P.W.M.'s.. a question about a dc pwm with a 100A capacity?

Thread Starter

SWARF

Joined May 7, 2019
5
I am new to the forum and I need some advice.
I have been making a very small electric car using bicycle parts and powered by two 12v brushed and geared down motors in series driving 2 separate wheels.
All works great on 2 car batteries, but only either on or off,... I have no controller.
The motors draw max approx' 50A at 24v (tested on full load with a DC clamp meter)
My question is... can I find a reliable PWM to drive it ?.
The reviews on these are not good. Possibly because of users overloading and the specs' on them are exaggerated.
I have been looking at the below Link.
https://ebay.us/848eZi
Any comments would be greatly appreciated. Chris.
 

oz93666

Joined Sep 7, 2010
737
Nice work SWARF

As Alec has hinted at , the problem is with zero revs (stall) these motors will draw very high current ... in normal operation your clamp meter may not notice this peak ... but electronics will in the PWM and could blow ....

To measure stall current lock one of these motors so it cannot turn , then measure current ( only for a few secs) ....

Car batteries are not designed for electric vehicles ... you may only get a few dozen life cycles ... for long term use you have to think about Li-po cells , a quarter of the weight for same capacity and 20 times life.
 

shortbus

Joined Sep 30, 2009
8,866
The only difference between a low power and high power PWM module is the output mosfets, IGBTs or transistors. The wave forming part is the same.
 

Thread Starter

SWARF

Joined May 7, 2019
5
Electric Golf Carts use PWM for their motor speed controller, and they are reliable
If Li-Ion Batteries are cost prohibitive ( most likely )
then use Deep Cycle Golf Cart Lead Acid Batteries from Trojan.
https://www.trojanbattery.com/markets/golf-utility-vehicle/

Making PWM speed controller for Electric bike
https://forum.allaboutcircuits.com/...eed-controller-for-electric-bike.26626/page-2
Nice work SWARF

As Alec has hinted at , the problem is with zero revs (stall) these motors will draw very high current ... in normal operation your clamp meter may not notice this peak ... but electronics will in the PWM and could blow ....

To measure stall current lock one of these motors so it cannot turn , then measure current ( only for a few secs) ....

Car batteries are not designed for electric vehicles ... you may only get a few dozen life cycles ... for long term use you have to think about Li-po cells , a quarter of the weight for same capacity and 20 times life.
Nice work SWARF

As Alec has hinted at , the problem is with zero revs (stall) these motors will draw very high current ... in normal operation your clamp meter may not notice this peak ... but electronics will in the PWM and could blow ....

To measure stall current lock one of these motors so it cannot turn , then measure current ( only for a few secs) ....

Car batteries are not designed for electric vehicles ... you may only get a few dozen life cycles ... for long term use you have to think about Li-po cells , a quarter of the weight for same capacity and 20 times life.
Nice work SWARF

As Alec has hinted at , the problem is with zero revs (stall) these motors will draw very high current ... in normal operation your clamp meter may not notice this peak ... but electronics will in the PWM and could blow ....

To measure stall current lock one of these motors so it cannot turn , then measure current ( only for a few secs) ....

Car batteries are not designed for electric vehicles ... you may only get a few dozen life cycles ... for long term use you have to think about Li-po cells , a quarter of the weight for same capacity and 20 times life.
Thanks OZ, The measurement was done at stall and it showed 51A with 24v to bopth motors in series.and both stalled.
I know car batteries are not deep cycling but are cheap from my scrap yard at £10. for one that has had a drop test so still good.
I have used those same motors before (30 years ago) om my kids go kart. the old tractor battery lasted well then.
Lithium ones are great but too expensive, that is for a "fun contraption" ! .
 

MisterBill2

Joined Jan 23, 2018
9,468
Aside from the PWM issue you have another problem, which is that friction driving the tires will rear them out, and the first time that the drive slips and the wheels do not turn you will wear through the tire in a hurry. PWM driver circuits do get published and some of them work well. With some complex switching you could also start out with both motors in series and fed with 12 volts. That would be a bit gentler, then switch to two in series and 24 volts, followed by 24 volts to each motor. 3 speeds starting and no power burned in a resistor. Just a few extra relays or a drum controller.
 

