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Old 06-20-2010, 09:12 PM   #1 (permalink)
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Is this EV AC vs DC for dummies? Or EV lessons from a Power Wheels?

From my reading on here, AC electric cars have regenerative cycle built in. However, I know for a fact a DC motor can generate electricity. So why don't more people take advantage of this? Is it simply too difficult to make a DC motor controller that can also have regen braking?

I was thinking these thoughts as I pushed my 2 year old around on her 6 volt power wheel. If I pushed her while she was pressing the go button for the motor, there was considerably more drag than if she let off the button and opened the circuit to the motor.

Can those who know explain the complications of using a DC motor in an electric car and wiring in some sort of regen braking?

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I think you missed the point I was trying to make, which is that it's not rational to do either speed or fuel economy mods for economic reasons. You do it as a form of recreation, for the fun and for the challenge.
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Old 06-20-2010, 09:25 PM   #2 (permalink)
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In a series wound motor, it has something to do with the brushes. They were talking about it on the EV Tech list a while back. I guess you would have to be able to dynamically change the brush locations while putting power in, and taking power out. I have no idea why though.
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Old 06-21-2010, 08:26 AM   #3 (permalink)
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Here is my rudimentary explanation.

There are different kinds of DC motors, shunt wound, series wound, permanent magnet, and more. I know you can regen with permanent magnet. I'm pretty sure you can't with series/shunt wound.

With a series motor, you have a coil on the case and a coil on the armature (shaft that spins). You put power through the case coils and the armature coils to produce power. You put in more power, you get more power out. Shunt wound motors are similar in that you have a coil on the armature and case I believe, but are wired differently. There are other incarnations of DC motors too, but I don't know enough to explain them all.

With a permanent magnet motor you have magnets on the armature and coils on the case (brushless), or magnets on the case and coils on the armature (brushed permanent magnet). So you put power into the coils and they push against the magnets. This limits the power of the motor since the magnets can only push back so hard, but makes it more efficient. But, it allows you to pickup the magnetic field when the magnets pass over coils.

Without a magnetic field to pass over a coil, you can't make electricity. So with a series wound you'd have to put power into one of the coils to get power out of the other coil. The net effect would be a loss in power since its not 100% efficient.
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Old 06-21-2010, 12:50 PM   #4 (permalink)
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You also need to create a higher voltage out of the motor than you already have in the batteries.

On DC motors, the higher the voltage, the faster it makes the motor spin.

I assume that you were pushing the PowerWheels faster than the Powerwheels itself could drive right then. Those little toy cars typically use a pair of very small permanent magnet motors.

When she would press the "GO" button, it completes the circuit between the motor and batteries. The drag of the magnets would increase resistance on pushing the car because you are now making a magnetic field in the motors and pushing a little power back into the batteries.

On Electric Vehicle Racing , when they were racing a car on the salt flats, it had a sepex motor in it. To recharge the car, they would drag it behind a pickup truck with the car in gear to regenerate and charge the battery pack!
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Old 06-21-2010, 05:15 PM   #5 (permalink)
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Here's a little talk about it too:

final batteries and controller? - DIY Electric Car Forums

The line about melting down the motor caught my eye....
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Old 06-21-2010, 07:50 PM   #6 (permalink)
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Certainly I'm thinking about this too simplistically, but why couldn't a regen circuit be activated that includes a transformer to step up the voltage being returned to the battery?

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I think you missed the point I was trying to make, which is that it's not rational to do either speed or fuel economy mods for economic reasons. You do it as a form of recreation, for the fun and for the challenge.
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