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How can I select a suitable bldc motor? Answered

We are designing an electric tricycle having rear wheel drive(26 inches wheel dia).It will weigh around 400kgs including drivers.

There are constraints for motor output power(max 400 watts) and voltage(max 48v).
What are the calculations to be done for the selection of motor.
The desired speed of the vehicle is 0 to 4m/s in 20 seconds.

How can I calculate the speed of the vehicle for hill climbing(max 7degree gradient)?


7 Replies

mpilchfamily (author)2015-07-03

Moving a vehicle that size that fast in that short of period of time may be outside the ability of 48V 400W motor. That's only giving the motor 8A. Given that constraint you only have one option in motor choices and you get what you can get. This seems to be about the best option.


Are you building this on an existing trike frame? Do you plan to use this with the existing axle and sprocket? If so that is a 12000 RPM motor. Under load that top speed will drop but you can kind of figure out what your top speed will be. As for how long it will take to get from zero to a given speed will depend on a lot of factors that you won't be able to calculate vary well. So you just have to test it and see what happens. There are online calc to help you will gear ratios so if you have 12000 RPMs at the shaft going to a 3 inch sprocket on the axle it will tell you how many RPMs to expect at the axle. Then find the circumference of the tires multiplied by the RPMs gives you the distance you can travel in a minute. Then you can break it down from there. But that would be under perfect conditions and zero load. Realistically you'll need to gear it down to offer enough torque to actually drive the vehicle.

Or go with a simpler solution. Get an in hub bicycle motor and stick it on the front wheel. It will need 48V and 1000W but will top out at around 30mph. But it won't get you from 0 to 8mph in 20 seconds.

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Wired_Mist (author)mpilchfamily2015-07-04

Think a Mid Drive Electric Bicycle motor might work better over a Hub?

Switching gears would eat up precious seconds, but you would think it might help give a higher top speed ?

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rickharris (author)mpilchfamily2015-07-03

+1 .

1. Your trike is far too heavy you should aim for less than 50 Kg

2. Study what others have done. Peddle assist is far more versatile

Google the ELF recumbent, ICE recumbents, Velomobiles

3. You can get to 30 MPH with 500 watts see my instructable


BUT that car would not start from stationary as it pulled too much current. We got 2 hours duration on the flat from 2 x 70Ahr car batteries.

Gears would have made starting from stationary possible. but we didn't need to do that.

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Wired_Mist (author)rickharris2015-07-03

Nice I'ble ! how was the Stability of the Unit ?

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rickharris (author)Wired_Mist2015-07-04

Very good - raced by kids so not many turn over unless they hit each other.

This OP has little chance with what they are quoting. Weight is critical, Even on 500 watts the performance is hardly startling. We were limited to SLA car battery but lighter weight lipos would have helped but 70Ahr is hard to get unless you spend a fortune.

There used to be a drag race for older kids where they used a car starter motor and aimed for a top speed over 100 mtrs but they stopped it because it cooks the battery and the top speeds got a bit scary.

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Wired_Mist (author)rickharris2015-07-04

I've been throwing around the Idea to build something Similar to cart my Lazzy Butt back and forth to work. (25min walk or 5min drive) Supose I could hide the motor behind panels like you did and avoid this Ignorant By-Law I have to work with :D

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iceng (author)2015-07-03

School work assignment is what I think.

Tell us how many HP is your 48V maximum output power ?

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