# Motor Selection

1. Nov 24, 2007

### z_24

Hi all, I need some help on how to select a DC motor for my project.
Basically, I want a 50kg mass to accelerate to ~1.5 m/s in ~2 seconds. So by F=ma, F = 38.1N.
Radius, R, of the wheel I am using is 0.0564m, thus torque required to move the is =FxR = 2.1Nm.
Using a formula I found online. Power = (nxM)/1350, where n=speed(RPM), M=torque(oz.in)
plug in the numbers with correct units. I got ~ 56 watts. So does that mean I need a motor of at least 56 watts?
I don't have any engineering background so not sure if I did these calculations correctly.
Thanks for the help

2. Nov 25, 2007

### z_24

Any one :(?

3. Nov 26, 2007

### FredGarvin

Power is one thing you need to look at. Another thing is you need to get a curve from the manufacturer that shows the motor's torque vs. speed. This will tell you if the motor will be able to move the item at the speeds you want it to. The overall power may be correct, but if you need a lot of torque at the low end, you need to make sure that your motor does that.

4. Nov 26, 2007

### stewartcs

Well in US units the Horse Power (HP) of a motor is equal to...

HP = (n*T)/5252

where,

n = rotor speed
T = torque

[5252 is a conversion constant and is equal to 33,000/(2*pi) exactly].

To find the power in watts multiply the HP by 746.

This will be how much power the motor uses while operating at rated conditions. However, like Fred pointed out, you'll need a motor curve to see how it will operate at other than rated conditions.

5. Dec 1, 2007

### 1Parsifal

DC motors

DC motors sre for precise torque/speed control or positioning systems...if you really need to select a DC motor, you need to find a catalogue for speed, W and A range you are working with...