// ELEKTROPROMREMONT
Burnt windings

Why do motor windings burn out?

Winding damage is one of the most common causes of an electric motor failing in an emergency. In production practice people often say that a winding has "burnt out," but this phrase can hide fundamentally different faults: prolonged overload, running on two phases, an interturn short circuit, a breakdown to the frame, rotor lock-up, a cooling fault or a repair mistake.

Externally the consequences can look similar: darkened insulation, a smell of burning, melted conductors or a fully charred winding. However, the nature and location of the damage often make it possible to identify the actual root cause of the failure.

Correct diagnostics must answer not only the question "what was damaged?" but also "why did it happen?" If the motor is simply rewound without eliminating the underlying problem in the supply network, the protection system, the driven mechanism or the process regime, the new winding can burn out again.

The short answer

Motor windings burn out when the temperature of the conductors and insulation exceeds the permissible values, or when the insulation breaks down under electrical, mechanical or chemical stress.

The most common causes are:

prolonged motor overload;
jamming or lock-up of the rotor;
loss of one phase;
voltage unbalance between phases;
reduced voltage;
increased voltage;
an interturn short circuit;
a phase-to-phase short circuit;
a breakdown of the winding to the frame;
frequent or prolonged starts;
a faulty cooling system;
high ambient temperature;
incorrect protection settings;
an incorrect star–delta connection;
rotor damage;
overload of the driven mechanism;
overvoltages and impulses from a variable frequency drive;
moisture, oil and conductive dust;
vibration and loosened coil fastenings;
ageing of the insulation;
process errors made after rewinding.

In most cases the winding does not fail instantly. First overheating or a local insulation defect appears, then a short circuit develops, after which the damage quickly spreads to the neighbouring turns, coils and core.

What "the winding burnt out" actually means

This phrase can refer to very different degrees of damage.

Thermal ageing

The winding can still work, but the insulation:

has darkened;
has lost its elasticity;
has become brittle;
has cracks;
has partly delaminated from the wire;
has lost its original dielectric strength.

A local burn

One area is damaged:

a single turn;
a coil;
a connection;
the point where a coil exits a slot;
an end winding.

This kind of defect can arise from insulation damage, mechanical rubbing or local overheating of a contact.

Complete thermal damage to a phase

The insulation of one or several phases has darkened, charred or melted evenly.

A phase-to-phase short circuit

An electric arc forms between the windings of different phases. The damage is often pronounced and local, with heavy melting of the conductors.

Breakdown to the frame

The winding comes into contact with the stator core or another earthed metal part.

Complete burnout of the stator

A significant part of the winding is destroyed, the wedges and slot insulation are damaged, and the core steel itself may also be affected.

How the winding heats up

During normal operation, electrical losses arise in the winding. The largest share consists of conductor losses, which grow in proportion to the square of the current.

This means that even a relatively small increase in current can significantly increase the heating.

For example, if the current increases by 20%, the thermal losses in the winding will rise by roughly 44%.

This is why a motor can run with a slight overload without tripping immediately, while the temperature of its winding gradually reaches a dangerous level.

Besides the current-related losses, the temperature is also affected by:

losses in the magnetic core;
the temperature of the cooling air;
the fan speed;
contamination of the ducts;
the duty cycle;
the number of starts;
heat transfer from the winding to the core and frame;
the quality of the impregnation.

Why high temperature destroys the insulation

The winding wire is coated with a thin layer of electrical-insulation enamel. Besides this, the construction also uses:

slot insulation;
interlayer insulation;
tapes;
spacers;
bandages;
impregnating varnishes or compounds;
insulation of the connections and leads.

At excessive temperature these materials:

lose their mechanical strength;
become brittle;
crack;
delaminate;
lose their elasticity;
char;
become electrically conductive;
stop reliably separating the conductors.

Once the insulation breaks down, electrical contact appears between turns, between phases, or between the winding and the frame.

In this way, a visible burn is often only the final stage of a process that began much earlier.

The main causes of winding damage

Prolonged overload

Overload occurs when the driven mechanism demands more torque or power than the motor can continuously deliver.

Causes:

an incorrectly selected motor power;
increased equipment throughput;
an overloaded conveyor;
increased pump or compressor pressure;
a change in the viscosity or density of the medium;
contamination of the driven mechanism;
gearbox wear;
jamming of the bearings;
excessive belt tension;
an incorrect process regime.

