Inverter acoustic noise

[abfer]

The 3 phase inverter is operating at 7KHz. When it starts, it begins to produce acoustic noise even no load is connected. It's coming from rectifier or inverter section i think; i couldn't exactly detect the source of the noise. Is this usual? What may cause this and how can i eliminate it?

Note: It's a home made inverter.

 

[itsmoked]

The transformer windings are the biggest source..

 

[abfer]

There is no transformer(except one for logic supply but it doesn't produce noise) and there is no load. It's directly connected to the utility grid.

 

[abfer]

The problem was the dead time. I've changed it to zero to test waveforms then forgot to restore. If there wasn't pre resistor it would burn. By the way, does dead time interval change with the load for igbt based inverters? How can we determine the min. dead time?

 

[electricuwe]

Congratulations abref, you solved the problem yourself.

But let me try to provide answers on the further questions.

Dead time is the time provided by the control unit to avoid short circuit within a phase leg. This time usually is choosen by the engineer designing the drive so that short circuits are avoided under worst case conditions.

The delay time of the drivers and the IGBTs have to be considered, distinguising between turn on and turn off. Usually ample safety margin is provided.

But if, for a given switching frequency, the dead time is set to high this will result in loosing headroom for modulation and, more severe, instable operation of induction motors under V/f control or nonlinearities in more sophisticated control schemes.