Voltage/Current variation in a motor

[rbarbali]

Hi all.
I have a question.
I have a WEG asynchronous 3-phase motor, 0.55 kW (0.75 HP), 380 V, 50 Hz, cos PHI 0.82, 1410 RPM, star / Wye connected, plate current 1.50 A.
It's mounted on a conveyer belt and powered simply trough a 3-phase contactor and a Thermal Magnetic GV2ME.
In the facility, there were 410 V line voltage and I measured a line current of 1.60 A, but when later the voltage down to 400 V the current measured was 1.35 A, the current was measured by an RMS current probe.

I thougth that greater line voltage reduces the line current working near motor's nominal power, but I don't understand the measurement. Any suggestions?

rbarbali

 

[jim hardy]

I thougth that greater line voltage reduces the line current working near motor's nominal power, but I don't understand the measurement. Any suggestions?

Your 410 volts is more than the motor's rated 380 by about 8%.

Which means you're driving its magnetic flux higher, nearer saturation
so it draws a bit more magnetizing current.

That's because of the ubiquitous B-H curve
from http://www.coolmagnetman.com/magfund07.htm

Since this plot has no numbers on it, we can say it's effectively amps horizontal & volts vertical

Flux will be in proportion to voltage
and once you're out past the knee it takes quite a bit more magnetizing current to push flux higher..

greater line voltage reduces the line current working near motor's nominal power,

If you drop volts low enough, the motor will begin to slow down and could even draw starting current.
That's why they burn up if you try to run them on an inadequate extension cord ..So there's a 'sweet spot' for voltage, and as you move away from it in either direction current goes up. ..

Any help?

old jim
edit PS i see Tom and i crossed in the mail again !

 

[Tom.G]

I thougth that greater line voltage reduces the line current working near motor's nominal power, but I don't understand the measurement. Any suggestions?

That is valid if the motor is operating at constant power to the load.

p.s. That is the short answer, I see Jim gave a long version as I was typing.

 

[jim hardy]

PS² A curve with magnetizing current vertical and flux(and volts) horizontal,

 

[rbarbali]

Thanks Jim and Tom.
I thought that the effect of the increment of the voltage over the magnetizing current was minor than I measured, but I understand, from your explanation, that the motor is going into saturation.Can we relate the effect of the voltage increment / current increment with the decrement of the power factor in the graph?

I can't measure the machine with 380 V (motor's nominal voltage): the present line voltage is, more or less, 400 V. From the measurement (1.60 A @ 410 V and 1.35 A @ 400V) would you tell (estimate) that the motor is not working at full load? (Working at full load, at nominal voltage 380 V, I would expect measure 1.5 A as per plate).

 

[jim hardy]

I would estimate that as you suspect it's lightly loaded.

I say that because your current drops below nameplate between 5% and 8% overvoltage

Is my arithmetic right -
dividing your readings by nameplate values
at 7.9% overvoltage you have 6.7% overcurrent
and at 5.2% overvoltage you have 10%undercurrent

Is that motor running pretty hot to the touch ? Its iron is being worked fairly hard because of the overvoltage.
Find an old solenoid coil, connect it to a DMM set for AC millivolts.
Wave the coil around your motor and the meter will give you an indication of leakage flux.
Try it on days when your house voltage is running high, and when it's running low.
They'll differ because as the core moves into saturation more flux will squeeze out into the air.
If you have one of those battery "ScopeMeter" gizmos, observe the waveform. You'll see a very peaky wave, and you should notice the peaks are exaggerated on high voltage days.

Magnetics is a LOT of fun !

Any Industrial Plant is a tinkerer's paradise and such experimenting solidifies your basics - i hope your management is enlightened enough to permit it.
Write up an experiment plan and approach the boss?

old jim

 

[jim hardy]

PS when it burns up , put in a 400 volt motor.

 

[jim hardy]

Hmmmm... a WEG you say ?
Does it have a little sticker saying it's wound with "Wise-Wire" ?
If so that's quality motor..
I found one, 30hp, in my favorite salvage yard(30 cents a pound) , bought and gave it to a friend.
Its construction was impressive.

old jim

 

[rbarbali]

Thanks Jim for your help.
I will check the temperature of the motor in a couple of weeks. Keep you informed.

Jose.