Why is an electric motor noisy?

Bearings are one of the most common sources of noise.

As wear progresses, the following increase:

• radial clearance;
• axial play;
• uneven rotation;
• contact impacts;
• vibration.

The character of the noise depends on the type of defect:

• damage to the outer raceway often creates repeating impacts;
• a defect in the inner raceway produces a sound linked to the shaft speed;
• damage to the rolling elements causes an uneven crunching sound;
• a cage defect can create grinding or an unstable knocking sound.

At an early stage the bearing may be noisy without significant heating.

When lubricant is lacking, the thickness of the lubricant film decreases, friction increases and the following appear:

• whistling;
• rustling;
• high-frequency noise;
• heating;
• accelerated wear.

Simply adding lubricant without checking the condition of the bearing does not always solve the problem.

If the raceways or rolling elements are already damaged, fresh lubricant may only reduce the noise temporarily.

An excess of lubricant is also dangerous.

The rolling elements churn the lubricant intensively, which causes:

• increased resistance;
• heating;
• hum;
• leakage of lubricant;
• seal damage.

Excess lubricant is especially dangerous in high-speed motors.

Using a lubricant with unsuitable:

• viscosity;
• temperature resistance;
• consistency;
• compatibility;
• speed rating.

This can cause noise and bearing overheating.

Mixing incompatible lubricants sometimes leads to thinning, thickening or loss of lubricating properties.

Dust, metal shavings, water and wear debris cause abrasive damage to the raceways.

Signs:

• uneven rustling;
• crunching;
• increasing vibration;
• accelerated heating;
• a change in the colour of the lubricant.

Damage can occur as early as during assembly.

Typical mistakes:

• transmitting force through the rolling elements;
• hammer blows;
• misalignment;
• an incorrect fit;
• excessive interference;
• insufficient interference;
• a contaminated mounting surface;
• an incorrect axial clearance;
• incorrect installation of the retaining parts.

A new bearing can start being noisy right after a repair.

If the inner or outer ring has a loose fit, it can spin in place.

This causes:

• metallic noise;
• heating;
• wear of the shaft or end shield;
• vibration;
• an increased clearance.

In motors, especially when operating from a variable frequency drive, electric currents can pass through the bearings.

The raceways develop:

• craters;
• frosted patches;
• characteristic fluted raceways;
• "fluting" damage.

The bearing starts to produce a characteristic hum that gradually intensifies.

Imbalance occurs when the centre of mass of the rotor does not coincide with the axis of rotation.

Causes:

• loss of a balancing weight;
• uneven deposits;
• fan damage;
• winding deformation;
• a repair carried out without rebalancing;
• uneven impregnation;
• displacement of parts.

Imbalance usually creates:

• hum;
• rhythmic vibration;
• noise that increases with speed;
• an increased load on the bearings.

A bent shaft causes periodic displacement of the rotor.

Possible consequences:

• knocking;
• hum;
• increased vibration;
• an uneven air gap;
• accelerated bearing wear;
• the rotor rubbing against the stator.

Excessive axial play can cause:

• impacts;
• the fan or coupling making contact;
• a shift in the position of the bearings;
• seal friction;
• unstable noise when the load changes.

In vertical motors the thrust bearings must be checked with particular care.

This is one of the most dangerous causes of metallic noise.

Contact can occur because of:

• bearing failure;
• shaft deformation;
• displaced end shields;
• incorrect assembly;
• frame deformation;
• loose stator fastening;
• a foreign object getting inside.

Signs:

• metallic grinding;
• a sharp increase in vibration;
• overheating;
• increased current;
• traces of rubbing;
• metal dust;
• damage to the stator or rotor core.

The fan can be noisy because of:

• a crack;
• a deformed blade;
• a lost part of a blade;
• contamination;
• loose fastening;
• incorrect installation;
• imbalance.

The sound often grows in proportion to the speed.

Causes:

• cowl deformation;
• loose fastening;
• axial shaft displacement;
• incorrect fan installation;
• a foreign object getting inside.

