DIY build with DC Brushed Motor

[Loker]

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

 

[Deleted member 4366]

All you need is a basic PWM control. Any high speed microprocessor can do it. You use the MP to switch a transistor that switches a bank of MOSFETs. The power is controlled by the % of PWM (duty cycle).

Typicaly, you cut down the 36v to 12v with a 12v regulator to switch the MOSFETs, then cut down a branch of the 12v again to 5v to power any sensors (PAS, throtte, speed and brakes) and the MP.

The main battery wires go to the MOSFETs, which control how much gets through to the motor. You need sizeable capacitors across the battery wires to deal with the high frequency switching.

There's normally a current limit set by using a shunt in one of the battery lines (negative). The MP can read the voltage drop across it to make decisions about power.

You need to research a bit about PWM control of motors because you can get some strange effects from inductance and capacitance.

 

[Benjahmin]

That's just wot I would have said

 

[Deleted member 4366]

If you want to be really clever, fit a wheel-speed sensor and allow a lower amount of power at low speed to stop the motor from burning out. Say you have a maximum current of 20A. Below 25% max rpm, allow only 7 amps, from 25% to 50% max rpm allow 14 amps and 50% to 100% allow 120 amps.

 

[Danidl]

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

In an engineering context, rather than a simplistic hobbyist viewpoint you have to be able to measure , analyse and construct and explain . The first thing you need to find out is the general specs for the motor. What kit do you have access to? Dynamotor? Lathe, the things you need to know are what is the intended operating voltage for the motor, whether it is shunt or series wound, does it use permanent magnets ?.
The math and the computer modelling of an DC electric motor is very simple and there are plenty of resources and spice type models for it. If you are a senior in engineering, you should be familiar with these . But none of the models work until you can give it the fundamental properties of resistance, back emf per rev so getting my these parameters is your first task.
After you know this you can then start looking at control technologies. The technique described by d8vh, is one such...