Speed Controller for my awesome wagon project

vskid · Post by **vskid** » Sat Mar 17, 2007 10:17 pm

I need a way to regulate voltage at fairly high amps (at least spikes of 20A) to control a motor's speed. The max voltage will be either 12v or 24v, depending on the motor I end up using. I was thinking of using a transistor or something, and efficiency will be needed. I need your input on other way or how to use a transistor to do what I want.

Its going to be for a motorized wagon that I'll be able to ride. Its still in the planning stages, so I'm trying to figure out how I'll do everything before I start spending money.

EDIT: Changed the title.

dragon · Post by **dragon** » Sun Mar 18, 2007 1:35 pm

I made a mini buggy a while ago using electrical golf buggy.

. But you could just use the speed controller on the golf buggy.

Kurt_ · Post by **Kurt_** » Sun Mar 18, 2007 1:53 pm

get some TI switching regulators. If you need more current, just buy 20 of em.

timmeh87 · Post by **timmeh87** » Sun Mar 18, 2007 4:26 pm

no, putting 20 switching regulators in parallel is not a good idea

what do you need variable voltage regulation for? speed control? if so, I suggest you do some more research into motor controllers, because thats not how its done usually. it turns out its much easier to use one constant voltage, and just switch it on and off really fast. (look up PWM, pulse width modulation)

if you are trying to design this yourself, be warned that large DC motors tend to exhibit a lot of transient behavior. what that means is that having 20v across a motor and switching it off all of a sudden can generate huge voltage spikes in the other direction (> 500V). Its not so easy to deal with motors, and its a topic I havent even really covered yet in university.

vskid · Post by **vskid** » Mon Mar 19, 2007 9:38 am

Thanks for making my life a lot harder timmeh.

So I'm now thinking it would be easier if I just bought a speed controller, does anyone know where I could get a cheap speed controller that can take 24v at at least 20A? Only needs one channel. I might take dragon's advice about the golf cart speed controller.

Post by **Triton** » Mon Mar 19, 2007 3:53 pm

ebay obviously, just search for speed controllers and see what comes up, a broken electric scooter or golf cart would be another option (so long as its not the speed controller thats dead lol)

timmeh87 · Post by **timmeh87** » Mon Mar 19, 2007 5:12 pm

The neat thing about controllers is that you can get ones with regen that charge your batteries when you brake

but they Arent very cheap. at least $100. $300 for a good one

I guess theoretically you could just make yourself like a super-high-power potentiometer and hook it up to the motor. It wouldnt be anywhere near efficient though. And you'd probably have to wind it yourself out of like steel wire or something, due to the fancy requirements (real low resistance, real high power). Make it like this (wire wound)

I dont think you need more than like 10 or 20 ohms to stop a motor in its tracks.

vskid · Post by **vskid** » Mon Mar 19, 2007 5:21 pm

So just how inefficient would the potentiometer be compared to a good speed controller? I think I'd rather just get another pair of batteries to make up for the lost efficiency. The ones I'm looking at are $15 for a 12v 7Ah Seal Lead Acid, so I'll have pairs in series to make 24v and have those pairs in parallel with other pairs to increase run time. So $30 for 2 more batteries would be better for me than a $100-300 controller.

timmeh87 · Post by **timmeh87** » Mon Mar 19, 2007 5:26 pm

It depends on a lot of things. in general 100% at full speed an 0% at full stop. Not necessarily linear in between

vskid · Post by **vskid** » Mon Mar 19, 2007 5:34 pm

timmeh87 wrote:It depends on a lot of things. in general 100% at full speed an 0% at full stop. Not necessarily linear in between

So I'd want a kill switch for when I'm stopped, so I don't drain the batteries. Maybe I'll look into snowmobile clutches, they create pretty much unlimited gears, so I could have the motor going at full speed all the time I'm moving. But I think a speed controller might be cheaper and easier than that.

Do you have any suggestions for what to look for in a speed controller? Or I could switch the power manually for the desired speed....

timmeh87 · Post by **timmeh87** » Mon Mar 19, 2007 5:49 pm

i dont really have much experience with controllers. cheap is a good thing to look for. well kinda

. There might be some requirements about motor geometry that you have to watch out for, theres tons of different ways to wind a motor.

The potentiometer idea might be impractical depending on the average current you have. it would be worst at nearly full speed, when the current was large (power = R*I^2). lets say you have 50 amps going to the motor and 0.5ohms on your potentiometer... thats 1250W... as much as a toaster or hair dryer

vskid · Post by **vskid** » Mon Mar 19, 2007 5:55 pm

timmeh87 wrote:thats 1250W... as much as a toaster or hair dryer

Maybe I will get a speed controller, instead of making a drivable toaster.

Do you know how I would control how fast the motor goes with a controller? Would it be like a monster transistor and have a trigger from an Xbox controller control the speed, or what?

timmeh87 · Post by **timmeh87** » Tue Mar 20, 2007 9:53 am

the only controller i have dealt with (solar car) uses a small pot set up as a voltage divider. it reads the voltage, converts it to a speed, and then makes the car drive at that speed for you. (the PWM part is always automatic

. no pressing buttons repeatedly). The one we have also has a serial port that tells you the RPM and temp of the motor and stuff (obvi the motor has to have those sensors in it to be able to show that data)

im assuming golf-cart ones work in a similar fashion. minus the serial port or sensors.

if you just want something small, look around hobby robotics sites.

dragon · Post by **dragon** » Tue Mar 20, 2007 3:06 pm

I would recommend a golf cart speed controller still maybe you could try a golf course if they have a old golf cart with a burnt out motor.
The golf cart speed controllers that i have use a potentiometer to control the speed. And im sure that you can get 24v golf carts. Thatâ€™s what i would recommend anyway.

vskid · Post by **vskid** » Tue Mar 20, 2007 4:34 pm

dragon wrote:I would recommend a golf cart speed controller still maybe you could try a golf course if they have a old golf cart with a burnt out motor.
The golf cart speed controllers that i have use a potentiometer to control the speed. And im sure that you can get 24v golf carts. Thatâ€™s what i would recommend anyway.

So about how much could I expect to pay for one of those?