Motor Exceeding Current

[Marktal]

We have a small (approx 120w) three phase, high frequency motor, recently repaired. We have re-wound the windings. It also needed replacement rotor bars, which were completed, offsite. The motor is rated at 200vac (2 pole) and 300 Hertz. spins upto 18,000 rpm. flc rated 3.8 amps
During testing we have only been able to raise the voltage applied to 30-40 volts ac due to high current (3-4 Amps). At 50 volts the motor pulls 6 amps. we do not want to test at 200 volts, as these previous readings are high, and the motor runs hot. All testing voltages are at 50 Hertz.
Windings are connected in star, INFO - 1 / .6 mm guage used. 45 turns. 6 single coils.
We are certain the windings are accurate, and correct. We are wondering if someone may know another reason for such high current.
Anything to do with the rotor bars?
Will current drop, once hertz is increased?
Any ideas will be much appreciated.

 

[motorspert]

Assuming you are running the motor at no load then you are only supplying nagnetising current. Inductive impedance varies with frequency therefor the motor impedance is 16% of what they should be and the no-load current will be 6x normal.

This is a quick and nasty calc as there will be stray flux paths etc that will vary the impedance at 300Hz, but i think you should keep V/f constant ie 33V @ 50Hz

 

[Marktal]

Thnks for youyr reply motorspert. I didnt quite understand all of it tho. i dont quite understand how u arrived at 16% motor impedanc, although more importantly, 6x normal current.

Do you also mean that what current the motor takes at 33 vac @ 50 hz, will resemble 200 vac @ 300 hz?

 

[CJCPE]

If you apply 33 vac at 50 hz, the current should be similar to the current with 200 vac applied at 300 hz. When you apply 50 volts at 50 hz, that is like applying 300 volts at 300 hz, 150% of rated voltage. The motor will be saturated and the current will be much higher than normal. If you are testing at 50 hz, 33 volts is the proper voltage to apply.

 

[edison123]

Maintain V/Hz (200/300)constant.

 

[waross]

I concur with Marktal
The volts per Hz ratio should be kept constant.
3-4 Amps at 30 - 40 Volts sounds good. That would extrapolate to 3.3 amps at 33 Volts. This would indicate 3.3 amps on 200 V. 300 Hz.
The troubling thing is the load. If 3.8 amps is no load current at 300 Hz. you're in good shape. The extra windage at 300 Hz. would account for the difference between 3.3 and 3.8 Amps.
If there is a load to consider you may have a problem. However if the motor has a very low power factor, there may be more capacity available for loading than is indicated by the no-load current. To get an accurate indication of the current at 300 Hz. You would have to load the motor with full load running torque in a dynomometer. Then add a little extra for increased windage at 18 000 RPM.
yours
50 Hz. / 300 Hz. = 16.7%

 

[aolalde]

At reduced frquency (f) the motor inductive reactance (XL)is reduced proportional to frequency, and so the total impedance (Z), following Ohm's low the current (I) increases.
I = V/Z were; Z = r +jXL and V= phase voltage
When the flux produced saturates the magnetic circuit, the permeability (u) tends to zero ( u->0), the inductance (L) is no longer constant
L= N^2*u*A/l and the reduced value of inductance reduces the reactance(XL) further more.
XL = j2*Pi*f*L
To avoid this phenomena, keep the V/HZ ratio constant as recomended by almost every body.
A real motor test requires 300 HZ and Full Voltage (200Vac).