3 Phase Induction Motor Running on VFD

[riyajahamad]

I am trying to run 3 phase Induction motor Rated 0.75 KW, 1.8 Amps, 1440 rpm. with Mitsubishi FR-500E Drive.
When I run motor at 17 Hz ( As process requires ) it takes 2.5 Amps current and getting hot on no load, And when I run at 50 Hz it takes only 1 Amps and runs ok.
Is it not recommended to run the motor on such low frequencies ?
I tried this on other motor with other drive, in that case current not increases as frequency decresses.

Anybody suggest me to run the motor on low frequency with low current, what will the probable causes for the same.

Thanks in advance.

 

[DickDV]

The motor's current characteristic is determined by the Volts per Hz curve that the drive produces on its output. From the motor's base speed of 50 Hz down to 17 Hz, the ratio of Volts to Hz should remain constant. It certainly should not increase.

You can check this by using the drive's display to show output voltage and frequency. Start at the motor base speed and record the values. Then reduce to, say, 40 Hz and record again. Then reduce to 30 Hz, then 20 Hz, and finally 17 Hz. If the ratio is not constant, the drive is programmed wrong. Study the Instruction Manual carefully to find the parameters that control the output voltage as speed changes. Especially, look for parameters named Torque Boost or IR Compensation or something similar that add extra voltage at slow speeds to improve starting torque. If these are set too high, the motor will saturate and actually develop less rather than more torque.

If motor current still goes up as speed falls, and you are sure that the above ratio is constant, then you can be sure that the motor is seeing increasing torque in the load. You haven't told us what the load is but, while rare, there are some loads that do increase load as speed falls.

If you are seeing increasing load, then your motor needs to be larger or, you could change the power train by increasing the speed reduction ratio. Then operate the motor overspeed to get full speed back. The increased ratio will both improve slow speed cooling and increase output torque. You will not hurt the motor by operating up to 70 or 80 Hz.

 

[riyajahamad]

Thanks a lot DickDV

First I thanks heartly to DickDV for replying so details, and our problem of motor over current solved.

As DickDV suggested I checked Voltage and frequency found constant.

Parameter No. 0 setting found 14 which was causing over current problem.

Pramater No. 0 is for "Torque Boost" I reduced to 2 and motor started to drow less current, 50 % w.r.t. before.
This motor is connected to drive conveyor, after connected to gearbox.

 

[stasis]

also keep in mind that motors are cooled by drawing air into the back of the frame...if you run 'most' three phase motors at less than 25% of it's rated RPM, you run the risk of cooking the motor because it can't cool itself. This will also cause the heat you mentioned & will also make the motor draw more amps.