# Can I use variable resistor to control motor speed without using a controller



## bsamarah (Sep 16, 2014)

Hi 
I am planning on doing a car conversion but wanted to understand a few things before buying the car, motor and electronics. I am a newbie to electronics, so forgive the dumb or repeat questions. (PS. I did search the forum as best I can for this before posting). 

Can I use a potential divider (or variable resistor) to control speed of the motor, without a controller ? If so how can it be done? Meaning what parts / part numbers to I need and how do I connect them assuming a simple circuit that consists of batteris motor and the parts I need.


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## Coulomb (Apr 22, 2009)

> Can I use a potential divider (or variable resistor) to control speed of the motor, without a controller ?

You could, but it would have to be rated for half the power of the motor (so tens of kilowatts), and at other than full speed, it would waste horrendous amounts of power. It would need prodigious cooling, and range would be impacted mightily.

Ancient EVs used to use resistors to get motors started, then run at full power (no resistance), but it's just not practical these days.

Just the 5% of power that a modern controller wastes as heat is enough of a headache. At 10% throttle, nine times more power would be wasted as heat as would go to the motor (then there would be the losses in the motor on top of that). So 10% efficient (90% loss) at 10% throttle.


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## bsamarah (Sep 16, 2014)

ah yess. thanks for the clarification. I was just asking as a quick test before I get a controller. Good points again I appreciate the reply. 

Thanks

Basem.


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## david85 (Nov 12, 2007)

moved to correct forum...


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## PhantomPholly (Aug 20, 2008)

Some folks have talked about using a trick used by early trolleys and cranes to get variable voltage in steps, although the cost in relays might be as bad as a controller.

By breaking up your battery pack into 12 modules, you can wire it multiple ways to get different voltages. 


Putting all in parallel yields 1/12 of maximum voltage with gobs of amps available
Wiring 2 banks of 6 in parallel then putting the two sets in series yields 1/6 of maximum voltage
Wiring 3 banks of 4 in parallel then putting the 3 sets in series yields 1/3 of maximum voltage
Etc.
All of this wiring would have to have some relatively easy-to-use switching system to re-configure from one layout to the next, using contactors as "clutches"



However, this type of poor-man's throttle would be both heavy and ugly as sin...


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## ga2500ev (Apr 20, 2008)

PhantomPholly said:


> Some folks have talked about using a trick used by early trolleys and cranes to get variable voltage in steps, although the cost in relays might be as bad as a controller.
> 
> By breaking up your battery pack into 12 modules, you can wire it multiple ways to get different voltages.
> 
> ...


I started a long thread long ago on the subject. Take a read when you get a chance.

ga2500ev


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## lithiumlogic (Aug 24, 2011)

If you're doing a really basic test just to make sure that the adapter plate etc work, before splashing out on controller and most of the batteries, you could do what the FavElec guy did on the forums, hooked the motor up to 12V and used the solenoid from the ICE's old starter motor to switch on/off. Allows a low speed crawl around the block. He said by the end of the trip though, the solenoid was very hot, it's not designed to be continuously engaged.


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## david85 (Nov 12, 2007)

PhantomPholly said:


> Some folks have talked about using a trick used by early trolleys and cranes to get variable voltage in steps, although the cost in relays might be as bad as a controller.
> 
> By breaking up your battery pack into 12 modules, you can wire it multiple ways to get different voltages.
> 
> ...


Wow, this takes me back to my earliest research in EV propulsion...good times...

The mars2 electric car from the 1960s used a system like that. Explanation of the propulsion system and paralleling of battery strings is explained beginning on page 4:

http://www.electricauto.com/files/67627171.pdf

Using contactors instead of solid state PWM was how the early EMD diesel locomotives worked too. Some older turbo-electric battleships also had the ability to change pole count on the generator to achieve electric gear ratios while maintaining a mechanically decoupled drive system. Is it practical today? Probably not, but its still pretty cool stuff.


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## Ziggythewiz (May 16, 2010)

lithiumlogic said:


> hooked the motor up to 12V and used the solenoid from the ICE's old starter motor to switch on/off. Allows a low speed crawl around the block. He said by the end of the trip though, the solenoid was very hot, it's not designed to be continuously engaged.


Why bother using a solenoid? Your contactor will turn 12v on and off with no issues and is designed to be continuosly engaged.


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## PhantomPholly (Aug 20, 2008)

david85 said:


> Wow, this takes me back to my earliest research in EV propulsion...good times...
> 
> The mars2 electric car from the 1960s used a system like that. Explanation of the propulsion system and paralleling of battery strings is explained beginning on page 4:
> 
> ...


Hehe - I may not understand everything you guys say, but I do enjoy learning stuff. About 38 years ago I worked in a steel mill in Gary and had an opportunity to operate an overhead crane (frame about 150' wide, lift trolleyed back and forth across the span and the whole contraption could navigate over 1/2 mile up and down the building section). It is clear to me now that it operated on a principle similar to this - moving the motion handles for left/right; forward/back; up/down all resulted in a shower of sparks and definite "notched" settings. Operating a cable with 40 tons hitched and over a 100' drop smoothly required a lot of planning ahead and careful watching of sway to bring a load smoothly to rest without developing a dangerous oscillation.


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## Sunking (Aug 10, 2009)

You can certainly do that for series wound motors, and that is how it was done decades ago in electric vehicles like golf carts and the first EV's before gas engines took over in the early 1900's. 

But as pointed out would be very foolish and wasteful if you did. But if you insist the resistors are still around in old resistor golf carts. People by the old carts for pennies on the dollar, and throw away the existing throttle/Resistor accelerator pedal and motor and replace them with a Shunt motor and electronic PWM controller. 

Go over to Buggies Unlimited and you can shop around for the resistors pedals. My bet is some one will give it to you if you pay shipping so they do not have to haul them off to the dump or recycle center.


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