Ques. What precaution should be taken with the brush holders?
Ans. Since, they are liable to be short circuited through the rocker by metallic dust lodging in the insulating washers, they should be kept clean.
Ques. How are the brush holders tested?
Ans. A galvanometer and battery are connected in series with one terminal of the galvanometer connected to one set of brushes; the unconnected terminal of the battery is then connected with the other set of brushes. A deflection of the needle will indicate a short circuit.
Fig. 698.--Field coil testing with telephone receiver. In the method here shown, a telephone receiver is connected in series with two symmetrically placed coils A and B. Very little sound will be heard when the flux through the two coils AB is the same; but if a short-circuited coil is being tested, the fluxes through the coils A, B will not be equal and a noise can be heard in the receiver.
Ques. What is the effect of a short circuit in the field coils or field circuit?
Ans. The machine generally refuses to excite.
Ques. How are the field coils tested for short circuit?
Ans. By measuring the resistance of each coil with an ohmmeter or Wheatstone bridge. The faulty coils will show a much less resistance than the perfect coils. The fault may also be discovered and located by passing a strong current from a battery or another dynamo through each of the coils in turn, and observing the relative magnetic effects produced by each upon a bar of iron held in their vicinity.