Motor insulation resistance

[mike3671]

What is an acceptable limit for insulation resistance for a motor. I've always gone with 1MegOhm from table 61 although to be fair if its that low I would do a bit more investigation or replace/send for repair.

However I've come across some literature with a new motor that states to check its above 10meg which seems slightly high to me. I've tried to pin down a definitive answer but keep coming back to 1Meg

Does anyone know for sure what the limit is and where it states it.

Thanks

Mike

 

[rtonne10]

Mike,

I had a problem with a motor popping fuses and meggered it and found the ir was 1.3Mohm. Thought that was ok so started to check that the impeller for the pump wasn't seized. It wasn't. We then swapped both pumps over in the panel to check that it wasn't fault on panel. The other pump ran away fine and the other pump popped the fuse on the other starter section. We then meggered the other pump and got 199Mohm on the megger. I now only accept 199Mohm on my megger (99% of tests show this). Anything lower would indicate to me that the insulation is starting to break down.

That may be a bit over the top but at least the client would Know that there may be a fault about to develop and the motor may need to be replaced or re-wound soon.

 

[mike3671]

I agree with what you are saying.Unfortunately the food factory where I work loves to hose everything down so even if the motor is not directly soaked there is a lot of moisture in the air hence a lot of our motors tend to have less than 199Mohm, so we tend to change only if below 1meg or if it causes problems otherwise the rewind company would be making a fortune of us

I was more curious as to what the legal limit according to the regs is as I'm sure I'm right with 1 Meg its just the motor literature thats confused me a little.

 

[rtonne10]

Yeh thats what I taught to take as a minimum value. I think that is what is stated in the regs. If the manufacturer specifies 10Meg then I would take that as the minimum. If it is below that I would contact them and then have it replaced if they say it has to be at least 10Meg.

They may ask what type of environment it is installed in and try and use that as an excuse for not replacing it.

I know this will sound like an obvious answer but check the IP rating of the terminal box and make sure all seals have not been broken.

 

[mike3671]

Thanks for confirming that. I don't think there is an IP rating high enough to defeat our hygiene team

Mike

 

[rtonne10]

Yeh know the feeling. We had porters who where cleaning a kitchen and hosed down the control panel for the walking freezers when they where cleaning. What a mess that left.

 

[Knobhead]

I'm an ex M&Q electrician, as an apprentice I was taught the the minimum IR to earth or between windings was 1/10,000th leakage based on the FLC of the motor. Simple to work out using Ohms law.
But remember
winding to winding is calculated at phase – phase voltage
winding to earth is calculated at phase - neutral voltage

Just use phase – phase voltage it gives you just one reading to aim for, and it's quicker

 

[Knobhead]

Thanks for confirming that. I don't think there is an IP rating high enough to defeat our hygiene team

Mike

There is, I can’t remember the IP rating.
Look for “Deluge Proof” motors, switchgear etc

I worked as a supervisor in a frozen chip factory for a while. The hygiene mob were pretty indiscriminate about what got washed down. Including me after I upset them

 

[pmac10]

i also work in a food factory and the policy in our joint is 1M.ohm + 1K.ohm per volt, or 1.415M.ohm for 3 phase motors. i have read that elsewhere but cant mind where. Hygiene teams must be same the world over.

 

[silva.foxx]

A little off topic but for you guys suffering washdown syndrome:

Marlin Stainless - Metric Stainless Steel Motors


Circuits up to 500v must have a minimum insulation resistance of 1.0 Mega ohms when tested at 500v dc with all current using equipment disconnected. 612.3.2

from 17th Edition Insulation

from http://www.greenageproducts.com/resources/05-010/SEW%20BRAKE%20MOTORS.pdf related to Long Term Storage, but why should it be any different to in-service...

If the motor must be stored for a long period of time without operating,
the motor must be stored in a dry, protected area, and in the mounting position
indicated on the unit nameplate. In order to ensure that the motor
has not been damaged by moisture after a prolonged storage, the insulation
resistance should be checked. An insulation tester with a measurement
voltage of at least 500V (e.g. magneto generator) should be used for
this purpose. The insulation resistance is sufficient if it has an ohmic
value of at least 1000 x V​

​

​

​

N (e.g. at VN = 230VAC: Rinsul 230000 ohms =
0.23M ohms). If the measured value is smaller, the motor should be dried
before use (for example, with hot air up to a maximum of 90°C or by resistance
heating with an auxiliary AC voltage of 10% of VN via an isolating
transformer). Care should be taken to ensure that the motor is heated
with not more than 20% of its rated current and that the rise in temperature
is not more than 90°C. The drying procedure can be stopped when​

the insulation resistance has reached 500000 = 0.5M ohms.

 

[mike3671]

Thanks again for all the replies.Good to know we are not alone in our hygiene problem.Unfortunately due to our budget upgrading to deluge proof isn't ever going to happen.

 

[ordnator]

I work in an Abattoir and water ingress is a pain as are those responsible for operating the jet hoses! I used to work on ships and ofcourse it was the same story there. I have found the guidance in the regs as far as minimum values go to be lower than what I would normally be satisfied with, if I measured 1M ohm I would be suspicous of a latent defect. Certainly I would be looking for much greater than 10M. Not practical always I know, I simply base this on expieriance from working in wet electrial enviorments for a fair while. As has been suggested I would apply some heat so I was sure the entire unit was devoid of all moisture.