Motor Troubleshooting

The troubleshooting guide outlines a comprehensive variety of motor problems. Generally the categories are arranged according to symptoms offering brief suggestions concerning what to look for when investigating motor failures (why motor fails?) and often providing advice on how to correct the problem once it has been identified.

Motor Shuts Off Before Full Speed

A. Motor also hums (single phase motor)

Run winding may have a loose connection - disassemble motor in order to make the appropriate repair

Run winding is burned out - motor must be replaced or rewound

B. Overloaded

Reduce the load.

C. Excessively low voltage.

A reasonable overload or voltage drop of 10-15% will reduce speed only 1 to2 percent.

A report of any greater drop would be questionable.

D. Inaccurate method of measuring RPM

Check meter using another device or method.

E. Wrong connections

F. Bad capacitor

Motor Shuts Off After Reaching Full Speed

A. Overload

B. Bearings

C. Defective overload protection

D. Fuses

Motor Starts Slowly

10 or more seconds on small motors - 15 or more seconds on large motors

A. Bad capacitor

B. Overloaded

C. Low voltage

D. Wrong connections

E. Across the line start.

Excessive voltage drop
(5-10% voltage drop causes 10-20% or more drop in starting torque).

• Check connections between power supply and motor.
  
• Check conductor size between power supply and motor.
  
• Consult power company/check system.

High Inertia Load.

• Reduce starting load.
  
• Increase motor size.

F. Reduced voltage starter

Loss of starting torque
a. Y-Delta - starting torque reduced to 33%.

• Reduce starting load
  
• Increase motor size.
  
• Choose type of starter with higher starting torque.
  
• Reduce time delay between 1st and 2nd step on starter

b. Part winding starter - starting torque reduced to 50%.

• Reduce starting load
  
• Increase motor size.
  
• Choose type of starter with higher starting torque.
  
• Reduce time delay between 1st and 2nd step on starter.

c. Auto transformer starter - starting torque reduced 25% to 64%.

• Reduce the starting load
  
• Increase the motor size.
  
• Choose a different model of starter with higher starting torque.
  
• Reduce the time delay between the 1st and 2nd starter steps.
  
• Get the motor across the line sooner.

Motor Runs Excessively Hot

A. Overloaded

Reduce load or load peaks

Reduce number of load starts/cycles

Increase motor size

B. Blocked Ventilation:

Fan cooled motors

• Clean external ventilation system
  
• Check fan operation

Open motors

• Blow out internal ventilation passages.
  
• Eliminate external interference to motor ventilation.

C. High ambient temperature over 40°C (104°F)

Reduce ambient temperature or provide outside source of cooler air.

D. Unbalanced input current.

Balance supply voltage.

Check motor leads for tightness.

E. Single phased

Eliminate the single phase condition.

F. Drive belt too tight
G. Bad capacitor

H. Worn bearings

I. Wrong connections

J. Dirt, dust, trash

Excessive Vibration

A. Check alignment of motor and driven load - check and realign motor with the driven machine.

B. Check for Unbalance in Motor and Driven Load

Check Motor Mounting

• Be sure motor mounting/shims are tight and solid.
  
• Check grouting for cracks

Check Driven Load

• Disconnect belt or coupling
  
• Restart motor
  
• If vibration stops, the unbalance is in the driven load.

Check Sheaves or Coupling

• Remove sheave or coupling
  
• Securely tape 1/2 key in shaft key way
  
• Restart motor
  
• If vibration stops, the unbalance is in the sheave or coupling.

Check the Motor

• If the vibration does not stop after checking a, b and c above, the unbalance is in the motor
  
• Replace the motor. Make sure the new mounting is proper.

C. Tightness of mounting
D. Bearings?

E. Load and motor

F. Grounds or shorts?

Motor Runs Noisily Under Load

A. Motor hums excessively

The motor may not be connected properly

Motor is single phased

Improper overload relays

There may be a winding fault or a short circuit in the motor

Air gap between rotor and field stators may be uneven

The rotor is unbalanced or the PTO shaft is bent

B. Noisy bearings (listen to bearings)

Smooth mid range hum - OK.

