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Discussion Starter · #1 ·
In an electric vehicle application, what happens to the current generated by a DC motor as the vehicle is going downhill?
A mechanically driven motor becomes a generator, should I be concerned about that current?

For a DC motor without regen, (and I understand they can be rigged so), what is the best practice for going downhill? Depress the clutch, shift into neutral? Let it ride?
So many thanks for sharing your knowledge.
 

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A motor only works as a generator when mechanically driven if it has a working field, and a series-wound motor won't have that because the field current is the armature current. It is possible to make a series-wound motor brake - not generate - but you need to set up wiring to do that (and it's generally not a good plan).

I don't see any need to declutch or shift to neutral, as the motor with no current will just spin without problems.
 

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It is possible to make a series-wound motor brake - not generate - but you need to set up wiring to do that (and it's generally not a good plan).
Every electric forklift does this. It's called "plug braking". Even my 1969 York forklift with SCR and capacitor snuffing speed controller has automated plug braking.

More or less, you slam the motor into reverse. The motor doesn't really care which way it's spinning, it just cares how you're trying to accelerate it. If you're going forward and you put it in reverse, it'll decelerate down to a dead stop and then start reversing. I think there was some handy circuit to have a reasonable amount of braking (so you're not actually putting it into reverse, the controller understood the concept of braking) by default.

In this sense, not only is the braking not regenerative, you're actually spending new energy to counter the old energy. The result is the motor getting, I dunno, presumably twice the heat dump on it as plain acceleration.

You can also do resistive braking, where you use the motor windings as a resistor to slow down, but without adding new power into them. I don't think that's technically called "plug" braking at that point. You'd never come to a complete stop, but it would bleed some energy off as heat. Less effective but more efficient than plug braking because at least you're not spending new energy on it.

All in all, okay, yeah, probably not a good plan when your car is going to have brakes anyways. Brake pads are cheap. If you're not getting any extra energy out of the braking, I'd say spend your brake pads, don't risk your motor and controller smoking. It's all going to end up as heat anyway, might as well put it into a system that has a 100 years of engineering behind being safe and effective at turning motion into heat.
 

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Annndd from controller point of view: generally on power silicon there is a diode bypass scheme that is reverse to the flow under power. In the case of my KOSTOV, the interpoles are exciting the field slightly so there is some current running backwards through the controller back into the battery. Since it is less than battery voltage it is kinda meaningless, but measureable. It made Tessaract, the designer of the soliton, slightly crazy once when I asked WTF do I get regen on a series DC motor. Documented somewhere in this forum.

Series DC has no way to prevent over excitation and theoretical infinate voltage/current without separating and controlling the field windings. Sepex does do regen.
 

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In this example, would a brushed DC motor behave differently than a brushless DC motor?
Yes, because a "brushless DC motor" is really an AC motor with a non-sine-wave inverter.

Would an AC motor wired without regen capabilty behave similarly?
Regeneration is not a wiring feature in AC motors - it is a capability of the controller (inverter). There are no extra wiring features for regeneration.
 

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Discussion Starter · #9 ·
Regarding best practices for going downhill with a DC motor/manual transmission combo…
If you stay in gear, Watch your RPMs. You may mechanically spin the motor in excess of of it’s RPM design specs.
My car was once towed in first gear and left a trail of copper bits a hundred yards long.
 

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Not so much a problem with going downhill
But if you motor a DC motor without any current going through the brushes you will destroy the "film" on the commutator and get accelerated wear on the brushes and commutator

So towing or a very very long downhill drive could wear out the brushes/comm
 

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How does regen work in reluctance motors ?
There are no switched reluctance motors in electric cars - just some industrial equipment. There are synchronous reluctance motors available in sizes suitable for cars, but none are used in production EVs and I've never heard of one in a conversion.

All interior permanent magnet synchronous motors have a reluctance torque component, which becomes significant at high speed. It doesn't really change the PM AC behaviour as far as spinning freely or regenerative braking is concerned.

Edit note: for the truly dense, the first paragraph above means that there are no switched reluctance motors in cars, and the second paragraph means that a synchronous reluctance motor works for regen just like a synchronous permanent magnet motor. Of course regen would work in a switched reluctance motor if you used one.
 

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Blah blah blah...didn't answer the question.

It can be done:

 
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