Frequency Drive Question

I agree with what Doug Adams wrote except about reducing harmonics. Changing carrier frequency can SHIFT harmonics but rarely reduces them.

As I mentioned earlier, the general rule is to use the lowest carrier frequency you can without getting objectionable motor whine.

There is one other benefit for using higher carrier frequencies that hasn't been mentioned. If you are going to operate at a very low motor speed, a higher carrier will provide smoother shaft rotation (less cogging) and somewhat better torque, but only at very low speeds.

With today's technology, struggling to make a V/Hz or scalar drive produce low speed torque seems almost primitive. Far better to use a sensorless vector drive and, even better, an ABB drive with DTC. I use these frequently and routinely develop full motor torque at zero speed without any speed feedback device.

Back to shaft cogging, I've run into a few buffing and polishing applications where the slightest variation in shaft speed puts a pattern in the polished finish. I've even seen precision DC drives fail in these applications due to the commutator bars being reflected in the polished pattern. Usually, an AC motor with a high frequency carrier inverter is good enough to do this tough job.

Excellent feed back guys, only thing I might add, without going into detail about all the different harmonic frequencies bouncing around inside you VFD panel. As a practise, we will have our PLC's mounted as far away from VFD's as possible, isolation is the best form of filtering. We also use line filters for bus filtering and load reactors if our motors are farther away.

cheers

As a matter of principal, Shaun, I also try to get separation between the PLC and the VFD. However, when backed in a corner, I have put PLCs right in the VFD enclosure and they work just fine.

Never say never, I guess.

To Steve Bailey

I know of only two reasons to increase the carrier frequency. First, as has been mentioned, audible motor noise can be shifted to above the hearing range making the motor seem to run quieter.

Second, motor torque production is improved and shaft cogging is reduced at very low speeds. Back when v/Hz drives were the only show in town, this was a big deal but today seems almost primitive. Modern sensorless vector drives accomplish more at low shaft speeds than higher carrier frequencies could ever hope to do. Beyond that, ABB's DTC (Direct Torque Control) gives you full rated torque and precise speed control even at zero speed--that's right, zero speed. And without any type of encoder or speed feedback device. I use this system often and am astounded at the level of performance possible from this system. I know of no other open loop drive that has this level of performance. (I once did a hydraulic test cell job with ABB's DTC and, at 10000 rpm, got only 8 rpm speed error from no load to full load!)

One other rather obscure reason for increasing the carrier frequency would be where you have wave traps or sine filters in the motor leads. Some mfgr's of this equipment specify a minimum carrier frequency for their equipment to work. You won't run into this very often.

Otherwise, I choose the lowest carrier available that doesn't cause excessive motor noise. Drive efficiency is better and the stress on motor insulation is much less.