|Use a low-reading ohmmeter, Kelvin bridge, or Wheatstone
bridge to test for opens and shorts. The figure at right shows standard compound motor
terminal markings and connections.
The shunt field leads are normally the
smallest diameter leads in the motor connection box. They can be quickly identified in
this manner if not marked.
Test the shunt field for opens and shorts by reading across the F1 and F2 or F1 and A2
leads in the connection box.
Compare internal resistance readings to those listed. The total shunt field reading
will normally be only a few ohms. If the resistance value of only one field winding is
listed, it will be necessary to multiply by the number of windings in series to determine
the approximate total internal field resistance.
A very high or infinity reading will indicate an open field winding, open connection
box leads, or open winding interconnection lead(s).
Take corrective action if readings are not satisfactory. This will include comparison
readings of individual windings and replacing or rewinding faulty winding(s).
Testing series field/interpoles
series field windings and interpole (IP) windings, if applicable, will sometimes be
internally connected in series between the A2 connection box lead and the A2 terminal on
the brush rigging.
Test the series field for opens and shorts by reading across S1 and S2 leads in
connection box, if applicable.
armature lead that runs from the brush rigging to the series field or interpole windings
is normally designated A2. The lead from the motor connection box to the brush rigging is
normally designated A1. The armature leads (A1 and A2) can be identified in this manner if
In cases where only the A2 lead is brought out to the connection box, test the series
field and interpoles, if applicable, by readings across the A2 connection box lead and the
A2 lead on the brush rigging.
If necessary, disconnect lead A2 from the brush rigging or lift brushes from the
commutator prior to reading series field/interpoles. Continuity of the A1 lead can now be
read between A1 in the connection box and the incoming brush rigging lead connection
Compare internal resistance readings to those listed. The total series field/interpole
reading will normally be less than 1 ohm. Determine total design internal resistance in
the same manner as per shunt field.
A very high or infinity reading will indicate an open series field or interpole
winding, open connection box leads, or open winding interconnection lead(s).
If necessary, take the same corrective action as for the shunt fields.
Testing the armature
The armature will be tested for opens, shorts, and grounds by the bar-to-bar method.
The endbell on the commutator end may require removal to allow for complete access to
the commutator. Refer to the manufacturer's technical manual and the lesson on inplace repair of dc
motors for disassembly procedures.
The alternate inplace bar-to-bar test procedure below can be accomplished if
motor accessibility permits.
Lift or remove all brushes except for one set of positive and one set of negative
brushes (adjacent sets). Apply battery potential to the brushes.
Use the same circuit configuration as in the shop method.
Test between the adjacent bars spanned by the brushes. Test between alternate bars, if
necessary. Rotate the armature to place untested bars between the brushes.
Continue this procedure until all bars have been tested. Analyze the results. Take
corrective action, as necessary, after completion of the bar-to-bar test.
If results of all tests are satisfactory, an operational test will be conducted.
Reassemble the motor as necessary. Reconnect and insulate motor leads. Reconnect motor
leads in the controller, if applicable. Inform personnel responsible for the driven unit
that the motor is to be operated. Have tended ship's personnel conduct tag-in and
reenergize the power supply.
voltage should be ±5 percent of rated voltage.
Test incoming voltage to the controller to determine if it is correct. Take action to
determine the cause of improper voltage, if it is present.
Motor test run
the motor if heavy arcing is present, motor speed seems too fast or slow, or if unusual
noise or vibration is present.
Start motor at no load, if possible. Observe operation. Pay particular attention to the
commutator and brushes.
If there is a malfunction, take action to determine the cause. Inspect the brushes for
contact and tension. Ensure that the brushholders are set 1/16 to 1/8 inch from the
commutator surface. Inspect the controller for proper operation.
Conduct a no-load current test, if applicable, with a dc clamp-on ammeter. Compare the
reading to information listed in the manufacturer's technical manual or on the motor
Test-run the motor for 15 minutes at no-load condition. Observe motor operation as
previously discussed. Secure the test immediately if improper operation is noted or
Have a full load applied to the motor. Conduct current reading with a dc clamp-on
ammeter. Compare the readings as previously discussed.
Clean the commutator with a canvas wiper, if necessary, and if the test run is
progressing satisfactorily. Run the motor at full load for 15 minutes and observe its
If tests prove satisfactory
Have the circuit secured and tagged out by tended unit personnel.
Replace the connection box cover.
Inform tended unit personnel that the motor can be placed back in service.
If tests prove unsatisfactory
Further testing/disassembly will be necessary if any tests
or motor operation prove unsatisfactory. Refer to the lesson on inplace repair of dc motors.