Insulation Test

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nazirie

Member
Location
Malaysia
Hi all,

I'm Nazirie. I have a question about the insulation testing. Actually, i facing a problem with our submersible pump (brand:Grindex). Before we start our operation onboard drilling rig, my trusted technician do the insulation testing on the motor. Without any supervision from someone else, the test conducted wrongly as i saw the picture how he do that thing. 2 test probe of the megger, connected to the end of U1 & U2 with injected volt 1000VDC (Kyoritsu 3007A). Same for all phases.

Before this thing happen, the pump already operate and no problem occur. But when this pump has been backload to onshore, and the testing was conducted, the problem occur. Our personnel that operate the pump reported the pump trip after start about a minutes. The rig electrician clamp the cable and found the ampere 400A.

When i investigate this things, no problem on mechanical parts. I conducted the insulation test and the value shown up around 200-300MOhm. I used same meter, Kyoritsu 3007A with selected range 2000MOhm instead of 20MOhm and 200MOhm range.

The question is, why the Ampere too large. According in the manual, the insulation should more than 1MOhm. Is't 200-300MOhm consider bad insulation? Is't the wrong doing by my technician damage the insulation?


Regards,
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Two reasons for high current with no fault to ground showing are a turn to turn short in one winding and a winding to winding short in a dual volt or three phase motor.
 

just the cowboy

Inactive, Email Never Verified
Location
newburgh,ny
No the tech did not do it.

No the tech did not do it.

The test the tech did, did not screw anything up if the motor was disconnected. The megger does not generate enough current to damage the windings that way. On the other hand if this motor is controlled with a VFD or soft starter and the test was done with the motor connected then you may have a problem with the drive.
 

nazirie

Member
Location
Malaysia
Thanks for the info. We just start this pump by using DOL starter. When the insulation was conducted, all connection open. Only the leads of the winding are left. I get remind that the starting current maybe high but, with just a few miliseconds the current will stable. In my case, the current remain the same after 30-60s and the thermal overload relay tripped that pump. Any idea guys?..
 

nazirie

Member
Location
Malaysia
I'll gather all the information and discuss for further details. Maybe i need to check back the continuity or the resistance value for each phase
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I'll gather all the information and discuss for further details. Maybe i need to check back the continuity or the resistance value for each phase
A short between adjacent turns in a winding will not show up in winding resistance measurements made with DC.
An impedance meter that uses an AC test signal can detect shorted windings.
 

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
Digital Low Resistance Ohmmeter

Digital Low Resistance Ohmmeter

A short between adjacent turns in a winding will not show up in winding resistance measurements made with DC.
An impedance meter that uses an AC test signal can detect shorted windings.

Also, if the resistance range is in milli-ohms or micro-ohms then a DLRO (10A
DC) will measure shorted turns. There are many brands on the market but I use a Biddle Cat# 247001. Very compact and portable that can find shorted turns on motors and transformers.

https://www.instrumart.com/assets/Megger-DLRO-247000-Manual.pdf
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Also, if the resistance range is in milli-ohms or micro-ohms then a DLRO (10A
DC) will measure shorted turns. There are many brands on the market but I use a Biddle Cat# 247001. Very compact and portable that can find shorted turns on motors and transformers.

https://www.instrumart.com/assets/Megger-DLRO-247000-Manual.pdf

If you have 100 turns in a winding and there is a short between adjacent turns this will only make a 1% difference in winding resistance. If the three windings of the motor are initially matched to better than .5% or if you have had the foresight to take reference readings of the three windings when the motor was new, then the DC test can indicate that shorted turn. Minimizing variance in the connection resistance of the test lead to winding connection is critical for this method to work.
On the other hand an impedance meter that uses AC may show a factor of two difference in impedance between the damaged and good windings.
 

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
If you have 100 turns in a winding and there is a short between adjacent turns this will only make a 1% difference in winding resistance. If the three windings of the motor are initially matched to better than .5% or if you have had the foresight to take reference readings of the three windings when the motor was new, then the DC test can indicate that shorted turn. Minimizing variance in the connection resistance of the test lead to winding connection is critical for this method to work.
On the other hand an impedance meter that uses AC may show a factor of two difference in impedance between the damaged and good windings.

