The 6 Most Common Electric Motor Failures - How to Recognize and Resolve Them
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The 6 Most Common Electric Motor Failures - How to Recognize and Resolve Them

  • Katarina Knafelj Jakovac

    December 7, 2023

Electric motors are the most commonly used drive machines; they are everywhere around us. Low-voltage electric motors consume up to 45% of the world's produced electrical energy, and it is predicted that, by 2040, the number of electric motors in daily use will double.

We will explore why electric motors fail, the most common motor failures encountered by maintenance personnel in everyday work, and the types of motor testing that should be regularly performed to assess their condition and prevent potential disasters.

When looking at why a driven machine has malfunctioned from a mechanical perspective, we often focus so much on mechanical causes that we tend to overlook potential electrical failures and their impact on the continuous operation of various machines and devices in the manufacturing industry.

Electric motor driving a pump in the process industry
Image: Electric motor driving a pump in the process industry (Source)

Basic Characteristics of Electric Motors

Rotational magnetic fields are generated in the presence of two spatially displaced windings through which phase-shifted currents flow.

A single-phase asynchronous motor with a capacitor on the stator, in addition to the main winding or phase, has an auxiliary winding. The auxiliary winding has higher resistance. The main phase is directly connected to the alternating current network, while the auxiliary phase is connected through a series-connected capacitor that provides a phase shift relative to the current of the main phase.

The next image schematically shows a single-phase electric motor with a capacitor.

Screenshot 2024-02-03 at 16.23.00.png
Picture: Electric motor driving a pump in the process industry (Source)

The rotating magnetic field of the stator intersects the rotor conductors, inducing a voltage that directs the current. A force acts on the conductor in the magnetic field, causing the rotor to rotate in the direction of the rotational magnetic field if a current flows through it.

Single-phase motors with capacitors typically have at most two built-in capacitors.

In practice, this is implemented by having the electric motor and main components in one housing, as shown in the picture.

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Image: Parts of an electric motor

Single-phase motors, or as they are commonly called, monophase motors, are designed as single-speed, while three-phase motors are designed as single-speed or multi-speed, depending on their purpose. The power range of single-speed motors complies with the regulations of IEC 60034-1.

The dimensions of shafts made of special steel in electric motors depend on their nominal characteristics and are adapted to the sizes of electric motors.

The connection box of single-phase electric motors is made of plastic, and in addition to the connection plate, the box contains capacitors.

Each connection box is equipped with a cable gland for the entry of the connecting cable.

The nominal values of all capacitors apply to a network with a nominal voltage of 230V.

Electric motors of lower power and dimensions generally have permanently lubricated bearings, while electric motors of higher power have bearings with built-in lubrication fittings. Lubrication intervals, the quantity of lubricant, and the type of lubricant are specified on the additional plate of the electric motor.

Low-voltage electric motors mostly use ball bearings, an example of which is shown in the picture, which have high rotation speeds and low vibrations.

Closed ball bearing 6205-2Z/C3 with seals
Image: Closed ball bearing 6205-2Z/C3 with seals (Source)

Ball bearings carry radial and axial loads in both directions, are robust, easy to assemble and disassemble, and have seals and plates on the outer side to prevent the ingress of dirt.

The lifespan of ball bearings can reach up to 100,000 hours of operation under average working conditions without sudden changes.

The lifespan of bearings mounted on horizontally constructed electric motors ranges from 40,000 hours to 60,000 hours if there is no additional axial force. If an allowable axial force is present, the lifespan of the bearings must be at least 25,000 hours.

Cooling of a single-phase or three-phase asynchronous electric motor is done using an axial fan located in the fan cap.

Single-phase electric motors should not be started more than 20 times in one hour to protect the capacitor; in other words, you can start it every 3 minutes at most.

Three-phase cage asynchronous electric motors are allowed to be started a maximum of 3 times in one hour if the electric motor is heated to operating temperature.

Electric motors must meet the requirements of standards IEC 60034, IEC 60072, IEC 60038, and IEC 60085, with a voltage and frequency of 230V ± 5% and 50Hz.

Why Do Electric Motors Fail?

The failure of electric motors is not solely due to their age or the number of working hours.

Numerous other causes, to which we often pay less attention, influence the operation of electric motors.

