INTRODUCTION

ELECTRIC MOTOR MAINTENANCE,
GENERAL

This section describes brush installation, inspection and
checks for malfunction of DC motors. Inspect the com-
mutator and brushes every 350 hours of operation. The
commutator is the rotating electric connection between
the armature and the electric power supplied by the bat-
tery. Brushes made of carbon compounds slide on the
rotating commutator and are the path for electricity from
the battery to the commutator and the armature. The
maintenance of the commutator and the brushes is im-
portant to the good operation of a DC motor.

A 36 volt battery (24 volt for the E30FR-24) is the power
source for the drive motors, the hydraulic pump motor
and the power steering pump motor. The drive system is
regulated by the transistorized system controller. Each
lift truck has two drive motors that are each controlled
by its own traction amplifier. The controller senses
when the control handle is moved and sends a pulse
modulated signal to the traction amplifier. The traction
amplifiers then convert these signals which are sent to
the motors.

The hydraulic pump motor and the power steering pump
motors are located in the front frame area, above the tilt
cylinders. The hydraulic pump motor is connected di-
rectly to the hydraulic pump. The power steering pump
motor is connected to the steering pump. The power
steering pump motor is a permanent magnet type motor.

The hydraulic pump motor is activated through signals
received from the lift potentiometer. (or by a contactor
on the E30FR-24). When the lift or tilt control lever is
moved, a pulse width modulated signal is sent to the am-
plifier. The lift amplifier converts the data signal into a

36VDC power signal and activates the hydraulic pump
motor.

The power steering pump motor is controlled by the lift
amplifier (or by a contactor on the E30FR-24). The lift
amplifier acts as an electronic relay which will turn the
motor on when the brake pedal is depressed or the con-
trol handle is moved for forward or reverse.

The drive motors and hydraulic pump motors are similar
in design. The hydraulic pump motors are smaller than
the drive motors, but the disassembly and maintenance
of these motors are similar. The cooling fan in the drive
motors is fastened to the armature with three capscrews
and can be removed from the armature. The cooling fan
can be removed during disassembly of the drive motor.

The cooling fan in the hydraulic pump motor is nor-
mally a press fit on the armature shaft and is not easily
removed during disassembly of the motor. The armature
and cooling fan must be removed from the drive end of
the motor during disassembly.

1. ARMATURE SHAFT
2. COOLING FAN
3. BRUSH COVER

FIGURE 1 – DRIVE MOTOR

REPAIRS

DRIVE MOTOR AND HYDRAULIC PUMP
MOTOR

See the section, HYDRAULIC SYSTEM, 1900 SRM
894 for instructions to remove and install the hydraulic

pump and motor and the power steering pump and mo-
tor. The power steering pump motor is a permanent
magnet motor.

See the section, DRIVE UNIT, 630 SRM 890 for
instructions to remove and install the drive motor.

See the BRUSH AND COMMUTATOR INSPECTION
and BRUSH INSTALLATION, DRIVE AND HY-
DRAULIC MOTORS in this section for more informa-
tion on these components of the motor.

Disassembly
(See FIGURE 2 or FIGURE 3)

1. Clean the outside surfaces of the motor before disas-

sembly. Make index marks on the ends of the motor and
the motor field ring so that correct assembly is possible.

2. Remove the brush cover. Remove the brushes and
spring assemblies.

3. Remove the capscrews from the commutator end of
the motor. Carefully slide the end cover from the the mo-
tor and armature shaft. Do not damage the parts. A puller
is often necessary to separate the pinion end cover from
the motor field ring.

1. HALF MOON KEY
2. PINION END COVER–COMPLETE
3. OIL SEAL
4. ALLEN HEAD BOLT AND WASHER
5. 2 POLE BRACKET
6. B-BEARING
7. END COVER–COMPLETE
8. SQUARE NUT
9. CAPSCREW AND WASHER

10. SPACER

11. KEY

12. WASHER
13. HEX. NUT
14. SNAP RING
15. CLAMP
16. PIPE INSULATION
17. PLATE RATING WITH GROOVED NAIL
18. WASHER

19. CAPSCREW WITH WASHER
20. BRUSH
21. SNAP RING
22. BALL BEARING
23. SNAP RING
24. BRUSH CARRIER–COMPLETE
25. BRUSH COVER
26. ALLEN HEAD BOLT WITH WASHER
27. HOUSING
28. ROTOR
29. POLE SHOE
30. COUNTERSUNK SCREW
31. FIELD COIL SET
32. CAPSCREW WITH WASHER
33. O-RING
34. SHIM WASHER
35. BALL BEARING

FIGURE 2 – TYPICAL DRIVE MOTOR ASSEMBLY

CAUTION

The pinion end cover and the armature are heavy
components. Work carefully so that the field coils,
pole pieces, and the armature are not damaged dur-
ing disassembly and assembly.

