Bypassing Limitations

by admin on February 9, 2009

It was as warm as one could hope for in Wisconsin winter this weekend.

Which of course meant that I would have the spring urge to tinker.

For a while, I have had an idea about a controller bypass for the car.

While PWM (Pulse-Width-Modulation) electronic motor controllers are great, they do get expensive fast at higher voltages. I bought the highest voltage I could afford, which was a Curtis 72V controller.

The car performs reasonably well for a “grocery-getter”, but simply doesn’t have the speed for the larger country roads, including the 45 mph road right outside my house. My house is also just on the other side of a hill, with a 55 mph speed limit on the other side of it, so guess how fast people drive past my place.

Well, they drive faster than I want to pull out in front of with my little 72v eco-beater.

I really don’t need a lot of control, just higher speed for one little section of road. too bad buying the next controller up would be so much money. If only there was a way to use my controller at low and medium speed, and just get some higher voltage straight into the motor for selected stretches of road.

Off to my box of miscellaneous forklift parts.
A common electro-mechanical part is a “contactor”. Contactors are basically very beefy mechanical on/off switches. A specialized type, a reversing contactor, completes one circuit a split second after disconnecting the first one. It’s a great way to switch high-amperage circuits.

I put together two reversing contactors, screwed them to a hunk o’ plywood, and started cabling them up. The idea is that normally the contactors keep the original controller in the circuit, but when powered up, they disconnect that and connect the batteries, plus two ADDITIONAL BATTERIES directly to the motor – TURBO MODE!

In case you didn’t know, the more voltage, the faster the motor spins.

Once I got everything cabled up, I tested it in the driveway with the car’s front drive wheels jacked off the ground. Everything seemed to work fine, so it was off to road testing.

I got out on the road, and drove along, going through a couple of gears to get it up to about 40 mph. Then I let off the go pedal, and hit the TURBO.

It’s like there was a whole ‘nuther gear – but amazingly, it had lots of power, like a lower gear, but at the same time, more speed, like a higher gear! This is only possible through the power of higher voltage.

The car briskly accelerated to 55 and then crept up to just past 60. My ammeter was pegged out. It’s only a 300 amp, and the PWM controller is rated at 400. I really have no idea how many amps I was pulling, but it was plenty!

The Turbo Bypass really opens up a lot of possibilities. I could now drive a short hop on the freeway. I can power up that last stretch of road, just before my house, where the speed limit hops up. Maybe I will even try that one really big hill.

Still, before doing much more of anything, I need to properly rig up some diodes and other little bits that will make the system work better. (And not FRY the controller!) Trying to hand-hold a momentary-on switch while driving and then hitting the main contactor isn’t exactly elegant.

What I do love about this is that I was able to go beyond the voltage limitation of the PWM controller, just by using a little imagination, and some parts I already had around.

Without spending a cent, I was able to experience how my car could perform as a 96V system. Not bad for “try before you buy”.

I can now only imagine what the car would be like at 108, 120, or 144 volts.

Of course I am now bitten and hunger for more voltage. It really is hard to describe the feeling of flying down the road propelled by nothing more than electro-magnetism.

Ok, I guess there is a standard line to describe it.

It’s electrifying.

EV grins,


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