Why does an electric motor get hot?

Overload is one of the most common causes of overheating.

When the driven machine demands more torque than the motor’s rated characteristics allow, the current rises. As a result the electrical losses in the windings and the motor temperature increase.

Overload can be caused by:

• jamming or a stiff-running machine;
• a clogged pump;
• excessive pressure;
• exceeding the rated throughput;
• an overfilled conveyor;
• excessive belt tension;
• a faulty gearbox;
• thickening of the process medium;
• a motor selected with insufficient rated power;
• an increased load after equipment upgrades.

A typical symptom is a motor that runs normally with no load but heats up quickly once the driven machine is connected.

Increased current can be either a consequence of overload or a sign of an electrical fault.

It is caused by:

• mechanical overload;
• reduced voltage;
• voltage unbalance between phases;
• a winding defect;
• rotor problems;
• a prolonged start;
• incorrect winding connection;
• unsuitable variable frequency drive settings;
• operation with increased slip.

The current must be measured not in one phase only, but in all phases and in different operating regimes.

At reduced voltage the motor may try to maintain the required torque by drawing more current.

This is especially dangerous under load, because:

• the winding current rises;
• the motor accelerates more slowly;
• the slip increases;
• the thermal load on the stator and rotor grows;
• the protection may trip.

The voltage drop can be caused by:

• an overloaded supply network;
• insufficient cable cross-section;
• a long supply line;
• poor contacts;
• faulty starting equipment;
• an undersized transformer;
• simultaneous starting of high-power equipment.

Increased voltage can also lead to overheating.

Excessive voltage raises the magnetic flux and increases the core losses, the no-load current and the heating of the magnetic core.

Possible symptoms:

• the motor gets hot even at low load;
• increased no-load current;
• louder electromagnetic noise;
• stator heating without any obvious mechanical overload.

Even a relatively small voltage unbalance can cause a significant current unbalance.

As a result:

• individual winding phases heat up more;
• additional losses appear;
• the torque drops;
• vibration increases;
• the thermal regime of the rotor worsens.

Measuring a single line-to-line voltage is therefore not enough. All three values must be checked and the phase currents compared.

If a phase is lost, a three-phase motor may fail to start or, if it was already running, keep rotating on two phases.

In this regime the currents in the remaining phases rise sharply and the temperature increases rapidly.

The causes may be:

• a blown fuse;
• a faulty contactor;
• a broken cable;
• a loose terminal;
• a burnt contact;
• a damaged connection inside the motor.

Running with a lost phase is an emergency regime and requires an immediate stop.

During every start the motor draws a significant current, and the stator and rotor take an increased thermal load.

If starts follow each other too often, the motor has no time to cool down.

Particularly dangerous are:

• repeated unsuccessful starts;
• frequent reversals;
• switching on again immediately after the protection has tripped;
• operation with frequent stops and starts;
• starting the machine under load;
• starting an already hot motor.

The permissible number of starts depends on the design of the motor and the thermal state of the machine.

If the motor takes a long time to reach speed, it runs at increased current for an extended period.

A prolonged start can be caused by:

• excessive load;
• reduced voltage;
• a jammed machine;
• a rotor defect;
• faulty bearings;
• incorrect settings of the starting equipment;
• an incorrectly selected motor;
• incorrect star-delta changeover;
• insufficient starting torque.

A prolonged start is especially dangerous for the rotor, because significant currents and thermal losses arise in it at that moment.

Dust, oil, fibres, process deposits and other contamination can block:

• the external cooling fins;
• the ventilation ducts;
• the fan cowl grille;
• the internal air passages;
• the filters;
• the heat exchangers.

Even a sound motor will overheat if the heat is not dissipated properly.

This problem is especially typical of:

• cement plants;
• metallurgy;
• the mining industry;
• woodworking;
• chemical production;
• the textile industry;
• rooms with an oil aerosol in the air.

The fan may:

• be broken;
• have damaged blades;
• rotate in the wrong direction;
• be contaminated;
• have worked loose on the shaft;
• have insufficient capacity;
• not work at reduced motor speed.

On motors with forced ventilation the separate blower, its power supply, filters and the direction of the airflow are checked as well.