4. Remove the screws that fasten the pinion end cover
to the motor field ring. Remove the end cover and arma-

ture. Use a plastic or rubber hammer as necessary to
loosen the end cover.

5. Remove the pinion end cover from the armature.

6. Remove the screws that fasten the brush holder as-
sembly to the commutator end cover.

7. Disassemble the components of the motor as neces-
sary to make repairs.

1. PINNION END COVER
2. BRUSH PLATE
3. SPRING WASHER
4. BEARING
5. FIELD COIL PACKAGE
6. BRUSH COVER
7. BRUSH SPRING KIT
8. BRUSH SET

9. SNAP RING

10. END HEAD ASSEMBLY

11. BEARING

12. FAN
13. ARMATURE
14. POLE SHOE SCREW PACKAGE
15. MOTOR HOUSING

FIGURE 3 – HYDRAULIC PUMP MOTOR ASSEMBLY

Assembly
(See FIGURE 2 or FIGURE 3)

1. Make sure all of the components are clean. If the
bearings are worn, install new bearings in the end covers.

2. If the cooling fan was removed, install it on the ar-
mature. Make sure the field coils and pole pieces are
installed correctly in the motor field ring.

3. Install the brush holder assembly in the commutator
end cover.

4. Install the pinion end cover on the armature.

5. Carefully install the armature and pinion end cover
in the motor field ring. Make sure the index marks are
aligned. Install and tighten the screws.

6. Carefully install the commutator end cover on the
armature and fasten it to the motor field ring. Install and
tighten the capscrews.

7. Install the brushes and spring assemblies. See
DRIVE AND HYDRAULIC MOTORS–BRUSH
INSTALLATION in this section.

POWER STEERING PUMP MOTOR
(See FIGURE 4)

CAUTION

The motor used on this assembly is the permanent
magnet type. It is recommended that all jewelry be
removed. Use care when disassembling. Keep fingers
from between shell and magnet assembly and the
rear end bell assembly.

Disassembly

1. Clean the outside surfaces of the motor before disas-
sembly. Make index marks on the end bell assemblies of
the motor and the shell and magnet assembly so that cor-
rect assembly is possible.

2. Remove the capscrews retaining the mounting
bracket to the shell and magnet assembly. Remove the
mounting bracket.

3. Remove the tie bolts and the terminal cover.

4. Remove the rear end bell assembly with the brush
assembly and springs.

5. Remove the front end bell cover assembly.

6. Carefully remove the armature assembly from the
shell and magnet assembly.

1. FRONT END BELL ASSEMBLY
2. BEARING
3. ARMATURE ASSEMBLY
4. WAVE WASHER
5. SHELL AND MAGNET ASSEMBLY
6. BRUSH SPRING

7. BRUSH ASSEMBLY
8. REAR END BELL ASSEMBLY
9. TERMINAL COVER

10. TIE BOLT

11. MOUNTING BRACKET

12. CAPSCREW

FIGURE 4 – POWER STEERING PUMP MOTOR

7. If necessary remove the bearings from either the
front and/or rear end bell assemblies. On the rear end
bell assembly, if the bearing is removed, make certain to
remove the wave washer.

8. Repair or replace the components as necessary.

Assembly

1. Make certain all the components are clean. If the
bearings were removed, install the wave washer and
bearing into the rear end bell assembly. Install the bear-
ing in the front end bell assembly.

2. Carefully install the armature assembly in the shell
and magnet assembly.

3. If the brushes and brush springs are being replaced,
install them in the rear end bell assembly. Align the rear
end bell and carefully install it on the armature assem-
bly. Align the front end bell assembly with the index
marks on the shell and magnet assembly and install the
front end bell assembly on the opposite end of the arma-
ture assembly.

4. Align the terminal cover and install the tie bolts to
retain the cover to the motor.

5. Install the mounting bracket on the shell and magnet
assembly using the capscrews.

6. Check the brush and spring assemblies. See
BRUSH AND COMMUTATOR INSPECTION in this
section.

BRUSH AND COMMUTATOR INSPECTION

WARNING

Do not work under a raised carriage. Lower the car-
riage before servicing the lift truck.

