Low insulation readings on small motors

[Pitt123]

We have several small motors for a new plant that we are finding to have low insulation readings. Most of these motors are in the 1hp - 10hp range. Some of the readings are below 1Megohm however others are around 5Megohm or so. The standard for the job is we only accept motors with readings 50Megohms or over, but I have a feeling that was written from someone who didn't know much about the subject.

I know as a rule of thumb that you dont want to energize anything reading less than 1Megohm however what about the ones where we are reading 5Megohms?

Even if we are getting low 5Megaohm readings, if we get a good DAR, or PI value should these still be o.k. to accept? What values would we want to see? I find it strange that soo many motors are reading low, and now we are either having to dry them out or replace them.

Would you say that for the ones less than 1Meg we should replace but the ones around 5Meg we should do second DAR or PI test for acceptance.

Also on a 6 lead motor we had on of the phases read 20meg to ground while the other two read 5megs to ground when wired in delta, but when wired in wye all three legs read 5megs to ground. What would cause this high 20meg reading when wired in Delta?

 

[steve66]

What are you using to get the readings? I'm not an expert on insulation readings, but I'm wondering if you are using a voltage that is too high.

Also on a 6 lead motor we had on of the phases read 20meg to ground while the other two read 5megs to ground when wired in delta, but when wired in wye all three legs read 5megs to ground. What would cause this high 20meg reading when wired in Delta?

That sounds reasonable. Each winding have a very small series resistance. And you connect them all together at one point for a Wye. So the resistance to ground for a wye (measured from any of the terminals) is going to be about the same, and somewhat lower than the lowest resistance of any single winding.

Steve

 

[RichB]

Here is what Naval Sea Systems uses and Iv'e always used it as a base if I don'tr have any history

1.1 tmes the applied voltage

so, a 480 volt motor would be 1.1(480)=4.8Megs minimun

 

[zog]

We have several small motors for a new plant that we are finding to have low insulation readings. Most of these motors are in the 1hp - 10hp range. Some of the readings are below 1Megohm however others are around 5Megohm or so. The standard for the job is we only accept motors with readings 50Megohms or over, but I have a feeling that was written from someone who didn't know much about the subject.

I know as a rule of thumb that you dont want to energize anything reading less than 1Megohm however what about the ones where we are reading 5Megohms?

Even if we are getting low 5Megaohm readings, if we get a good DAR, or PI value should these still be o.k. to accept? What values would we want to see? I find it strange that soo many motors are reading low, and now we are either having to dry them out or replace them.

From current NETA MTS
The recommended minimum insulation resistance (IR1 min) test results in megohms should be corrected to 40? C and read as follows:
1. IR 1 min = kV + 1 for most windings made before 1970, all field windings, and others not
described in 2.2 and 2.3.
(kV is the rated machine terminal-to-terminal voltage, in rms kV)
2. IR 1 min = 100 megohms for most dc armature and ac windings built after 1970 (formwound
coils).
3. IR 1 min = 5 megohms for most machines and random-wound stator coils and formwound
coils rated below 1 kV.
NOTE: Overpotential, high-potential, and surge comparison tests shall not be
performed on machines having values lower than those indicated above.

Would you say that for the ones less than 1Meg we should replace but the ones around 5Meg we should do second DAR or PI test for acceptance.

We always DAR or PI every motor, you can have acceptable IR readings and still have a bad DAR or PI that signifies insulation damage, they test different properties of the insulation.

Also on a 6 lead motor we had on of the phases read 20meg to ground while the other two read 5megs to ground when wired in delta, but when wired in wye all three legs read 5megs to ground. What would cause this high 20meg reading when wired in Delta?

Sketch out a wye and delta winding configuration and assign 5 ohms to each, then calulate the values you measured, series or paralell resistances.

 

[Pitt123]

From current NETA MTS
The recommended minimum insulation resistance (IR1 min) test results in megohms should be corrected to 40? C and read as follows:
1. IR 1 min = kV + 1 for most windings made before 1970, all field windings, and others not
described in 2.2 and 2.3.
(kV is the rated machine terminal-to-terminal voltage, in rms kV)
2. IR 1 min = 100 megohms for most dc armature and ac windings built after 1970 (formwound
coils).
3. IR 1 min = 5 megohms for most machines and random-wound stator coils and formwound
coils rated below 1 kV.
NOTE: Overpotential, high-potential, and surge comparison tests shall not be
performed on machines having values lower than those indicated above.

So for a 10 hp motor built after 1970 and less than 1kV would you use the case #3 value of 5 megohms be the minumum value. Would this be the minimum for accepting the new motor, or for energizing the motor under any circumstance?

Even if we had as low as 1megohm wouldn't this still only lead to 277V/1megohm = .00027A of ground current?

We always DAR or PI every motor, you can have acceptable IR readings and still have a bad DAR or PI that signifies insulation damage, they test different properties of the insulation.
.