Thread Starter

SWARF

Joined May 7, 2019
5
Aside from the PWM issue you have another problem, which is that friction driving the tires will rear them out, and the first time that the drive slips and the wheels do not turn you will wear through the tire in a hurry. PWM driver circuits do get published and some of them work well. With some complex switching you could also start out with both motors in series and fed with 12 volts. That would be a bit gentler, then switch to two in series and 24 volts, followed by 24 volts to each motor. 3 speeds starting and no power burned in a resistor. Just a few extra relays or a drum controller.
Yes,, MisterBill, I like your alternative thinking. I have drawn out a switching circuit to do your suggested battery switching using a rotary switch. Or as you call it a drum. I did order a multi terminal switch. Then wired it up temporary with 12v to two motors in series. This gave insufficient power to pull away properly. Then again temporary with both motors on a separate 12v battery. That was good but too much power (65A on each starting up motor) giving a 10 mph drag race. Then both motors in series off 24v. this was best but still to much power but the current was down to a level where I was hoping a PWM would drive it.??
The tires take the friction with no problem and this slip has the advantage of not letting the motors reach a full stall which reduces the current at start.
It is interesting that PWM circuits have been posted on this site. I will search again.
 

jpanhalt

Joined Jan 18, 2008
11,088
I agree with the comment about using a friction wheel to drive. Also, when power is off, you would need a way to allow coasting, as the motors can act as brakes.

A chain drive with or without an electric clutch or coaster mechanism as with bicycles might be a better approach. 4QD has extensive experience in DC powered carts ( http://www.4qdtec.com/ ). As for your controller, you should probably consider softstart. That can be done with a microcontroller or a purpose-designed chip like the TPIC2101 (http://www.ti.com/lit/an/slit110/slit110.pdf ).
 
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Thread Starter

SWARF

Joined May 7, 2019
5
These worked quite well for many years. 120 years ago.

https://en.wikipedia.org/wiki/Baker_Motor_Vehicle

Start at 3:15.


Imagine the improvements that could be made to it's construction.
Oh, Fantastic, That one is a lovely thing. It seems to be worked without a speed control. At least they don't show anyone using one.
That Wikipedia link is interesting. I know someone who still services working Nickle/iron batteries that were first in use about 1940.
I have a boat with banks of NiCad's that were bought in 1983... Capacity well down now though.
 

MisterBill2

Joined Jan 23, 2018
9,468
Yes,, MisterBill, I like your alternative thinking. I have drawn out a switching circuit to do your suggested battery switching using a rotary switch. Or as you call it a drum. I did order a multi terminal switch. Then wired it up temporary with 12v to two motors in series. This gave insufficient power to pull away properly. Then again temporary with both motors on a separate 12v battery. That was good but too much power (65A on each starting up motor) giving a 10 mph drag race. Then both motors in series off 24v. this was best but still to much power but the current was down to a level where I was hoping a PWM would drive it.??
The tires take the friction with no problem and this slip has the advantage of not letting the motors reach a full stall which reduces the current at start.
It is interesting that PWM circuits have been posted on this site. I will search again.
A hundred amp switch would be very expensive, and is quite different from a "drum", which the ones that I have seen were a cylinder of insulating material with copper patterns and fairly heavy duty copper contacts on spring loaded arms that rode against the cylinder and contacted the copper conductors. That was an item that my grandfather had collected quite a while ago. So it must have been at least 40 years ago that I saw it.
Certainly an adequate PWM controller would be a lot smaller and even simpler to wire in. The big challenge with a PWM controller will be the power dissipated in the switching transistors as they change from off to on and pass through the linear control range where they act like a resistor.
 

Alec_t

Joined Sep 17, 2013
12,248
The tires take the friction with no problem
That should be ok for a fun vehicle, despite the migivings above. Back in my youth I had a petrol engine accessory for my pedal cycle. The engine shaft had a ribbed 'cog' about 2" diameter which was a friction drive for an ordinary cycle tyre. Despite the fact that the 'clutch mechanism' simply pulled the cog down onto the tyre so that the engine was started up by pedal power, the tyre lasted surprisingly well (a couple of years of trips to and from school and around town).
 

shortbus

Joined Sep 30, 2009
8,866
These worked quite well for many years. 120 years ago.

https://en.wikipedia.org/wiki/Baker_Motor_Vehicle

Start at 3:15.


Imagine the improvements that could be made to it's construction.
Growing up there was an old lady a few blocks over, that had one of them in a basement garage, it had been her mothers. Wouldn't let any of use kids in the garage but would let us look through the windows at it. Don't know what ever happened to it.
 
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