Efficiency of a D.C. Electric Motor (Brake Test Experiment)

The motor is connected to the supply line and the current and the voltage supplied to it are measured with the help of an ammeter (A) and a volunteer (V). The two ends of a belt are connected to two spring balances. The belt passes over a pulley attached to the main shaft. When the motor is started the pulley rotates. One of the spring balances exerts more force than the other because of the friction of the belt and pulley. The friction is equal to the difference between the readings of the two spring balances.

Let n = motor or pulley speed in r.p.s.
l = circumference of the pulley
Then, the distance moved = n x l.
Let F be the difference in the readings of the two spring balances.
Work done in one second = W = F x n x l watts
. :    Output = F x n x l watts

If the voltmeter reading = E volts and the ammeter reading = I amperes,
Input        = E x I watts
Efficiency    η = output/input x 100 = F x n x l / EI x 100%

Thus, efficiency of an electric motor can be determined.


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