1. Lower the forks completely.

2. Move the mast to the full forward position.

3. Turn the key switch to the off “O” position.

WARNING

To prevent electrical shock and injury, disconnect
the battery, separate the connector and discharge
the capacitor bank before inspecting or repairing
any component. DO NOT short the capacitor bank
to the frame or ground.

4. Disconnect the battery and separate the battery con-
nector.

5. Turn the key switch to the “I” position. The operator
display will light and go off. After the display is com-
pletely dark, the capacitor bank is fully discharged.

6. Turn the key switch to the off “O” position.

7. Remove the battery. See the section, PERIODIC
MAINTENANCE, 8000 SRM 900.

8. Raise the drive wheels from the surface. See PUTT-
ING THE LIFT TRUCK ON BLOCKS in the section
PERIODIC MAINTENANCE, 8000 SRM 900.

FIGURE 5 – RAISE THE DRIVE WHEELS FROM

THE SURFACE

9. Remove the front cover. Refer to FRONT COVER–
REMOVAL. See the section, FRAME, 100 SRM 888.
Remove the brush covers to the motor. Wear eye protec-
tion. Use a vacuum cleaner or compressed air to remove
dirt and “brush dust” from the commutator area.

NOTE:

Vacuum cleaning, when possible, is the rec-

ommendation of manufacturers of electric motors. The
use of compressed air can send dirt particles into the
bearings and other areas of the motor that can cause pos-
sible damage.

10. Inspect the commutator surface. Carefully rotate
the armature. DO NOT damage the commutator if you
use a tool to rotate the armature.

11. Inspect the white or gray insulation (mica) between
the commutator bars. The mica must not touch the
brushes or the brushes will wear very rapidly.

The commutator wears slowly in normal service. The
mica must be cut below the surface of the commutator

bars after a long service period or after a commutator has
been turned in a lathe.

Normal Commutator Surface

A commutator that has been in service will have a
smooth and polished surface with a darker brown color
where it rotates under the brushes. A variation of color
on the commutator surface between light brown and
darker brown is normal. This surface condition is the lu-
brication between the commutator and the brushes. The
brushes will wear rapidly if this surface condition does
not develop during the first six to ten hours of operation
after a commutator with a new surface is installed.

A light brown surface of the commutator where
it rotates on the brushes is a normal condition.
The surface of the commutator must be smooth.

Variations between light brown and dark brown
colors are also normal. The surface of the com-
mutator must be smooth.

A condition called “slot bar marking” is also nor-
mal if the commutator surface is smooth. The
variable color occurs in a pattern according to the
number of conductors per slot.

A very dark surface is also a normal and an accept-
able condition if the commutator surface is
smooth.

FIGURE 6 – NORMAL COMMUTATOR SURFACES

Commutator Problems

Commutator and motor problems are shown in FIGURE 7 through FIGURE 14.

Heavy streaks and fine grooves indicate the beginning of
damage to the commutator.

Possible Causes:

•

Operation of the motor in dirty and abrasive conditions

•

Continuous operation of a motor with a light load

•

Brush pressure is too low

•

Worn brushes

FIGURE 7 – COMMUTATOR PROBLEMS

Grooves and lines that have followed the heavy streaks
and fine grooves shown in FIGURE 8. The armature must
be removed from the motor so that the commutator can be
repaired. A commutator with this condition will cause the
brushes to wear rapidly.

Possible Causes:

•

Operation of the motor in dirty and abrasive conditions

•

Continuous operation of a motor with a light load

•

Brush pressure is too low

•

Worn brushes

FIGURE 8 – COMMUTATOR PROBLEMS

Grooves that are the width of the brushes.

Possible Causes:

•

Operation of the motor in dirty and abrasive conditions

•

Wrong type of brushes for this motor and operation

•

Brush pressure is too high

FIGURE 9 – COMMUTATOR PROBLEMS

A condition called “copper drag” occurs when copper from
the commutator bar is pulled into the slot between the
commutator bars. This condition will cause a short-circuit
between the commutator bars if it is not corrected. The
brushes will wear rapidly.

Possible Causes:

•

Operation of the motor in dirty and abrasive
conditions

•

Brush holder is not adjusted electrically correct for the
motor

•

Wrong type of brushes for this motor and operation

•

Brush pressure is wrong (too high or too low)

FIGURE 10 – COMMUTATOR PROBLEMS

Electrical burns on commutator bars on opposite sides
of the commutator.

