View Single Post
Old 12-16-2015, 10:00 PM   #2483 (permalink)
Permanent Apprentice
Join Date: Jul 2010
Location: norcal oosae
Posts: 523
Thanks: 351
Thanked 314 Times in 215 Posts
Originally Posted by thingstodo View Post
Yes. The negative torque command (throttle or potentiometer) slows down rotation using the AC motor, then the direction reverses and the AC motor continues to 'accelerate' in the negative direction

In the first part of the test, the 24V pack is putting power into the DC motor, which drives both motors. Using the throttle, I command negative torque on the AC controller, which is done by lowering the frequency put out by the AC motor. That puts the motor into a 'generator' mode. That energy output from the generator flows back into the AC controller, becomes DC, charges up the capacitors to a higher voltags, and supplies the power used by the electronics on the AC controller. Since the voltage is now above the voltage of the battery pack, the surplus power flows back into the high voltage pack.

When the 24V pack is turned off and the inertia of the motors turning is overcome by the negative torque on the AC controller .. there is no more energy to feed 'back' into the AC controller. The DC bus voltage drops back to the battery pack voltage. The AC controller starts to consume energy from the battery pack again.

That likely does not make much sense .. but that's how I understand it.
Ok, it seems to be doing what I thought it was - thank you.

What's interesting is that when commanded negative torque when the motor is turning a particular speed can result in energy being fed back to the AC controller's supply. So it appears that the system doesn't really "care" if there's a "regen braking" command or if the controller's basically given a "reverse" command.

So it seems that somewhere around 300rpm, the BEMF produced by the motor reduces to some insignificant amount and the controller has to supply power in order to maintain the negative torque command, thus "regen" turns into "reverse" at that point. If the system is purely regen ( power fed back to the supply ) then that would be helpful for extending range. However, if battery power is used to stop the vehicle, then it seems more efficient to use friction brakes at that point.

It seems a set-up like this would be nice:
When torque commanded is equal to or greater than the torque required to maintain speed, the controller would consume power from the supply. When the torque commanded is less than the torque required to maintain speed, then then the controller would "regen" power back to the supply. This is essentially what is happenning with a "positive" throttle. It would also "feel" similar to an IC car when downshifting.

If more negative torque is needed, then the driver can hit the brakes. It may be possible to put a sensor on the brakes - either a pressure sensor or a brake pedal position sensor. When the brake pedal is pushed, then the regen could be increased. This reverse torque would only be available when energy flows to the supply. Perhaps the controller could monitor the current flow and basically "ignore" the regen command when it results in power flowing from the supply.

However - and this is where things get tough from a user-interface perspective - the regen torque will be proportional to speed. As this decreases due to the reducing speed, the braking torque from the friction brakes would have to be increased. Most drivers want very predictable brakes, where the negative torque is proportional to brake pedal pressure. Balancing/tuning this will be a challenge, but very rewarding.

- E*clipse
  Reply With Quote