Drive Question

arocon

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Join Date
Oct 2006
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Dubai
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A drive is intermittently tripping. It is vector control drive. The motor is 3 phase induction motor. My question is how much current variation can be each phase of the motor from the other phase. Similarly voltage too.

Also the measurement of current in a phase is by CT. My question is instantaneous measurement of current is peak current or rms current ?

Your reply will help me understand better.
 
A drive is intermittently tripping. It is vector control drive.

I think you are going to need to give us a few more details
- brand and model of drive
- fault code

My question is how much current variation can be each phase of the motor from the other phase. Similarly voltage too.

Phase currents and phase voltages can vary a bit. Our older drives are Siemens CUVC, mitsubishi A560. Newer are Mitsubishi A760 and A860. Motor ground faults don't trigger until 33% - 50% ground fault current. Phase imbalance is in the range of 10% of rated drive output before it trips.

It is tough to measure the voltage and current while the drive is running, particularly at low speeds. I have not found a meter that will average consistently enough to give me a decent reading

Also the measurement of current in a phase is by CT. My question is instantaneous measurement of current is peak current or rms current ?

Your reply will help me understand better.

With no info on the drive you have, I would expect instantaneous to be peak, or instant. RMS stands for Root Mean Square, and requires averaging by the meter.

The CTs on a drive are designed to respond to the frequencies generated by the drive. These are filtered before being displayed on the keypad. Tthe carrier frequency, or the frequency of the pulses generated by the drive, is normally 500 Hz for large drives, up to 16 Khz ... depending on efficiency and design

Does that help at all?

If this is causing you downtime, I'd start with replacing the motor. We have motors that test fine, but that cause problems with our drives. I have changed out maybe 10 or 12 motors for this reason in the past 10 years.
 
I think you are going to need to give us a few more details
- brand and model of drive
- fault code



Phase currents and phase voltages can vary a bit. Our older drives are Siemens CUVC, mitsubishi A560. Newer are Mitsubishi A760 and A860. Motor ground faults don't trigger until 33% - 50% ground fault current. Phase imbalance is in the range of 10% of rated drive output before it trips.

It is tough to measure the voltage and current while the drive is running, particularly at low speeds. I have not found a meter that will average consistently enough to give me a decent reading



With no info on the drive you have, I would expect instantaneous to be peak, or instant. RMS stands for Root Mean Square, and requires averaging by the meter.

The CTs on a drive are designed to respond to the frequencies generated by the drive. These are filtered before being displayed on the keypad. Tthe carrier frequency, or the frequency of the pulses generated by the drive, is normally 500 Hz for large drives, up to 16 Khz ... depending on efficiency and design

Does that help at all?

If this is causing you downtime, I'd start with replacing the motor. We have motors that test fine, but that cause problems with our drives. I have changed out maybe 10 or 12 motors for this reason in the past 10 years.

Thank you for your reply. Does it means CT measures instantaneous current not rams current right?
 
I have seldom seen drives trip due to a phase imbalance.
Many drives will still run the motor on only 2 phases.
Typical reasons I have seen for tripping are incorrectly entered motor parameters, under size drive, or mechanical problem with load causing an overload.
 
I have seldom seen drives trip due to a phase imbalance.
Many drives will still run the motor on only 2 phases.
Typical reasons I have seen for tripping are incorrectly entered motor parameters, under size drive, or mechanical problem with load causing an overload.
I agree, I don't even know of a VFD that has output current imbalance protection.

Focusing on what a meter reads on the output of a drive is a generally fruitless endeavor if you don't first KNOW there is an issue to look for. Where you start is by looking at what the trip indication of the drive is telling you. Being that drives generally don't trip on phase current imbalance per se, I'm curious why this is what you are looking at.
 
Hello Jref,

I have little knowledge but sometimes we all stumble upon.

When trip Drive shows only AC over-current. With that meaning it can be many things : Either motor, Drives control or IGBT or even can cause CT/CT interface.

This is intermittent problem , running for 24 hours then trip once and again after 1 hour or 2 hour... When tripped there is nothing abnormal change of load.
If you have suggestion i am open to learn.
 
Hello Jref,

I have little knowledge but sometimes we all stumble upon.

When trip Drive shows only AC over-current. With that meaning it can be many things : Either motor, Drives control or IGBT or even can cause CT/CT interface.

This is intermittent problem , running for 24 hours then trip once and again after 1 hour or 2 hour... When tripped there is nothing abnormal change of load.
If you have suggestion i am open to learn.

That information is useful. If the drive is tripping on AC Over-current, I would start with the mechanical stuff - as GTUnit suggested.

I know nothing of your process, so it is difficult to suggest anything to check. If you talk to your mechanical, process, and operations people - perhaps they can suggest possible issues. An intermittent problem with the drive itself is quite unlikely. It puts out the current and torque that is required to turn the motor at the speed requested by the 4-20 ma signal, or network signal. The current will rise and fall slightly even when things are running smoothly.

If you need to investigate the electrical part for your due diligence:
- Problems with the IGBT's, in my experience, show up as specific faults. my guess is the problem is not there
- Overload of the motor is possible, but the motor does not use more or less current on it's own. The process and the mechanical systems need to change the load on the motor.
- CT's are a specific problem. While anything is possible, I don't see how a CT problem can 'fix' itself after a trip. I would expect a different fault if there was a CT problem.

Given your situation, I would change out the motor first. After that does not help, I'd change out the drive. That is your last step before changing out the cabling. I would not mention that one. After you have changed out the motor and the drive you can reasonably state that it is not your problem. Have the operations and mechanical people look at what they are doing.
 
Again, it would be useful to know the make and model of the drive. In drives would, there are often several "types" of over current trip, such as drive over current, motor over current, motor over load, hardware over current, ground over current etc. Most of the time the drive mfr will provide you with more in depth descriptions of the differences in their manual.

But in general, it appears you are trying to head down the road of their being something wrong with the drive's current sensors. That's like having your car stop going down the road and you start off by assuming the speedometer is broken; but it's more likely that you just ran out of gas. In my 30 years of experience with thousands of drives, I've only come across a bad power component that was not immediately annuncusted AS a bad component exactly once, when an IGBT was simultaneously cracked and contaminated so that the crack was allowing current to leak to ground but because of where it happened, you couldn't see it until the transistor was removed and flipped over. I found it after literally a weeks worth of testing on a bench (I had already replaced in on the line). Yet even to this day, I don't start there, I start with the load really having an issue that other meters cannot see. VFDs, because of what is needed to make them work, have very sophisticated sensing systems that end up finding issues that other meters or sensors don't.

So back to the over arching issue; if you want more help, provide more information. Many of the denizens of this forum have a lot of valuable experience with a lot of machines and processes and may have encountered your issue before. But with no idea what you have or what it's doing, that is not available to you.
 
Last edited:
Why use CT's on the motor leads?. They expect sinewave power which certainly isn't there. As a result, the information you are getting from them is entirely meaningless---all error.

Get output current from an analog output on the VFD programmed to output current.
 

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