Paul and Sabrina's Cheap 3 Phase Inverter (AC Controller) with Field Oriented Control

[thingstodo]

Originally Posted by MPaulHolmes

The nice thing is we'll have the encoder where we can test the real speed and the sensorless speed, and try weird varying loads and stuff to see if we can trick it.

Good point. We can throw every weird and wonderful situation at the setup and log whether there is ever a time when the calculated frequency does not line up with the measured speed from the encoder.

I think I have found my calling ... I break things even while TRYING to get them running correctly. IMAGINE what stuff I could come up with if I TRY to break it!

[Piotrsko]

Unfortunately FWIW, when you try to break things they work to specifications and beyond. Murphy.

[e*clipse]

Originally Posted by thingstodo

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 PowerElectronicsSpecifier.com where they compared performance of a 30hp motor/inverter using SiC switches vs similarly rated IGBT's:
http://www.cree.com/~/media/Files/Cr...ThreePhase.pdf

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.

[thingstodo]

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.

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.

[thingstodo]

Update for Sep 15

No pictures since not much changed.

- got the DC motor removed from the test base, placed on the floor for a couple of days
- soldered the small circuit to send 24V to the encoder, get 4.95V signals back

I should get the encoder mounted tomorrow, and hopefully the new code loaded and tested. We'll see

[MPaulHolmes]

Originally Posted by e*clipse

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?

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.

The advantage of doing a single A/D read sync'ed to the PWM period is that the measurements take place when the IGBTs are not switching (center aligned mode). So, you are always taking a current snapshot at the same point in the pwm duty waveform. So there is no concern about noise from the IGBT switching on reading #1, no igbt switching noise on reading #2, and averaging apples to oranges.... And these LEM swiss sensors are pretty dang quiet. I did a buck converter charger that was using a single current sample and got 0.2% accuracy for commanding the output current over most of the current range. Their datasheet says 1%.

[thingstodo]

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?

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.

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.

[e*clipse]

I'd second that, for my sample of 1 project. LOL!

It's a Sumitomo VFD, (I'm very impressed w/ it and Sumitomo's support - I think I'll get a couple more for my Lathe & Mill ) Anyway, the drive is 7.5kW, the switching frequency defaults at 5kHz, and allows up to 15kHz. At 5 kHz the noise is very noticable & annoying. At 15 kHz it's mostly noticable only at startup. There are two small fans for the control, and the heatsink/fans are a little under 1/2 of what's in the VFD box.

- E*clipse

[e*clipse]

So, according to Paul, it sounds very possible to not have a whiny car. It also sounds like one could easily adjust frequency according to operating parameters. Thus, one could have a daily driver/ track car that could be quiet for commuting, and (in another mode) make use off all power potential (but be whiney and less efficient). Awesome!

thingstodo:

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.

The best info I can find is the inductance for the motors I'm using is very low - 1.4mH to about 3mH ( It varies due to phase angle, I think )

What's the inductance on the big ( 75hp ) motors, in general? Isn't motor inductance lower at higher power levels ( in general ) It seems as the inductance decreases, the smoothing would work better at higher frequencies. True??

- Thanks,a bunch,
E*clipse

[MPaulHolmes]

My motor's stator inductance is about 30mH, and it's an 8kW motor I think (I forgot). I read about some tests in some app notes done on 1HP ACIM motors whose stator inductance was 300mH. So I would say that it does seem to get smaller the higher the power.