VFD increasing the motor winding temperature

[edison123]

I tested a 415 V, 350 KW, 2 pole motor on no-load (open shaft).

With utility supply at 50 Hz, the motor winding temperature was 64 deg C with 130 A current.

With VFD supply at 50 Hz, the winding temperature went up to 97 deg C with the same 130 A current.

The drive is Yaskawa GA 700 model.

Where is the additional heat coming from? From the stator iron due to drive switching frequency?

Will using a sine filter on VFD output reduce the winding temperature?

 

[jhnblgr]

Did you check the hz independently of each other? Also what was the shaft speed? Could be poor wave form on the vfd output otherwise.

 

[mparenteau]

Distance from drive to motor? Is the motor inverter duty rated?

Mike

 

[edison123]

Shaft speed is directly measured as 3000 RPM in both cases by independent digital non-contact speed sensor.

The motor is VFD duty rated, and the distance is 5 meters.

 

[jhnblgr]

Ok so shaft speed rules out ineffective cooling. What were the voltages? Might want to check your sine waves

 

[waross]

Have you ruled out measurement error?
The switching frequency may be affecting your temperature measurement.
Check the temperature immediately after stopping the motor as a quick check.

 

[Gr8blu]

Really stupid question #1 - did the motor turn the same rotation with utility and drive supplies? With a 2-p machine, chances are good the fan is directions - which means adequate cooling in one direction and not so much in the other.

Really stupid question #2 - how is the temperature being monitored? Are both sets of readings (64 C and 97 C) from the same data acquisition? or is the 97 reading from the drive (which may not be set for the correct detector type)?

Heating from harmonic content is going to be related to the square of the RMS current (including harmonics) - which will be considerably higher than the RMS voltage (including harmonics). For example: a drive may output only 3% voltage THD, but the resulting current could be at 10-12% THD. If we choose 10%, the resulting increase in heat is going to be at least (1.1 * 1.1 = 1.21 pu) * the "design" rise above ambient. Most machines are rated for 80 C rise above 40 C max ambient ... so having 10% current harmonic can result in an additional 16 C temperature rise at full load condition.

 

[EdStainless]

Some harmonic noise actually causing higher temp, but I would wager that part of this is noise on the temp sensors.
What is the measured temp 10sec after you turn the VFD off?