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Selecting DC Motor Power rating for an electrical-bike project? Answered

I want to select a dc motor for my e bike project. I want to move a mass of 140 kg including cycle weight at top speed of 25km/h.

I am confused with selecting motor specification, on Power rating. How can I calculate the required Power to drive a weight of 140kg with a speed of 25km/h. I have tried to use the formula that I found in the website ( http://www.kreuzotter.de/english/espeed.… ) .

The power rating that I got is about 179W. But on reading of the articles on (investigating the human cycling performance), I found that the average people cycling power is less than 100W.

I also have see the commercial e-bike specification, the commercial e-bike used 24V 250W DC motor to power their e-bike.

Please help me in this matter, I am very confused with this, if there is any formula or calculation...please guide me ...I am appreciate your helps.

9 Replies

SumantaN (author)2017-01-29

Hi, I make a e-bicycle project but my project failed for which reason I don't know. I Have used a DC motor (My 1016, 250 W )and using 2 nos of EXCIDE battery 2 x 12 V (7 ah/Battery). Here the problem is that the motor rotate the back wheel of bicycle(28")easily without any load but when the load applied to it don't enough power to rotate the wheel and create more heat .my weight is 78 Kg.Will you help me?

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Manimass (author)SumantaN2018-02-27

Now clear the problem same problem I had this please say the solution

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SS133 (author)SumantaN2018-02-05

If you use high speed motor it doesn't rotates when on heavy load.so you can use slow speed motor and just move the bicycle or any for rotating the motor to continue riding

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venkats29 (author)SumantaN2017-04-12


According to your details you are not supplying the required power for your 250w motor the battery you mentioned generates only 168 watts (24volts*7AH=168 watts) you need to increase your voltage to 35 volts or amps to 11Ah to make your motor run up to its max capacity. 78 kg weight is way to much load for 250 watts motor if possible increase your motor capacity to 1-2kw for better perfomance.

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MohdA64 (author)2016-03-30

how to know Grade motor in percentage?

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rickharris (author)2013-06-14

1. Have you checked that you are allowed by Law to ride an electric bike at 25Kph.

2. The answers will still be the same as your previous question.

3. A fit human puts out about 1/2 Hp over a reasonable time - Peaking at 1 Hp for short distances. 1 Hp = 746 watts. A bit more than your estimate. This will push you along at about 20Kph'ish on the flat

4. Your going to want to match at least that in power to assist your peddle power. 20 your looking at 200 to 300 watts for your motor to give you the speed you need.

There are lots of variations on lightweight electric vehicles. Just because you say bike doesn't necessary mean 2 wheel. Above about 12 Kph air drag becomes significant and your going to need to either over come that or your going to need an aerodynamic vehicle - Hence the links I directed you to.

With a modestly aerodynamic body and an all up weight with driver of about 200 Kg our electric car would do 30 Mph 50 Kph. With no body on it it would top speed at about 20 Kph and have a reduced range.

There aren't really any precise formulae. you can calculate drag etc but this isn't going to help you without some data to match it against.

Your best bet is scale models - in a wind tunnel - Not hard to make. to assess the effects of design on drag.

After that you may well have to take the plunge and just decide what your going to use and see how it goes.

Something running at between 250 to 500 watts at 24 volts will most probably do for your project -

Bear in mind what I said about how commercial suppliers rate their motors. you need to know the voltage and current.

Our motor was rated at 250 watts continious.

As ICENg says brushed PM motors can generally be over run by a lot.

Running the motor at 24 volts 20 amps gives us 480 watts.

The cct breaker was 70 amps and from a standing start many cars that were over geared would pop the breaker that's 24 volts at 70 amps = 1680 watts!

Later some cars fitted 100 amp breakers. I didn't see any of those pop but I did see a number of burned out motors.

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TushharN (author)rickharris2016-03-17

hey, I have some doubt.. can u clear? can I have ur mail id or any othe contact?

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rickharris (author)TushharN2016-03-18

NO. I am unable to communicate other then through the Instructables site.

You all ready have all the information you need in these 2 answers (and the other question)

Now it is down to you to build and experiment..

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The MadScientist (author)2013-06-14

I wouldn't bother with motor matching for the job in this case, it's just a pain. I would recommend looking at your states laws on what the maximum wattage is for a electric bike and using the max (for me it's 200W pfff). I would aim for 300-400W that ought to do the job however with assistance 200W can do.

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