Irregularities in power supply, strain due to elevated operating temperatures, constant and increased moisture presence, lack of lubrication, dirt, and variable operational loads over time lead to the weakening of components and the subsequent failure of electric motors.

Studies have shown that the lifespan of electric motors can increase by several hundred thousand working hours when such operational issues are diagnosed and resolved in a timely manner.

Regulation on electrical equipment intended for use within certain voltage limits

and Regulation on electromagnetic compatibility extensively prescribe obligations and requirements for electric motor manufacturers to meet conformity conditions for using equipment on the market.

Irregularities in power supply cause 80% of problems in the operation of electric motors in refinery and petrochemical plants.

During the power supply of process plants, some common problems occur, such as: harmonics causing overheating and reduced efficiency, excessive voltage reducing efficiency and lowering power factor, low voltage increasing current intensity and causing overheating, and reducing operational efficiency under full load.

Ideal power supply manifests as a perfect sine wave for each phase at the rated current and frequency, which is difficult to achieve under real conditions.

Voltage imbalance also leads to overheating and reduced efficiency, where an imbalance greater than 1% slows down the operation of electric motors. Efficiency must be determined according to IEC 60034-2-1.

In this case, electric motors should not operate in a supply system with more than 5% voltage imbalance.

Voltage surges are caused by the action of capacitors or standing waves traveling through cables and originate from electric motors with variable frequency drives (VFD).

Voltage surges often cause insulation damage.

When electric motors with variable frequency drives operate at a frequency lower than 60 Hz, it is necessary to reduce the torque or provide additional cooling because the electric motor overheats.

Electric motors with variable frequency drives can also cause the generation of eddy currents that damage the bearings.

In such cases, it is recommended to check with the electric motor manufacturer if they can provide bearings made of material that acts as an insulator, recommend lubricating grease with no electrical conductivity, or install a specially adapted grounding system.

It is impossible to completely prevent dirt from entering the electric motor housing, regardless of the construction design of the housing.

Dirt is harmful because it causes corrosion and abrasive action on internal parts and leads to overheating as it behaves like a layer of thermal insulator.

The coils of electric motors bend during operation, and dirt particles damage the coating on the wires. Some substances like salt or graphite become electrically conductive, and electricity then acts through cracks in the insulation, accelerating if moisture is present.

A large amount of dirt particles also blocks cooling passages from the outside or inside, leading to overheating.

Improper lubrication is a fairly standard cause of failures, where the grease may contain dirt particles and contaminate the bearings if not carefully dosed and using clean lubricating greases.

There are manual mechanical and electric lubricators; the choice depends on the lubrication requirements.

Image: Manual mechanical lubricator for lubricating electric motor bearings
Image: Manual mechanical lubricator for lubricating electric motor bearings (Source)

Different electric motors have different requirements for lubrication and the removal of old grease.

Variable mechanical loads can increase the stress on bearings or deform the housing, causing an increase in clearance, leading to vibrations or coil overheating.

It is essential to avoid coupling eccentricity, overly tightened belt if the electric motor is connected via a belt drive to a driven machine, eccentric pulleys, soft foot, incorrectly placed shims or bailage, dynamic load imbalance, or rotor imbalance, mounting the wrong type of bearings, and unresolved resonance in electric motors with variable frequency drives (VFD).

Moisture becomes a problem when the electric motor is turned off for an extended period, and its temperature equals the ambient temperature.

Condensation of moisture from the environment occurs on the internal or external parts of the electric motor.

Moisture weakens the dielectric strength of insulation and causes corrosion of bearings and other mechanical parts.

Condensation of moisture can be prevented if the electric motor is constantly warm and operates at a specific temperature.

Other methods of eliminating moisture include: maintaining the environment in which the electric motor is below 80% relative humidity by heating or dehumidifying, providing additional heating when the electric motor is not in operation, and regularly turning the shaft by hand when the electric motor is off and idle to evenly distribute the lubricant on the surfaces of the bearings.

The 6 Most Common Electric Motor Failures

1. Overheating

The electric motor has a plastic cooling fan designed for air circulation to cool it during operation, located in the cap.

The path through which air circulates can be filled with dust and dirt particles, limiting air circulation.

Accumulations of dirt over time create insulation, causing overheating and damage to the wiring.

Other causes of overheating may include overloading of the driven machine, the electric motor not being suitable for operational needs due to insufficient power, or an open phase in the supply.