Under overload, typically:

the currents in all three phases are elevated;
the heating is distributed relatively evenly;
the insulation of all phases shows a similar degree of darkening;
the motor runs for a long time at a temperature above the permissible level.

If the thermal protection is set up incorrectly or is absent, the winding gradually ages and eventually breaks down.

Rotor lock-up

Lock-up occurs when the rotor cannot turn, or comes to an abrupt stop while running.

Causes:

jamming of the mechanism;
bearing failure;
a foreign object getting in;
jamming of the gearbox;
shaft deformation;
the rotor contacting the stator;
lock-up of the pump or compressor;
freezing or hardening of the material.

With the rotor stationary, the motor draws a current close to the starting current. Cooling is much worse at the same time, so the temperature of the winding and rotor rises very quickly.

If the lock-up protection fails to trip, the winding can burn out within a short time.

The pattern of damage often resembles a severe general overload, but the failure develops much faster.

Prolonged start

During a normal start, the increased current acts for only a limited time.

A start becomes prolonged if:

the mechanism has a large inertia;
the voltage is reduced;
the load is not disconnected before starting;
the motor is incorrectly matched;
the rotor has a defect;
the star–delta circuit operates incorrectly;
a soft starter limits the current excessively;
the variable frequency drive has too short a ramp or too low a torque;
the mechanism is partly jammed.

The motor may still accelerate, but every such start delivers a severe thermal shock to the winding.

Frequent starts

Even if every individual start is successful, an excessive number of starts can cause overheating.

Between starts the motor must have enough time to cool down. If this does not happen:

the average winding temperature keeps rising;
the insulation ages quickly;
the end windings suffer repeated electrodynamic shocks;
the coil fastenings work loose;
the risk of an interturn short circuit increases.

Repeated starts right after an emergency trip, while the motor is still hot, are especially dangerous.

Loss of one phase

When one phase disappears, a motor that was already running sometimes keeps operating on the two remaining phases.

In this regime:

the currents in the two live phases increase significantly;
the motor torque decreases;
the speed can drop;
a loud hum appears;
the winding temperature rises quickly;
a subsequent restart is often impossible.

A characteristic sign is heavy damage to two phases with lighter damage to the third.

The causes may be:

a blown fuse;
a burnt contactor contact;
a broken cable;
a loose terminal;
a damaged lead;
a faulty circuit breaker.

Correctly configured phase-loss and unbalance protection should trip the motor quickly.

Voltage unbalance between phases

A small voltage unbalance can cause a much larger current unbalance.

One or two phases begin operating overloaded, as a result of which the winding heats up unevenly.

Causes:

uneven loading of the supply network;
loose contacts;
differing contact resistance;
cable damage;
a faulty contactor;
a transformer defect;
problems with the generator.

Unlike a complete phase loss, the motor can keep running for a long time while the damage develops gradually.

Reduced voltage

Reduced voltage does not always mean reduced current.

If the mechanical load stays the same, the motor tries to produce the required torque by increasing its slip and current.

The consequences:

slow acceleration;
reduced speed;
increased current;
overheating;
unstable operation;
protection tripping.

The causes of low voltage:

an overloaded supply network;
insufficient cable cross-section;
a long supply line;
poor contacts;
an undersized transformer or generator;
voltage dips when other loads start.

The voltage must be measured directly at the motor terminals under load.

Increased voltage

At increased voltage the magnetic flux grows. The core can go into saturation.

This causes:

an increase in the no-load current;
higher losses in the steel;
stator overheating;
increased noise;
additional stress on the insulation.

A combination of increased voltage with a reduced supply frequency is especially dangerous.

Incorrect star–delta connection

The connection diagram must match the motor’s nameplate voltage.

If a winding designed for star operation at a given line voltage is connected in delta, each phase receives an excessive voltage.

The consequences:

a sharp increase in current;
saturation of the magnetic core;
rapid overheating;
damage to the winding.

The opposite mistake is leaving the motor running in star when it should switch to delta. In that case the torque will be insufficient, the start will be prolonged, and the motor can overheat from overload.

