Troubleshoot AC Motors

Troubleshoot AC Motors

The following charts will give you multiple pathways to assist when you are attempting to troubleshoot AC Motors. The 4 problems covered are:

  • Problem A – Motor won’t start or motor accelerates too slowly
  • Problem B – Motor runs noisy
  • Problem C – Motor overheats
  • Problem D – Motor bearings run hot or noisy

#1 Troubleshoot AC Motors – Motor won’t start or motor accelerates too slowly

A1: Check input power to starter. Is there power on all lines? (Three-phase motors won’t start on one-phase.)Restore power on all lines
A2: Check starter. Is overload protection   device opened?Replace or reset device. Does it open again when starting?
A3: Is there power on all lines to motor?Repair starter
A4: Is voltage to motor more than 10% below nameplate voltage?Restore proper voltage.
A5: Check motor terminal connections. Are any loose or broken?Repair connections.
A6: May be wrong motor for application. Is starting load too high?Install Design C or Design D motor. Install larger motor.
A7: Is driven machine jammed or overloaded?Remove jam or overload.
A8: Are misalignments, bad bearings or damaged components causing excessive friction in driven machine or power transmission system?Repair or replace component.
A9: Are bad bearings, bent shaft, damaged end bells, rubbing fan or rotor or other problem causing excessive friction in the motor?Repair or replace motor.
A10: Check stator. Are any coils open, shored or grounded?Repair coil or replace motor.
A11: Check commutator. Are any bars or rings broken?Replace rotor.

#2 Troubleshoot AC Motors – Motor runs noisy

B1: 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.
B2: Is a hollow motor foundation acting as a sounding board?Redesign mounting. Coat foundation underside with sound dampening material.
B3: Check motor mounting. Is it loose?Tighten. Be sure shaft is aligned.
B4: Is motor mounting even and shaft properly aligned?Shim feet for even mounting and align shaft.
B5: 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.
B6: Is air gap nonuniform or rotor rubbing on stator?Recenter rotor rubbing on worn bearings or relocate pedestal bearings.
B7: Listen to bearings. Are they noisy?Lubricate bearings. If still noisy, replace.
B8: Is voltage between phases (three-phase motors) unbalanced?Balance voltages.
B9: Is three-phase motor operating on one-phase? (Won’t start on single-phase.)Restore power on three-phases.

#3 Troubleshoot AC Motors – Motor overheats

C1: Is ambient temperature too high?Reduce ambient, increase ventilation or install larger motor.
C2: Is motor too small for present operating conditions?Install larger motor.
C3: Is motor started too frequently?Reduce starting cycle or use larger motor.
C4: Check external frame. Is it covered with dirt which acts as insulation and prevents proper cooling?Wipe, scrape or vacuum accumulated dirt from frame.
C5: Feel output from air exhaust openings. Is flow light or inconsistent indicating poor ventilation?Remove obstructions or dirt preventing free circulation of air flow. If needed,                        clean internal air passages.
C6: Check input current while driving load. Is it excessive indicating an overload?Go to Step C11.
C7: Is the driven equipment overload?Reduce load or install larger motor.
C8: Are misalignments, bad bearings or damaged component causing excessive friction in driven machine or power transmission system?Repair or replace bad components.
C9: Are motor bearings dry?Lubricate. Does motor still draw excessive current?
C10: Are damaged end bells, rubbing fan, bent shaft or rubbing rotor causing excessive internal friction?Repair or replace motor.
C11: Are bad bearings causing excessive friction?Determine cause of bad bearings (See Problem D).
C12: Check phase voltage. Does it vary between phases?Restore equal voltage on all phases.
C13: Is voltage more than 10% above or 10% below nameplate?Restore proper voltage or install motor built for the voltage.
C14: Check stator. Are any coils grounded or shorted?Repair coils or replace motor.

#4 Troubleshoot AC Motors – Motor bearings run hot or noisy

D1: Check loading. Is excessive side pressure, end loading or vibration overloading bearings?Reduce overloading.* Install larger motor.
D2: Is sleeve bearing motor mounted on a slant causing end thrust?Mount horizontally* or install ball bearing motor.
D3: Is bent or misaligned shaft overloading bearings?Replace bent shaft or align shaft.*
D4: Is loose or damaged end bell overloading shaft?Tighten or replace end bell.*
D5: Are bearings dry?Lubricate.*
D6: Is bearing lubricant dirty, contaminated or of wrong grade?Clean bearings and lubricate with proper grade*
D7: Remove end bells. Are bearings misaligned, worn or damaged?Replace.

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

Need more assistance troubleshooting you electric motor? Here are two great resources:

Maintenance and Troubleshooting of Electric Motors

Electric Motor Problems




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