Chuck Yung 
EASA Technical Support Specialist 

EASA’s Engineering and Technical Support Department has received many calls over the past few months concerning repairs on DC motors. Most callers have asked for tips on troubleshoot­ing to make sure repairs are handled correctly. 

Prior to assembly, all windings should be tested for shorts, grounds and correct polarity. 

After a DC motor is assembled and ready to test run, a few simple checks will greatly reduce the chance of motor problems.  The following procedures should be especially helpful to those shops that don’t have a dyna­mometer. 

Adjusting Brush Box Height 
Brush boxes should be adjusted so that all are the same height above the commuta­tor.  On most ma­chines, this height should be between 1/16 inch and 1/8 inch, although some larger machines may specify clearances as great as 1/4 inch. Consult the manufacturer specifications for the particular machine if in doubt. Use a spacer of the desired thickness to set the brush-box clear­ance. Teflon will not scratch the commutator and it conforms to the shape, although other material also will work. 

Seating Brushes 
First, fully seat the brushes using fine (100 grit), non-conductive sandpaper.  It may be easier to seat the brushes a few at a time, especially with large machines with a lot of brushes (16 or more). In that case, be sure to install each brush in the box it was seated in. 

Remember, the brushes should be sanded until contact with the commutator is 100 percent. 

The neutral position is in the center of the brush contact zone with the commutator.  As a partially seated brush wears in, the contact zone broadens and shifts the center of the contact area away from the neutral position. 

After the brushes are fully seated, use dry compressed air to blow any carbon dust out of the motor.  (If possible, cover the armature windings while seating the brushes.) 

Adjusting Neutral 
Connect a volt­meter across two adjacent brush-holder studs and apply 110 volts AC to the shunt fields. The better the neutral setting, the lower the indicated voltage will be. Shift the brushholder assembly back and forth to obtain the lowest possible reading.  It is some­times possible to obtain a zero voltage reading, and readings below 0.010 volt are normal. 

Once in a great while, a DC machine has a “false” or “soft” neutral.  A low reading may be obtained, but it is not the true neutral position.  At the false neutral position, the voltage reading is not as low as that of the true neutral.

Interpole Polarity 
Mark the neutral position, then shift the brush rigging assembly an inch or so. Apply low DC voltage to the A-leads (A I+/ A2-). Apply just enough voltage to tum the a1mature. The armature should rotate in the direction the brush rigging was shifted. If the armature rotates the opposite direction, the polarity of the interpoles relative to the armature is wrong. Switch the leads at the brushholders, and repeat this test.

• Be sure to move all leads one position. If the motor has only 2 brush studs, interchange the leads. In a 4-pole machine with multiple circuits, the leads at all 4 brush studs must be moved (likewise for 6-pole machines, etc.). 
• On some machines, the brushholder position is fixed. Apply AC voltage to the brushholder leads, and measure the voltage across the A-leads. If interpole polarity is correct, the voltage reading will be less than the input voltage. 

Reset neutral using the AC method. Next prepare to run the motor, using the correct lead polarities per the EASA Technical Manual. After assembly and before attempting to run the motor, be sure to meg all windings. 

Series Field Connection in Compound Machines 
If you're dealing with a compound machine, connect only the F-leads and A-leads and run the motor as a shunt motor. Note the direction of rotation, and verify that it corresponds with the diagram in the EASA Technical Manual.

Now connect the S-leads instead of the F-leads, and nm it as a series motor. A series motor will continue to accelerate without a load, so do not apply full voltage. The armature should rotate the same direction as it did when operated as a shunt motor. 

• If the armature rotates the opposite direction, it is differentially connected. Most applications require that the series and shunts be connected cumulatively. Change the lead markings of whichever set of fields does not correspond to the diagram for rotation. Be sure to make your customer aware that you have corrected the lead markings.

​Test-run the motor with it connected for the customer's application (shunt, series or compound). No arcing should be observed. If arcing is noted, these further tests should be done: 

• Measure brush spacing around the commutator. The easiest way to do this is to wrap a piece of adding machine tape tightly around the commutator. Mark the point at which the ends of the paper overlap. Mark the same side of each brush in a single brush path (CW side or CCW side.) Remove the paper, lay it on a flat surface and measure the distance between marks. They should be equal. They must be within 3/64" for most DC machines to perform properly. If not, the individual boxes must be adjusted. Counting the bars between boxes is not a good substitute. 
• Check commutator run-out with a profilometer or a dial indicator. Runout tolerances depend upon rpm, and should be within manufacturers specifications. (Very rarely, intentional commutator runout may be desired to reduce the possibility of brushes sticking in the brush-boxes.) 
• If arcing is noted at one brush stud, verify brush spacing and neutral setting.
• If arcing is more evident in one brush path, verify commutator runout in that path. A bent shaft or improperly machined commutator may be to blame. If the arcing seems greater on the brush paths furthest from the brush stud mounting, check for looseness at the brush studs and check for excess vibration. 
• For larger machines with several brushboxes mounted on each stud, measure the spacing at each brush path. A brushholder stud may be cocked by improper assembly, or the insulator may be damaged. Causes of damage include electrical arcing under an insulated block, physical damage from careless handling, etc. 
• Spring tension should be between 4-6 psi for normal DC machines. For traction motors, it should be between 6-8 psi. Use a spring-tension scale (or a fish scale) to measure the spring tension. Measure the cross-sectional area of a brush before calculating spring pressure. A ½" by 1" brush only has ½ square inch of cross-sectional area; a 1 ½" by 2" brush has 3 square inches.

If motor performance looks good and no arcing is visible, record the field and armature voltage, as well as the rpm. Connect and run the motor in the opposite rotation. At the same exact field and annature voltage, the rpm should also match.

Categories: DC motors, Testing, Motor testing

Tags: Motor testing

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