Testing of DC Machines


Testing of DC Machine

When construction of DC motor or DC generator is completed. To perform its electrical characteristics, several tests are conducted for testing of a dc machine (Generator or Motor).

Testing of machines is used for finding Losses, Temperature rise and Efficiency. The efficiency of a d.c. machine depends on its losses. When losses are decrease, The efficiency is increase and it is vice-versa.

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

efficiency of DC machine

ccc16Tests of DC Machines

The important Tests of DC machines are classified to

  1. Direct test method (break over test)
  2. Swinburne Test (Indirect test Method )
  3. Hopkinson’s-Test ( Regenerative Method )
  4. Retardation or Running down Test
  • Direct test method (break over test)

In this method of testing the machine is put on full load and whole developed power is wasted by applying the break. Direct test method is used, for small machines. Because of in case of large motors, it is difficult to dissipate the large amount of heat generated at the brake. It is also called break over test.

Though this method is simple, but complication in the measurement of mechanical power input in the case of generator and output in case of a motor. If brake is to be applied to a series motor, the brake must be tight before the motor is started, otherwise the armature may gets damaged and winding burned.

break over test

The main draw back 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.


  • Swinburne Test (Indirect test Method )

Swinburne’s test is the simplest indirect method of testing of dc machines. In this method, the dc machine is run as a motor at no-load and losses of the machine are determined. The iron and friction losses are determined by measuring the input to the machine on no-load, the machine is run as a motor at normal voltage and speed. The copper losses are calculated from measured values of the various resistances. Swinburne test also known as no load test. Indirect test method is used for large shunt machines.

swinburne test

Advantages :

  • The efficiency can be determined at any load because constant losses are known.
  • Power required for this test is small.
  • The power required to carry out the test is small because it is a no-load test
  • This method is quite economical and convenient.


  • It does not take into account the stray load losses that occur when the machine is loaded.
  • This test is not give accurate efficiency of the machine. because of iron losses under actual load are greater than the measured efficiency. It is mainly due to armature reaction distorting the field flux.
  • At full-load, due to armature reaction flux is distorted, In some cases which is increases the iron losses as much as 50%.


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