It usually produces:

• grinding;
• periodic catching;
• metallic knocking;
• traces of rubbing on the cowl.

The following can get into the ventilation ducts or under the cowl:

• a nut;
• a washer;
• a piece of insulation;
• part of a fan blade;
• a fastener;
• a wire;
• a stone;
• process debris.

The noise can be unstable and change when the motor is stopped and restarted.

Loose foot or flange bolts cause the frame to shift.

Signs:

• knocking;
• hum;
• resonance;
• a change in noise under load;
• traces of movement near the feet;
• foundation damage.

"Soft foot" occurs when one or more of the motor’s feet cannot sit flush on the foundation without deforming the frame.

When the bolts are tightened, the frame becomes twisted.

Consequences:

• a change in alignment;
• deformation of the end shields;
• an uneven air gap;
• vibration;
• noise;
• accelerated bearing wear.

The foundation may have:

• cracks;
• loose anchor bolts;
• insufficient rigidity;
• voids under the frame;
• corrosion;
• deformation.

In that case even a sound motor can run with increased noise.

Resonance occurs when the excitation frequency approaches the natural frequency of:

• the motor;
• the frame;
• the pipeline;
• the cowl;
• the foundation;
• the guard;
• an adjacent structure.

Signs:

• a sharp increase in noise within a limited speed range;
• a decrease in noise as the speed increases further;
• strong vibration of an individual part;
• a change in noise after pressing on the cowl or a panel.

Resonance is particularly common in variable-frequency drives.

Angular or parallel shaft misalignment creates periodic forces.

Consequences:

• hum;
• knocking;
• increased vibration;
• bearing heating;
• coupling wear;
• noise that increases under load.

After the unit heats up, the alignment can change, so thermal growth must be taken into account.

Possible:

• wear of the elastic elements;
• a crack;
• loose bolts;
• tooth damage;
• insufficient lubrication of a gear coupling;
• an incorrect clearance;
• displacement of a coupling half.

Characteristic sound:

• rhythmic knocking;
• metallic clicking;
• noise when the load changes;
• impacts on starting and stopping.

If a coupling half has play on the shaft or the key is damaged, the following occur:

• impacts;
• clicking;
• a change in noise on reversal;
• damage to the keyway;
• an uneven torque.

Excessive tension:

• overloads the bearings;
• deforms the shaft;
• causes hum;
• accelerates wear;
• changes the alignment.

Slipping produces:

• whistling;
• squealing;
• heating of the belts;
• a smell of rubber;
• unstable speed.

Misaligned pulleys cause:

• sideways movement of the belt;
• whistling;
• impacts;
• edge wear;
• vibration.

Noise that seems to be motor noise may actually come from the gearbox.

Causes:

• tooth wear;
• insufficient lubricant;
• incorrect meshing;
• bearing damage;
• loose shafts;
• a coupling defect;
• overload.

To separate the sources it is useful to test the motor without the gearbox, if this is technically permissible.

The alternating magnetic field creates periodic forces in the core and the air gap.

This is why an asynchronous motor has a characteristic steady hum linked to the supply frequency and its harmonics.

Normal hum should be:

• stable;
• even;
• free of impacts;
• free of sharp increases;
• approximately the same under identical operating conditions.

Even a small voltage asymmetry causes negative-sequence components to appear.

They create a magnetic field that rotates relative to the rotor in the opposite direction.

Consequences:

• increased hum;
• torque pulsations;
• vibration;
• uneven currents;
• overheating.

A motor that keeps running on two phases usually:

• hums loudly;
• loses torque;
• draws increased current in the two remaining phases;
• overheats;
• cannot restart normally.

This is an emergency regime.

At low voltage the motor may:

• take longer to accelerate;
• increase its slip;
• draw increased current under load;
• create a stronger hum;
• run unstably.

The noise is especially pronounced during a prolonged start.

Increased voltage raises the magnetic flux and can saturate the core.

Signs:

• strong electromagnetic hum;
• increased no-load current;
• stator heating;
• increased vibration.

If the motor is incorrectly connected:

• it can hum loudly;
• accelerate slowly;
• draw excessive current;
• fail to develop the required torque;
• overheat.

