Freq Drives

[E.C.]

We are doing an airhandler retro to frew drives. The are all installed by the contractor but when they take the drive out of bypass the motor screams at a high pitch. So bad we get calls to turn them back to bypass. What gives?

 

[ptonsparky]

Freq drives or softstarts?

The freq drives I have put in did not use a bypass, but the softstart did. The SCRs of the softstart are not supposed to be in the circuit once the motor has reached full speed.

 

[boater bill]

I would change the carrier frequency of the VFD. It is probably 2K Hz go to 8-10 KHz if the lead length is less than about 75 feet. Also, was the motor replaced with the retrofit? Sometimes the insulation of motors aren't rated for VFD's.

Good luck

 

[weressl]

Freq drives or softstarts?

The freq drives I have put in did not use a bypass, but the softstart did. The SCRs of the softstart are not supposed to be in the circuit once the motor has reached full speed.

This is not necessarily true for all SSRV's.

There is a legacy practice to put in bypass contractors for ASD's. It became the practice in the stone age of drives where their reliability was far from stellar and failure was frequent and common. The only reasons when this would/could be justified today are:
- prolonged operation at full speed would result in energy saving
- 100% availability is a must

As far as the OP question, the carrier frequency may be adjustable on most drives(as pointed out by others), the higher the selected frequency is the less noise will be generated. There may be additional noise mitigating software settings available where various algorithms will find the optimal settings and perform the adjustment continously, dependent on speed and feedback signal.

 

[charlie tuna]

are there reactors built into the drives or installed on the line side???

 

[tom baker]

I use a 2 kW carrier frequency as its better for the motor but makes more noise.

 

[don_resqcapt19]

Tom,
I have heard that before but is it really true? It is my understanding that the damaging voltage spikes are caused by the very fast switching time of the transistors in the drive. Changing the carrier frequency does not change the switching time. Does this work only because it reduces the number of high voltage spikes that occur?

 

[tom baker]

The first VFD I installed in about 1994 had the motor fail two weeks later. It wasn't inverter spike resistant, the windings in the first few turns failed. But Don what I've read is that reducing the carrier frequency the motors will not be as likely to be damaged. I have a white paper and I will try and find it.

 

[noonan]

I agree with the suggestion to change the carrier freq.

Speaking of prehistoric, our corporate spec still calls for bypasses on VFDs, unless we can show there is enough redundancy in the system.

OP, have you had a chance to try anything?