# finding the right torque and motors

I am trying to find the right motors for the following case:

I want to have a total of two motors on opposite sides of a board. The board will have a maximum weight of 150kg (330 lbs) and needs to have a maximum speed of 20km/h (12mph). Time for the motors to reach this spead is trivial, but anywhere between 5-10 seconds looks about right.

Note that those numbers can be exceeded but I need to be able to go at least this fast at peak RPM.

I am trying to find the Torque the motors will need in order to move this weight and any other specification such as Voltage and Current, in order to find the right motors.

The diameter of the wheel should be anywhere between 10 - 20 cm (4 - 8 in)

I also don't care if the motor is brushed or brushless. I would prefer to have a brushed ones since I value cost more then lifespan.

What I tried:

Horse power = kg.m/s = 150kg/2 * 20km/h = 5400kg.m/s = 72HP (sounds unreal)

which then by the use of online calculator was converted to Torque to be equal to

T = 2.57 kg.m^2/s^2 (sounds ok? or is it too small?)

[first to imperial HP = 71, then to imperial T = 18.6 - only calculators I found were using imperial]

In this scenario RPM was chosen to be 20000 since I saw a motor that had that much and since raise in RPM means decrease in torque I figured the bigger the better in order to not get a crazy high torque which I expected provided that I have 72HP

Another approach:

Downwards force due to the weight = m.g = 150kg.g

its split between two wheels so F at each wheel is half of that assumming weight falls in the center

F1=F2 = 75kg.g where F1 and F2 are forces at each wheel.

Torque = F1.d where d is the radius of the wheel (in m)

T = 75kg.g.0.075m = 55,2 kg.m^2/s^2

Those two numbers are very differnet though.

What should I do?

• Torque is related to acceleration, so we need to know either the required acceleration, or time required to get to required speed. – Mattman944 Feb 12 at 2:02
• You should also include a diagram of your arrangement including any gearing or pulleys. You need to account for losses if any are significant. – K H Feb 12 at 2:37
• @KH weight of those are accounted for in the 150kg with a bit over so they wouldn't have any impact – Miroslav Blagoev Feb 12 at 8:32
• @Mattman944 it is given. I want to be able to reach 20km/h at peak rpm, any normal time to reach that is fine. Since its a DC motor, am i not reaching this spead almost instantly anyway? ( for a chosen rpm) – Miroslav Blagoev Feb 12 at 8:38
• What about losses? You need to fully describe your system for someone to really guess at a solution to help you. It takes more power and a more powerful motor to accelerate things more quickly. 1 second, 5 seconds or 1 minute are all "normal" acceleration times depending on the application. It's necessary to know how it will be arranged and the efficiency of any attached gearing systems. – K H Feb 12 at 8:51