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Old 10-15-2014, 09:23 PM   #1211 (permalink)
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Continuing on from this morning (I had to go to work)

I edited the last post and put a link to an explanation of how induction motors work that I think is pretty good. i duplicated it here

** Sorry for the long post **

Originally Posted by e*clipse View Post
A analogy may be how an unloaded ac induction motor responds when it is first plugged into a 60hz wall socket. Initially the rotor is at zero rpm and the field is rotating at full speed. The current (and torque) is at a maximum because the slip is at a maximum.
That does not quite match my understanding. There is a torque curve on the motor that is related to slip. 100% rated torque is at rated slip. As the slip rises to somewhere between 3.5X and 5X rated slip you reach the 'knee of the curve' at maximum torque, anywhere from 4X to 7X rated current. To get the fastest acceleration out of an induction motor, you need to keep the slip as high up that curve without going over the top. Torque after the peak drops on a curve to about 50% of the rated (depending on motor design) as a minimum and then rises as the slip increases until at 0 rpm is maybe 65 - 70% of rated torque.

As the motor speeds up, the slip decreases and the torque decreases.
Again, not quite how I understand it.

Eventually the motor gets close to the field speed, but will never match it because there always needs to be enough slip to keep the rotor spinning and overcoming the drag in bearings, the fan, etc.
And by the load it is driving

The torque curve starts with HUGE spike (limited by winding resistance, inductuance, etc.), then settles to near zero, while the speed curve gently approaches a value close the the field speed. All this happens relatively slowly, as it generally takes a couple of seconds for an induction motor to come up to speed.
A portion of the spike comes from the additional current that the motor absorbs at 0 rpm. Modern high efficiency motors can take 10X rated current for 10s of ms. And more current means more torque, even with the lower torque curve.

So, in a way, the torque is somewhat speed related - I'm trying to clarify - I'm NOT saying that speed is the control request.

The problem Paul is running into is that at high speeds, control gets more difficult and rough. I'm guessing here, but physically what may be happening is that as the motor spins faster, it generates more back-EMF. So maybe the stator field has to spin proportionally faster than the rotor to compensate and produce the desired torque.
As I understand it, the back-EMF limits the current, which limits the torque. So if you can't push the amps, you don't maintain the torque. The idea is that if you take a motor wound for 100V and drive it at 4X the speed, you need 400V instead. I have not been able to test that so far. I guess I'm not creative enough

As for the control getting more difficult, I don't know. Getting more rough ... that's not what I see in commercial controllers. Is it possible the feedback is getting more coarse as you have fewer samples per motor rotation? Above perhaps 20 Hz output, I've never seen a motor run rough for electrical reasons ... bearing issues, alignment issues, load issues - sure. But not controller issues.

The difference between the bus voltage and the back-EMF probably doesn't reduce in a nice, linear fasion.
I thought that it did. The impedance of the motor winding is related directly to frequency?

With both IC motors and electric motors, the torque reduces as the speed increases. Electric motors are cool because up to a certain speed, the torque is constant. However, after that point, all motors see their torque drop in a decaying exponential. So, Vd and Vq can be approached from a linear perspective as long as the torque output can be constant.
On the induction motor, I've seen graphs in presentations on VFDs (controller sales guys) below rated speed called Constant Torque and above rated speed called Constant Horsepower, as the torque degrades linearly in an ideal motor, minus the wind resistance, bearing friction, etc. The graphs look linear to me ..

Thus, for an induction motor, the changes in Vd and Vq need to be proportionally larger when operating at a speed where back-EMF becomes significant. In other words, this is a very non-linear problem, and attempting to solve a non-linear problem with a constant value solution will be nearly impossible. It does seem that it might help to give the "motor men" some parameters to work from depending on operating conditions may help the situation where one or the other attempts to grab all the resources...
I think Vq, or rather Iq, needs to remain at the desired setpoint and Vd must be adjusted according to available voltage. But that's only my opinion until I see some data ... data usually changes my first opinions to better opinions.

One thing I am pretty sure about is that shooting for a really fast convergence is a nice, interesting challenge, BUT it can lead to it's own problems. I think Paul was able to get the motor to converge in about 1ms with a 5amp step. While it's cool that the control system can get the motor to converge that fast, it's probably not necessary. There is soooo much inertia in an automobile that requiring a 1ms convergence to a step input would require HUGE amounts of torque. F=ma.
1 ms convergence is perhaps a bit aggressive. At 8 Khz carrier frequency, that's only 8 pulses ... right?

In order to avoid twisting the transmission input shaft, perhaps something in the order of 100 - 200 ms would be a better target? The ICE world ramps up in 500 - 750ish ms for a more normal car .. perhaps 100 - 200 ms if you rev it and pop the clutch?

It would probably help a lot to figure out some reasonable ballpark numbers for mass and desired accelleration (not just of the car, but how the motor/drivetrain responds with inertia) This would help produce some reasonable specifications for the control loops (like convergence time) to shoot for. Perhaps this could be broken down with different specifications over the speed range, depending on the motor's torque output curves.
My car is likely close to typical - 2500 lbs curb weight. 92 Mazda MX-6
The low end would be more like the WIKISPEED car - 1400 lbs, no roof, roadster
The high end ... perhaps a half ton truck or SUV? 4500 lbs or so.

Performance ... that I don't have a good grasp of. I'm looking at starting in second gear, doing one shift to fourth gear, and getting to 60 mph in maybe 15 seconds ... just grabbing numbers. I'll have to look into what that would require.

The torque required to slip the clutch on my Mazda (clutch has 360K on it) is something I need to measure.. well, measure the DC going in and calculate about what the torque would be. I'll be using a netgain warp 11 (series DC motor) for that. Using 5th gear to ensure a large load on the motor. Limit the current to 100, 200, 300 ... 1100 amps and log times, rpm, currents, voltage sag, etc for perhaps 0 - 60 kph. When it starts to slip I'm told I'll be able to smell the clutch (sounds like a bad idea, but I have not heard a better one so far)

After the test (likely a few tests to verify that the smell comes back at about the same torque) I replace the clutch and the front axles for sure since they need replacing anyway. Plus anything I break during the test

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