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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. 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?
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Originally Posted by MPaulHolmes
The frequency change would be seamless to the motor. When I was doing the hissing spread spectrum switching with the DC motor, it was completely indistinguishable as far as how the acceleration and constant speed felt, even though the switching frequency was randomly spread across like 7kHz to 12KHz, changing every cycle. You could change to 16KHz once the rpm was above a particular value. Then, the added heat from the switching losses would only be under the unusual circumstance of 12,000 RPM. You could even have the switching frequency be set by the RPM. Maybe some minimum value of 8KHz for 0rpm - ??? rpm, and then linear ramp of frequency from 8KHz to whateverKHz from RPM_START_RAMP to RPM_MAX.
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I did some checking on the controllers we use. Interesting results:
First brand
- under 5 HP, default switching speed is 8 Khz
- 5 - 10 HP, default switching speed is 4 Khz
- 10 - 25 HP, default switching speed is 2 Khz
- 25 - 75 HP, default switching speed is 1 Khz
- 75 HP and over, default switching speed is 500 Hz (!!)
Second brand (we have fewer of these)
- 25 HP, default switching speed of 4 Khz
- 400 HP and above, default switching speed of 1 Khz
Third brand (again, not many of these)
- Under 10 HP, default switching speed is 4 Khz
- 10 HP - 40 HP, default switching speed is 2 Khz
The size of the air-cooled heat sinks on these controllers, the large fans (some of them have multiple fans) leads me to believe that the amount of heat that is generated by the IGBTs (which are at least a couple of generations old) far outweighs any control consideration. We have no liquid-cooled controllers.
The higher switching frequency on the smaller controllers suggests that there is some advantage. The heat sinks on the smaller controllers are proportionately *MUCH* larger than on the larger controllers.
All of the controllers allow the switching frequency to be changed. The larger controllers have warnings about derating the maximum current when using higher frequencies.