Testing of Dc Machines

Testing of Dc Machines Points : Testing of Dc Machines, Direct Method, Indirect Method, Regenerative Method The most important performance tests to be conducted on dc machine are:
(i) The magnetization of open-circuit test.
(ii) The load characteristics.
(iii) The determination of efficiency curve.
(iv) The temperature rise test.

In addition, when a dc machine is first put on test at the manufacturer’s end, it must be given an insulation test must also be made to ensure that commutation is satisfactory. For determination of efficiency of a dc machine it is necessary to have a knowledge of the power input and power output. There are three different methods of determining the efficiency of a dc machine, namely, direct method, indirect method and regenerative method. 1. Direct Method By this method the efficiency and losses of only small machines can be determined. In this method full load is applied to the machine and output is directly measured. Though this method is very simple in looking but involves complication in the measurement of mechanical power input in the case of generator, and output in the case of a motor. In the case of a generator, the input mechanical power is measured by connecting some form of dynamometer to the prime mover and water resistance load is applied. In the case of small motor, some form of the brake is applied to water pulley for measuring the output. If brake is to be applied to a series motor, the brake must be tight before the motor is started, otherwise the armature may get damaged and fly to pieces. In case of large motors the motor is coupled to a generator and generator is loaded by means of a resistance. This method id wasteful, since all the input is to be wasted and can only be used incase of small machines due to the difficulty of having enough power and suitable brake arrangements in case of larger machines. 2. Indirect Method By this method, efficiency of shunt and compound dc machines can be determined. This method enables the determination of losses without actually loading the machine. The power is required to supply the losses only, so there is no difficulty in applying this method even to very large machines. Although the efficiency can be calculated with fair accuracy from the results obtained with this method, the disadvantage of this method is that the machine is run light during the test which gives no indication as to the temperature rise on load or to the commutating qualities of the machines. 3. Regenerative Method It requires two identical machines, one of them work as a motor and drives the other, which is mechanically coupled to it. The other machine works as generator and feed back power into the supply. Thus the total power drawn from the supply is only for supplying the internal losses of the two machines. Thus even very large machines may be tested as the power required is small. The machines can also be tested under full load conditions and for long duration to study their performance regarding commutation, temperature rise etc. conveniently.