Possible Causes:

•

Open armature winding

•

Motor has been stalled

FIGURE 11 – COMMUTATOR PROBLEMS

Copper wears rapidly at the edge of the commutator
bars.

Possible Causes:

•

Operation of the motor in dirty and abrasive
conditions

•

Wrong type of brushes for this motor and operation

FIGURE 12 – COMMUTATOR PROBLEMS

Flashover causes burning of the ends of the

commutator bar.

Open circuit in winding causes deep burning of

adjacent commutator bars.

Overheating causes damage to varnish insulation.

Possible Causes:

•

Motor has been too hot–Wrong lift truck for the
application

•

Motor has been stalled

•

Open armature winding

•

Open field coil

FIGURE 13 – COMMUTATOR PROBLEMS

High commutator
bars at each brush
position can occur if
the motor is stalled.

Rapid brush wear from
high commutator bars

Check for loose or high commutator bars.

Possible Causes:

•

Motor has been stalled

•

Motor has been too hot

FIGURE 14 – COMMUTATOR PROBLEMS

DRIVE MOTOR BRUSHES

NOTE:

Specialized tools, fixtures, and training are

required to service the drive motor. Refer to DRIVE
UNIT REBUILD. See the section, DRIVE UNIT, 630
SRM 890.

Removal

1. Put the truck on blocks or stands so the drive wheels
are off the ground. See the section, PERIODIC MAIN-
TENANCE, 8000 SRM 900.

WARNING

Do not work under a raised carriage. Lower the car-
riage before servicing the lift truck.

2. Lower the forks completely.

3. Move the mast to the full forward position.

4. Turn the key switch to the off “O” position.

WARNING

5. Disconnect the battery and separate the battery
connector.

6. Turn the key switch to the “I” position. The operator
display will light and go off. After the display is com-
pletely dark, the capacitor bank is fully discharged.

7. Turn the key switch to the off “O” position.

8. Remove the front cover. Refer to FRONT COVER–
REMOVAL. See the section, PERIODIC MAINTE-
NANCE, 8000 SRM 900.

9. Remove the mast assembly from the truck. See the
section, MAST REPAIR, 4000 SRM 881.

10. Disconnect the brake line from the disc brake unit.

11. Disconnect the wire harness and four power cables
from the drive motor, noting their location for correct
replacement.

12. Remove the four capscrews securing the drive unit
to the truck frame.

13. Tilt the drive unit forward approximately 90

, until

the capscrews can be installed. Install two capscrews to
secure the drive unit in the tilted position.

14. Remove the screws securing the brush leads to the
brush holder.

15. Rotate the brush holder spring away from the brush.

16. Remove the brush.

1. TAPERED END
2. WEAR END

MINIMUM BRUSH LENGTH

ÎÎÎ

MOTOR

Brush Length

Max./Min.

Spring Tension

Max./Min.

5.5” diameter

drive

28/15 mm

(1.10/0.60 inch)

15/10 Newtons

(55/35 ozf)

6.5” diameter

drive

38/20 mm

(1.50/0.80 inch)

11/6 Newtons

(38/ 22 ozf)

7.15” diameter

drive

38/20 mm

(1.50/.80 inch)

10/4.5 Newtons

(36/16 ozf)

8.0” diameter

drive

33/16 mm

(1.30/0.62 inch)

18/11 Newtons

(65/40 ozf)

36 volt lift pump

38/20 mm

(1.50/0.80 inch)

10/4.45 Newtons

(36/16 ozf)

24 volt lift pump

19/22 mm

(0.75/0.38 inch)

11/5.5 Newtons

(40/20 ozf)

TABLE 1 – CHECKING THE BRUSHES

Installation

1. Insert the brush into the brush holder in the correct
orientation.

2. Reposition the end of the brush spring on the brush
and attach the brush lead to the brush terminal. Tighten
the screw to 1.7-2 N

m (15-18 lbf in) torque.

3. Remove the two capscrews securing the drive unit
in the tilted position and return the drive unit to the oper-
ating position.

NOTE:

The drive unit is mounted to the frame with

four capscrews. Three capscrews are 38 mm (1-1/2 in)
long and one is 32 mm (1-1/4 in) long. The 32 mm (1-1/4
in) capscrew goes in the lower front hole.

4. Install the four capscrews securing the drive unit to
the frame and tighten to 108 N

m (80 lbf ft) torque.

5. Connect the brake line to the disc brake unit and
bleed the brakes. See BLEEDING THE BRAKE SYS-
TEM in section, BRAKE SYSTEM, 1800 SRM 893.