Fault in the starting circuit

The causes of damage may be:

a faulty contactor;
burnt power contacts;
an incorrect switchover time;
simultaneous closing of contactors;
an incomplete transition to the running connection;
an incorrect phase sequence;
a faulty soft starter;
incorrect protection settings.

An intermittent contactor defect can be especially difficult to diagnose: the motor sometimes starts normally, and sometimes runs with an unbalanced supply.

Interturn short circuit

An interturn short circuit occurs when the insulation between adjacent turns is damaged.

A significant circulating current appears in the shorted part of the coil. At the same time the external phase current may initially increase only slightly.

The defect causes:

local overheating;
charring of the insulation;
expansion of the damaged area;
asymmetry of the magnetic field;
a drop in torque;
progression into a phase-to-phase short circuit or a breakdown to the frame.

An interturn short circuit often starts as a small local spot, but very quickly destroys the whole coil.

Phase-to-phase short circuit

A phase-to-phase short circuit occurs when the insulation between the windings of different phases is damaged.

Typical locations:

the end windings;
crossings between coils;
phase-to-phase connections;
the point where a coil exits a slot;
a damaged insulating spacer.

When the short circuit occurs, an electric arc forms at a very high temperature.

The consequences:

melting of the copper;
charring of the insulation;
damage to neighbouring coils;
deformation of the end windings;
damage to the core;
tripping of the overcurrent or differential protection.

Breakdown of the winding to the frame

A breakdown occurs between a current-carrying part and the earthed core or frame.

Causes:

damage to the slot insulation;
sharp slot edges;
movement of a coil;
ageing;
moisture;
contamination;
overvoltage;
mechanical damage during a repair;
local overheating.

The damage is often localised in the slot or at the point where a coil exits it.

If earth-fault protection or differential protection is absent or insensitive, the arc can seriously damage the core steel.

Cooling system fault

Even at a normal current, the winding can overheat if the heat is not being removed.

Causes:

a damaged fan;
the fan turning in the wrong direction;
contaminated frame fins;
blocked ventilation ducts;
contaminated filters;
a faulty heat exchanger;
the absence of a water flow;
recirculation of hot air;
a blocked fan cowl;
insufficient capacity of the external fan.

In large machines it is necessary to check not only the presence of an air flow, but also its distribution across the individual zones of the winding.

Operation at low speed without forced ventilation

In motors with a shaft-mounted fan, the cooling intensity depends on the rotation speed.

When running from a variable frequency drive at low speed:

the airflow decreases;
the motor can produce a significant torque for a long time;
the current remains high;
the winding temperature rises quickly.

A forced-ventilation fan may be needed for prolonged operation at high torque and low speed.

High ambient temperature

A motor is designed to operate under specific cooling conditions.

At an elevated ambient temperature:

the temperature difference between the motor and the air decreases;
heat dissipation worsens;
the winding reaches its limit temperature sooner;
the permissible load may have to be reduced.

Particularly dangerous are:

machine rooms without ventilation;
operation next to furnaces;
closed enclosures;
summer overheating of premises;
recirculation of hot air.

Contamination

Dust, oil and process deposits act in several ways at once:

they block the ventilation ducts;
they worsen heat transfer;
they retain moisture;
they create conductive tracking paths;
they cause surface discharges;
they degrade the impregnating material.

Particularly dangerous are:

metal dust;
coal and graphite dust;
cement dust;
salts;
chemical deposits;
oil containing wear particles.

Moisture and condensation

Moisture reduces the insulation resistance and accelerates electrical ageing.

It gets into the motor through:

damaged seals;
prolonged storage;
temperature fluctuations;
flooding;
washing of the equipment;
operation in damp premises;
faulty space heaters.

Starting a moistened motor can cause a surface breakdown or a short circuit to the frame.

Overvoltages

Short voltage impulses can damage the insulation even without any noticeable general overheating.

Sources:

lightning activity;
switching operations;
vacuum circuit breakers;
emergency conditions in the network;
long cables;
incorrect earthing;
a variable frequency drive;
reflections of voltage waves.

The turns closest to a coil’s line lead are often the first to be damaged.

Impulses from a variable frequency drive

A drive shapes the voltage using fast switching pulses. High values of voltage rise rate can occur at the motor terminals.

