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.
[Illustration: 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.
[Illustration: 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:
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.
[Illustration: Fig. 694.--Method of testing dynamo for short circuits.
In the figure, one pole of the battery B is placed in contact with the
frame of the machine at a point which has previously been well scraped and
cleaned; the other pole is connected to one of the galvanometer terminals
as shown. The other terminal of the galvanometer is connected to each of
the dynamo terminals T T under test in turn. If a deflection of the needle
be produced when the galvanometer terminal is in contact with either, the
terminals are in contact with the frame, and they should then be removed,
and the fault repaired by additional insulation or by reinsulating.]
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.
[Illustration: Fig. 695.--Watson armature discs. Each lamination is made
from low carbon electrical steel of high magnetic permeability. Each disc
is annealed and afterwards varnished.]
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.
[Illustration: Fig. 696.--Fort Wayne pedestal type commutator truing
device. When this device is used, the armature is revolved in its own
bearings by means of a handle clamped to the pulley. The tool has a
horizontal travel of 21 ins., (being 3 ins. wide inside the fastening
bolt in the base), and a vertical adjustment of 12 ins., adapting it to
machines with commutators up to 36 ins. in diameter.]
[Illustration: Fig. 697.--Fort Wayne yoke type commutator truing device
for machines having brush mechanism mounted on a yoke carried by the field
frame. It consists of a carriage for the tool holder having a screw feed
and a bracket for attaching to the brush yoke. The bracket replaces two
brush holder brackets on the brush yoke, and is made to fit the yoke of
the particular machine on which it is to be used.]
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.
[Illustration: 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.
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.
[Illustration: Fig. 699.--Watson armature complete. The armature coils
are form wound, heavily insulated and so mounted on the core as to insure
rapid dissipation of heat by ventilation. Each coil is protected by an
insulating sheath and tape covering before mounting. The armature is baked
after the coils are mounted to drive out all moisture, then, while hot,
is treated with insulating compound and again baked twelve hours.]
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.