Even a sound cooling system has limited capability.

If the motor runs:

• next to a furnace;
• in a boiler room;
• inside a closed enclosure;
• in direct sunlight;
• in a room without ventilation;
• at a high process air temperature,

the thermal margin decreases.

It is especially important to check whether the motor is rated for the actual installation conditions.

Overheating can be caused by:

• an interturn short circuit;
• a phase-to-phase short circuit;
• a short circuit to the frame;
• loose connections;
• a soldering defect;
• local insulation damage;
• moisture ingress;
• contamination;
• ageing of the varnish;
• mechanical movement of the coils.

An interturn short circuit often starts locally. The overall insulation resistance may still be acceptable at that point, so a single megohmmeter measurement is not enough.

Typical symptoms:

• local heating;
• current unbalance;
• the smell of insulation;
• repeated protection tripping;
• increased noise;
• worse performance under load.

In squirrel-cage asynchronous motors the following are possible:

• cracked bars;
• broken bars;
• end-ring defects;
• loss of contact between a bar and the ring;
• local overheating;
• deformation;
• the rotor rubbing against the stator.

With a damaged rotor the motor may:

• accelerate slowly;
• lose torque;
• run with pulsations;
• overheat under load;
• produce a characteristic noise;
• have increased vibration.

A faulty bearing increases the mechanical resistance to rotation and the load on the motor.

The causes may be:

• insufficient lubricant;
• excess lubricant;
• the wrong lubricant;
• contamination;
• an incorrect internal clearance;
• an excessively tight fit;
• misalignment;
• defects of the raceways and rolling elements;
• bearing currents;
• excessive axial or radial load.

Bearing overheating is often accompanied by noise, vibration and local heating of the end shield.

Incorrect alignment of the motor and the driven machine puts additional loads on:

• the bearings;
• the shaft;
• the coupling;
• the frame;
• the foundation.

As a result friction, vibration and temperature all increase.

After replacing the motor, repairing the foundation, removing the coupling or changing the bearings, the alignment must be checked again.

Imbalance produces cyclic centrifugal forces that load the bearings and the structure of the motor.

The causes may be:

• a lost balancing weight;
• a damaged fan;
• a contaminated rotor;
• deformation;
• poor-quality repair;
• uneven application of a coating;
• damage to the rotor winding;
• assembly errors.

Replacing the bearings without eliminating the imbalance does not solve the problem.

The rotor must be correctly positioned relative to the stator.

An uneven air gap can result from:

• bearing wear;
• misaligned end shields;
• a bent shaft;
• frame deformation;
• incorrect assembly;
• wear of the bearing seats;
• loose fastenings.

This leads to an asymmetric magnetic field, additional forces, vibration and local heating.

A loose terminal or a burnt contact creates an increased contact resistance.

Local heating appears in the contact area, which can:

• damage the cable;
• melt the terminal;
• cause voltage unbalance;
• lead to the loss of a phase;
• damage the winding.

During inspection, check:

• the tightness of the terminals;
• the condition of the links;
• traces of darkening;
• melting;
• that the cable cross-section is adequate;
• the condition of the cable lugs;
• the quality of the crimping.

Mistakes in the star or delta connection can lead to:

• insufficient torque;
• increased current;
• a prolonged start;
• overheating;
• protection tripping.

The connection must match the motor’s nameplate data and the supply voltage.

A variable frequency drive makes speed control possible, but incorrect settings can cause overheating.

Possible causes:

• operation at low speed without forced ventilation;
• excessive torque at low speed;
• an incorrect control characteristic;
• an unsuitable switching frequency;
• harmonic components;
• excessive cable length;
• bearing currents;
• incorrect acceleration and braking settings;
• a motor unsuitable for inverter operation.

At low speed the shaft-mounted fan turns more slowly and may not provide the required cooling.

The motor nameplate may specify a duty type — for example continuous, short-time or intermittent periodic duty.

If a motor intended for one duty is used in another, its thermal load may exceed the permissible value.

Examples:

• a short-time duty motor runs continuously;
• the number of starts exceeds the permissible value;
• the cycle duration has been increased;
• the pauses between switch-ons have been shortened;
• the motor runs at low speed for long periods;
• the motor is used in braking mode or with frequent reversals.