The risk increases with:

a long cable;
a high switching frequency;
the absence of an output choke;
the absence of a dU/dt filter;
old insulation;
the use of a motor not rated for inverter supply;
incorrect earthing and shielding.

The winding can gradually be destroyed by repeated impulse loading, even when the RMS voltage matches the rated value.

Damage to the squirrel-cage rotor

Broken bars or cracked end rings cause:

a reduction in starting torque;
a longer acceleration time;
torque pulsations;
increased slip;
an increase in stator current;
local heating of the rotor;
overloading of the stator winding.

As a result, the stator can burn out as a secondary consequence of the rotor defect.

Uneven air gap

Rotor eccentricity causes asymmetry of the magnetic field.

Causes:

worn bearings;
a bent shaft;
frame deformation;
incorrect assembly;
worn bearing seats;
displacement of the end shields.

The consequences:

uneven heating;
vibration;
electromagnetic noise;
increased currents;
possible contact between the rotor and the stator.

Stator core damage

Interlaminar short circuits create local eddy currents in the core steel.

Causes:

an electric arc during a previous burn;
mechanical damage to the slots;
overheating while burning out the old winding;
incorrect dismantling technique;
welding near the core stack;
contact between the rotor and the stator.

If the core is not checked and restored, the new winding can overheat in the same zone.

Loose slot wedges

When the wedges are loose, the conductors and coils can move under the action of electromagnetic forces.

This causes:

abrasion of the enamel;
destruction of the slot insulation;
cracking of the impregnating varnish;
an interturn short circuit;
a breakdown to the frame.

The signs can include friction dust, vibrating conductors and traces of mechanical movement.

Vibration of the end windings

The end windings are subjected to significant electrodynamic loads during starts and short circuits.

If the bracing is weak, they can:

rub against each other;
come into contact with metal parts;
displace the spacers;
damage the phase-to-phase insulation;
damage the leads.

A local defect in an end winding is often linked precisely to this kind of mechanical movement.

Ageing of the insulation

Insulation has a limited service life.

Its ageing rate is affected by:

temperature;
the number of thermal cycles;
vibration;
moisture;
the chemical environment;
electrical impulses;
the quality of the materials;
previous instances of overheating.

An old winding can pass a standard insulation resistance measurement while still having a low resistance to impulse voltage or mechanical stress.

Fault in the temperature protection

The winding can overheat if:

the sensors are installed incorrectly;
the sensors are not connected;
the sensor cable is damaged;
the setting is too high;
the signal is ignored by the control system;
the thermal relay is set up incorrectly;
the protection does not account for repeated starts;
the protection applied does not match the duty type.

It is important not only that the protection is installed, but that it actually trips the motor in a dangerous regime.

Incorrect choice of protection

An ordinary circuit breaker mainly protects the cable and the equipment against a short circuit. It does not always provide full protection of the motor against prolonged overload, phase loss or rotor lock-up.

Critical drives may require:

thermal protection;
overload protection;
phase-loss and phase-unbalance protection;
rotor lock-up protection;
starting-time protection;
earth-fault protection;
differential protection;
monitoring of the temperature sensors;
monitoring of the number of starts.

Mistakes after rewinding

A new winding can burn out because the repair technology was not followed correctly.

Typical causes:

an incorrect number of turns;
an incorrect wire cross-section;
an incorrect connection diagram;
an incorrect winding pitch;
incorrect phasing;
weak interturn insulation;
damage to the enamel;
poor-quality slot insulation;
weak coil fastening;
insufficient impregnation;
an incorrect drying regime;
residual moisture;
incompatibility of materials;
the absence of verification tests.

Incorrect impregnation

Impregnation is meant to:

fix the conductors in place;
fill the voids;
improve heat transfer;
protect against moisture;
increase the dielectric strength.

With insufficient impregnation:

the coils vibrate;
air voids remain inside;
heat removal worsens;
the risk of partial discharges increases;
the insulation deteriorates faster.

An excessive or improperly cured compound can also create problems if it obstructs heat removal or cracks.

Wire damage during winding

The enamel can be damaged by:

sharp guides;
incorrect tension;
impacts;
kinking;
contaminated tooling;
rough coil shaping;
insertion into the slot without proper protection.

The defect may not show up immediately, but can become the site of a future interturn short circuit.

