# Sizing a brushed DC motor

#### Scubasteves

Joined Dec 11, 2015
20
Hello!
I am wanting to use a brushed DC motor as a generator/braking system on a bicycke! It will be "engagable". I have the mounting figured out but I need to know how to size my future motor/generator!

Based on my max potential momentum, and I want to reach a complete stop in ~15seconds, I need 110 Newtons per second of braking power.

Any ideas, big motor or small with a gearbox. The idea is recharge a 48v battery via diode or such
Examples are best! Thanks!

#### Kermit2

Joined Feb 5, 2010
4,162
You should research what work the average human is capable of doing measured in watts. Hint: 1 Horsepower is 745 watts and humans on average produce less than the 1/10th of that.
Don't oversize your generator because a human has limits on his power generating ability

#### wayneh

Joined Sep 9, 2010
17,493
Kermit2, I don't think he intends to ever be pedaling against the generator. He wants regenerative braking. I'd look at the starter/generator off a garden tractor. Just a hunch that it would be about the right size. An automotive starter motor might be gotten for next to nothing.

Joined Jul 18, 2013
28,534
It would have to be an automotive starter with a P.M. field as the older series wound field type do not generate.
Max.

#### Scubasteves

Joined Dec 11, 2015
20
So things the motor needs:
1.~500 w +/- 250w (.5-.75 HP seems like the force I would want. I'm not sure the conversion of newtons to watts?)
2. No emf.
3. Permanent magnet? (Not exactly familiar with what max is speaking of to avoid)
4. Brushed
5. To be 12-48v with the appropriate charger/ voltage changer for 48v battery
6. Reasonable weight

Am I missing anything?

Joined Jul 18, 2013
28,534
You cannot use a wound field series motor for re-generation, some later type starter motors are Perm. Mag. which you would need.
Max.

#### wayneh

Joined Sep 9, 2010
17,493
You want it wound to produce the most power at the rpm range it will spin at. In other words you don't want it to be designed for 5000 rpm if you intend it to run at 500 or less.

This can be a problem when using a "motor" as a generator. The motor ratings may not tell much about how it will perform as a generator.

I don't understand your #2 point above.

#### Scubasteves

Joined Dec 11, 2015
20
You want it wound to produce the most power at the rpm range it will spin at. In other words you don't want it to be designed for 5000 rpm if you intend it to run at 500 or less.

This can be a problem when using a "motor" as a generator. The motor ratings may not tell much about how it will perform as a generator.

I don't understand your #2 point above.
In my previous thread or somewgere in my research someone mentioned an emf circuit which prevents feedback?

Good point on the rpm!