Section 5. Troubleshooting Model 8210/8250 Maintenance Manual
DC Electric Motors
5-12 Publication: 1146945/001, Revised: 25 Sep 2018
Open Circuit Motor Test
An open circuit is an electrical circuit within the
motor that is broken. This can be caused by:
• bad brushes or brush springs
• a broken wire in the field or armature
winding
• a loose or bad connection
To test a motor for an open circuit:
1. Isolate the motor from the truck circuit by
removing the power cables. Use two
wrenches to avoid twisting the terminal
stud.
2. With the motor at room temperature,
connect the leads of a digital ohmmeter
between the individual circuits in the
motor.
3. Observe the measurements given in
Table 5-2.
4. If the meter indicates high resistance in
the armature, check the condition of the
brushes before replacing the motor.
5. If an open circuit is found in a motor, the
motor must be disassembled by a motor
rebuilding facility to isolate the problem to
the field or armature circuit.
Table 5-2. Lift Motor Resistance Readings
Grounded Motor Test
In a grounded motor, an unintentional electrical
connection exists between the current-carrying
conductors and the motor housing. This can be
caused either by direct contact or through
conductive foreign material.
The ground may be caused by:
• insulation breakdown
• brush leads touching the motor housing
• build-up of carbon dust or other materials
To test a motor for grounds:
1. Isolate the motor from the truck circuit by
removing the power cables. Use two
wrenches to avoid twisting the terminal
studs.
2. Attach one lead of a megohm meter or a
digital ohmmeter to a motor terminal and
the other lead to an unpainted surface of
the motor housing. Set the ohmmeter to
the highest scale.
3. If the ohmmeter reads resistance of less
than 100,000 ohms, the motor is
grounded. Clean, repair, or replace the
motor as necessary.
Short-Circuited Armature
A short circuit in the armature causes heating,
and can result in burning of:
• Armature coil
•Brush wires
• Commutator segments
Visual inspection may reveal this condition.
Positive determination of a short-circuited
armature requires special equipment at a motor
rebuilding facility.
Terminals
Acceptable
Resistance
Readings
Unacceptable
Resistance
Readings
A to S 0.020 to 0.030
ohms
Greater than
0.1 ohms
NOTE: Measure all readings at room
temperature. See Figure 5-2 on page 5-8.
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