Motor Controller Saga

When I first started the conversion project, I had chosen to build my own controller from an open source kit (Open Revolt Cougar Controller). The controller has been successfully used by many members on the ecomodder forum and in the conversion community, but I had some bad luck with mine.

I’ll start with a bit on the controller’s architecture. There is a control section powered by an Atemel atmega8 which takes in throttle position from a potentiometer and current readings from a LEM HASS current transducer and appropriately throttles 10 mosfets in parallel to deliver power to the motor. the mosfets are paralleled with 10 freewheel diodes to prevent inductive kickback and 16, 820μF capacitors to prevent spikes and dips in input current. A more detailed explanation along with schematics can be found here.

When I first built the controller and tested it, I was really excited that it worked. I had a 5K pot, a small 12v SLA battery, and a brushed DC motor. Turning the pot yielded more motor speed and I was impressed with myself. Fast forward a few months and the controller is installed in the car. when everything was just about right, I took it out for a spin and it worked great.


Unfortunately, the controller blew up quite spectacularly, blowing s shower of sparks across my floor and sending my 9″ motor to full speed with no load. At the time this happened, I was under the car undoing the propeller shaft to finally remove the gas tank (quite an involved process). I jumped out from under the car and hit the emergency stop switch I had installed in the car for this reason (runaway motor) and my main contactor very willingly cut power to the controller and isolated my high voltage pack. I’m very thankful that I got out of that unscratched and that my car didn’t fly though my garage (it was in neutral and chocked on jack stands).

After this event, I gained a lot of respect for properly engineered power electronics. In tearing down the controller, it was concluded that one of the bolts securing the heat sink to the motor negative bus bar as well as holding down the power section of the controller shorted to the negative battery input.

blowup 017

blowup 018

blowup 016

The power section of the controller is held together with the 4 bolts covered in heatshrink shown above. It goeas through the following “layers” of the power section from top to bottom:

  • Motor negative output
  • Fibreglass isolation strip
  • Negative battery input
  • Positive battery input
  • Fibreglass isolation strip
  • Heatsink

There has been 2 hardware revisions of the controller since I built mine including the addition of nylon bushings instead of just heat shrink to isolate layers of the power section and changing from a milled copper board to a printed circuit board. The current revision (rev 2D) is much more refined and better designed than the version I had.

So after figuring out that my life is worth more than the $2000 it would take for me to buy an off the shelf motor controller, I got myself a Soliton Jr. from Evnetics. This controller, and Evnetics, initially started on diyelectriccar forums out of the fact there were no EV conversion specific controllers. The build quality of the controller is solid (heavy, lots of aluminum, very shiny). I would highly recommend their products if you are doing a DC conversion.


The Soliton Jr. uses IGBTs, which are safer in the sense they tend to fail off. The controller also has a number of safety features like an internal contactor which cut power in failure conditions (ie no throttle, full current). There is also built in rev limiting, a brake input, along with 3 programmable inputs and 3 programmable outputs. The controller is interfaced with via ethernet and settings can be tuned through a web interface.

I’ve put a fair number of hours on the Soliton Jr. and it has worked flawlessly. In the end, I would recommend that anyone doing a conversion buy the highest quality components you can, otherwise you’re probably not in the right mindset to do a conversion.

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