6. Install the wire harness and power cables to the
drive motor in the correct locations.

7. Install the mast assembly. See the section, MAST
REPAIR, 4000 SRM 881.

8. Install the front cover. Refer to FRONT COVER–
INSTALLATION. See the section, PERIODIC
MAINTENANCE, 8000 SRM 900.

9. Connect the battery.

LIFT PUMP MOTOR BRUSHES

Removal

NOTE:

Two of the lift pump motor brushes may be

inspected without removing the lift pump motor from
the truck.

WARNING

Do not work under a raised carriage. Lower the car-
riage before servicing the lift truck.

1. Lower the forks completely.

2. Move the mast to the full forward position.

3. Turn the key switch to the off “O” position.

WARNING

4. Disconnect the battery and separate the battery
connector.

5. Turn the key switch to the “I” position. The operator
display will light and go off. After the display is com-
pletely dark, the capacitor bank is fully discharged.

6. Turn the key switch to the off “O” position.

7. Remove the battery.

8. Remove the front cover. Refer to FRONT COVER–
REMOVAL. See the section, FRAME, 100 SRM 888.

9. Remove the lift pump and motor assembly. Refer to
LIFT PUMP AND MOTOR ASSEMBLY–RE-

MOVAL. See the section, HYDRAULIC SYSTEM,
1900 SRM 894.

10. Release the spring tension on the brush by rotating
the brush holder spring away from the brush.

11. Remove the screw securing the brush lead to the
brush holder.

12. Remove the brush.

Installation

NOTE:

If any brush needs replacing, replace all the

brushes on the motor.

1. Insert the brush assembly into the brush holder.

2. Position the end of the brush spring on the brush.

3. Attach the brush lead to the brush terminal and
tighten the screw.

4. Install the lift pump and motor assembly. Refer to
LIFT PUMP AND MOTOR ASSEMBLY–INSTAL-
LATION. See the section, HYDRAULIC SYSTEM,
1900 SRM 894.

5. Replace the front cover. Refer to FRONT COVER–
INSTALLATION. See the section, FRAME, 100 SRM
888.

6. Install and connect the battery.

STONING THE COMMUTATOR

WARNING

Wear eye protection. Raise the drive wheels. Operate
the motor at low speed. Protect your fingers. You are
doing work close to moving parts of the motor. Do
not use a brush seater stone less than 60 mm (2.5 in)
in length.

New brushes made by the manufacturer are normally
made to fit the surface of the commutator when they are
installed. This contact surface must be checked when
new brushes are installed. If the contact surface is not
approximately 85% of the brush surface where it
touches the commutator, an abrasive process called
“Stoning the Commutator” is used to make the brushes
fit the commutator better.

If the commutator has grooves or other

damage, the armature must be removed so that
the commutator can be repaired. Motors are
normally repaired by service persons that have
the special equipment required. Connect the
battery so that the motor can be operated. Use a
jumper cable if necessary to connect the battery
to the lift truck. Raise the drive wheels. See
“How to Raise the Drive Wheels” in the
OPERATOR’S MANUAL.

Close the seat switch and key switch so that

the motor will operate.

Operate the motor so that the commutator

rotates slowly. Apply the brush seater stone to
the moving commutator with light pressure.
Move the brush seater stone backward and
forward across the surface of the commutator
until the marks on the commutator are removed.
Apply the brush seater stone to the commutator
before each set of brushes so that the brush
seater particles have an even distribution.

Do not permit the brush seater stone to stay
in contact with the commutator too long and
cause more wear than necessary.

Turn the key switch to the OFF position and

disconnect the battery. Check all brush contact
surfaces. The brushes fit correctly when 85% of
the brush contact surface touches the
commutator. This stoning procedure normally
requires approximately 15 to 45 minutes.

When the brushes have the correct contact

surface with the commutator, use a vacuum
cleaner to remove the abrasive dust from the
commutator area and the motor.

When the installation and checks are

complete, install the brush cover. Make sure that
the wires to the brushes do not touch any part of
the motor case and cause a short-circuit.

NOTE:

A brush seater stone can also be fastened

to a wood stick with glue as shown in the illustration.
This arrangement makes it easier to apply a brush
seater stone in small spaces.

11418

1. RAISE DRIVE WHEELS
2. JUMPER
3. BATTERY
4. BRUSH SEATER STONE

FIGURE 15 – STONING THE COMMUTATOR

If new brushes do not fit the surface of the commutator
when they are installed, the contact surface can be small
until the brushes wear to fit the commutator. This small
surface can cause burns and a rough surface on the com-
mutator. “Stoning the Commutator” is most easily done
when the motor rotates slowly. When the motor rotates
at higher speeds, the centrifugal force removes the abra-
sive particles from the commutator more quickly. See
FIGURE 15.