Overheating during drying or burnout

Violating the temperature regime during a repair can:

damage the new insulation;
worsen the properties of the steel;
cause deformation of parts;
destroy the interlaminar insulation of the core.

The temperature must be monitored not only from the oven readings, but also with regard to the actual temperature of the object itself.

How to identify the cause from the burn pattern

The external appearance of the winding does not always give an unambiguous answer, but it often helps narrow down the list of causes.

All three phases uniformly overheated

Likely causes:

prolonged overload;
poor cooling;
high ambient temperature;
frequent starts;
an incorrect duty cycle;
reduced voltage under a steady load.

Two phases heavily damaged

This points to operation with one phase lost.

It is necessary to check:

the fuses;
the contactors;
the terminals;
the cable;
the unbalance protection.

One phase damaged more heavily

Possible causes:

voltage unbalance;
a poor contact;
an internal defect of that phase;
an interturn short circuit;
an error in connecting the parallel branches.

A local burn on a single coil

Likely causes:

an interturn short circuit;
damaged enamel;
weak impregnation;
coil vibration;
a local core defect;
mechanical damage in the slot.

Damage in an end winding

Possible causes:

a phase-to-phase short circuit;
vibration;
loosened bandages;
rubbing between coils;
damaged connections;
overvoltage.

Burning in a slot

Likely causes:

damage to the slot insulation;
a sharp edge;
a loose wedge;
movement of a coil;
a breakdown to the frame;
local damage to the core.

Damage near the leads

Possible causes:

overvoltages;
a poor connection;
local overheating of a contact;
mechanical stress on a lead;
impulses from a drive.

Complete charring of the winding

This is often already a secondary consequence of a sustained electric arc or the absence of timely protection. With this kind of state it is harder to identify the original cause, so the following are especially important:

the protection data;
the current readings;
the fault log;
the condition of the contactors;
an inspection of the mechanism;
a check of the rotor and the core.

Signs that winding damage is approaching

Early signs include:

an increase in the operating current;
unbalance of the phase currents;
a gradual rise in temperature;
a smell of heated insulation;
a change in the colour of the varnish;
a longer acceleration time;
a drop in speed under load;
increased electromagnetic hum;
unstable operation;
frequent protection tripping;
a decrease in insulation resistance;
a change in the phase inductance;
differences between the surge-test curves;
an increase in the level of partial discharges;
local hot spots shown by a thermal imager;
friction dust in the area of the winding.

Why a motor can burn out even though the current was "normal"

This is possible in several cases.

A local interturn short circuit

A high current circulates inside a few shorted turns and does not fully pass through the external measuring instruments.

Insufficient cooling

At a normal current, the temperature can exceed the permissible level because of the absence of an airflow.

High ambient temperature

The rated current does not guarantee a permissible temperature under poor cooling conditions.

Incorrect measurement

The current may have been measured:

in only one phase;
without taking the start into account;
before the machine warmed up;
not at the moment of maximum load;
with an inaccurate instrument;
at the input of the drive instead of its output to the motor.

Overvoltages

An impulse breakdown can occur without any prolonged excess of the average current.

Diagnostics after winding damage

1. Do not begin dismantling without recording the condition

Before cleaning and disassembly, it is necessary to:

photograph the winding;
record the location of the burn;
mark the phases;
record the condition of the leads;
check the contactors and the cable;
save the protection data;
interview the personnel;
record the circumstances of the failure.

After dismantling, an important part of the information can be lost.

2. Check the supply system

It is necessary to assess:

the voltages;
the unbalance;
the contacts;
the fuses;
the contactors;
the circuit breaker;
the cable;
the protection settings;
the fault log of the variable frequency drive.

3. Check the mechanism

The following are checked:

free rotation;
the bearings;
the gearbox;
the pump;
the compressor;
the conveyor;
the coupling;
the belts;
the actual load.

4. Measure the winding resistance

If the condition of the winding allows it, the following are compared:

the phase resistance;
the parallel branches;
the connections;
the leads.

5. Check the insulation resistance

The following are measured:

phase to frame;
phase to phase;
winding to auxiliary circuits.

6. Carry out a surge test

If the damage is not yet complete, a surge test can help detect an interturn defect and compare the phases.

