# Electric motors

1. Nov 12, 2013

hello

im really new to engineering. i know the rock bottom basics of these stuff but really rock bottom.

i have a couple of questions regarding electric motors:

1)torque- what makes a motor have more torque?
2)rpm- what does that effect? and if im not mistaken higher voltage= higher rpm and visa versa?
3)amp- what does it effect?
4)other factors about motors i should know.

an example motor you could look at is a goat-cart motor with a 200lb guy on it.

im in a really bad high school so im learning a lot of this stuff on my own. i learnt already about transformers, inductors, capacitators, basic motor, etc. but even those just the basic of what they do.

hope this is the right place to learn :) sorry if these questions bother anyone.

thanks

2. Nov 12, 2013

### Staff: Mentor

Welcome to the PF.

This link should get you started: http://en.wikipedia.org/wiki/Electric_motor

3. Nov 19, 2013

hey

berkeman thanks for the link. im still reading the page on motors but there is a million "sublinks" in the page of motors which i also have to read. while im reading i would like to continue working on my project so i really only need to know a little info.

i want a high torque motor that could pull about 200lb and yet be fast like 20mph. (didnt get up to it in the reading yet.)
i also want it to be able to control its speed. i learned a couple of ways like using forms of inverters (VFD) or a variable resistor. i saw on hobby websites that for their remote control cars they use a ESC?

i want these parts to last long and work good.
if anyone could give me a suggestion i would appreciate it. if you know any good motors i could buy, post a link. thanks

4. Nov 19, 2013

### pixelpuffin

well it quite depends on the means of pulling the 200 lb
the way its worded it sounds like pulling with a rope with a 200 lb load at the end
in that case you'd need enough torque to accelerate the load and enough rpms to pull 20 miles of rope per hour
the math behind that is fairly simple for rpms
rpms = rope pulled per minute / circumference of spool
at 20 mph rpms = 1760 / circumference of spool
for torque
torque = mass * acceleration * radius of spool / 32.174
at 200 lb torque = 200 * acceleration * radius of spool / 32.174
why the 32.174? because a lb of force will accelerate an object at 32.174 ft per second per second and thus we can keep constant units throughout the calculation

tell us more about the project

5. Nov 19, 2013

Hey sorry for the confusion

I'm working on a electric skateboard. I don't like to look up on youtube how to build it, I like to figure it out on my own.
The only things as of now that I need help on is the motor and speed control.

Everything Else I think I could do.
The brakes and how to attach the a gear to the motor I'm not sure but ill figure it out when I get to it.

6. Nov 19, 2013

### Staff: Mentor

7. Nov 20, 2013

### Staff: Mentor

8. Nov 20, 2013

### pixelpuffin

the math stays the same just replace radius of spool with radius of wheel and likewise circumference of spool with circumference of wheel
just be sure to use feet and not inches

9. Nov 22, 2013

But how do you find how much torque the motor has? The motors that I see on hobby websites don't say anything About torque all they speak about is RPM and KV?

10. Nov 22, 2013

### Travis_King

you could contact the manufacturer. Typically one can calculate torque in N-m with the formula
Torque (N-m) = kW * 9550 / rpm
or T = Hp * 5250 / rpm