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Originally Posted by e*clipse
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.
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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.
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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 stepwise 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.
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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.