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Old 09-16-2015, 12:34 AM   #2014 (permalink)
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Originally Posted by e*clipse View Post
However, running at a more normal 8kHz, a standard IGBT is only 1% worse, according to their tests.
So, am I missing something? Perhaps running faster has other advantages??
The only advantage (besides efficiency) that I know of is that higher switching frequencies get rid of the annoying high pitched carrier noise. Not much, I'm afraid.

My motor is a 4 pole pair motor, so the **electrical frequency** will have to be 2X that of the Siemans motor for the same speed, right? That would be 4 electrical revolutions for every mechanical revolution. So, at 12,000rpm the mechanical speed would be 200Hz and the electrical frequency would be 800Hz, right?
4 pole pairs is 8 poles, rated at 750 rpm for 50 Hz or 900 rpm for 60 Hz.
12,000 rpm / 750 rpm * 50 Hz gives 800 Hz - agreed.

Ok, so if the switching frequency is 8kHz, that 800Hz sine wave would be broken into 10 pieces - right? It seems to me - and this is really a guess - that breaking the sine wave into only 10 parts would be rather rough, kind of like 6 step switching.
8000 Hz / 800 Hz = 10 steps per sine wave - check.
10 step switching is not a great description ... it's not like a step-wise signal from a D/A converter. It's ON or it's OFF, and the proportions of on versus off adjust proportionately. If you look at it on a scope, it does NOT look like a sine wave until you add the motor as a load. The inductor in the motor smooths out the signal, averages it if you like.

10 steps is still identifiable as a sine wave at the motor leads.

Are there any efficiency advantages to a closer approximation? Also, this is a 12,000 RPM - much more normal speeds would be half that. Running at 16kHz would double that resolution - how would the motor respond?
The motor inductance, actually the impedance since there is resistance as well, is a pretty effective low pass filter. The control *SHOULD* not be much different at 16K versus 8K.
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