# Controlling a 500A DC motor ?

A while ago I built a pc board that serves as a 3 channel SS relay. The output thyristors are 25A. This circuit uses a zero cross opto coupler to fire the thyristor and turn it off (DigiKey MOC3042). I've used many of these to control aa AC wash machine motor (on/off). The first question is if I could substitute this thyristor (DigiKey Q4015L6) with a big monster one like a 520A RMS (DigiKey ST330S04P0-ND) ?

The second question has to do with controlling the PWM circuit. I think I need to reverse polorize the thyristor to shut it off. If so, then a second reverse polorized thyristor (accross the motor)could have the inverted gate polarity with respect to the first thyristor. When the PWM signal drops out then Thyristor 2 reverse polorized thyristor 1 to shut it off and there is no current flow. Then when the PWM comes on thyristor 1 comes on again (T2 now off), and the motor has current. Is this possible to make work?

By the way, this is for my electric car, a 66 MGB.
Roger

## Answers and Replies

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rbj
there's something called a Jones Chopper circuit that uses SCRs and somehow reverse biases them to turn them off. long ago (1979 or 1980), in an electric car project that my school was doing, we built a Jones Chopper, but couldn't get it to work because the capacitors used to help with the SCR reverse biasing, had too much internal inductance (too low of quality). we ended up buying a working chopper circuit.

After considring my options, I think I will just buy a 400A, 120V Curtis EV controller. It costs just over $1000. My MG has 9 12V deep cycle batteries (108v), so this should work well. With gas almost at$4.00/gallon, I think it is a wise investment. If my MG does not perform as desired (my old controller and 8 batteries gave me about 20 mile range), I am looking into converting an S-10 or Ranger and I could double the battery payload and should be able to get more than 40 mile range. Eventually retrofit with Lithium Ion batteries and tripple that range.