7. Check the rotor

It is necessary to rule out:

broken bars;
cracked rings;
overheating;
deformation;
traces of contact with the stator;
a fault in the wound-rotor winding.

8. Check the stator core

The following are assessed:

interlaminar short circuits;
local hot spots;
melting;
burrs;
deformation of the teeth;
the condition of the slots.

9. Establish the root cause

The decision to rewind should be made only after the entire system has been assessed.

Step-by-step diagnostics of the cause of failure

01Record the circumstances of the shutdown.
02Obtain the current, voltage, temperature and protection data.
03Inspect the terminal box and the power switchgear.
04Check that all three phases are present.
05Assess the mechanical load.
06Inspect the winding before cleaning it.
07Determine the nature and location of the burn.
08Check the stator, the rotor, the bearings and the air gap.
09Check the core.
10Analyse the diagram and technology of the previous repair.
11Check the settings of the protection and the drive.
12Draw up a technical report stating the root cause and the contributing factors.

A typical diagnostic table

Nature of the damage	Likely cause	What to check
All three phases uniformly overheated	Overload, poor cooling	Current, load, ventilation
Two phases heavily damaged	Loss of one phase	Fuses, contactors, cable
One phase overheated more	Unbalance, poor contact, internal defect	Voltages, contacts, phase resistance
Local burn on a coil	Interturn short circuit	Surge test, inductance
Burning in a slot to the frame	Damage to the slot insulation	Slot, wedge, core
Burning in the end windings	Vibration or a phase-to-phase breakdown	Bandages, spacers, fastenings
Damage near a lead	Overvoltage or a poor contact	Connections, cable, drive
Winding burnt out after a long start	Overload or low torque	Starting time, voltage, mechanism
Repeated burn in the same place	A core or design defect	Core-steel test
Winding burnt out after rewinding	A process error or an uncorrected cause	Diagram, turns, impregnation, network
Burn during operation from a VFD	Impulse overvoltages	Cable, filter, switching frequency
Current normal, burn local	An internal insulation defect	Surge test, thermal imaging

When the motor must be stopped immediately

The motor should be switched off if any of the following signs appear:

a smell of burning insulation;
smoke;
a sharp rise in temperature;
a loud hum;
a drop in speed;
running on two phases;
current unbalance;
repeated protection tripping;
a sharp increase in current;
jamming;
sparking;
traces of an electric arc;
local overheating of the frame;
a confirmed interturn short circuit;
damage to the cooling system;
a sharp increase in vibration.

Repeated trial starts must not be carried out until the cause of the failure has been established.

What should not be done

Do not increase the protection rating

If the protection trips, that does not mean it is "too weak." Raising the setting can lead to complete burnout of the winding.

Do not bridge a fuse with a jumper

This creates a direct risk of fire and equipment destruction.

Do not perform repeated starts

Every repeated start sharply heats up the damaged winding.

Do not limit the repair to rewinding

It is necessary to establish why the winding burnt out.

Do not ignore the rotor

A damaged rotor can overload the new winding all over again.

Do not ignore the core

Local damage to the core steel can cause a repeat burn.

Do not assess the condition with a megohmmeter alone

A high resistance to the frame does not rule out an interturn short circuit.

Do not start the motor after drying without diagnostics

If the insulation is already damaged, drying will not restore it.

Do not change the variable frequency drive parameters at random

Raising the current limit or the voltage boost can accelerate a failure.

Do not draw conclusions from a photograph alone

The nature of the burn is important, but the final conclusion must take into account the electrical measurements, the condition of the mechanism and the protection data.

Typical mistakes made during the investigation

01Cleaning the winding before photographing it.
02Not recording the location of the damaged phases.
03Not checking the power contactors.
04Not analysing the fault log.
05Not measuring the voltage under load.
06Not checking the mechanism for jamming.
07Treating any burn as the result of overload.
08Not checking the rotor.
09Not checking the core after an electric arc.
10Not analysing the number and duration of the starts.
11Not checking the actual operation of the fan.
12Not taking the ambient temperature into account.
13Not checking the star–delta connection.
14Not checking the variable frequency drive parameters.
15Not establishing the cause of repeated damage.

Practical repair experience

In practice a winding is often damaged by the simultaneous action of several factors.

