Contactor vs SCR

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BM1
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Joined: 04/07/2010
Points: 6

Hey everyone,

I'm currently working on an electric motorcycle with a 72v pack and was wondering if there are any benefits of using an SCR or high power Thyristor over a contactor as the primary switch that turns the power system on. The only negative thing about SCR's ive come across is that they tend to fail closed while an electromechanical switch fails open. I don't see this as being much of a problem as long as there's a fuse in line with it. Has anyone else used an SCR in their EV?

- Michael

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Spaceangel
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Joined: 07/15/2009
Points: 497
Re: Contactor vs SCR

I first used an SCR on my Aurenthentic 24 volt scooter to turn it on. Oops. I forgot about off setting and redesigned it. Now back to a contactor NC. at least that contactor was smaller. Then when I went down a hill the motor turn into a generator and dropped out because of reversing biasing. After a while of riding the very large SCR's did fail and you are right they ALWAYS fail short. Same thing with 24 volt Triac's using opto. I used that scenario on generator for there was significant savings in weight. At least most AC circuits use fuses or circuit breakers so you might be OK doing that. I got my Triacs from surplus in Nebraska.
I love the learning curve!

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marylandbob
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Joined: 06/22/2009
Points: 524
Re: Contactor vs SCR

It would seem that power FETs would be most practical, as they are available in sizes to handle hundreds of amps, and can easily be paralled to switch thousands of amperes! I would choose a combination rated for at least TWICE the maximum voltage and amperage used, so as to lessen failure from transient voltage spikes, or current surges, and I wouls include "snubbing" circuitry, to reduce inductive spikes. (gate voltage can easily turn the FET switch ON or OFF in a D.C. circuit-SCR devices will remain ON once triggered.)--Bob

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Robert M. Curry

BM1
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Joined: 04/07/2010
Points: 6
Re: Contactor vs SCR

Almost all BMS designs ive seen use FET's to control the current flow. Wouldn't it be practical to have them double as both the power cutoff in the event of overdischarging or failure and at the same time be used to turn the power system on. It just seems redundant to also have a contactor.

- Michael

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