Power, speed, and torque... AGH

[Ken Leung]

Ah, there come a point in a student's search of knownledge about speed, torque, and power of motor when he finally ask the question, "what is this power thing?"... I know this because I asked the same question last year.

As you said it, power is work over time. Its a unit of how much work can be done over certain amount of time. But Power, is also equal to force times velocity, and torque times angular velocity. Play with the equations, and you will see how those two are the same.

So, power is a measurement of how force and speed you have. Its a combination of both things. It is a measurement of how much work you are doing at each condition of speed/torque. Just look at the speed torque curve. At the end point of the curve, where the stall torque is or free speed is, you can calculate that both ends have power of zero, simply by seeing that Stall torque * zero speed and free speed * zero torque = to zero power.
In the middle is where you find maximum amount of power you can get out of a motor working in its best condition.

So what's important about power you ask? Well, with the calculation you showed, you can clearly figure out how you want to gear the motor... (you should gear it to had a load of about 1/2 stall torque for max power). But in general, when you are faced with a problem, you should figure out how much power you need to accomplish a task, and see if the motor have enough power to do it. After all, if FIRST switch to different motors, how do you know which one to use?

In your example. You know the torque you need to do the task... But at the load the motor is facing, is it moving at a speed fast enough? You have to go through the calculation and speed torque curve just to see how fast its spinning. If you do this with power, you first figure out how much torque you need to do this, and how fast you want the thing to move... Multiply those numbers, and you get the amount of power you need. Then you look up which motor have enough power for that, and you can see which motors you can and cannot use.

The important about the power curve, is that you can see where you should gear your motor at. Near the peak of the power curve is what load you want your motor feeling. The further you are away from the peak, the more power you are wasting. The wasted power usually goes into Heat and heat up your motor.


I have a lecture note from the motor workshop I give for WRRF teams in the white paper. Go through the calculation at the end, and you will see my process of figuring out the motor and gearing.

Its on the fifth page @ http://www.chiefdelphi.com/forums/pa...C&pagenumber=5