Sometimes the motor is sound but incorrectly matched to the driven machine.

The mistakes may concern:

• the rated power;
• the starting torque;
• the speed;
• the duty type;
• the degree of protection;
• the cooling method;
• the ambient temperature;
• the installation altitude;
• explosion protection;
• operation from a variable frequency drive.

In that case repairing the motor again will not solve the problem — the drive parameters have to be reviewed.

It is necessary to establish:

• when the problem appeared;
• whether the motor gets hot with no load;
• whether the temperature changes with the load;
• whether a repair was carried out;
• whether the bearings were replaced;
• whether the lubricant was changed;
• whether a variable frequency drive was installed;
• whether the process regime has changed;
• whether there were emergency stops;
• whether the motor was restarted after the protection tripped.

The history of the fault often points to the cause faster than an individual measurement.

Check:

• the cleanliness of the frame;
• the condition of the fan;
• the ventilation openings;
• the filters;
• the terminal box;
• the cables;
• the fastenings;
• the foundation;
• the coupling;
• the belts;
• traces of lubricant;
• smell;
• darkening;
• traces of rubbing or overheating.

The temperature should preferably be measured:

• on the stator frame;
• near the drive-end bearing;
• near the non-drive-end bearing;
• in the terminal box;
• on the adjacent machine;
• at the inlet and outlet of the cooling system.

It is important to record not only the value, but also the time of the measurement and the load.

You need to check:

• the no-load current;
• the current under load;
• the current of each phase;
• the starting current;
• how the current behaves during acceleration.

Comparing the currents often makes it possible to tell mechanical overload from electrical unbalance.

All line-to-line voltages are measured:

• before starting;
• during the start;
• in the steady state;
• at maximum load.

It is especially important to monitor the voltage dip during the start.

Establish whether the following turn freely:

• the pump;
• the fan;
• the gearbox;
• the compressor;
• the conveyor;
• the drum;
• any other unit.

Where possible, the motor is tested uncoupled from the driven machine.

Assess:

• the temperature;
• the noise;
• the vibration;
• the lubricant;
• the play;
• the condition of the seals;
• the alignment;
• the axial and radial load.

Vibration can indicate:

• imbalance;
• misalignment;
• bearing defects;
• a loose foundation;
• a coupling defect;
• an uneven air gap;
• rotor defects;
• mechanical rubbing.

Depending on the type and rating of the machine, the following may be performed:

• insulation resistance measurement;
• determination of the absorption ratio;
• measurement of the phase resistance;
• comparison between phases;
• high-voltage withstand testing;
• interturn insulation testing;
• assessment of the winding condition;
• visual inspection;
• partial discharge monitoring for the relevant equipment.

The scope of testing is determined by the design of the machine and its technical condition.

The following may be used to assess the rotor:

• start-up analysis;
• slip measurement;
• current spectrum analysis;
• vibration analysis;
• inspection after dismantling;
• inspection of the bars and end rings;
• testing of the wound-rotor winding;
• inspection of the slip rings and brushes.

You need to check:

• the rated current;
• the thermal protection class;
• the starting time;
• the voltage-unbalance protection;
• the phase-loss protection;
• the temperature sensors;
• the variable frequency drive settings;
• the acceleration and braking times;
• the permissible number of starts.

The protection must not compensate for a fault. Raising the setting without establishing the cause can lead to serious damage to the motor.

Overheating can be caused by the mechanical load, the bearings, the ventilation, the supply or a rotor defect.

The frame temperature does not always accurately reflect the temperature of the winding, the rotor or a bearing.

A single current or voltage reading does not reveal unbalance.

The fault is often not in the motor but in the pump, gearbox, compressor or conveyor.

Without comprehensive diagnostics you can pay for an expensive repair that does not remove the real cause of the overheating.

Some defects show themselves best precisely during acceleration.

If the heating appeared right after a bearing replacement, reassembly or reconnection, that is an important diagnostic clue.

Excess lubricant also causes overheating.

The protection may be tripping because of a real fault. Raising the setting only hides the problem.

The trend in temperature, current and vibration is often more valuable than a single measurement.