coils and interpoles (overhead crane P&H CR frame split case motors) rewound. My motor man then assembles and tests these motors.

And obviously my big DC mill motors (800,900,1200 HP) get re-worked from a competent area rewind shop.
 

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THE "BIG RED MACHINE"
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Al smelter, Big John where do you guys work?
 
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