# PWM Control for Electric Vehicle

Last year I built an electric car. I converted a 1966 MG to electric using 8 led acid batteries (96V bus), a DC motor coupled to the transmission, and a large motor controller. The vehicle was working fine. Top speeds of 55 mpg, but with many led acid vehicles, only a 20 miles range. The DC motor controller is a Kelly, purchased from China, and now sems to be disfunctional. It is also limited to the 96V I have. I would like to get this vehicle operational again with more capacity and without spending too much. This is where I need some help.

I was wondering if I could build my own PWM motor controller. There are plenty of small PWM motor controllers around, but I need to control hundreds of amps. I also need some current limit protection (I do have a fusable link, but the drive ought to limit prior to that). The idea I have is to use a low end DC motor controller (many on the market for about $100 or less), then take the output stage and connect it to the gate of a large SCR. The SCR would of course be a series connection between the 96V (or higher) bus and the DC motor. For current limiting I have a current monitor device that is in line with the 96V bus that is fed to my on board PLC. I use a small touch screen to display battery voltage, RPM, temperature, and running current (among other things). I could put a setpoint on the touch screen and when the current exceeds that simply take a PLC output and tie it to the inhibit line of the small DC controller. I think I have the basics covered, but my biggest question is how exactly to connect the output stage of the small PWM controller to the gate of the SCR. The device I am looking at is a Digi number ST330S04P0-ND, good for 520A RMS and 400V. I could then add another couple led acid cells for a 120V bus for better performance and range. I would appreciate input about this idea, and specifically how to make the PWM connection from a small DC drive to a large SCR. Thanks, Roger ## Answers and Replies RJK, Welcome to PF! Controlling "hundreds of Amps" to a DC motor is problematic and expensive, but certainly possible. I wouldn't go ordering any SCRs until you have fully investigated how an SCR actually works; the short version is that while turning an SCR "ON" is fairly trivial, turning one off is a bit more of a challenge. You certainly aren't going to do anything useful by connecting a high current SCR to a "small DC controller". Let's put things in perspective: 200A @ 120V is 24kW, this is essentially the same as a 100A 240V AC service in many older homes. It is a LOT of energy. I have a 250A TIG welder; it uses a shoe box full of IGBTs to turn 240Vac @~25A into a constant current source up to 250A. My point is that it takes a$2k machine to control roughly 1/4 the power you are talking about, and it can only operate at full power with a duty cycle of ~40%.

OK, with that little disclaimer out of the way, you should investigate IGBTs for your circuit. Calculating a SOA for them and a suitable drive circuit will be a complicated engineering feat, but it can certainly be done.

It may prove easier to investigate a commercial three phase VFD and a matching three phase motor. This link:

http://www.dealerselectric.com/mfg-subcat-item.asp?cID=28&scID=166&mID=-1

Has both single phase and three phase input models, suggesting it might be possible to simply use a DC input, though you would certainly want to contact their engineering department prior to purchasing.

What ever direction you decide to go, good luck!

Fish

Thanks for that reply. EV motor controllers of this size cost about $1500 for a name brand. The one I bought from China was$800, but looks to have failed. I figured the challenge of this sort of project would be controlling the SCR or IGBT. I am an electrical engineer, but most of my career has been with industrial controls and I do not do much on the component level. Therefore I will probably shy away from this idea unless someone has done this and has a circuit. It does seem like too much time and risk otherwise. I probably just have to pay for the name brand controller.