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E-Bike project (How to choose a suitable DC motor) Answered

Hi Guys, I am currently doing my school project on electrical bicycle. Mind teach me how to choose a suitable e-bike motor to be used and what is the calculation that I needed. I plan to enable the e-bike to support the weight of 100kg with people riding on it.



5 years ago

What kind of motors do you have access to ?

I am now finding a suitable rating of motor to drive my bicycle. I have not access to any dc motor yet. Do you know how to calculate the power rating of the motor needed? I have try using P=F*V, but it seems that my calculation is not correct. I have tried using this online calculator http://www.kreuzotter.de/english/espeed.htm , I found the power is about 179W. But I heard that 200W motor is too powerful to drive a ebike, and they told me that 50W should be more than enough to drive the ebike....so I quite confuse now. Because the specification sheet for commercial ebike motor rating is about 24V and 250W rating. Could you help me in this matter?...Thanks I am appreciate it...

Your problem lies in the way firms quote their motor power.

To make things look good! They take the voltage and MAX current and multiply together giving a figure for max watts.

1. Running the motor flat out will make it hot. If you can't dissipate that heat then the motor will burn out very quickly.

2. Running the motor flat out will rapidly discharge the power supply giving a short range.

3. In reality there is some compromise - A motor that will offer enough speed - with or without gears - and yet run cool enough, (you may need to help it with artificial cooling), so it can withstand the strain.

4. Typically You need for a kids scooter something rated around 150 watts (this will be a max rating if your looking on Ebay) - You will need at least 24 volts from 2 12 volt lead acid batteries.

These come in various physical sizes. You will need a suitable speed controller for a brushed motor. You will need to gear down the motor output either with fixed gears or variable gears.

Typically these motors will; runt at around 2000 to 4000 RPM so work out how fast you wheel needs to be going to give a reasonable speed. A reasonable speed on a bike is 12 to 20 MPH!!! NOT 50 MPH.

Motors are readily available on the web as replacements for scooters and bikes golf carts etc. Ebay carries many - Expect to pay upwards of $300 to $500 for a halfway reasonable motor.

A lot depends on what your building as a frame.






All true !
What a grand collection of frames, the E-Bike-Train looked great for a flash mob.

For some reason my wife cannot ( not comfortable ) ride a bike for two
in either position ? !


A single horsepower is equal to 746 Watts
Rick made do with 250 Watts or 1/3 HP DC I'm guessing permanent magnet stator.
His and most DC motors can accept 5 to 10 times over the rated power for a
short time, making it an ideal vehicle  propulsion machine.

Most electric cars use an PM rotor and inverter switch the field for speed and torque
like an inverted series DC brush motor and a gearbox to operate in the peak eff area..

Although the leading Electric car company in the world right now The Tesla uses
a liquid cooled Induction motor because they get overall increased efficiency
over a PM machine.

EV hub bike motors are touting 250W which is more then you may really want 
for your first DIY.

I would use a PM 150W myself as a 15 mph straight and level but could help
me on a hill and peddling could recharge the battery.


Yes a PM brushed motor - Originally intended for powered wheel chairs. In an amateur electric vehicle the motor will be abused and needs to be very robust unless you go to great lengths to protect it.


You need to think:

1. What is offered commercially what are the specifications of bikes, scooters and small cars.

2. What performance do I expect from the bike

3. How will it be powered (battery pack) this weighs a LOT

4. What budget will you have - Big motor BIG price.

Our Motor was 250 watts CONTINUOUS rating. We ran it at around 20 Amps with a Max current of 70 Amps at 24 volts so Max power was about 20A x 24V  480 watts.

This would give us 30 MPH and about 2 hours running on the flat from a pair of car batteries.

See the related panel at the right.

For a school project this is HUGE and a big big commitment in time and effort. Make sure you get at it early and allocate time every day/week beyond what your school provide.

Don't attempt unless your convinced your teachers are behind you and have the expertise your going to need to support your efforts. Most ambitious school projects fail because the staff don't have enough knowledge to sort t things out and they have unrealistic expectations.



Hi, thanks for the advice, I will try my best in completing this project. Thanks for the references