CHAPTER XXXI

DYNAMO FAILS TO EXCITE

This trouble is of frequent occurrence in both old and new machines. If a dynamo fail to excite, the operator should first see that the brushes are in the proper position and making good contact, and that the external circuit is open if the machine be shunt wound, and closed if series wound.

In starting a dynamo it should be remembered that shunt and compound machines require an appreciable time to build up, hence, it is best not to be too hasty in hunting for faults.

The principal causes which prevent a dynamo building up are:

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;

a. In external circuits;

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.

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.

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:

1. The armature circuit;

2. The field circuit;

3. 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?

Ans. In the hand regulator through a broken resistance coil or bad contact.

Very frequently the fault occurs in the connecting wires leading from the machine to the hand regulator fixed upon the switchboard, or in the short wires connecting the field coils to the terminals or brushes.

The insulation of a broken wire will sometimes hold the two ends together so as to defy any but the most careful inspection or examination; therefore, in order to avoid loss of time, it is advisable to disconnect the wires if possible, and test each separately for conductivity with a battery and galvanometer connected, as in fig. 694. If the fault be not located in the various connections, the magnet coils should be tested with the battery and galvanometer coupled up as in fig. 706, care being first taken to disconnect the ends of each of the coils. A faulty coil will not show any deflection of the galvanometer.

Ques. At what point of a shunt coil does a break usually occur?

Ans. At the point where the wire passes through the flanges of the spool or bobbin.

Ques. How should the coil be repaired?

Ans. In most cases a little of the wood or metal of which the flange is made can be gouged or chipped out, and a new connecting wire soldered on to the broken end of the coil without much difficulty.

If it be necessary to take the magnets apart at any time, care should be taken in putting them together again to wipe all faces perfectly clean, and screw up firmly into contact, and to see that the connections of the coils are made as they were before being taken apart.

If the faulty coil cannot be repaired quickly, and the machine is urgently required, the coil may be cut out of circuit entirely, or short circuited, and the remaining coils coupled up so as to produce the correct polarity in the pole pieces.

Ques. What trouble is liable to be encountered in operating after cutting out a coil?

Ans. The remaining coils are liable to heat up to a greater extent than formerly, owing to the increased current, hence it is advisable to proceed cautiously in starting the dynamo, since the temperature may exceed a safe limit. If this occur, a resistance may be put in circuit with the field coils, or the speed of the dynamo reduced.

Ques. What kind of dynamo is affected by breaks in the external circuit?

Ans. A series dynamo.

Ques. Name the kind of break that is difficult to locate.

Ans. A partial break.

Short Circuits.--In a series or compound dynamo a short circuit or heavy load will overload the machine and cause the fuses to blow. A shunt machine will not excite under these circumstances, for the reason that practically the whole of the current generated in the armature passes direct to the external circuit, and the difference of potential between the shunt terminals is practically nil.

Ques. What should be done if it be suspected that the failure to excite arises from this cause?

Ans. The main leads should be taken out of the dynamo terminals, then, if due to this cause, the machine will excite.

Ques. What parts of a dynamo are specially liable to be short circuited?

Ans. The terminals, brush holders, commutator, armature coils and field coils.

Ques. How are the terminals liable to be short circuited?

Ans. The terminals of the various circuits of the machine are liable to be short circuited, either through metallic dust bridging across the insulation, or through the terminals making direct contact with the frame of the machine.

The various terminals should be examined, and if the fault cannot be located by inspection, they should each be disconnected from their circuits and tested with a battery and galvanometer arranged as in fig. 694.

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.

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.

The short circuit may be in the terminals or connections, and these should first be examined and tested.

Some series dynamos are provided with a resistance, arranged in parallel or shunt with the field coils, to divert a portion of the current therefrom, and thus regulate the output.

When making a series dynamo excite, all resistances and controlling devices should be temporarily cut out of circuit by opening the shunt circuit. Series machines have frequently a switch which short circuits the field coils. Care should be taken that this is open, or otherwise the machine will not excite.

Wrong Connections.--When a machine is first erected, the failure to build up may be due to incorrect connections. The whole of these latter should therefore be traced or followed out, and compared with the diagrams of dynamo connections given in figs. 190 to 198.

Sometimes errors are made in connecting the field coils, causing them to act in opposition. This may occur when the dynamo is a new one or the coils have been removed for repairs. It may be caused either through the coils having been put on the field cores the wrong way, or through incorrect coupling up. Under these circumstances, the dynamo, if bipolar, will fail to excite; and if multipolar, poles will be produced in the yokes, etc. It may be remedied by removing one of the coils from the core and putting it on the reverse way, or by reversing its connections. The correctness of connections of all the coils should be verified.

In compound dynamos it sometimes happens that the machine will excite properly, but that the series coils tend to reverse the polarity of the dynamo, thus reducing the voltage as the load upon the machine increases. This may be detected when the machine is loaded by short circuiting the _series coils_, not the _terminals_. If the voltage rise in doing this, the series coils are acting in opposition to the shunt coils, and the connections of the _series coils_ must be reversed.

Reversed Field Magnetism.--This is sometimes caused by the nearness of other dynamos, but is generally due to reversed connections of the field coils. Under such conditions the field coils tend to produce a polarity opposed to the magnetization to which they owe their current, and therefore the machine will refuse to excite until the field connections are reversed, or a current is sent from another dynamo or a battery through the field coils in a direction to produce the correct polarity in the pole pieces.