We have one more item to learn before we come to the end of this lesson. That item is
All machines lose some power by heat and friction. If they didn't, they would be 100
percent efficient and the output would be equal to the input. For instance, suppose a one-
horsepower motor only required 746 watts to operate at its rated horsepower. We know that
one horsepower is equal to 746 watts, so the output would equal the input and the motor
would be 100 percent efficient.
To find out just how efficient an electrical device is, we use this formula.
Efficiency = output (watts) x 100.
Let us suppose we have a 10-horsepower motor connected to a 400-volt source at 20
amps. The output then is 10 x 746 (watts in one hp) or 7460 watts. The input is P - E x I - 400
x 20 = 8,000 watts. Now, let's see how efficient this motor is.
Efficiency = output x 100 = 7460 = .9325 x 100 = 93.25 percent,
or rounded off 93.3 percent.
Now you try one.
What is the efficiency of a 1-horsepower motor that requires an input of 960 watts?
77.7 percent. frame 20A
128.7 percent. frame 15B