High No-load current of Induction motor

[kada]

Hello everybody!

This is for the first time I am using this forum to solve my actual problem.

Well, I've got an AC drive and Induction motor.

IM name-plate:

P=1.5kW (2HP)

U=230V AC (line-to-line; in delta connection)

I=6.9A (in delta connection)

6-pole

AC drive:

3-phase output

V/Hz control (open loop, no optimization, no slip or IR drop compensation)

SVM Modulation

 

Motor is coupled to a freely rotating DC motor (not powered).

I apply 230V AC (rms) on the output of inverter (line-to-line voltage) and 50Hz. I would expect a no-load current about 30% of nominal (maybe 40% for a 6-pole motor) - it means about 2.5A. But the current RMS is 5A (70%)!

Is that normal? I think it isn't. And that's the reason I am asking WHY.

Some questions and suggestions:

Motor is dedicated for operation condition described above.

PWM is 16kHz, but the situation is the same for any PWM frequency.

I think, the motor is overexcited. Line currents are not purely sinusoidal but distorted on the top of sine-waves.

The 1-st harmonic of AC (line-to-line) voltage applied to the motor is indeed 230V AC. I checked it using RC filter and scope.

Hope, my description is not too confusing.

Thanks for any comment.

[marke]

Hello kada

 

Welcome to the forum.

 

First, three questions.

 

• What is the frequency rating of the motor? 50Hz or 60Hz?

• How did you measure the current?

• How hot does the motor run?

  These answers may help us give a better answer.

   

  Best regards,

  Mark.

[kada]

Thanks for response.

Here are answers to your 3 questiones:

 

1. Motor frequency rating: 50Hz (Siemens IM)

2. Current was measured by analogue A-m (proportional to r.m.s. values) and by scope (using LEM probe).

3. The motor is cold because it doesn't work for a long time. I think you should know I am not talking about industrial installation but laboratory testbench.

[Guest]

Hi Kada:

 

I suggest you to decouple DC machine from induction motor and then no note down induction motor current. Reason why i am asking you to do this is, i just do not know the rating of DC machine. If your DC machine is quite big, then residual magnetism inside DC machine may causing high current in your Induction motor. Anyway, if possible just try it out.

 

Please try to explain bit more on your point # 3.

 

i.e.,I think, the motor is overexcited. Line currents are not purely sinusoidal but distorted on the top of sine-waves.

 

Thanks

Sri

[jraef]

Even True RMS meters will read higher than expected currents because of the nature of the newer higher carrier frequency PWM signal output from the VFD. Most are not sampling fast enough to interpolate the signals properly. Here is a decent paper on the subject.

http://www.ohiosemitronics.com/support/AppendixF.pdf

 

Ammended:

Actually, in reading that in more detail it doesn't really support what I was saying, except in the end where they describe thier products as being capable of frequencies up to 1kHz, saying that this was adequate for VFDs. That was probably true when VFDs used Darlington transistors, but newer IGBTs are being switched MUCH faster than that now.

[kada]

To jraef:

I didn't use any digital A-meter, just an old good electromagnetic measuring device for AC current measurement. Moreover I have checked the phase current by quite new Osciloscope + Hall effect current probe, and I can trust them. I don't assume any measuring pell-mell. Current is simply much more higher than anyone would expect (70% of rated).

Could there be a problem related to higher harmonics? Does that problem escalate in the case of delta connected motor?

 

To Anonymous:

The PWM is quite high - 16kHz, so the phase currents are expected to be sinusoidal for a healthy motor. But they are not.

VHz characteristics is linear - 0 Hertz relates to 0 Volts and 50 Hertz relates to 230 Volts. Currents are distorted not only for 50Hz but also for 10Hz and above.

Why do I think the motor is overexcited? When 60Hz and 230V is applied (field weakening region), the currents become purely sinusoidal. Would you like to see oscilographs?

[marke]

Hello kada

 

Small motors operate with a higher flux in the iron than large motors. This results in increased iron loss and increased magnetising current, but helps reduce the copper loss - less turns of larger wire.

On large motors, it is important to minimise the losses in order to prevent excess temperature rise with a given frame size. This is not such an issue with small motors.

I would try to run the motor open shaft to ensure that you are measuring the magnetising current only. It is possible that there is sufficient shaft torque to increase the current.

I have seen many small motors with the iron loss as high as 60%. I have also experienced US made motors designed for 60Hz but stamped 50Hz. These do have a very high magnetising current and iron loss and usually overheat.

 

I would run the motor, preferably open shaft, and monitor the temperature rise. If the flux is excessive, the motor will heat significantly under open shaft conditions. If the temperature rise is acceptable, there is no problem.

Remember that the cooling is proportional to the surface area of the motor, but the power is proportional to the volume. Small motors can tolerate a higher iron loss.

 

Best regards,

[kada]

OK, thanks for all comments.

I will do ASAP recommended tests and measurements. I will prepare a brief report that would clearly describe my problem with overexcitation and will attach it. Then (as I hope) will start next turn of discussions.

[marke]

Hi kada

 

You may want to check the current waveform into an unloaded transformer as well. You will see that the magnetising current is always distorted due to the non linear nature of the iron, so do not assume a distorted current waveform on an unloaded motor means that it is over fluxed. The real question is the heat in the iron. If the temperature rise is reasonable, then the flux level is fine for that frame.

 

Best regards,