Testing of D.C. Motors

Testing of D.C. Motors

The efficiency of a d.c. motor is given by,

Efficiency =  Power output / Power input = Power input -  Losses / Power input

• The various losses taking place in a d.c. motor and efficiency can be calculated by carrying out testing of d.c. motors. There are different methods of testing d.c. motors. These methods are broadly classified as :

i) Direct method of testing ii) Indirect method of testing.

• Now let us study direct method of testing d.c. motors.

1. Direct Method of Testing

• In this method the d.c. motor which is to be tested is actually loaded and input and output are measured. The efficiency is given by

Efficiency η = Output / Input

• Generally this method is employed to small motors. The motor is loaded by means of a brake applied to the water cooled pulleys.

• The main drawback of this method is that the accuracy in determining the mechanical power output of the motor is limited. Alternately it is difficult to provide full load for the large capacity motor.

2. Indirect Method of Testing

•  In these methods the motor is not loaded directly but the losses and efficiency at different loads can be estimated. Out of the different methods available for testing of d.c. motors, Swinburne's test and Hopkinson's test are commonly used in practice on shunt-motors. Since series motors cannot be started without load, the no load tests cannot be performed do on d.c. series motors.