Model 6210 Maintenance Manual
Section 5. Troubleshooting DC Electric Motor
Open Circuit Motor Test
Grounded Motor Test
An open circuit is an electrical circuit within the motor that is broken. This can be caused by:
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.
•
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.
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:
2. With the motor at room temperature, connect the leads of a digital ohmmeter between the individual circuits in the motor.
1. Isolate the motor from the truck circuit by removing the power cables. Use two wrenches to avoid twisting the terminal studs.
3. Observe the measurements in Table 5-1.
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.
Table 5-1. DC Motor Resistance Readings Motor Lift Motor
Probe Points
Acceptable Resistance Readings
A to Frame MP1 to Frame
Greater than 100,000 ohms
MP1 to MP2
0 ohms
Measure all readings at room temperature.
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.
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 that could 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.
Publication: 1146947, Revised: 12 Jan 2017
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