Unusual noise
A change in a motor’s sound is often the first sign that something has changed. What the noise sounds like, and when it happens, can help a technician narrow down the possible causes.
Possible causes
Mechanical
- Bearing damage or a lubrication problem
- A fan rubbing on its cover or the end shield
- Loose mounting bolts on the base
- Rotor imbalance
Electrical or magnetic
- An uneven air gap (the small space between the turning rotor and the fixed stator), for example from worn end shield or bearing seats
- Humming from an open circuit (a break in the circuit), especially when the motor will not start
From the installation
- Noise carried from the driven equipment, coupling or belts
Worth noting before you call
- Whether the noise is constant or changes with load
- Whether it stops as soon as the power is switched off, or continues while the motor coasts down
- Where it seems to come from: the drive end (the end connected to the load), the other end, or the driven machine
What a qualified technician may check
- Vibration and bearing temperature measurement
- Bearing and lubrication inspection
- Fan, cover and mounting checks
- Air gap and fit checks during a strip-down
Vibration
Some vibration is normal. A change, or vibration that is clearly higher than usual, needs attention because it wears out bearings, windings and the driven equipment faster.
Possible causes
Alignment and mounting
- Misalignment between motor and driven machine
- A weak or loose base
- Belts that are too tight
Balance
- An unbalanced coupling, pulley or driven equipment
- Rotor imbalance or shifted balancing weights
- A mismatch between how the rotor and the coupling were balanced (half key versus full key)
Motor condition
- Defective bearings
- Bearings out of line
- Too much end play (the shaft can slide too far lengthwise)
- A three-phase motor running with one phase missing
Worth noting before you call
- Whether vibration appeared suddenly or grew gradually
- Any recent work: new belts, a new coupling, moving the motor
What a qualified technician may check
- Vibration measurement and analysis
- Alignment check
- Base and mounting check
- Bearing and shaft inspection, and balancing
Overheating
Heat ages winding insulation and grease. If a motor runs hotter than usual, find out why, even if it is still running.
Possible causes
Load
- Overload: the driven equipment needs more power than the motor is rated for
Cooling
- Dirt in the frame or ventilation openings
- A damaged fan or fan cover
- Too little space for air around the motor or the fan cover inlet
Supply and windings
- One phase open (disconnected) or a poor connection
- Uneven voltage between the phases at the motor terminals
- A winding fault, such as a coil shorted to earth
Worth noting before you call
- When it started and whether the load or process changed
- Whether the fins and grille are clean (looking only, with the motor stopped)
- Any burnt smell or discolouration
What a qualified technician may check
- Measuring the current on each phase and comparing it with the nameplate
- Supply voltage and balance checks
- Insulation resistance and winding tests, with the motor disconnected
- Cooling system inspection
Fails to start
A motor that will not start, or that hums without turning, should not be left energised. Switch it off and have the cause found.
Possible causes
Supply and protection
- Blown fuses or a tripped overload
- A supply that does not match the rating plate
- Incorrect connections
- An open circuit in the supply, control circuit or winding (often heard as a hum)
Load and mechanics
- The motor or driven machine cannot turn freely, for example a bearing or lubrication problem
- Starting load too high
Motor internals
- A short-circuited stator winding
- A poor stator coil connection
- Broken rotor bars or a loose rotor
Worth noting before you call
- Whether it hums, does nothing, or trips immediately
- Whether any other equipment on the same supply is affected
What a qualified technician may check
- Supply, fuse and protection checks
- Control circuit checks
- Insulation resistance and winding tests, with the supply disconnected
- Mechanical checks of bearings and driven load
Protective device trips
A trip means a breaker, fuse or other protective device detected something it was set to act on. Resetting it again and again without finding the cause risks damaging the motor and the wiring, and hurting people nearby.
Possible causes
Overload
- The driven load is too high
- Mechanical binding in the motor or driven equipment
Supply
- An open phase
- Low voltage
- A loose connection
Insulation and windings
- A short-circuited or earthed winding
- Low insulation resistance, for example from dampness after a long standstill
Worth noting before you call
- Which device tripped (overload relay, breaker, fuse, earth-leakage device) and how quickly
- Whether the trip happens at start or after running for a while
- Any recent change, such as wet weather, a new load or a moved motor
What a qualified technician may check
- Finding out why the device tripped before the motor goes back into use
- Insulation resistance testing with the motor disconnected
- Current and supply measurements
- Checks of the protection settings against the motor rating
