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10-15-2014, 12:53 AM   #1208 (permalink)
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Quote:
 Originally Posted by thingstodo As I understand it, that's what is being done already. The setpoint is a torque request, which is translated into Id and Iq. Vd and Vq are set based on what current you want. And current is proportional to torque (I know that's a simplification for the AC case ... but I think it helps the explanation) Or perhaps I'm off base here. In order to set the current, I thought you had to keep increasing the speed until the output torque (current) is where the setpoint (accelerator pedal) asked it to be?
Ok, you're right. - It is the current that is being measured and compared to the request value. I was looking at the photobucket graph of convergence and screwed up.

Again, I think part of my misunderstanding is directly related to the induction motor. In an induction motor, the rotor's current is proportional to the slip - right? So more slip, then more rotor current, which produces the rotor's field, which interacts with the rotating stator's field to finally produce some torque . . . right?

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. As the motor speeds up, the slip decreases and the torque decreases. 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. 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.

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. The difference between the bus voltage and the back-EMF probably doesn't reduce in a nice, linear fasion.

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.

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...

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.

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.

 The Following User Says Thank You to e*clipse For This Useful Post: thingstodo (10-15-2014)