Operating Frequency of Vector VFDs 1

[abfer]

What should we care about when we decide the switching frequency of the vector drives? One aspect is, switching losses. What are the other aspects? One of my friend said that it is better to run a Siemens (also omron, brand is not important) drive at 4KHz without saying a solid reason. He said that when we choose it, we have more available power etc. Or we may translate it as efficiency is better. Is this true? I've read once, that if there is a certain coefficient between the carrier frequecy and output frequency, then some of the harmonics are eliminated. Is this something like that?

 

[Skogsgurra]

Go with as low a switching frequency as you can. It is good for EMI and it is good for efficiency. It also reduces bearing damages.

There are two reasons to increase the switching (or carrier) frequency: To reduce sound and to get better dynamic performance. If dynamic performance is OK (it usually is) and the sound doesn't irritate (it often does), then go with lowest frequency.

The ratio between carrier frequency and output frequency shouldn't make much difference - if it isn't too small, that is - I have run many inverters with 1.6 kHz carrier without any problems - except sound level. As long as you have carrier at least some twenty times output frequency, you shouldn't have any real problems.

Gunnar Englund

http://www.gke.org

 

[jraef]

Eliminated? If it were that easy, don't you think everyone would do it?

The primary reasons for INCREASING the carrier frequency is to smooth out the pseudo-sine wave to reduce harmonically induced vibration in the stator windings, which has the side effect of reducing the noise made by the motor. Higher CF however means more switching, which means more switching losses. The lower the CF, the better the drive throughput efficiency and the lower the motor heating effect. So there is a compromise CF that is different for everyone. If for instance, your motor is on a high speed spindle on an industrial saw, the whine from the motor is going to be irrelevant, so I would select the lowest CF available. If it is on an in-duct vent fan feeding air to a conference room, it is totally relevant and may call for a high CF to avoid complaints.

Some manufacturers now also have a modulated CF that varies within each 1/2 sine wave as well, optimizing the vibration/noise vs efficiency balance


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[abfer]

I think coeffcient is valid for low ratio betwwen CF and output frequency. Thanks for replies.