Whooo boy, where to start. Thank you for the corrections, however I feel that the main point I was trying to express was missed.
Yes, I did get it wrong about an induction motor's torque curve. I for some reason remembered a high peak in the beginning. In reality, it has a lot to do with the motor's resistance and other design characteristics. So, I stand corrected and completely agree with your description of an induction motor's torque curve.
Here are some standard torque curves for induction motors based on their duty classes (a,b,c,d, etc)
I'm talking about an uncontrolled, unloaded motor or an uncontrolled motor with a constant load. If it has a VFD, it is a controlled motor, and someone worked pretty hard to get that nice, predictable performance.
Controlled AC motors (both brushless DC and induction) can see nice torque curves like this:
Here's the Prius MG2 Buried Permanent Magnet Synchronous motor:
I'm pretty sure that curves similar to the ones above are what the VFD salesmen showed in their presentations.
Notice how curves like the one above are basically composites of many curves:
So, if you have a VFD and a motor with a base speed of 1800rpm, then you can get the characteristic AC motor curves by fixing the speed to any speed less than the base speed. Let me repeat - you are using the VFD to produce a constantly rotating magnetic field - like a 60hz line frequency - for speeds up to the motor's base speed. This is NOT using any of the VFD's feedback capabilities, whether a rotor position sensor or "sensorless" control that uses current sensors. You're merely providing different constant speed rotating magnetic fields that are NOT adjusted to provide optimal slip as the motor's speed increased. This is merely a set of tests to provide fixed conditions for the motor to respond to.
Beyond the base speed, notice that the curves don't peak as high - those peaks fall off in a decaying exponential no matter how good the control system is. Now we're getting into the nonlinear part of the torque curve I was talking about. Also note that if you put a dot at the peak of all those curves, then drew a line from dot to dot, you would end up with something like the Prius torque curve. This part of the torque curve is very non-linear, however it's an important to use this region to take full advantage of the motor's capabilities.
That's what a nice FOC AC motor controller can get you and that's why it's worth it to do all this work.
I'm going to stop here. I need some sleep.
Quote:
Originally Posted by thingstodo
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
http://www.explainthatstuff.com/induction-motors.htm
** Sorry for the long post **
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.
Again, not quite how I understand it.
And by the load it is driving
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.
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.
I thought that it did. The impedance of the motor winding is related directly to frequency?
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 .. |