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?

 

[Gr8blu]

Higher switching frequency increases the impedance of ANY circuit due to skin effect. Noticeable changes in impedance can occur at switching frequencies in the 2 kHz range, and definitely at values above that. In essence, the area available to carry current becomes much smaller - which means copper losses become higher and temperature rise increases (further increasing the copper losses).

Add in the rotor effects noted by Marke, and the total loss package climbs significantly.
And add in some additional iron (core) losses due to the higher frequency - because the lamination design and thickness was predicated on a typical line frequency in the 10s of Hz, not a switching carrier at several thousand Hz.

Anecdote: Had a wound rotor induction design where cable was routed from the outboard collector ring(s) to the inboard winding(s) by passing AWG 3/0 cable (1 per phase) under the bearing inside the shaft. Cable should have seen a "high" temperature of about 85 C under full rotor current. However, the rotor was fed from an inverter operating with a 4 kHz switching frequency. The cable failed (dramatically) on overtemperature in a relatively short time - less than two weeks. Replicating the failure in the lab required a conductor temperature in excess of 260 C - or an equivalent increase in current to more than 2.8 per unit.