Testing is usually performed on DC machines to establish efficiency and power losses. A brake test is a direct technique that involves applying a brake to a water-cooled pulley mounted on the motor shaft as demonstrated in Figure 1.0. This brake test method is typically used on small motors.
The brake band is fixed with the help of wooden blocks gripping the pulley. One end of the band is fixed to the earth via a spring balance S and the other is connected to a suspended weight W1. The motor is running and the load on the motor is adjusted till it carries its full load current.
To calculate the efficiency of the motor:
Let W1 = suspended weight in kg
W2 = reading on the spring balance in newtons
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The net pull on the band due to friction at the pulley is
9.81 (W1 – W2) newton
If R = radius of the pulley in metre and,
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N = motor or pulley speed in revolutions per second (rps)
Therefore, shaft torque Ts developed by the motor
= 9.81 (W1 – W2) R Nm
The motor output power
Let V = supply voltage, I = full load current taken by the motor, therefore, input power = VI watt
Then
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