TESTS FOR A DAMAGED FIELD AND
ARMATURE

The tests described in the following paragraphs are to
help a service person check a motor for damage and de-
termine if it must be sent to a repair service for rebuilt
motors. The resistance checks will not normally indi-
cate a short-circuit in a motor winding. A resistance
greater than 1 to 2 ohms can indicate a damaged wind-
ing. The motor must be removed from the lift truck and
disassembled as shown in the illustrations before the
tests can be done.

Test for an Open-Circuit in the Armature

The armature windings in large electric motors nor-
mally have less than one ohm of resistance. The two
commutator bars for a winding are found 180

_ apart on

the commutator. If an ohmmeter (R

1 scale) is used to

check the resistance between the two commutator bars
of the winding, a resistance of more than one ohm indi-
cates a problem in that winding. A resistance of infinity
(

∞

) indicates an open (damaged) winding.

If the armature has an open circuit, there will normally
be two burned commutator bars on opposite sides of the
commutator. These burned areas will cause the brushes
to wear rapidly. When the motor operates, large electric
sparks and arcs occur as the damaged commutator bars
rotate under each brush. See FIGURE 11.

Test for a Short-Circuit in an Armature
Winding

A short-circuit in a motor winding is difficult to test be-
cause of the normal low resistance (less than one ohm)
of a good armature. Special equipment is necessary to
check for a short-circuit in a motor winding. A motor
with a short-circuit in an armature winding will have a
different sound when it begins to operate, but a service

person must have experience to hear and understand the
difference in sound. A winding with a short-circuit will
also run hotter than a good winding and can have indica-
tions of heat damage. A winding that shows heat dam-
age when the other windings are normal can have a
short-circuit.

Another indication of a short-circuit will be a higher
than normal current draw by the motor. A higher than
normal current draw can also indicate other problems or
needed adjustments and does not always indicate a
short-circuit in a motor winding.

1. ARMATURE SHAFT
2. COMMUTATOR
3. PROBES TO OHMMETER

FIGURE 16 – TEST FOR AN ARMATURE

SHORT-CIRCUIT

Test for a Short-Circuit in the Armature

An ohmmeter (R

10,000 scale) can be used to test for a

short-circuit between an armature winding and the ar-
mature shaft. Put one probe on the armature shaft and
the other probe on a commutator bar. See FIGURE 16.
A resistance of less than one megohm indicates a prob-
lem between a winding and the armature shaft.

NOTE:

Clean the dirt and “brush dust” from the com-

mutator area before making a test for a short-circuit.

Test for an Open-Circuit in a Field Coil

The field windings in large electric motors normally
have less than one ohm of resistance. If an ohmmeter
(R

1 scale) is used to check the resistance between the

two terminals of the winding, a resistance greater than
one ohm indicates a problem in that winding or cor-
rosion in the terminal connection. See FIGURE 17.

Test for a Short-Circuit in a Field Coil

A short-circuit in a motor field winding is difficult to test
because of the normal low resistance (less than one
ohm) of a good field. Special equipment is necessary to
check for a short-circuit in a motor winding. A motor
with a short-circuit in a field winding will have a differ-
ent sound when it begins to operate, but a service person
must have experience to hear and understand the differ-
ence in sound. A winding with a short-circuit will also
run hotter than a good winding and can have indications
of heat damage.

1. FIELD COIL TERMINAL
2. OHMMETER

FIGURE 17 – TEST FOR AN OPEN FIELD

CIRCUIT

Test for a Short-Circuit Between the Field and
the Motor Case

An ohmmeter (R

10,000 scale) can be used to test for a

short-circuit between the field and the motor case. Put
one probe on the motor case and the other probe on a
field terminal. Check the resistance between the field
terminal and the motor case. A resistance of less than
one megohm indicates a problem between the field ter-
minal and the motor case. Make sure that carbon dust
has been cleaned from the motor before making this test.

Brush Holder Test

Make sure the carbon dust has been removed from the
brush holders. Use an ohmmeter (R

10,000 scale) to

measure the resistance between the brush holder and the
motor case. The correct resistance is an indication of in-
finity (

∞

Document Outline

INTRODUCTION
- ELECTRIC MOTOR MAINTENANCE, GENERAL
REPAIR

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