MOTOR OPEN, SHORTED, OR GROUNDED CIRCUIT MEASUREMENT PROCEDURE

StepExpected Result/Action
6. Turn power off. Use the capocitor discharge tool (Figure SP-8-6) to discharge
the run capacitor and any other capacitors that may be used in the unit.
Locate the wiring connected to the motor windings. (Refer to the unit wiring
diagram.)
The wiring related to the motor is identified. All high-voltage capacitors used in
the equipment are discharged, including the run capacitor.
7. Isolate the motor from the remainder of the circuit by disconnecting the
motor leads from all the related components, including the run capacitor.
The motor leads are disconnected from other components to measure motor
resistance only.
8. Check the motor for shorted or open windings as follows:
Set up the VOM/DMM to measure resistance on the R x 1 ohm scale. If using
a VOM, make sure that it is zeroed. Connect one lead of the VOM/DMM to
the motor winding common lead as shown in Figure SP-10-8, View A. Touch
the other meter lead to the remaining motor leads, one lead at a time, and
observe the meter indication.
VOM/DMM indicates a measurable resistance for all measurements. When
measuring the motor run winding leads, the highest resistance is normally
measured between the common lead and the low (LO) speed run winding lead.
The lowest resistance is measured between the common lead and the high (HI)
speed run winding lead.
VOM/DMM indicates zero resistance for one or more measurements. This
indicates a completely shorted winding. Replace the motor.
VOM/DMM indicates infinity for one or more measurements. This indicates that
one or more motor windings are open. If checking a motor with an internal
motor protection device, make sure the motor has had adequate time to cool off
so that the protective device has reset. It may require an hour or more after the
motor has been turned off before the internal protection device resets. Replace
the motor if the motor is cool and the internal protection device has not reset.
9. Check the motor for grounded windings as follows:
Set up the VOM/DMM to measure resistance on the R x 10,000 ohm scale.
Connect one Eead of the VOM/DMM to a good ground connection, such as the
motor frame shown in Figure SP-10-8, View B. Poor electrical contact
because of a coat of paint, layer of dirt, or corrosion can cause an inaccurate
measurement and hide a grounded winding. Touch the other meter lead to
all of the motor leads, one lead at a time, and observe the meter indication.
VOM/DMM indicates infinity or high resistance. If a resistance reading is
indicated, it should not be less than 1,000 ohms per volt. For example, on a 230-
volt motor, the resistance should not be less than 230,000 ohms (230 volts x
1,000 ohms/volt = 230,000). This indicates that the motor windings are not
grounded.
VOM/DMM indicates low or zero resistance, or a measurable resistance that is
less than 1,000 ohms per volt. This indicates that the motor windings are
grounded. Replace the motor.

pic1 2 MOTOR OPEN, SHORTED, OR GROUNDED CIRCUIT MEASUREMENT PROCEDURE


Categories: Service Procedures | Leave a comment