The nameplate data, the line voltage and the actual winding connection must be checked.

Possible:

• failure to switch over to "delta";
• incorrect switching time;
• a broken contact;
• the contactors closing at the same time;
• a phasing error.

The noise often changes at the moment of switching or right after it.

An interturn defect creates:

• asymmetry of the magnetic field;
• local overheating;
• increased current;
• hum;
• torque pulsations;
• vibration.

At an early stage the motor may still run, but the sound becomes uneven.

Such damage is usually accompanied by:

• a sharp crackling sound;
• an electric arc;
• protection tripping;
• a smell of burning;
• smoke;
• a loud hum before disconnection.

A broken bar or a cracked end ring causes:

• torque pulsations;
• an uneven hum;
• vibration;
• an increased acceleration time;
• loss of power;
• increased current;
• rotor heating.

The noise may have a characteristic periodic "beat" that changes with the load.

Rotor eccentricity creates uneven magnetic forces.

Causes:

• worn bearings;
• a bent shaft;
• displaced end shields;
• frame deformation;
• incorrect assembly;
• worn bearing seats.

Consequences:

• electromagnetic hum;
• vibration;
• local heating;
• a risk of the rotor rubbing against the stator.

The laminations of the active steel must be reliably clamped together.

When they loosen, they can vibrate under the alternating magnetic field.

Signs:

• a characteristic buzzing;
• localised noise in the frame;
• a change in sound under load;
• heating;
• metal dust or traces of movement.

Slot wedges hold the winding in the slots.

When they loosen:

• the wedges can vibrate;
• the coils shift;
• hum or knocking appears;
• the insulation is damaged;
• the risk of an interturn short circuit increases.

Electrodynamic forces are especially high:

• at starting;
• under overload;
• during a short circuit;
• when operating with current pulsations.

If the bracing or spacers are loose, the end windings can create:

• hum;
• buzzing;
• knocking;
• crackling.

An error made during a repair changes the magnetic flux.

Possible consequences:

• core saturation;
• increased no-load current;
• a strong hum;
• heating;
• reduced torque.

An incorrect pitch, phasing, or connection of the coils or parallel branches can cause:

• field asymmetry;
• hum;
• torque pulsations;
• increased current;
• unstable operation.

In high-speed machines the fan creates a significant part of the total noise.

The noise increases with the rotation speed and can dominate over the electromagnetic and bearing noise.

An uneven airflow through contaminated ducts creates:

• whistling;
• turbulent noise;
• local eddies;
• a rise in temperature.

After a repair, a fan of a different geometry is sometimes installed.

This can lead to:

• increased noise;
• insufficient cooling;
• overload;
• a change in the direction of the flow;
• cowl resonance.

In some ventilation systems, an incorrect direction of rotation of the motor significantly reduces cooling efficiency and changes the character of the noise.

This is especially important for:

• external fans;
• pumps;
• centrifugal fans;
• machines with a directed airflow.

A thin metal cowl can vibrate at a certain frequency.

Signs:

• a ringing hum;
• localisation near the cowl;
• a change in sound when lightly pressed;
• a strong dependence on speed.

Pulse-width modulation (PWM) creates high-frequency components of voltage and current.

They cause mechanical vibration of the core and windings, which can be perceived as:

• a squeal;
• a whistle;
• buzzing;
• a changing tone.

Changing the switching frequency can change the sound, but it also affects:

• motor heating;
• drive losses;
• electromagnetic interference;
• the load on the insulation.

Therefore the parameter should not be changed only to reduce the noise, without a technical analysis.

Non-sinusoidal current creates additional pulsating electromagnetic forces.

Consequences:

• hum;
• increased vibration;
• additional heating;
• bearing and structural noise.

A variable frequency drive allows the motor to pass through a wide range of speeds.

At a certain point the rotation frequency or one of its harmonics can coincide with the natural frequency of the unit.

Signs:

• strong noise only within a narrow frequency range;
• normal operation above and below that zone;
• a sharp increase in vibration.

