3 phase motor

[mcclary's electrical]

I know it's possible to find and verify leakage current by using an amp clamp around both leads of a single phase motor(or circuit). My question is, can you do the same with a 3 phase motor? If you have an amp clamp around all 3 leads of a 3 phase motor, will they cancel each other to zero? with the exception of the amount that is leaking? If you had a motor pulling 44.5 amps per phase, but around all 3 you read 3.1 amps, and there is 3.1 amps flowing on the EGC, what does that tell you? if anything

 

[eric9822]

You are correct. A clamp on amp meter around all threee phases will read 0 amps since the three phase currents will cancel each other out. If you are reading 3.1 amps you have a ground fault of 3.1 amps.

 

[LarryFine]

My question is, can you do the same with a 3 phase motor? If you have an amp clamp around all 3 leads of a 3 phase motor, will they cancel each other to zero? with the exception of the amount that is leaking?

Yes, yes, and yes. Single-phase or three-phase, the science is the same. It's just like grouping phases and the recent 240v-GFCI thread.

If you had a motor pulling 44.5 amps per phase, but around all 3 you read 3.1 amps, and there is 3.1 amps flowing on the EGC, what does that tell you?

Exactly what you think. Unless there's some weird induction going on, you're measuring leakage current, as matching 3.1a numbers show.

 

[mcclary's electrical]

Does the motor being controlled by a VFD change either of your opinions'?

I was called to this utility well pump by the town I live in. The motor has an hour meter in the controller, so they have verified that the motor is not running anymore then usual. The past year the utility bill has increased by 50% for the same amount of run time as the previous years, and at the same KWH charge. 200 amp 3 phase 208 volt service, one well pump, numerous controllers, one small heater, 3 strip lights. I think the motor is on it's way down.

Does a drive change your opinion? still accurate readings?

 

[LarryFine]

Does a drive change your opinion? still accurate readings?

Hmmm. Have you tried this on both the input and output of the drive?

 

[eric9822]

Hmm is right, never tried that. Clamp ons are not reliable on VFD outputs unless they are high dollar true RMS models and even then I usually don't use them and rely on the drive display. I would think that the error would be consistent though so you would read leakage current although the reading may be off. Can you run the drive at 60 Hz while checking? Have you meggered the motor and feeders?

 

[Jraef]

99.9% of modern VFDs on the market have output Ground Fault monitoring (residual current method) built-in and would shut down if there was not balanced current in all 3 legs of the output. They need that to protect the transistors.

Ditto on using clamp-ons to read the output of a VFD, the likelihood of you having one capable of accurately doing that are remote, so most of the time anything you read is unreliable because of the output harmonics.

Other possibilities:
1) Someone put a VFD in where it wasn't going to do any good. A Salesman sold them on the "energy savings" concept, without explaining that if you run the thing full speed all the time, it will actually use MORE energy because the VFD is only 97% efficient at best. So unless there is a significant speed turn-down, there may not have been a good energy savings justification to do it. Happens all the time unfortunately.

2) There is a problem in the front-end of the VFD at the bridge rectifier. A high resistance GF there may not be enough to cause the OCPD to clear, but just pumps energy into the ground. That would show up as an imbalance on the input current, but again, be careful of false readings from cheap meters. A Power Quality Analyzer would be a better choice.

3) The VFD's harmonics are causing the utility's meter to go squirrely and record more energy use than you really have. A recording meter would verify this.

 

[mcclary's electrical]

Hmmm. Have you tried this on both the input and output of the drive?

Sadly, No. The panel is really tight, and the guy was looking over my shoulder, didn't want to disturb too much.

 

[mcclary's electrical]

Hmm is right, never tried that. Clamp ons are not reliable on VFD outputs unless they are high dollar true RMS models and even then I usually don't use them and rely on the drive display. I would think that the error would be consistent though so you would read leakage current although the reading may be off. Can you run the drive at 60 Hz while checking? Have you meggered the motor and feeders?

This was a spur of the moment call, and it was online when I got there. It would take alot of coordinating of downtime to be able to meg it. This is a midway pumping station between two towns, being shared by two towns and two pumping stations. I will be megging the motor and wires, but haven't yet.