Which property does each one measure?

Sketch out a wye and delta winding configuration and assign 5 ohms to each, then calulate the values you measured, series or paralell resistances.

But even with the different configurations, each phase should read the same and shouldn't have on odd ball high reading such as the 20 megohm value listed above. I'm also having a hard time visualizing how why and delta configurations will combine differenty to ground as a parallel circuit. Would they even be parallel at all since we are measuing to ground and not across the windings?

 

[Cow]

I'm not an expert at insulation testing like Zog, but those readings sound very low to me. I hope you have spares ready to go!

 

[zog]

So for a 10 hp motor built after 1970 and less than 1kV would you use the case #3 value of 5 megohms be the minumum value. Would this be the minimum for accepting the new motor, or for energizing the motor under any circumstance?

These are the minimum acceptable values for ANSI, NETA, and NEMA standards. As a certified test tech my job is to inform the owner of the equipment of the hazards of running a motor less than these values. What they actually do is thier call. Keep in mind these are tempature corrected values, you did not state if you had temature corrected your readings to 40C. Makes a huge difference.

Even if we had as low as 1megohm wouldn't this still only lead to 277V/1megohm = .00027A of ground current?

That is current flowing through the insulation, and is enough leakage current to cause insulation failure to be imminent.

Which property does each one measure?

When a high DC voltage is first applied the total current consists of three current components;

1.Leakage current
2.Capacitance charging current
3.Absorption current

Conduction/Leakage Current - This current passes through the surface of the insulation. The magnitude of current flow depends on the resistance of the insulation. Surface leakage is usually not a problem because it can be eliminated through external cleaning.

Capacitive Current - The insulating specimen appears, ideally, as a capacitor. As a DC voltage is applied to a capacitor initial charging current flows until the voltage drop across the component equals the source voltage. As the capacitor charges, its charging current decreases to a minimum. This is called its steady-state value.
Dielectric Absorption Current - This current also appears at the initial application of test voltage the same as capacitance current. This current is required to polarize the insulating medium. In other words it is energy absorbed by the insulating system.

These 3 different currents measure the 3 main properties of the insulation. While you cannot seperate the currents they decay at different times and are measured by calculation the DAR and PI of the insulation.

 

[zog]

But even with the different configurations, each phase should read the same and shouldn't have on odd ball high reading such as the 20 megohm value listed above. I'm also having a hard time visualizing how why and delta configurations will combine differenty to ground as a parallel circuit. Would they even be parallel at all since we are measuing to ground and not across the windings?

You know, I brain farted there and was thinking winding resistance. But all 3 phases should be the same and the 20M odd ball makes no sense. Unless you have an open winding. Have you done winding resistance?

 

[Pitt123]

These are the minimum acceptable values for ANSI, NETA, and NEMA standards. As a certified test tech my job is to inform the owner of the equipment of the hazards of running a motor less than these values. What they actually do is thier call. Keep in mind these are tempature corrected values, you did not state if you had temature corrected your readings to 40C. Makes a huge difference.

That is current flowing through the insulation, and is enough leakage current to cause insulation failure to be imminent.




When a high DC voltage is first applied the total current consists of three current components;

1.Leakage current
2.Capacitance charging current
3.Absorption current

Conduction/Leakage Current - This current passes through the surface of the insulation. The magnitude of current flow depends on the resistance of the insulation. Surface leakage is usually not a problem because it can be eliminated through external cleaning.

Capacitive Current - The insulating specimen appears, ideally, as a capacitor. As a DC voltage is applied to a capacitor initial charging current flows until the voltage drop across the component equals the source voltage. As the capacitor charges, its charging current decreases to a minimum. This is called its steady-state value.
Dielectric Absorption Current - This current also appears at the initial application of test voltage the same as capacitance current. This current is required to polarize the insulating medium. In other words it is energy absorbed by the insulating system.

These 3 different currents measure the 3 main properties of the insulation. While you cannot seperate the currents they decay at different times and are measured by calculation the DAR and PI of the insulation.

Well I guess my delima is that as I mentioned earlier we have several motors as part of a new construction project that have been sitting in place for well over a year now. Now that we are getting ready to energize them we are performing insulation tests on them and finding that several of them are reading low and well below our spec of 50megaohms. People here are very quick to jump the gun and say that because it is below 50megaohms that we need to send them out to be dried or repaired. As you can guess, this turns out to be a pretty costly process, as well as delays the startup of equipment while vendors are on-site.

I want to be able to make a better educated decision on weather the motors should be accepted or not rather then just say below 50megs is the cutoff and have to send every motor (over 10) out for repair wasting everyones time and money. If you were in my shoes how would you approach this? For the smaller 10-15 hp motors would you say that 5-10 megs is an acceptable value? Would you back this up by doing a DAR or PI? What if readings were below 5 megs?