The drive can sometimes be set to skip the resonant frequency, but the mechanical condition of the unit must be checked first.

Incorrectly entered:

• rated current;
• voltage;
• frequency;
• speed;
• power factor;
• model parameters,

can cause unstable control, torque pulsations and noise.

With vector control, an incorrect motor model can cause:

• hum;
• jerking;
• speed pulsations;
• unstable torque;
• increased current.

With incorrect gain, slip-compensation or voltage-boost settings, the motor can:

• hum;
• run jerkily;
• create torque pulsations;
• overheat.

Brushes can create:

• rustling;
• whistling;
• crackling;
• rhythmic impacts.

Causes:

• commutator runout;
• incorrect pressure;
• jamming;
• excessive clearance;
• an incorrect grade;
• a rough surface.

Heavy sparking is accompanied by:

• crackling;
• hissing;
• a smell of ozone;
• scorching of the surface;
• unstable noise.

Protruding bars, out-of-roundness, runout or poor soldering of a section can create a sound that repeats with every revolution.

Cavitation creates a sound similar to:

• crackling;
• crunching;
• stones moving around;
• impacts.

Causes:

• insufficient suction pressure;
• a clogged filter;
• an incorrect operating regime;
• high liquid temperature;
• a closed valve;
• air ingress.

Noise can be transmitted to the motor through the pump housing and the foundation.

Causes:

• contaminated blades;
• imbalance;
• impeller damage;
• flow separation;
• operation away from the optimum point;
• the impeller rubbing against the housing.

Possible:

• valve impacts;
• liquid slugging;
• mechanism wear;
• pressure pulsations;
• coupling defects.

Noise can be created by:

• rollers;
• the gearbox;
• the chain;
• the belt;
• a foreign object;
• jamming.

It is necessary to establish:

• when the noise appeared;
• whether a repair was carried out;
• whether the bearings were replaced;
• whether the load has changed;
• whether alignment was performed;
• whether the drive parameters were changed;
• whether there were emergency stops;
• whether the noise occurs on a cold or a hot motor.

Record:

• hum;
• whistling;
• knocking;
• grinding;
• squealing;
• crackling;
• rustling;
• periodicity.

It is advisable to make an audio recording from the same point for later comparison, but a recording does not replace instrumental diagnostics.

Check:

• the drive-end shield;
• the non-drive-end shield;
• the stator frame;
• the ventilation cowl;
• the terminal box;
• the coupling;
• the gearbox;
• the driven mechanism.

A technical stethoscope or a vibroacoustic instrument can be used for localisation.

Measure:

• phase voltages;
• phase currents;
• unbalance;
• active power;
• power factor;
• frequency;
• speed;
• current pulsations.

Monitor:

• the bearings;
• the frame;
• the stator;
• the terminals;
• the coupling;
• the gearbox.

Vibration helps distinguish:

• imbalance;
• misalignment;
• bearing defects;
• mechanical play;
• resonance;
• electromagnetic forces.

The vibration and noise spectrum can reveal components linked to:

• the rotation frequency;
• twice the rotation frequency;
• the mains frequency;
• bearing frequencies;
• gear meshing;
• the fan blade-passing frequency;
• the switching frequency of the drive.

Spectral analysis must be interpreted together with other data.

Monitor:

• the bolts;
• the anchors;
• the feet;
• the frame;
• cracks;
• a soft foot;
• the rigidity of the base.

Alignment is best carried out with an instrument that can account for:

• angular offset;
• parallel offset;
• thermal growth;
• movement of the unit as the bolts are tightened.

Assess:

• the temperature;
• the lubricant;
• the clearances;
• the axial play;
• the vibration spectrum;
• the condition of the fits.

The following must be ruled out:

• eccentricity;
• deformation;
• contact;
• broken bars;
• imbalance;
• fan damage.

Check:

• the winding;
• the wedges;
• the steel core;
• the fastening;
• traces of rubbing;
• local overheating.

This is only carried out where technically possible and in compliance with safety requirements.

Monitor how the following change:

• noise;
• current;
• speed;
• vibration;
• temperature.