Drive and motor

we had an motor that was 1.1 kw and working under 380 v current. this motor was using 0.8 A current while it is idle, while on full duty it was using 2.0 A current. same motor connected to a drive, that is 4 KW and output voltage is 380 volt 50 Hz. same motor had started to use 7.8 A. after that we tried same drive with some other similar motors. but situation didn't changed. what could be the problem?

I have ac drives running in vector mode with encoder feedback that will pull unusually high currents and run erratically when there is a problem with the feedback encoder. The drive won't fault if just one channel on the encoder fails. Don't know if this is relevant to your problem. What kind of drive?

You probably have the motor nameplate details set to 4kW (7.8A FLC) instead of 1.1 kW. Set the motor details in the drive to 1.1 kW and current limit to 100%.

JohnW said:

You probably have the motor nameplate details set to 4kW (7.8A FLC) instead of 1.1 kW. Set the motor details in the drive to 1.1 kW and current limit to 100%.

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sorry but problem is that this motor using 2.0 A if it is working without the driver. with driver it is using 7.8 A. and the same driver is controlling 3 other motors. other motors are 4 kw and 3 kw. also there are 8more drives like that and they are working properly.

I suspect that someone is trying to measure motor amps using a clamp-on ammeter on the motor leads. The result will be huge errors.

An accurate way to determine motor running amps is to set the drive to display output amps and then read the difference with the motor offline and the motor online. The drive will display true amps but the hand-held meter cannot handle the pulses and high frequency components and will measure very inaccurately.

How long is the cable between the drive and motor?

I have seen long motor lead lengths cause much the same condition if you are ACTUALLY reading 7.8 amps. Most drives with a built in keypad can display actual motor amps. The drive display is probably the most accurate means of measuring motor current these days.


If you have long leads and are pulling 7.8 amps, set the carrier freq to 2k hz. You may also need to add an output reactor.

Are you checking current on the drive input. Checking current or voltage on the output can give you false results. Try checking input drive voltage with the wires unhooked to the drive. EX. phase A to B. If this is okay check the amps on each drive input with the motor running are the same.

Pat

Where are you measuring the current? Is it just one motor and one drive together? what current do you get with a different motor? Can you confirm the drive setting as to the motor rating?

I don't think I am understanding the whole problem.

DickDV said:

I suspect that someone is trying to measure motor amps using a clamp-on ammeter on the motor leads. The result will be huge errors.

An accurate way to determine motor running amps is to set the drive to display output amps and then read the difference with the motor offline and the motor online. The drive will display true amps but the hand-held meter cannot handle the pulses and high frequency components and will measure very inaccurately.

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Dick I agree with that in general because most people just obtain an inexpensive amp meter but a TRUE-RMS clamp meter should not have a problem measuring motor leads from a drive. In most cases the drive has this capability built in (if the OP is there to use) so it is a good idea to become familiar with it. I have worked in environments where there were situations where it was easier to test at the motor then it was from the panel so all my meters have been true rms for a long time.

Fluke is not the only company that sells true-rms meters but here is a little tip file for clamp meters they have provided, clikc on the one title Clamp meters: http://us.fluke.com/usen/apps/electrical/appnotes/default.htm#

There are other articles there that may be of interest to people that do maintenance or need to "log" data pertaining to circuits, motors etc.

oh... i'm sorry i forgot to write that, our motor warms very quickly and coils burned out while working with driver. but without driver ther is no problem as i told before...

cavusogullari said:

sorry but problem is that this motor using 2.0 A if it is working without the driver. with driver it is using 7.8 A. and the same driver is controlling 3 other motors. other motors are 4 kw and 3 kw. also there are 8more drives like that and they are working properly.

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As I said before - the drive is giving you 7.8A regardless of the motor size because IN THE DRIVE it is set to 4 kW. You have to set the drive to tell it that it has 1.1kW motor connected, otherwise it will try and deliver 7.8A regardless of the motor size. It will just ramp up the output voltage until it achieves 7.8A and burns out the motor.

I agree with JohnW to a point. Granted, if the drive current limit were set correctly the current to the motor would be limited and the motor wouldn't burn out. But JohnW's point is only an issue if the drive is set up in specific ways or the motor is too small for the mechanical load.

If the drive is set up in V/Hz mode without any of the offsets voltages or custom profiles active then the motor will draw current from the drive 'similar' to what the motor does across the line. If the drive is configured with start boost or run boost voltages these can cause high currents if not set up correctly.

Also, if the drive is set up in one of the vector modes, either sensorless or with a sensor, the drive will try to develop a specific level of rotor flux. If the drive is configured to run a larger motor than it is connected to the rotor flux generating current alone, which is often 40% - 60% of nameplate current, will be driven through the motor regardless of load.

The fact that the drive can operate a larger motor without a problem leads me to believe that it is set up in a vector mode and, as JohnW said, it is set up for a motor larger than the one connected to it.

Keith

Are you using a 1 fase inverter or a 3 fase inverter? 1 fase inverter needs a motor in delta. 3 fase drive needs most of the time a start connection in the motor.

Confused ...

In post #4 its mentioned:

the same driver is controlling 3 other motors. other motors are 4 kw and 3 kw

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Won't this overload the 3kw drive if it's set to 4kw? I thought motors had to be sized the same to use the same vfd.


Mike

Motors do not have to be sized the same to be driven by a single VFD.