One example was a 350hp motor that was reading 11 megs. The DAR on this motor was 1.0. What kind of judgement call would you make here. Keep it or have to hassle with sending this large motor out?

 

[Pitt123]

That is current flowing through the insulation, and is enough leakage current to cause insulation failure to be imminent.

Yes I realize it will cause imminent insulation failure, but if the motor is energized I wouldn't think it woud fault or explode right away would it?

I'm not saying I would try this, I just want to understand.

 

[zog]

Yes I realize it will cause imminent insulation failure, but if the motor is energized I wouldn't think it woud fault or explode right away would it?

I'm not saying I would try this, I just want to understand.

Some time period between right away and a couple years.

 

[zog]

Well I guess my delima is that as I mentioned earlier we have several motors as part of a new construction project that have been sitting in place for well over a year now. Now that we are getting ready to energize them we are performing insulation tests on them and finding that several of them are reading low and well below our spec of 50megaohms. People here are very quick to jump the gun and say that because it is below 50megaohms that we need to send them out to be dried or repaired. As you can guess, this turns out to be a pretty costly process, as well as delays the startup of equipment while vendors are on-site.

They have been sitting that long? Well if you meggered at ambeint temp of 77F your corrected values will be exactly half of your measured value. So if you did not correct them they are twice as bad as you thought.

I want to be able to make a better educated decision on weather the motors should be accepted or not rather then just say below 50megs is the cutoff and have to send every motor (over 10) out for repair wasting everyones time and money. If you were in my shoes how would you approach this? For the smaller 10-15 hp motors would you say that 5-10 megs is an acceptable value? Would you back this up by doing a DAR or PI? What if readings were below 5 megs?

Yes I would do DAR, always. If the IR is less than the specs I posted I would bake them.

One example was a 350hp motor that was reading 11 megs. The DAR on this motor was 1.0. What kind of judgement call would you make here. Keep it or have to hassle with sending this large motor out?

DAR of 1.0 is unacceptable. Replace that one, baking won't help much.

 

[Pitt123]

They have been sitting that long? Well if you meggered at ambeint temp of 77F your corrected values will be exactly half of your measured value. So if you did not correct them they are twice as bad as you thought.
.

75 deg F converts to about 24deg C. So to correct this value to 40deg C I would need to halve my readings due to the fact that when you increase in correction you divide and when you decrease to correct you multiply?

Yes I would do DAR, always. If the IR is less than the specs I posted I would bake them.

DAR of 1.0 is unacceptable. Replace that one, baking won't help much.

What would you use as an acceptable DAR? Do you have ranges for what will be an acceptable DAR?

Why would a DAR of 1 indicate replacing motor? What does this indicate that baking wont help?

So do the the different time values for the duration of these tests corrospond to the three different types of currents that you posted earlier?

 

[Pitt123]

On somewhat of a different note, we are starting a PM program on several existing motors we have and just purchased a new meter that automatically produces DAR and PI values.

When doing a PM program as such I wouldn't think that the initial IR values would mean as much as they would during new acceptance tests of the motors. For instance using the below 50megaohm critera for existing motor really wont mean much. I would think in this case it would be how these readings changed over time that would be important and recording these values would be important.

However with a program like this, are there minimum numbers for IR and DAR that should raise a flag and say we better look at replacing this motor now!

 

[zog]

75 deg F converts to about 24deg C. So to correct this value to 40deg C I would need to halve my readings due to the fact that when you increase in correction you divide and when you decrease to correct you multiply?

I have attached the correction factors for 40C.

What would you use as an acceptable DAR? Do you have ranges for what will be an acceptable DAR?

1.0-1.25 - Questionable
1.4-1.6 - Good
>1.6 Excellent

Why would a DAR of 1 indicate replacing motor? What does this indicate that baking wont help?

A low DAR indicates brittle insulation, most likely cause by overheating at some point. A low IR can be caused by moisture in the insulation which can be baked out, but baking won't help brittle insulation.

So do the the different time values for the duration of these tests corrospond to the three different types of currents that you posted earlier?

Yes, I have attached a graph illistrating the currents measured.

 

[zog]

On somewhat of a different note, we are starting a PM program on several existing motors we have and just purchased a new meter that automatically produces DAR and PI values.

When doing a PM program as such I wouldn't think that the initial IR values would mean as much as they would during new acceptance tests of the motors. For instance using the below 50megaohm critera for existing motor really wont mean much. I would think in this case it would be how these readings changed over time that would be important and recording these values would be important.

However with a program like this, are there minimum numbers for IR and DAR that should raise a flag and say we better look at replacing this motor now!

Trending of IR test results is a key part of your predictive maintenence. It allows you to predict failures before they happen, but if you do not temp correct your values the trending is meaningless. I suggest you get a copy of "A stich in time" or some training, my job used to be providing custom maintenence training for power systems in industrial plants, there are still some good companies that provide this training.