Regenerative or Hopkinson’s Test (Back-to-Back Test)
By this method, full-load test can be carried out on two shunt machines, preferably identical ones, without wasting their outputs. The two machines are mechanically coupled and are so adjusted electrically that one of them runs as a motor and the other as a generator. The mechanical output of the motor drives the generator and the electrical output of generator is used in supplying the greater part of input to the motor. If there were no losses in the machines, they would have run without any external power supply. But due to these losses, generator output is not sufficient to drive the motor and vice-versa. The losses are supplied either by an extra motor which is belt-connected to the motor-generator set or as suggested by Kapp, electrically from the supply mains.
Essential connections for the test are shown in Fig. 31.6. The two shunt machines are connected in parallel. They are, to begin with, started as un- loaded motors. Then, the field of one is weakened and that of the other is strengthened so that the former runs as a motor and the latter as a genera- tor. The usual method of procedure is as follows :
Machine M is started up from the supply mains with the help of a starter (not shown) whereas main switch S of the other machine is kept open. Its speed is adjusted to normal value by means of its shield regulator. Machine M drives machine G as a generator and its voltage is read on volt- meter V1. The voltage of G is adjusted by its field regulator until voltmeter V1 reads zero, thereby showing that its volt- age is the same, both in polarity and magnitude as that of the main supply. There- after, S is closed to parallel the machines. By adjusting the respective field regulators, any load can now be thrown on to the machines. Generator current I1 can Motor-cum generator set be adjusted to any desired value by in- creasing the excitation of G or by reducing the excitation of M and the corresponding values of different ammeters are read.
The electrical output of the generator plus the small power taken from the supply, is taken by the motor and is given out as a mechanical power after supplying the motor losses.
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