The circuit shows in figure 1 is used to duplicate the required flux and speed for any specific load. The rheostat in series with the armature is adjusted until the voltmeter reads the same as the induced voltage at the specific load (generated voltage if the dynamo is a generator, counter emf if the dynamo is a motor. Since the I a R a drop of the dynamo as connected is very small, it can be neglected and the voltage across the armature is assumed to be the counter emf. The field rheostat is now adjusted to obtain the same speed at the specific load condition. The voltage induced in a dynamo is given by the equation E=KΦS. Inasmuch as the induced voltage as well as the speed have now be duplicated, it is apparent from the formula that the flux must now be the same as the flux at the load condition. Having duplicated the flux and the speed, the stray power is now determined by subtracting the copper loss from the power input to the armature. ...
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