Interesting analysis. The 1% applies to adjacent turns. I doubt a short this small would cause any operational problems. But in the real world we find that most problems occur when comparison readings are less than a certain percentage (5, 10% ?) or open circuit. Some manufacturers can provide the new baseline values, especially in the larger HP units for a more precise analysis.
In the case of power transformers a Turns Ratio Tester that employs the Wheatstone bridge (null balance) principle is a more accurate instrument to determine the integrity of the windings by comparing ratio percentages.
 

nazirie

Member
Location
Malaysia
Interesting analysis. The 1% applies to adjacent turns. I doubt a short this small would cause any operational problems. But in the real world we find that most problems occur when comparison readings are less than a certain percentage (5, 10% ?) or open circuit. Some manufacturers can provide the new baseline values, especially in the larger HP units for a more precise analysis.
In the case of power transformers a Turns Ratio Tester that employs the Wheatstone bridge (null balance) principle is a more accurate instrument to determine the integrity of the windings by comparing ratio percentages.

Means that, even with 1% of short, the problem will occur such as high running amp? My insulation tester have a value 0.98 ohm when both of the probe short together. So, my tester can't detect the short winding right? I'm already inspect the winding and nothing seems a problem. Also, the insulation resistance still showed the good result between 300-500 MOhm. Clearly, I need a new testing meter.
 

kwired

Electron manager
Location
NE Nebraska
Turn to turn short that only results in 1% change in impedance probably doesn't present an immediately noticeable problem.

However it likely does introduce additional heat at that fault point which will eventually lead to further damage whether it be short to additional turns or even burn itself into an open circuit - eventually becoming a noticeable problem.
 
Bad Motor Test

Bad Motor Test

It sounds like you have other problems, like a bad bearing or a siezed shaft through bearing. If the coil's insulation tests good, the only thing that could be happening is an improper connection, or, siezed or close to siezed bearings making it too hard to spin. If it is a pump, maybe something is caught inside preventing it from spinning correctly.

Hi all,

I'm Nazirie. I have a question about the insulation testing. Actually, i facing a problem with our submersible pump (brand:Grindex). Before we start our operation onboard drilling rig, my trusted technician do the insulation testing on the motor. Without any supervision from someone else, the test conducted wrongly as i saw the picture how he do that thing. 2 test probe of the megger, connected to the end of U1 & U2 with injected volt 1000VDC (Kyoritsu 3007A). Same for all phases.

Before this thing happen, the pump already operate and no problem occur. But when this pump has been backload to onshore, and the testing was conducted, the problem occur. Our personnel that operate the pump reported the pump trip after start about a minutes. The rig electrician clamp the cable and found the ampere 400A.

When i investigate this things, no problem on mechanical parts. I conducted the insulation test and the value shown up around 200-300MOhm. I used same meter, Kyoritsu 3007A with selected range 2000MOhm instead of 20MOhm and 200MOhm range.

The question is, why the Ampere too large. According in the manual, the insulation should more than 1MOhm. Is't 200-300MOhm consider bad insulation? Is't the wrong doing by my technician damage the insulation?


Regards,
 

Saturn_Europa

Senior Member
Location
Fishing Industry
Occupation
Electrician Limited License NC
What are the resistance readings from phase to phase? They should be very low, just a couple of ohms. I usually use a regular multimeter for this reading.
 

MattS87

Senior Member
Location
Yakima, WA
It sounds like you have other problems, like a bad bearing or a siezed shaft through bearing. If the coil's insulation tests good, the only thing that could be happening is an improper connection, or, siezed or close to siezed bearings making it too hard to spin. If it is a pump, maybe something is caught inside preventing it from spinning correctly.

I have had multiple motors meggar good and the motor was still the problem. One in particular was on a VFD which I could run manually. Motor worked just fine until the drive got above 12-13Hz then it would kick out on ground fault.

Can you un-couple the motor from the gearbox or take off the belts to see if the motor runs ok by itself? If so, take amp readings with no load to see if they are high.
 

Saturn_Europa

Senior Member
Location
Fishing Industry
Occupation
Electrician Limited License NC
I have had multiple motors meggar good and the motor was still the problem. One in particular was on a VFD which I could run manually. Motor worked just fine until the drive got above 12-13Hz then it would kick out on ground fault.

Can you un-couple the motor from the gearbox or take off the belts to see if the motor runs ok by itself? If so, take amp readings with no load to see if they are high.


Excellent advice. But I don't think it's applical in this situation. OP mentioned that it's a submersible pump. The pump impeller is pressed/fastened directly to the rotor shaft.

Welcome to the forum!
 
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