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Tuesday December 12th 2017
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Lessons Learned Using Industrial VFD for Motor Controller

This post contains a breif overview of what we learned trying to use an Industrial VFD as the Motor Controller.

We purchased two used Industrial Variable Frequency Drives on eBay. The first one was a 30 HP 460 AC Benshaw Sensorless Vector Drive RSi030SX4B which turned out not have enough current output to get the car rolling consistently from a dead stop without tripping the hardware over-current protection circuit (~80 Amp). We were able to drive the car, once the car started rolling. It just could not supply enough current to move the car without tripping the hardware protection limit. The second one which we are using now is a 100 HP 460VAC Benshaw Sensorless Vector Drive RSi100SG-4B (~150 Amp).

If you are planning to try to use an Industrial Motor Control in your electric vehicle project here are some lessons learned:

  • Most units above 30 HP are only available in the higher 460 VAC. Unless you plan to run a 600 VDC system you will need to modify the unit to operate at a lower voltage.
  • Make sure it has a sensorless vector torque producing mode of operation, or you will need an external shaft position sensor to get high torque at low speeds.
  • Make sure the maximum AC frequency output of the controller is known and is high enough to support the the max motor rpm desired. The 100 HP unit we have is limited to 120 hz which sets our max to 2500 rpms, 40 mph in second gear. To get more out of our motor, it should be more like 300 Hz.
  • Make sure the controller has the ability to turn off the phase loss detection. If not you will need to “fake out” the detection circuits.
  • Make sure that it has a control mode which lets you command an output torque not speed. Our present controller only lets us command a target motor speed with timed speed ramps which results in difficult operating conditions when stopping.
  • If the controller does not have a command torque mode, it needs to have someway to associate an external switch input to the “COAST TO STOP/RUN control input to turn off the drive output when coming to a stop if you do not have a clutch. This input is needed to circumvent the controller speed control loop which is trying to hold the rpm at the commanded speed.
  • If the controller does not have a commanded torque mode it needs a “flying start” mode which keeps the the Controller’s output current low until the motor and commanded rpm are the same. Unfortunately our experience to date is that this does not work well in our electric vehicle application and is the only reason we cannot drive the car around town safely. If “flying start” mode does not work well, the only way to accelerate after a “COAST TO STOP/RUN” command is issued, is to come to a complete stop, and then accelerate. The alternative to a full stop and then start is a screaming controller and a bucking car while the controller tries to sync the speed. This does not work in traffic.

If any one has some ideas or experience on how to overcome the issues associated with the transition from Coast to Stop, to Run, and then to Flying Start discussed above I would appriciate the help.