Torque and horsepower are very important motor characteristics that determine the size of the motor for a particular application. The difference between the two can be explained using a hand grinding wheel.
Torque is merely a turning effort. Suppose we have a grinding wheel with a crank one foot long. It takes a force of one pound to turn the wheel at a steady rate. We say the torque is one pound times one foot or one pound-foot. Now let us turn the crank twice as fast. The torque remains the same. Regardless of how fast we turn the crank, as long as we turn it at a steady rate, the torque is unchanged.
Horsepower, on the other hand, takes into account "how fast" we turn the crank. Turning the crank rapidly takes more horsepower than turning it slowly.
Horsepower is a rate of doing work. By definition, one horsepower equals 33,000 foot-pounds/min. In other words, if we used a perfectly efficient crank to lift a 33,000 pound weight one foot in height in one minute, that would be equivalent to one horsepower of work.
Let's find out how fast we would have to turn the crank to produce one horsepower. In one revolution the one pound force moves a distance of 2 x (pie) x 1 foot or 2 (pie) feet. The work done in one revolution is 2 (pie) feet times 1 pound or 2 (pie) foot-pounds. Thus, to produce one horsepower we would have to turn the crank at a rate of:
From the example above we can derive a formula for determining the horsepower from the speed and torque:
By transposition:
This steady constant torque at rated load is called the Rated Load Torque.
