Maintenance and Troubleshooting
of Electric Motors

CHAPTER 3 - Troubleshooting DC Motors

Problem E - Motor won't start

E1: Check main input power to controller. Is there power on the lines? Are contacts closed? Restore input power.
   
E2: Check controller. Is the overload protective device open? Reset or replace device. Does it open again when starting motor?
E3: Check controller. Is there voltage available at output terminals? Check controller for open starting resistor, broken leads and connections or other malfunctions. Repair.
   
E4: Set the controller for full speed. Is the voltage for field or armature circuits too low? Check voltage from power source. Correct if too low. Check controller for malfunction. Repair.
   
E5: Check for weak or non-existent field. Is motor field open? Has one field coil shorted? Repair broken leads or connections. Rewind or replace open or shorted coil.
   
E6: Check for open armature circuit. Is voltage at motor armature terminals zero when starting? Repair damaged armature circuit.
   
E7: Is driven machine jammed or overloaded? Remove jam or overload or install larger motor.
   
E8: Are misalignments, bad bearings or worn components causing excessive friction in driven machine or power transmission system? Correct misalignment or repair or replace worn component.
   
E9: Are bad bearings, bent shaft, rubbing fan or rotor, damaged end bells, or other mechanical problems causing excessive friction in motor? Repair or replace damaged motor components or install new motor.

Problem F - Motor starts but stops and reverses direction

F1: Check polarity of power source. Did it reverse? Determine why power supply reversed polarity and repair.
 
F2: Shunt and series field may be bucking each other. To check and correct:

Reconnect the shunt or series field to correct polarity. Connect armature for desired rotation direction. Try fields separately to determine rotation direction and connect so both give the same rotation.

    

 

Problem G - Motor runs but overload protective device trips too often.

G1: Is motor too small for load? Have loading conditions changed? Reduce load or install larger motor.
   
G2: Check controller. Is overload device set too low for application? Increase overload setting. NEVER exceed safe limits specified by codes or equipment maker.
   
G3: Is motor overheating? See Problem H.

 

Problem H - Motor overheats

H1: Is ambient temperature too high? Reduce ambient, increase ventilation or install larger motor.
 
H2: Check external frame. Is it covered with layer of dirt which acts as insulation and prevents proper cooling? Wipe, scrape or vacuum accumulated dirt from frame.
   
H3: Feel output from air exhaust openings. Is flow light or inconsistent indicating poor ventilation? Remove obstructions or dirt preventing free of air flow. If needed, clean internal air passages.
   
H4: High load speed consumes extra horsepower overloading motor. Is motor operating above normal speed? See Problem J.
   
H5: Check for overload. See Steps E7 thru E9.

 

Problem I - Motor runs too slowly.

I1: Is motor overloaded? See Steps E7 thru E9.
 
I2: Is the field resistance too low? Add proper resistance.
   
I3: Check for shorts in armature or between commutator bars. Are armature coils or wedges burned? Are any commutator bars burned? Replace or replace coils or bars.
   
I4: Check brush holders. Are brushes set ahead of neutral? Reset brushes to neutral.
   
I5: Voltage to armature too low. Set controller for full speed. Is voltage at output terminals below nameplate voltage? Check power source output voltage. Raise if too low. Check controller for malfunction. Repair.
   
I6: DC motors may run 20% slower on light loads when they don't heat up. Is motor operating cold? Increase load or reduce ventilation to increase heating. Install new motor.

Problem J - Motor runs too fast.

J1: Is driven load too light allowing motor to run fast? Increase load or install smaller motor.
   
J2: Check for a weak field per Steps J3
through J6.		    
   
J3: Are shunt or series coils reversed? Reconnect reversed coils for proper polarity.
   
J4: Is there excessive resistance in shunt field circuit? Remove excessive resistance.
   
J5: Is excessive heat causing higher resistance in shunt field circuit? Increase ventilation or correct other cause of overheating.
   
J6: No field causes unbalanced shunt motor to race. Is field circuit open? Repair broken lead or connection. Replace open coil.
   
J7: Set controller for full speed. Is voltage at output terminals of controller above nameplate voltage? Reduce output voltage. Check controller for malfunction. Repair.
   
J8: Check brush holders. Are brushes set behind neutral? Reset brushes to neutral.

 

Problem K - Motor runs noisy

K1: Are vibrations and noise from driven machine or power transmission system being transmitted to motor? Locate source of noise and reduce. Isolate motor with belt drive or elastomeric coupling.
   
K2: Is a hollow motor foundation acting as a sounding board? Redesign mounting. Coat foundation underside with sound dampening material.
   

K3: Check motor mounting. Is it loose?

Tighten. Be sure shaft is aligned.
   
K4: Is motor mounting even and shaft properly aligned? Shim feet for even mounting and align shaft.
   