High whine - internal fit of bearing too tight.

Low rumble - internal fit of bearing too loose.

Rough clatter

• Make sure mounting is solid level.
  
• The bearing may have been destroyed.

C. Mechanical noise

Driven machine or motor noise

Motor noise amplified by resonant mounting

Driven machine noise transmitted to motor through drive

Misalignment on close coupled application - improve alignment.

D. Clicking sound is regularly heard

Check for foreign matter in the air gap

The cooling fan may be hitting the end bell or housing

E. Rapid knocking sound heard on all types of motors - motor is out alignment causing excessive end play.
F. Check for brush chatter.

Never Reaches Normal Operating Speed (RPM)

A. Motor also hums (single phase motor)

Run winding may have loose connection

Run winding may have burned out

B. Possible overload
C. Excessively low voltage

D. Inaccurate method of measuring RPM

E. Wrong connections

F. Bad capacitor

Smells Funny / Smokes

A. Smell something burning or overheating or see smoke

The motor is overheating.

The cooling air flow may be restricted.

• Clean motor by wiping off excess dirt
  
• Use compressed air to blow dust particles out of passageways

Bearing problem may be causing an overload

• Bearings are overheated
  
• Too much lubrication of the bearing
  
• Bearings are bad

Short in the windings

B. Smell of ozone - burning windings

High Input Current

A. Accuracy of ammeter readings

B. Motor running idle

C. Motor running loaded

Motor overloaded

Motor voltage rating does not match power system voltage.

Unbalanced Input Current

5% or more deviation from the average input current

A. Unbalanced line voltage due to:

Power supply

Unbalanced system loading

High resistance connection

Undersized supply lines

B. Defective motor

If there is doubt as to whether the trouble lies with the power supply or the motor, perform the following test:
Rotate all three input power lines to the motor by one position. (i.e. move line #1 to #2 motor lead, line #2 to #3 motor lead and line #3 to #1 motor lead).

If the unbalanced current pattern follows the input power lines, the problem is in the power supply.
If the unbalanced current pattern follows the motor leads, the problem is in the motor.

{NOTE: A small voltage unbalance will produce a large current unbalance.}

Excessive Voltage Drop

More than 2 or 3% of nominal supply voltage

A. Excessive starting or running load

• Reduce load.

B. Inadequate power supply

• Consult power company.

C. Undersized supply conductors

• Increase supply conductor line sizes.

D. High resistance connections

• Check motor leads and eliminate poor connections.

E. Each phase lead run in separate conduits

• All 3 phase leads shall be in a single conduit, per National Electrical Code. (This applies only to metal conduit with magnetic properties.)

Overload Relays Tripping

A. During motor starting

Slow starting (10 - 15 seconds or more) due to high inertia load.)

• Reduce starting load.
  
• Increase motor size if necessary.

Low voltage at motor terminals

• Improve power supply.
  
• Increase motor size if necessary.

B. When running loaded

Overloaded

Unbalanced input current

Single phasing

Excessive voltage drop

Starting or overloading too frequent.

Ambient starter temperatures too high.

Wrong size or type of relays

Sparks

A. Sparking and flashing coming from the brushes

Roughened commutator

B. The commutator is blackened

Commutator may have a low bar

Commutator may have a high bar

May have a high mica

Worn or short brushes

Not enough tension on the brushes

Brush springs may be weak

Brushes stick in holders, do not fully surface on commutator

Commutator has dirt or oil on it

Water dripping onto the commutator

Short in the armature or field windings

Wound Rotor Motor Problems

A. Sparking and flashing from brushes

B. Brushes are worn, stuck, not set

C. Commutator is dirty, rough, has high bars, or is out of round.

D. Voltage is either too high or too low.

E. Governor is either stuck or out of adjustment.