VFD increasing the motor winding temperature 1

[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?

 

[edison123]

The winding RTD's were connected to the same temp scanner in both the cases and hence no metering error. The winding temperatures did not drop immediately after switching off power in both the cases.

The motor has an external blower which was off in both the cases. No shaft mounted cooling fan.

The rotation direction was the same in both the cases.

Another thing I noted was the motor bearings temperatures also increased with VFD by about 20 deg C. The ball bearing is insulated.

 

[waross]

I think that you may be seeing RF induction heating from the switching frequency.
I defer to jraef or Marke for comments about a filter.

 

[jraef]

Harmonics on the output of a VFD is generally not corrected by the VFD, so it absolutely increases the motor temperature. This is why, on NEMA based inverter duty motors, you often see a Service Factor of 1.15, but a note saying that the SF is 1.0 if run on an inverter. The inverter output basically "consumes" the extra thermal capacity in the motor that would have given it the 1.15SF.

 

[edison123]

Thanks, Jeff.

Do the harmonics overheat the core or the winding mainly?

And do they heat to the extent of over 30 deg C, which seems very high?

Will adding a sine filter reduce the heating to normal values? Is the cost justified?

The motor is running now on load and based on my advice, the client is loading the motor to 2/3rd capacity to keep winding temp under 140 deg C.

They want to use 100% capacity and if the 33 deg C temp rise due to VFD is removed, I am sure they can.

 

[cherryg]

You can specify more expensive VFD with restrictions on the THD on the load side. You need to discuss with the VFD vendor and read the manual. Load side filters are available for a price.

 

[electricuwe]

Surprised how you can discuss this topic with a lot of posts without even asking for the switching frequency. In nearly all VFDs the switching frequency is user-adjustable.

low fs --> reduces inverter losses, reduces EMI
high fs --> reduces motor winding losses, reduces acoustic noise, improves drives dynamics

 

[waross]

Alternating current sets up circulating currents in conductors.
Skin effect also comes into play.
The higher the frequency the greater the losses for a given current in a given conductor.
As an illustration of the effect at 60 Hz, I was at one time responsible for a 600 KVA, 277/480 Volt diesel generator.
Rated current was 722 Amps.
The windings were a stack of flat copper strips about 5/8" wide. Memory fails but there may have been 5 or 6 or more strips in each stack.
This was to mitigate eddy currents and skin effect at 722 Amps at 60 Hz.
I suggest that skin effect and the resulting increase in effective resistance and losses will be greater at VFD switching frequencies.
Yes, 130 Amps is less than 722 Amps but the switching frequency is much greater than 50 Hz.
Try changing the switching frequency and tell us what happens. Thanks.

 

[edison123]

Thanks electricuwe.

I contacted Yaskawa and they said the carrier frequency is 2 to 10 kHz with 2 kHz as default.

They advised to increase it to 5 kHz and try again to reduce winding temperature.

Apparently, the drive has to be derated above 5 kHz.

 

[waross]

I contacted Yaskawa and they said the carrier frequency is 2 to 10 kHz with 2 kHz as default.

They advised to increase it to 5 kHz and try again to reduce winding temperature.

ry changing the switching frequency and tell us what happens. Thanks.

It's nice to be supported by "The Big Guys".