is applied, and the speed is again measured. When you
seating one or two brushes, examine them to see if the
shift the brushes and the speed of the motor is the same
seater is being applied long enough to give a full seat.
in both directions, the brushes will be in the neutral
After seating the brush, if white dust is plainly visible
plain Generators are run at the same field strength and
on the seat, you have applied insufficient pressure to the
the same speed in both directions, and the brushes are
brush, or applied the brush seater too heavily or too far
shifted until the full-load terminal voltage is the same
from the brush. Be careful not to remove the copper
for both directions of rotation. To ensure accuracy, you
oxide film from the commutator surface. If you remove
must use a reliable tachometer to measure the speed of
this film, you must restore it, as described later in this
the machines.
chapter.
Use a vacuum cleaner during the seating operation
Inductive Kick Method
to prevent dust from reaching the machine windings and
bearings. After seating all the brushes, blow out the
machine with a power blower or completely dry
The kick method is used only when other methods
compressed air, or clean thoroughly with a vacuum
are inadequate and the conditions are such as to warrant
cleaner.
the risks involved. You must now connect sufficient
resistance in series with the field coils to reduce the field
SETTING ON NEUTRAL
current to about 10 percent of normal value.
With a lead pencil or other means that will not
When a machine is running without a load and with
damage the surface, mark A on a commutator bar under
only the main pole field windings excited,the point on
one set of brushes. Mark B on another bar one pole pitch
the commutator at which minimum voltage is induced
away from the center of the bar marked A. A pole pitch
between adjacent commutator bars is the no-load neutral
is the angular distance from the center of one main pole
point. This is the best operating position of the brushes
to the center of the next main pole. Raise all brushes.
on most commutating-pole machines. Usually, the
Connect bars A and B to a low-range voltmeter having
brush studs are doweled in the proper position. The
two or three scales (for example, 0 0.5, 0 1.5, or 0
correct setting is indicated on a stationary part of the
15 volts). Use leads with pointed prongs to connect the
machine by a chisel mark or an arrow. In some cases
bars. Separately excite the shunt field winding from a
commutation may be improved by shifting the brushes
dc source connected to the winding in series with a high
slightly from the marked position.
resistance and a quick-break switch. Start with the
The three methods to determine the correct neutral
minimum obtainable value of field current and the
position are (1) mechanical, (2) reversed rotation, and
high-range scale of the voltmeter.
(3) the inductive kick.
Close the knife switch and wait for the momentary
Mechanical Method
deflection to disappear; open the knife switch and note
the momentary deflection or kick of the voltmeter. If
The mechanical method is an approximate method.
insufficient deflection is observed on the lowest range
Turn the armature until the two coil sides of the SAME
scale of the voltmeter, decrease the resistance connected
armature coil are equidistant from the center line of one
in series with the shunt field winding and repeat the
MAIN field pole. The commutator bars to which the
procedure until an adequate deflection is obtained on the
coil is connected give the position of the mechanical
voltmeter when the switch is opened. Retain this setting
neutral.
of the resistor for the remainder of the test. Turn the
armature slightly until the position is found at which the
Reversed Rotation Method
minimum kick is produced when the field current is
broken. Bars A and B will then be on neutral. If one pole
pitch from the center of bar A does not fall on a bar but
Use of the reversed rotation method is possible only
where it is practicable to run a machine in either
on the mica between two bars, mark the bars next to the
mica, C and D. Then measure the kick when bar A and
direction of rotation, with rated load applied. This
method differs for motors and generators. For motors,
bar C are connected to the voltmeter, and again when A
and D are connected to the voltmeter. Adjust the position
the speed of the motor is accurately measured when the
field current becomes constant under full load at line
of the armature until these two deflections are equal and
voltage with the motor running in the normal direction.
opposite. The center line of bar A and the mica between
Then, the rotation of the motor is reversed, the full load
bars C and D will then be on neutral.
7-12