For example:

the mechanism is gradually becoming overloaded;
the motor current is rising;
the ventilation ducts are contaminated;
the thermal protection setting is too high;
the insulation has already aged.

On its own, each of these factors might not cause an immediate failure. Together, however, they lead to accelerated ageing and breakdown of the winding.

Common practical situations:

The motor burnt out after a fuse was replaced

The cause may not be the fuse itself, but a fault in one phase, a contactor or a cable. If a fuse with a higher rating was fitted, the protection stopped tripping the motor in time.

The motor burnt out after rewinding

Two groups of causes are possible:

a mistake made during the repair;
an undetected external cause that burnt out the previous winding.

The motor overheated periodically but kept running

Every overheating cycle reduced the remaining life of the insulation. The final breakdown could then occur under an ordinary load.

The winding burnt out locally in one slot

It is necessary to check not only the conductors, but also the slot insulation, the wedge and the core. A repeated burn in the same spot often points to a defect in the core steel.

The motor burnt out while running from a drive

It is necessary to check:

the cable length;
the type of motor;
the switching frequency;
the cooling system;
the output filter;
the current and torque settings;
the operating regime at low frequency.

Repairing a damaged winding

The scope of the repair depends on:

the nature of the damage;
the age of the winding;
the condition of the neighbouring coils;
the condition of the core;
the design of the motor;
the availability of spare coils;
economic feasibility.

Local repair

Possible if:

the damage is limited;
the rest of the winding is in satisfactory condition;
the design allows a single coil to be replaced;
the core has no significant damage.

Partial replacement

Used to replace a coil group or part of the winding.

The difference between the old and new insulation must be taken into account.

Complete rewinding

Advisable if:

the insulation has aged overall;
several phases are damaged;
there are numerous weak spots;
the winding is contaminated or moistened;
a local repair will not provide reliability;
the motor has already been repaired more than once.

Core restoration

If the core steel is damaged, it is necessary to eliminate:

interlaminar short circuits;
melting;
burrs;
deformation of the teeth;
local hot spots.

Without this, the new winding can overheat again.

What to check after rewinding

After the repair it is advisable to carry out:

a visual inspection;
a check of the connection diagram;
a check of the phasing;
measurement of the phase resistance;
a check of the parallel branches;
measurement of the insulation resistance;
determination of the absorption ratio;
a dielectric strength test;
a surge test of the interturn insulation;
measurement of the inductance;
a check of the temperature sensors;
a check of the coil fastening;
a check of the slot wedges;
a no-load test;
measurement of the currents;
vibration monitoring;
a load test;
monitoring of the thermal regime.

How to prevent repeated damage

Set up the protection correctly

The settings must match:

the rated current;
the duty cycle;
the starting time;
the number of starts;
the thermal protection class;
the characteristics of the mechanism.

Monitor the three-phase supply

The following should be checked regularly:

the voltages;
the currents;
the unbalance;
the contacts;
the fuses;
the condition of the cable.

Monitor the load

It is important to keep trends of:

the current;
the active power;
the starting time;
the speed;
the throughput of the mechanism;
the temperature.

Maintain the cooling

It is necessary to clean:

the ventilation ducts;
the fins;
the filters;
the fans;
the heat exchangers.

Maintain the mechanical part

The following are checked:

the bearings;
the couplings;
the alignment;
the belts;
the gearboxes;
free rotation of the mechanism.

Monitor the condition of the insulation

For critical machines the following are used:

insulation resistance measurement;
the polarization index;
surge tests;
partial discharge measurement;
thermal imaging monitoring;
current analysis;
trend monitoring.

Recommendations for the chief power engineer’s department

For every critical motor it is advisable to keep:

the nameplate data;
the winding diagram;
the rated currents;
the actual operating currents;
the voltages;
the acceleration time;
the temperature trends;
the number of starts;
the results of the insulation measurements;
the results of the surge tests;
the vibration data;
the repair history;
the protection settings;
the variable frequency drive settings;
information about previous failures.

After every case of damage it is advisable to draw up a technical report that separates:

the immediate cause;
the root cause;
the contributing factors;
the consequences;
measures to prevent a repeat failure.

For example, the immediate cause may be a phase-to-phase short circuit, while the root cause is prolonged vibration of the end windings caused by loosened bandages.

Frequently asked questions

Why did the motor winding burn out without overload?

