1. Brushes not properly adjusted;
  2. Defective contacts;
  3. Incorrect adjustment of regulators;
  4. Speed too low;
  5. Insufficient residual magnetism;
  6. Open circuits;
  7. Short circuits;
    1. a. In external circuits;
    2. b. In dynamo.
  8. Wrong connections;
  9. Reversed field magnetism.

Brushes not Properly Adjusted.--If the brushes be not in or near their correct positions, the whole of the voltage of the armature will not be utilized, and will probably be insufficient to excite the machine. If in doubt as to the correct positions, the brushes should be rotated by means of the rocker into various points on the commutator, sufficient time being given the machine to excite before moving them into a new position.

Defective Contacts.--If the different points of contact of the connections of the machine be not kept thoroughly clean and free from oil, etc., it is probable that enough resistance will be interposed in the path of the exciting current to prevent the machine building or exciting. Each of the contacts should therefore be examined, cleaned, and screwed up tight.

Ques. Which of the contacts should receive special attention?

Ans. The contact faces of the brushes and surface of the commutator. These are very frequently covered with a slimy coating of oil and dirt, which is quite sufficient to prevent the machine exciting.

Incorrect Adjustment of Regulators.--When shunt and compound machines are provided with field regulators, it is possible that the resistance in circuit may be too great to permit the necessary strength of exciting current passing through the field windings. Accordingly, the fault is corrected by cutting out more or less of the resistance. The field coils of series machines are sometimes provided with short circuiting switches or resistances arranged to shunt the current across the field coils. If too much of the current be shunted across, the switch should be opened, or if there be a regulator, it should be so adjusted that it will pass enough current through the field windings to excite the machine.

Speed too Low.--In shunt and compound dynamos there is a certain critical speed below which they will not excite. If the normal speed of the machine be known, it can be seen whether the failure to excite arises from this cause, by measuring the speed of the armature with a speed indicator. In all cases it is advisable, if the machine do not excite in the course of a few minutes, to slightly increase the speed. As soon as the voltage rises, the speed may be reduced to its regular rate.

Fig. 692.--Method of testing for break by short circuiting the terminals of the machine. If the external circuit test out apparently all right, and there be no defective contacts in any part of the machine, and all short circuiting switches, etc., be cut out of circuit, the machine still refusing to excite, short circuiting the terminals of the machine should be tried. This should be done very cautiously, especially in case of a high tension machine. It is advisable to have, if possible, only a portion of the load in circuit, and the short circuit should be effected as shown in the figure. The short circuit may be made by momentarily bridging across the two terminals of the machine with a single piece of wire. As this, however, is liable to burn the terminals, a better plan is to fix a short piece of scrap wire in one terminal, and then with another piece of insulated wire make momentary contacts with the other terminal and the short piece of wire. If the machine excite, it will be at once evident by the arc which occurs between the two pieces of wire. As the voltage of a series machine when induced to build in this manner generally rises very rapidly, great care should be taken that the contact is at first only momentary, merely a rubbing or scraping touch of the wires. The contact may be prolonged if the machine do not excite at the first contact. Compound wound machines can often be made to excite quickly by short circuiting their terminals in this manner.

Insufficient Residual Magnetism.--This fault is not of frequent occurrence; it takes place chiefly when the dynamo is new, and may be remedied by passing the current from a few storage cells, or from another dynamo, for some time in the proper direction through the field coils. If a heavy current, such as is obtainable from a storage battery, be not available, and the machine be shunt or compound wound, a few primary cells arranged as in [fig. 693] will generally suffice.