Testing of DC Machines
Testing of machines is used for determining losses, efficiency and temperature rise. Direct method is used, for small machines. The indirect method is used for large shunt machines. In practice, Swinburne,s tests are mostly used.
1. Direct method of testing
Explanation: In this method of testing the machine is put on full load and the whole developed power is wasted by applying the break. The direct test method is used, for small machines. Because in the case of large motors, it is difficult to dissipate the large amount of heat generated at the brake. It is also called the 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. 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.
Disadvantage
1. Accuracy in determining the mechanical power output of the motor is limited.
2. It is difficult to provide full load for the large-capacity motor.
2. Indirect method of testing
This method consists of measuring the losses and then calculating the efficiency. The simplest of the indirect test is Swinburne’s test. Hopkinson test is a commonly used test under this method on shunt motors.
Swinburne’s test
Swinburne’s Test is an indirect method of testing of DC machines. In this method the losses are measured separately and the efficiency at any desired load is predetermined. Machines are tested for finding out losses, efficiency and temperature rise. For small machines direct loading test is performed. For large shunt machines, indirect methods are used like Swinburne’s or Hopkinson’s test.
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