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Old 09-15-2015, 07:52 PM   #2013 (permalink)
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Originally Posted by thingstodo View Post
Switching frequency discussion - just so I can follow along:
- a higher dsPIC frequency means more computation, better control
- higher dsPIC frequency may mean higher A/D frequency to keep the control algorithmn fed with good data

- a higher switching frequency means higher losses, but better control, maybe need SiC transistors?
- lower switching frequency means lower losses, maybe a lower max speed?

10,000 rpm on a 50 Hz, 1500 rpm motor (2 pairs of poles, like the Siemens) would be running at 330 Hz or so. 10 Khz A/D would be 600,000 samples per minute, 60 samples per revolution, right?

Is there an issue with only 60 samples, or perhaps that MAY not be enough samples?

I think that the loop may already be too fast. If the acceleration I saw with the motor 'hunting' during the rotor test is typical - 30+ amps on one meter update (about 1 sec) and -something on the next meter update ... and the meter averages a bunch to get a relatively stable display number, I think that mechanical parts of the car would be very stressed. Acceleration from 0 speed to 2500 rpm can take 1 or 2 seconds if you are not racing. That's comparable to a performance engine, right?
Thanks for joining this discussion Hopefully I can learn something from your experience.

I've seen a bunch of wonderful app notes and spec sheets about SiC switches, claiming their additional costs will be offset by their efficiency. I am interested in cuttiing edge tech, and efficiency gains always interest me. Certainly #'s like 98% or 99% efficiency @ switching frequencies of 16kHz raised by eyebrows. Right now I'm not so sure it works out to be cheaper on a system level, because of about a 3X cost of SiC vs IGBT. (just looking up some prices @ Digikey.)

Here's a test at where they compared performance of a 30hp motor/inverter using SiC switches vs similarly rated IGBT's:

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??

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?

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?

Also, this is merely the switching frequency - how many AtoD samples of the rotor position and current should be done per swich, just to get an average or clean data?

I'd like to get a better handle on this. Right now it seems it's worth it to invest in good capacitors (like the SBE) and a planer bus set-up. Fortunately, that will help future upgrades to SiC, if the prices drop a bit.
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