# DIY build with DC Brushed Motor



## Loker (19 Nov 2017)

Hi all,


I’m studying electrical engineering at university and I’m working on putting together an electric bike for my senior project . I have an old brushed DC hub motor and three new rechargeable 12V lead-acid batteries that have a capacity of 15Ah each. I also have a bike frame that fits my hub motor and an ebike throttle I inherited from one of my dad’s old projects. I’ve looked extensively but I can’t find any specs on my motor online.


The thing I’m struggling with is I have no idea how to build a control unit between the batteries and the motor. I think a buck converter whose output voltage is somehow regulated by the position of the throttle might work but I have a strong feeling that there’s more to it than that.


Does anyone know of any resources detailing the typical circuitry between batteries and a brushed DC hub motor in the context of electric bikes?


Any and all replies are appreciated!


-Sean


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## Tigerbiten (19 Nov 2017)

I think you'll find most motor speeds are now controlled by "Pulse Width Modulation" due to less power wastage.
But that's about as much as I know.

Luck ..........


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## raleighnut (19 Nov 2017)

Tigerbiten said:


> I think you'll find most motor speeds are now controlled by "Pulse Width Modulation" due to less power wastage.
> But that's about as much as I know.
> 
> Luck ..........


I think that's with a 3 phase brushless 'Hall Effect' motor.


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## Tigerbiten (19 Nov 2017)

raleighnut said:


> I think that's with a 3 phase brushless 'Hall Effect' motor.


I think it works for all motors.
Electronic tutorial on PWM & Motors -> http://www.electronics-tutorials.ws/blog/pulse-width-modulation.html
From the tutorial ......


> _Pulse width modulation_ is a great method of controlling the amount of power delivered to a load without dissipating any wasted power. The above circuit can also be used to control the speed of a fan or to dim the brightness of DC lamps or LED’s. If you need to control it, then use Pulse Width Modulation to do it.


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## PhilDawson8270 (19 Nov 2017)

The easiest way would be to use an off shelf electronic speed control unit


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## QFour (21 Nov 2017)

PhilDawson8270 said:


> The easiest way would be to use an off shelf electronic speed control unit



The OP is studying Electrical Engineering don't think the University would appreciate an off the shelf conversion.

Everything you are trying to do has already been done many years ago. They have certainly moved on from DC Brushed motors and Lead Acid batteries. If all you are doing is fitting a motor to a frame with a cog to drive the chain just fit a big switch between the batteries and the motor. You will not be able to control the speed unless you use either fit a very large variable resistor in the circuit which will heat up like an electric fire or a modulating controller something like you get on model railways where power is supplied as a variable square wave.

If it's a DC Motor just connect one 12v battery up and see what happens. If it's a bit slow try two in series. Still to slow use Three. Not many DC 36V motors out there probably 12v. Simple to find out amperage with a meter. Alternatively just connect a battery up and time how long it take to flatten it, 

Good luck with your project


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## PhilDawson8270 (21 Nov 2017)

QFour said:


> The OP is studying Electrical Engineering don't think the University would appreciate an off the shelf conversion.



They wouldn’t appreciate somebody wasting their home reinventing an off the shelf solution. Particularly without good reason or benefit.

Making bespoke versions when off the shelf stuff Works is seriously poor engineering.


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## raleighnut (21 Nov 2017)

Yep,


PhilDawson8270 said:


> They wouldn’t appreciate somebody wasting their home reinventing an off the shelf solution. Particularly without good reason or benefit.
> 
> Making bespoke versions when off the shelf stuff Works is seriously poor engineering.


 Yep it's not like he's wound the Armature on the motor himself.


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## Paul Bromley (21 Nov 2017)

Ned Mohan is a good source of all things power electronic. His books are usually regarded as the reference source


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## subaqua (21 Nov 2017)

QFour said:


> The OP is studying Electrical Engineering don't think the University would appreciate an off the shelf conversion.
> 
> Everything you are trying to do has already been done many years ago. They have certainly moved on from DC Brushed motors and Lead Acid batteries. If all you are doing is fitting a motor to a frame with a cog to drive the chain just fit a big switch between the batteries and the motor. You will not be able to control the speed unless you use either fit a very large variable resistor in the circuit which will heat up like an electric fire or a modulating controller something like you get on model railways where power is supplied as a variable square wave.
> 
> ...



Last time I checked 3no 12Volt batteries in parallel still gave 12V just lasted a lot longer .....


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## QFour (21 Nov 2017)

Ohhhhh .. Senior Moment ..


View: https://www.youtube.com/watch?v=pIC3oiddJM0


This guy has come up with a brilliant way of controlling the motor. Magnet on one pedal and a read switch on the frame. Provides power to the motor every time the pedal goes by the read switch. The faster you pedal the faster the motor goes. You are in effect generating a DC Square wave.


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## keithmac (22 Nov 2017)

Ebikes come with various motor control strategies, either crank motion (hall effect sensors) or crank torque sensors to decide on and off state for the motor.

They all use wheel speed feedback (remeber you are limited to 15.5mpg), having a bike triggered by the throttle only is technically illegal iirc, it has to be done by pedalling.

Have a good look round the ebike forums, the normal now is 36v or 48v lithium ion battery packs at least 15ah.

If you can design a programmable controller which can be reflashed it would be a good start, I would say writing the software would take more time than wiring the hardware to get a usable SAFE ebike out of the other end.

To be honest with what you are starting with you might not get a satifactory outcome, weight is a big factor on how useable a cycle is. My Gtech comes in around 16kg (36v motor, LiOn battery pack).


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## raleighnut (23 Nov 2017)

keithmac said:


> Ebikes come with various motor control strategies, either crank motion (hall effect sensors) or crank torque sensors to decide on and off state for the motor.
> 
> They all use wheel speed feedback (remeber you are limited to 15.5mpg), having a bike triggered by the throttle only is technically illegal iirc, it has to be done by pedalling.
> 
> ...



The throttle only option is now illegal, good job it wasn't when I built my trike (and Maz's Dawes)


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