Possible causes:

an interturn short circuit;
an insulation breakdown;
overvoltage;
poor cooling;
a local core defect;
an incorrect measurement of the load;
running on two phases;
a defect left over from a repair.

Why did the motor burn out even though the circuit breaker did not trip?

The circuit breaker may have been designed mainly to protect against a short circuit, not against prolonged overload of the motor. In addition, a local interturn current may not fully pass through the breaker.

Why did the motor burn out after a repeated start?

After the first emergency trip, the winding may have remained hot or already had damaged insulation. The repeated start then created the maximum current and thermal load.

Why does losing one phase damage two phases?

Current keeps flowing through the two live phases, which become overloaded and heat up strongly.

Can low voltage burn out a motor?

Yes. With an unchanged mechanical load, the motor can draw an increased current and overheat.

Can increased voltage damage the winding?

Yes. It causes saturation of the magnetic core, an increase in the no-load current and a higher electrical stress on the insulation.

Can the cause be determined from the colour of the winding?

The colour and pattern of the burn provide important information, but are not sufficient for a final conclusion. The network, the mechanism, the protection, the rotor and the core all need to be checked.

Why did the new winding burn out quickly?

Possible causes:

an uncorrected external cause;
an incorrect diagram;
an error in the number of turns;
poor-quality insulation;
a damaged core;
weak impregnation;
a rotor defect;
incorrect protection.

Can a damaged coil simply be replaced?

In some designs, yes. But the condition of the whole winding, the neighbouring coils and the core steel must be assessed.

Does drying restore damaged insulation?

Drying removes moisture, but it does not restore insulation that is charred, cracked or has suffered an electrical breakdown.

Why does a motor burn out when running from a variable frequency drive?

The causes may be:

operation at low speed without sufficient cooling;
impulse overvoltages;
a long cable;
the absence of an output filter;
incorrect motor parameters;
an excessive switching frequency;
incorrect current limits.

Can a damaged rotor burn out the stator?

Yes. A rotor defect increases the slip, the acceleration time and the stator current, causing the winding to overheat.

How can you tell that the cause lies in the mechanism?

You need to uncouple the unit and check the motor and the mechanism separately, assessing free rotation, the no-load current and the actual load.

Is it mandatory to check the core after a burn?

Yes, mandatory in the case of severe local damage, an electric arc, or contact between the rotor and the stator. Damaged core steel can overheat a new winding.

Services of ELEKTROPROMREMONT LLC

ELEKTROPROMREMONT LLC carries out diagnostics, repair and testing of industrial electric machines.

The scope of work includes:

determining the causes of winding damage;
fault detection on the stator and rotor;
measurement of the winding resistance;
measurement of the insulation resistance;
a surge test of the interturn insulation;
checking the phase-to-phase and frame insulation;
manufacturing new coils;
partial and complete rewinding;
restoring the slot insulation;
repairing and bracing the end windings;
impregnating and drying the windings;
checking the stator core steel;
checking the squirrel-cage rotor;
shaft and bearing-assembly repair;
balancing;
a no-load test;
a load test;
monitoring the currents, temperature and vibration after the repair.

Conclusion

Electric motor windings do not burn out only because of overload. The damage can be caused by a fault in the supply network, a lost phase, a jammed mechanism, a rotor defect, a cooling fault, overvoltage, ageing of the insulation or a repair mistake.

The nature of the burn often contains important information:

uniform overheating of all phases can point to overload;
damage to two phases points to the loss of the third;
a local burn on a coil points to an interturn defect;
damage in a slot points to a breakdown of the frame insulation;
a burn in an end winding points to vibration or a phase-to-phase short circuit.

However, the final conclusion must rest on a comprehensive check of:

the supply;
the protection;
the winding;
the rotor;
the core;
the bearings;
the cooling system;
the driven mechanism.

The main task of a repair is not simply to install a new winding, but to eliminate the root cause of the failure. That is exactly what determines whether the motor will run reliably after the repair.

Important disclaimer

This material is for informational purposes. The values, diagnostic methods, scope of work and recommendations given here are general and do not replace the manufacturer’s technical documentation. The final decision for a specific machine is made from its own diagnostics and inspection, taking into account its type, power, design, duty, operating history and applicable standards.

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