Electrical Machines

How to Perform a Brake Test on a DC Motor

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.

Brake test on a dc motor
Figure 1.0 Illustration of a brake test

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

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,

     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

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Also Read: Step by Step Guide: How to Check and Test a Power Transformer

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John Mulindi

John Mulindi is an Industrial Instrumentation and Control Professional with a wide range of experience in electrical and electronics, process measurement, control systems and automation. In free time he spends time reading, taking adventure walks and watching football.

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