ELECTRICAL
EQUIPMENT
(iv)
Field
Coils
Continuity Test
:
Fig.
18
Testing for Open Circuit in the field coils.
Connect a battery and suitable
bulb in series with two pointed
probes.
If the lamp fails to light in the
following test an open circuit in
the field coils is indicated and the
defective coils must be replaced.
Place the probes on the brush
tappings. The bdb should light.
Lighting of the lamps does not
necessarily indicate that the field
coils are in order.
It
is possible
that a field coil may be earthed to
a pole shoe or to the yoke.
Insulation Test
:
Connect an ohm meter or a
l
10-volt a.c. test lamp between
the terminal post and a clean part
of the yoke.
Lighting of the test lamp or a
low ohmic reading indicates that
the field coils are earthed to the
yoke and must be replaced.
Replacing the field coils
:
Unscrew the four pole-shoe re-
taining screws, using a wheel-
operated screwdriver.
Remove the insulation piece which
is fitted to
revent the inter-coil
F
connectors rom contacting with
the yoke. Mark the yoke and pole-
shoes
in
order that they may be
refitted in their original positions.
Draw the pole-shoes and coils out
of the yoke and lift off the coils.
Fit the new field coils over the
pole-shoes and place them in
position inside the yoke. Ensure
that the taping of the field coils is
not trapped between the pole-
shoes and the yoke.
Locate the pole-shoes and field
coils by lightly tightening the
fixing screws.
Replace the insulation piece'be-
tween the field coil connections
and the yoke.
Finally, tighten the screws by
means of the wheel-operated
screwdriver.
(v)
Bearings
Bearings which are worn to such
an extent that they will allow
excessive side play of the armature
shaft, must be replaced. To
replace the bearing bushes pro-
ceed as follows
:
Fig.
19
Method of
fitting
Bearing Bushes.
Press the bearing bush out of the
end bracket.
Press the new bearing bush into
the end bracket, using a should-
ered, highly polished mandrel of
the same diameter as the shaft
which is to fit in the bearing.
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