Insulation readings on small motor

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philly

Senior Member
I have a small 1.5hp 230/460V motor which is wired for 460V. Upon performing an IR test as part of the acceptance testing of this new motor we noticed that we were getting low readings when tested at 1000V. The highest readings that were reported were 13megaohms.

I instructed the technicians to then perform a DAR test on the motor. The results for this were as follows:

A = 30 sec =9.1
60 sec 14.2
DAR = 1.56

B= 30 sec = 14.1
60 sec = 16.3
DAR = 1.15

C= 30 sec = 14.0
60sec = 16.2
DAR = 1.15

Only the A phase DAR reading was over 1.4 which appeard to be good but the other two were below 1.4. I decided to do a PI test on one of the leads and the one minute and 10 minute were the same at about 14.2 for a PI of 1. I know a PI of one is low and it should be at least 2 or greater.

From this I am going to recommend that the motor is damaged and should not be accepted as is. Do you guys agree with this procedure and recommendation?

Just as a side note, it is very humid out today if that would have any great effect.
 

zog

Senior Member
Location
Charlotte, NC
Typically a PI is only done on motors >200Hp, the absorbsion and polarization currents disapate quicker in a smaller motor so PI readings on a small motor are irrevelant.

Your DAR's are questionable as you stated, that indicates the insulation may be getting brittle and the motor is near end of life, it will most likely fail during shock conditions like start up. For a larger motor it would be time to recondition the motor to restore winding flexiblilty but for a smaller motor like this most people just wait for failure and replace, might be time to order a replacement to have on hand if you dont already.

What was the winding temp when you tested the motor? A Megohm value dosent mean much without knowing that.5M is good for most motors <1kV at 40 degrees C.

Humidty has some effect, and there is no way to correct your readings, testing it after it has been running a while will eliminate that.

I dont think this motor is really damaged, but there is a good chance it has been in storage for a long time.
 

philly

Senior Member
Typically a PI is only done on motors >200Hp, the absorbsion and polarization currents disapate quicker in a smaller motor so PI readings on a small motor are irrevelant.

Your DAR's are questionable as you stated, that indicates the insulation may be getting brittle and the motor is near end of life, it will most likely fail during shock conditions like start up. For a larger motor it would be time to recondition the motor to restore winding flexiblilty but for a smaller motor like this most people just wait for failure and replace, might be time to order a replacement to have on hand if you dont already.

What was the winding temp when you tested the motor? A Megohm value dosent mean much without knowing that.5M is good for most motors <1kV at 40 degrees C.

Humidty has some effect, and there is no way to correct your readings, testing it after it has been running a while will eliminate that.

I dont think this motor is really damaged, but there is a good chance it has been in storage for a long time.

The motor had not yet been run when tested and was at ambient temperature which was about 95deg F so probably about 35 deg C or so.

We tried to start this motor today and when starting it found that the shaft would not move and that the motor appeard to be locked up. When investigating further we found the motor terminal box to be full of rust and appeard to have some water damage. We could not get the motor shaft to turn by hand either. We therefore pulled the motor out of service and are making the contractor find a replacement.

5M is good for most motors <1kV at 40 degrees C.
.

So as a rule of thumb you are saying a motor should be consided acceptable if reading 5 Megs when tested at 1000V and 40deg C. If it is below 5 Megs but a DAR or PI checks out o.k. then would you still call this acceptable?

Should a DAR or PI always be the first approach and hold more weight then then the IR readings?

So lets say the motor has a 10meg reading. Although this is above 5 megs is this a good reading that would indicate long life in the motor?
 

zog

Senior Member
Location
Charlotte, NC
So as a rule of thumb you are saying a motor should be consided acceptable if reading 5 Megs when tested at 1000V and 40deg C. If it is below 5 Megs but a DAR or PI checks out o.k. then would you still call this acceptable?

