PWM Control for Electric Vehicle

  • Thread starter RJK
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RJK
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Main Question or Discussion Point

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

  • #2
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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
 
  • #3
RJK
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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.
 

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