DC Motor Speed Controlers

[thescreensavers]

I got a quick question.

Lets say I get a 30amp Speed controller and my batteries are 36amps, the speed controller will only allow 30amps in? or will something wrong happen. I am not sure.

Thanks

 

[Leros]

Yeah, you're fine. The device will only pull as much as it needs from the batteries.

 

[thescreensavers]

I got two 36v 27 amp 750watt motors here and I am probably going to build a go cart from them for a fun project. AWD baby lol. Just need to figure out some other simple things like if the front motor at 10amps runs a bit faster then the rear at 10amps what would happen? Each motor will drive two wheels.

Add batteries, frame, speed controller, some way to turn and we got a go cart lol.

 

[herm0016]

batteries should always have a higer rate than what you are supplying. the controller will only use what it needs up to 30 amps.

 

[randay]

you may overload your speed controller and it may burn itself out. most speed controls can handle a small burst, but if you overload the motor and draw too much amps something will definitely fail. speed control, battery, and motor draw must all be measured and spec'd correctly.

 

[herm0016]

sounds... dangerous... do you understand how much damage 30 amps can do? to control 2 motors in this way correctly you should have encoders on each output to match speed. are you going to use differential gears on each motor? i am going to guess that those batteries will not last too long with those motors, what are the specs on the motors?

 

[herm0016]

Originally posted by: randay
you may overload your speed controller and it may burn itself out. most speed controls can handle a small burst, but if you overload the motor and draw too much amps something will definitely fail. speed control, battery, and motor draw must all be measured and spec'd correctly.

the current rating on the motor should be stop current and that is less than the 30 on the controller so he should be fine. not a lot of headroom, but probably ok.

 

[thescreensavers]

Originally posted by: herm0016
sounds... dangerous... do you understand how much damage 30 amps can do? to control 2 motors in this way correctly you should have encoders on each output to match speed. are you going to use differential gears on each motor? i am going to guess that those batteries will not last too long with those motors, what are the specs on the motors?

Well I was going to buy 2 speed controllers each one for a motor.

All I can tell you about the motors is

750 Watt - 36 Volt Motor (Style: MY1020)

MY1020 36 Volt, 750 Watt, 2800 RPM, 27.4 Amp, permanent-magnet motor. 11 tooth sprocket for #25 chain. 12 gauge power leads with standard 1/4" push-in connectors.
Motor Dimensions: 4-1/4'' W x 5-1/2'' L not including shaft. 4-1/4'' W x 6-1/2'' L including shaft.

Bought them for 50 bucks off ebay along time ago.

 

[herm0016]

what is going to be the input to your controllers? what control architecture are you going to use?

 

[thescreensavers]

Originally posted by: herm0016
what is going to be the input to your controllers? what control architecture are you going to use?

Input to the controllers will be 3 12v 12amp batteries(Equaling to 36v 36amps). I am not sure by what you mean by "Control architecture"

I dont want to give any more then 27 amps to the motors because then they will get hotter and that's not good


Here is a little diagram I made rough draft

 

[herm0016]

input= how you are controlling your speed. this is not like a machine that runs at a set speed. how are you going to change the speed using the controllers and lol at 36 volts and 36 amps.... in parallel you will get 12 volts 36 amp max and in series you will get 36 volts at something less than 12 amps because of the internal resistances of the batteries.


are the controllers using pulse width modulation or voltage or current control?

you should really do some research.

and another edit...

those batteries ( at 6 lbs?) will probably not last nearly that long. they will probably only supply 12 amps for a few moments and it will drop quickly. voltage under load will be somewhat less than 12 volts also.

 

[randay]

this is a bad idea, by 12 amp batteries i assume you mean 12ah, which is by no means a measure of discharge rate, but of capacity. also like herm pointed out you can only connect them in series or in parallel.