Will it still run, sure, for how long is the question. The minimum recommended IR values for motors(Per ANSI, NETA, and MENA), corrected to 40 degrees C are as follows:

1. IR (60 sec)=kV +1 for most windings made before 1970, all field windings, and others not described in #2 and #3

2. IR (60 Sec) = 100M for most DC armature and AC windings built after 1970 (form would coils)

3. IR (60 sec) = 5M for most machines and random wound stator coils and form wound coils rated <1kV.

Should a DAR or PI always be the first approach and hold more weight then then the IR readings?

THey are telling you 2 different things and both need to be considered. The IR value tells you the dielectric strength of the insulation. The DAR tells you the "flexibility" of the insulation. Brittle insulation will give you a low DAR. A pinhole or crack in the insulation can give you a good DAR but a low IR.

So lets say the motor has a 10meg reading. Although this is above 5 megs is this a good reading that would indicate long life in the motor?

Trending is the key here, is 10M good? Well if it was 100M last year when it was tested 10M is pretty bad. If it has been 10M for the last 5 years straight, then 10M is pretty good. The real value of IR readings is trending them over time to predict failures.
 

philly

Senior Member
Trending is the key here, is 10M good? Well if it was 100M last year when it was tested 10M is pretty bad. If it has been 10M for the last 5 years straight, then 10M is pretty good. The real value of IR readings is trending them over time to predict failures.

What about for a new motor that is being tested as part of a QA/QC procedure during new construction. What would you say to this 10M example?
 

zog

Senior Member
Location
Charlotte, NC
What would be the minimum you would set for new. Maybe 25M or 50M?

I dont have a ton of experience with small LV motors, I am a MV guy. Maybe someone else with more small motor experience can chime in here. But, comparing it to other similar motors is a no brainer.
 

philly

Senior Member
I dont have a ton of experience with small LV motors, I am a MV guy. Maybe someone else with more small motor experience can chime in here. But, comparing it to other similar motors is a no brainer.

We have other several other LV motors here all the way up to 200hp. So I was looking for a new acceptance testing rule of thumb for all these motors in this range.
 

philly

Senior Member
Well the specs I posted are from the NETA/ANSI Acceptance Testing Standard.

So if any LV motor was over 5M then you would accept it (although reading is somewhat low).

Anything under 5M then you would not accept. Or would you do a DAR and consider the results.
 

zog

Senior Member
Location
Charlotte, NC
So if any LV motor was over 5M then you would accept it (although reading is somewhat low).

Anything under 5M then you would not accept. Or would you do a DAR and consider the results.

I always do a DAR, and the results you gave are "Questionable" per NEMA standards, however the NETA ATS only says it need to be >5M and DAR >1.0. If I were doing the testing I would accept it but would note in the test report that both results borderline and not normal for a new motor, I would point this out to the "owner" of the equipment and it would be thier call, but I could not "fail" the motor.
 

philly

Senior Member
I always do a DAR, and the results you gave are "Questionable" per NEMA standards, however the NETA ATS only says it need to be >5M and DAR >1.0. If I were doing the testing I would accept it but would note in the test report that both results borderline and not normal for a new motor, I would point this out to the "owner" of the equipment and it would be thier call, but I could not "fail" the motor.

In this case I am the owner, and I have one of the plant electricans performing this acceptance tests. He comes to me for close calls like this so I am just trying to gain a better understanding of what I should accept and not accept and to make calls like this.
 

zog

Senior Member
Location
Charlotte, NC
In this case I am the owner, and I have one of the plant electricans performing this acceptance tests. He comes to me for close calls like this so I am just trying to gain a better understanding of what I should accept and not accept and to make calls like this.

I am not helping much am I? :)

I would try to reject it and have the suppier provide a replacement, but if they want to fight you on it you dont have much to stand on because it "Passes".
 

76nemo

Senior Member
Location
Ogdensburg, NY
I dont have a ton of experience with small LV motors, I am a MV guy. Maybe someone else with more small motor experience can chime in here. But, comparing it to other similar motors is a no brainer.


I was going to chime in, but after post# 5, there's not a whole lot left to say.
 
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