K5: Is fan hitting or rubbing on stationary part or is object caught in fan housing? Repair damaged fan, end bell or part causing contact. Remove trash from fan housing.
   
K6: Is air gap nonuniform or armature rotor rubbing on pole pieces? Tighten loose pole piece. Recenter armature by replacing worn bearings or relocating pedestal bearings.
   
K7: Listen to bearings. Are they noisy? Lubricate bearings. If still noisy, replace.
   
K8: Are bearings noisy or running hot? See Problem L.
   
K9: Are the brushes developing high or low frequency chatter? See Problem M.

 

Problem L - Motor bearings run hot or noisy

L1: Check loading. Is excessive side pressure, end loading or vibration overloading bearings? Reduce overloading.* Install large motor.
   
L2: Is sleeve bearing motor mounted on a slant causing end thrust? Mount horizontally* or install ball bearing motor.
   

L3: Is bent or misaligned shaft overloading bearings?

Replace bent shaft or align shaft.*
   
L4: Is loose or damaged end bell overloading shaft? Tighten or replace end bell.*
   
L5: Are bearings dry? Lubricate.*
   
L6: Is bearing lubricant dirty, contaminated or of wrong grade? Clean bearings and lubricate with proper grade.*
   
L7: Remove end bells. Are bearings misaligned, worn or damaged? Replace.

*Bearings may have been damaged. If motor still runs noisy or hot, replace bearings.

 

Problem M - Brushes sparking excessively; may be accompanied by brush chatter
and/or excessive wear and chipping.

M1: Is motor overloaded? Reduce overload or install larger motor.
   
M2: Is vibration from driven machine or motor present? Locate source of vibration and reduce.
   

M3: Check brushes and brush holders. Are brushes worn too short?

Replace brushes.
   
M4: Does each brush fit commutator as indicated by polished surface over entire brush face. Refit brushes to commutator.
   
M5: Are brushes hanging up in holders? Clean brushes and holders. Remove rough surfaces that cause extra friction.
   
M6: Are brush springs broken or is spring pressure too light? Replace spring or increase pressure. Be sure pressure is equal on all brushes.
   
M7: Is spring pressure to high? (May also cause brush chipping) Reduce pressure or replace with lighter spring.
   
M8: Are brush holders set off neutral? (May also cause brush chipping) Reset holders at neutral.
   
M9: Are brushes set a wrong angle? (May also cause brush chipping) Reset holders for brush angle recommended by motor manufacturer.
   
M10: Is brush holder set for more than 1/8" clearance above commutator? (May also cause brush chipping) Reset holder for 1/8" clearance.
   
M11: Chipping brushes may also indicate wrong brush material. Are brushes too weak for duty? Consult motor manufacturer for recommendations.
   

M12: Check commutator. Is commutator surface under brushes polished brown color?

Normal condition. Go to
Step M18.
   
M13: Is commutator surface black (generally caused by sparking)? Check for overloads, low spring tension, poorly undercut mica, loose commutator bars, etc. Correct sparking. Dress commutator.
   
M14: Is there thick film on commutator - may appear black? Use more abrasive brushes.
   
M15: Is commutator surface bright and brassy looking?

If humidity is below 2 grams per cu. ft., increase humidity
OR
reduce spring pressure, use low friction brushes or use less abrasive brushes.
   
M16: Is commutator surface contaminated from paint spray, oil or chemical fumes? Is there excessive moisture in air? Clean commutator and brushes and protect motor from contamination. Install motor with proper enclosure to protect commutator.
   
M17: Is commutator streaked or grooved under one or more brushes? Be sure all brushes same grade. Replace if some are too abrasive. Check for faulty shunt connections causing unbalanced load; repair.
   
M18: Is commutator rough or eccentric? Grind commutator round
Undercut mica
   
M19: Is mica above bar surface? Undercut mica.
   
M20: Are some commutator bars too high, too low or loose? Replace commutator or tighten V-ring bolts to tension recommended by manufacturer and grind commutator.
   

M21: Are there flat or burned spots on commutator bars caused by unbalanced load in armature circuit?

Balanced load. Grind commutator.
   
M22: Is conductive film carbon dust or copper flaking causing shorts between armature bars? Undercut mica.
   
M23: Are there any shorts or opens in armature circuits? Locate and repair.
   
M24: Are there any grounds, shorts or opens in the field wiring circuits? Locate and repair.
   
M25: Are connections to brush holder poor or broken? Locate and repair.
   
M26: Is the interpole current weak or the air gap too great? Increase interpole current or reduce gap.

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Note: This material is not intended to provide operational instructions.
Appropriate Reliance Industrial Company instruction manuals
and precautions should be studied prior to installation, operation, or
maintenance of equipment.