Fig. 693.--Method of overcoming insufficient residual magnetism. The flexible "lead" L of the dynamo D is disconnected from the positive terminal of the machine, and is connected to the negative or zinc pole of the battery B, the other or positive carbon pole being connected to the terminal, from which the lead was removed, and shunt circuit S. As thus arranged, it will be seen that the battery B is in series with the armature and shunt circuit, and therefore its voltage will be added to any small voltage generated in the armature. When the machine is started, the combined voltages will probably be able to send sufficient current through the shunt to excite the machine. As the voltage rises and the strength of the current in the shunt windings increases, the flexible lead may be again inserted into the terminal from which it was removed. The battery will thus be short circuited, and may be cut out of circuit without any danger of breaking the shunt circuit, and thus causing the machine to demagnetize.
Open Circuits.--Dynamos are affected by open circuits in different ways, depending upon the type. Series machines require closed circuit to build up, while an open circuit is necessary with the shunt machine. An open circuit may be due to: 1, broken wire or faulty connection in the machine; 2, brushes not in contact with commutator; 3, safety fuse blown or removed; 4, circuit breaker open; 5, switch open; 6, external circuit open. If the trouble be due merely to the switch or external circuit being open, the magnetism of a shunt machine may be at full strength, and the machine itself may be working perfectly, but if the trouble be in the machine, the field magnetism will probably be very weak. Open circuits are most likely to occur in:
- The armature circuit;
- The field circuit;
- The external circuit.
When the open circuit is due to the brushes not making good contact, simple examination generally reveals the fact.
Ques. What causes breaks in the field circuit?
Ans. Bad contacts at the terminals, broken connections, or fracture of the coil windings.
Ques. How is the field circuit tested for breaks?
Ans. The flexible leads attached to the brushes are removed from their connections with the field circuit, and the latter is then tested for conductivity with a galvanometer.
Ques. Where is a break likely to occur in a shunt machine?