 

[mcclary's electrical]

99.9% of modern VFDs on the market have output Ground Fault monitoring (residual current method) built-in and would shut down if there was not balanced current in all 3 legs of the output. They need that to protect the transistors.

Ditto on using clamp-ons to read the output of a VFD, the likelihood of you having one capable of accurately doing that are remote, so most of the time anything you read is unreliable because of the output harmonics.

Other possibilities:
1) Someone put a VFD in where it wasn't going to do any good. A Salesman sold them on the "energy savings" concept, without explaining that if you run the thing full speed all the time, it will actually use MORE energy because the VFD is only 97% efficient at best. So unless there is a significant speed turn-down, there may not have been a good energy savings justification to do it. Happens all the time unfortunately.

2) There is a problem in the front-end of the VFD at the bridge rectifier. A high resistance GF there may not be enough to cause the OCPD to clear, but just pumps energy into the ground. That would show up as an imbalance on the input current, but again, be careful of false readings from cheap meters. A Power Quality Analyzer would be a better choice.

3) The VFD's harmonics are causing the utility's meter to go squirrely and record more energy use than you really have. A recording meter would verify this.

This is exactly my worry, I tried several clamps I have, and they all seem to match the readout on the screen, but there's no way to capture all 3 from the screen instantaneously and compare. Even if my amp meter was not reading all 3 together due to harmonics, what about the EGC flowing current?

 

[mcclary's electrical]

99.9% of modern VFDs on the market have output Ground Fault monitoring (residual current method) built-in and would shut down if there was not balanced current in all 3 legs of the output. They need that to protect the transistors.

Ditto on using clamp-ons to read the output of a VFD, the likelihood of you having one capable of accurately doing that are remote, so most of the time anything you read is unreliable because of the output harmonics.

Other possibilities:
1) Someone put a VFD in where it wasn't going to do any good. A Salesman sold them on the "energy savings" concept, without explaining that if you run the thing full speed all the time, it will actually use MORE energy because the VFD is only 97% efficient at best. So unless there is a significant speed turn-down, there may not have been a good energy savings justification to do it. Happens all the time unfortunately.

2) There is a problem in the front-end of the VFD at the bridge rectifier. A high resistance GF there may not be enough to cause the OCPD to clear, but just pumps energy into the ground. That would show up as an imbalance on the input current, but again, be careful of false readings from cheap meters. A Power Quality Analyzer would be a better choice.

3) The VFD's harmonics are causing the utility's meter to go squirrely and record more energy use than you really have. A recording meter would verify this.

Even if I disregard the clamp on measurement around all three, is the reading on the EGC accurate? or could that be harmonics? There's no such thing as "induced" amperage??

 

[winnie]

One of the issues with VFDs is that the high switching frequency can induce capacitively coupled current flow between the stator windings and the rest of the machine.

The VFD switching frequency is much higher than the apparent drive frequency, and the voltage being switched is higher than the normal AC voltage, so this capacitively coupled current will be much greater than with 'across the line' drive.

Dunno how significant this is for the measurements that you are dealing with.

-Jon

 

[mcclary's electrical]

One of the issues with VFDs is that the high switching frequency can induce capacitively coupled current flow between the stator windings and the rest of the machine.

The VFD switching frequency is much higher than the apparent drive frequency, and the voltage being switched is higher than the normal AC voltage, so this capacitively coupled current will be much greater than with 'across the line' drive.

Dunno how significant this is for the measurements that you are dealing with.

-Jon

This is what concerns me, I don't know if what I'm seeing on the EGC is even "real". I thought about determining it and then reading, but if it's actually current flowing, this would probably damage the drive, so i decided against it.

 

[LarryFine]

As Eric suggested, can you run the motor directly from the line temporarily, bypassing the VFD, and see if you get the same numbers?

 

[mcclary's electrical]

As Eric suggested, can you run the motor directly from the line temporarily, bypassing the VFD, and see if you get the same numbers?

It could be bypassed, and tested, that's a great idea. I'll have to keep you guys posted on the results. It will take a little coordinating to get done.