Quote:
Originally Posted by thingstodo
If we can come up with a way to do something reasonable and the speed changes gradually then you have time to get to the side of the road, or use your brakes, or let off the throttle. That's sort of what I'm looking for. The car would definitely handle differently in V/Hz than it does in Field Oriented Control. Acceleration and deceleration would be less aggressive and less efficient. But the car should still be drive-able.
I wonder if the throttle input for V/Hz could be interpreted as amps? That is close to a torque setpoint, and there should be no big change when switching from Field Oriented control to V/Hz and back again ....
I need to do some reading ... Google to the rescue!
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A summary of what I've read, mixed with a bit of my own experience:
- The amps for an AC motor don't really go much below 25%, even if it is spinning slowly and unloaded, unless you are moving to regenerative braking from motoring.
- So the amps go from 25% to about 115% for the throttle signal, but it depends almost totally on the acceleration/deceleration
- Keeping the amps at 75% the car speeds up slowly (relatively) compared to keeping the amps at 100%. The controller needs to increase the output voltage as the frequency rises to keep the amps at 100%. At some point you run out of voltage and it can no longer maintain 100% current.
- Using Paul's wig-wag style throttle arrangement, and the amps as a setpoint from the throttle, there is a deadband from 25% down to -25% where the motor is not exactly coasting but is not really motoring or regen braking. Perhaps that works OK as a control philosophy?
Switching to using the throttle as a current command when the encoder signal goes away sounds interesting to me. I think Klaus (is that his name?) does a similar thing with the controller that Damien Maguire is using. I think Klaus refers to it as slip control instead of field oriented control. And it uses encoder feeedback .. so maybe I'm off base there?
Does this re-interpreting -the-throttle-signal-and-switching-to-V/Hz-control -with-amps sound like something that we should pursue? Comments?
Besides the math and tracking the rotor position/angle ... Field Oriented Control appears to allow for the voltage to adjust auto-magically as required to get the commanded torque. Where the V/Hz has a specific voltage programmed as an equation for each output frequency. If you want the current to rise, you increase the output frequency with your PID loop. Depending on the motor load, you could be applying 'TOO MUCH' voltage and wasting some power as motor heat .. or you could be applying 'TOO LITTLE' voltage and not getting the current (and the torque) that you want.
I'm getting excited about seeing the difference in the control schemes, and trying to determine how much farther you can go, or how much faster you can accelerate, with Field Oriented Control ... or specifically the encoder signal .. compared to essentially open loop control at V/Hz.
My my ... I do get long winded!