 

[thescreensavers]

Originally posted by: herm0016
input= how you are controlling your speed. this is not like a machine that runs at a set speed. how are you going to change the speed using the controllers and lol at 36 volts and 36 amps.... in parallel you will get 12 volts 36 amp max and in series you will get 36 volts at something less than 12 amps because of the internal resistances of the batteries.


are the controllers using pulse width modulation or voltage or current control?

you should really do some research.

and another edit...

those batteries ( at 6 lbs?) will probably not last nearly that long. they will probably only supply 12 amps for a few moments and it will drop quickly. voltage under load will be somewhat less than 12 volts also.

The resistance makes it drop that much?! wtf. How do 36volt scooters do it that can do 25 mph or so if resistance is so high.

 

[herm0016]

Originally posted by: thescreensavers

The resistance makes it drop that much?! wtf. How do 36volt scooters do it that can do 25 mph or so if resistance is so high.

basic electricity fail. you should stick to pedals for a while longer.

http://www.allaboutcircuits.com/vol_1/chpt_5/1.html

read and get back to me.

and what randey said.. good catch. at his first post i assumed he knew a lot more than he obviously does.

 

[ApacheXMD]

Originally posted by: thescreensavers
I got a quick question.

Lets say I get a 30amp Speed controller and my batteries are 36amps, the speed controller will only allow 30amps in? or will something wrong happen. I am not sure.

Thanks

I think you're confusing capacity with current. Are the batteries capable of deliver 12amps each? Or are they each rated at 12 Amp-hours?

Your math in your diagram is wrong. You let's say you have three batteries rated at 12volts, 12 amp-hours in series. 12v X 3 = 36 volts. But since you're in series, the array's capacity is the same as each cell individually. So you don't have 36 amp-hours, you still have 12 amp-hours.

Taking each battery individually: 12Volts @ 12Amp-hours = 144Watt-hours which is the amount of *energy* in *each* battery.

144Watt-hours * 3 batteries = 432 Watt-Hours.
36Volts @ 12 Amp-Hours = 432 Watt-hours. See? They match.

But NOT THIS: 36v @ 36Amp-hours = 1296 Watt-hours! = You just violated the laws of thermodynamics.


If you have a motor that's rated at 750watts and have 432 Watt-hours of energy, then how much run time will you have? 432/750=0.576 Hours under ideal conditions. If your batteries are lead acid batteries, your amp-hr rating is probably highly optimistic under high load conditions, so your runtime is going to suffer.

There are going to many problems you're going to have to solve with a go-kart. GOOD batteries (li-po, Li-FePo4) are going to cost more than everything else put together. Lead acid batts are cheap, but are heavy.

If you're looking at standard RC speed controllers (might be tough finding one to meet your needs) you can use a servo tester for a throtlle. RC ESCs just accept a standard RC servo pulstrain from RC receivers. Since you wont' have a receiver to generate the necessary pulse, a servo tester will do the job. Just remember that a servo tester needs power as well. Servo testers usually just have a knob connected to a potentiometer, and you can pretty easily remotely mount the pot. So that will take care of the "control architecture".

 

[frostedflakes]

Damn you guys are harsh. :laugh:

 

[herm0016]

Originally posted by: frostedflakes
Damn you guys are harsh. :laugh:

and 30 amps will kill someone if not handled properly. just keeping darwin at bay.



edit: git rid of FT!!!!

 

[thescreensavers]

ok then so what if I used a car battery?

24v instead of 36 though

 

[thescreensavers]

woops double post

 

[randay]

car batteries are more than capable of providing hundreds of amps of discharge current.

 

[ApacheXMD]

One car battery is 12volts, not 24.
Perhaps you mean two car batts in series. If you have a 36volt motor, and you run it at 24v, it's not going to be at maximum efficiency. You still need to choose a motor controller...

And once you have the electronics figured out, you'll then need to figure out a the chassis and drivetrain, which is probably more difficult a task... You're aiming for an awd setup, but are you going to have universal joints or CV joints at the front? Differentials so you can actually turn worth a crap?