2. Vibrations

Electric motors for industrial applications must have a balance between phases and amplitude of electrical current.

Unbalanced current will lead to electrically induced mechanical vibrations.

DC motors have an additional problem, as they have speed control via a variable resistor or frequency controller, so there is a high chance of the machine operating in the resonance zone.

A warning signal usually occurs when the machine exceeds allowable and specified speed ranges.

Vibrations also result from eccentricity of aggregates, unbalanced rotor assembly, damaged bearings, or damaged foundations or mounts of electric motors placed on foundations.

3. Rotor Overload

The rotor of the electric motor is constructed from a specific number of flat iron or aluminum bars parallel to the shaft, forming the rotor cage; the number of bars depends on the size of the electric motor.

Overloaded or damaged bars will eventually break due to the torque generated by the current flowing in the stator.

A broken bar reduces the rotation speed of the electric motor due to reduced action of electrical forces.

When one bar is broken, both adjacent bars are under increased load, which will subsequently lead to the breakage of other bars.

The electric motor will not start if it has 3 to 4 broken bars.

Inappropriate cross-sectional area of electrical cables will increase electrical resistance in the circuit and reduce current strength, affecting the operation of the electric motor.

4. Electric Motor Does Not Start

The cause of this situation is most commonly blown fuses or a lack of power through the cable.

Less common causes may include improperly connected phases, damaged switch, loosely connected cables causing an open circuit, short circuit in the stator, or mechanical damage to parts of the electric motor.

5. Insulation Damage

All parts of the electric motor coated with insulation, such as the stator, are susceptible to insulation damage caused by elevated temperatures.

Once the insulation of the wiring is damaged, a short circuit will occur, causing further damage and ultimately the failure of the electric motor.

Insulation damage also occurs during bending.

A short circuit can lead to fires and the burning of the electric motor.

6. Combined Failures

An example is increased noise during operation due to the scraping of the cooling fan against the protective cover or the cover of the electric motor, consuming large amounts of electricity due to inappropriate voltage or incorrect cable, power failure in the substation, and excessive shaft movement due to damaged or incorrectly mounted bearings.

How to Test an Electric Motor?

Efficient operation of electric motors involves low operating costs, energy-efficient energy consumption, reliability, and a long lifespan.

All these requirements are linked to technical correctness and adequate construction.

The condition of electric motors and their operating parameters is monitored and regularly tested in various ways.

Even basic tests of electric motor operation can save time and money on repairs, resources, and maintenance.

Tests are conducted according to the ANSI standard / EASA AR100-2105, which provides recommendations for repairing electrical equipment, improving operation, safety, and defines testing methods.

When repairing electric motors in a selected service workshop, it is recommended to check beforehand the standards according to which the electrical workshop or specialized service conducts tests. Motor testing adds value and extends the equipment's lifespan.

Regular diagnostics and thorough detection of irregularities in operation will identify the initial problems of electric motors and timely eliminate faults, thereby increasing the reliability of electric motor operation and the driven machine, ultimately improving the reliability of the entire process plant where the equipment is located.

Proper alignment, regular vibration checks, and proper lubrication are key elements that affect the lifespan of electric motors.

When an electric motor won't start, operates intermittently, is frequently tripped by voltage protection, generates excessive heat during operation, and is not particularly reliable, it's time to find the causes of such behavior through an assessment of operating conditions and technical correctness.

Sometimes problems in operation are caused by power supply issues, circuit interruptions, or improper installation.

There are numerous diagnostic devices to monitor the condition of electric motors, such as ammeters, temperature sensors, Megger insulation resistance measurements, oscilloscope analysis, etc. Each test complements the picture of the electrical equipment problem from a different perspective. The first diagnostic tool you should use is your senses.

Do you smell a strange burning odor in the air when you are near the electric motor, or do you hear unusual sounds?

Are there uncontrollable vibrations?

Is the electric motor too hot when you measure the temperature with a pyrometer?

For the initial diagnostics, start with simple elements: what is the voltage, what is the power supply, and the resistance, and use a multimeter for preliminary testing.

Digital Multimeter
Image: Digital Multimeter (Source)

Ways to Test Electric Motors and How They Can Help Increase Reliability:

Voltage Drop Test

The voltage drop test is the simplest, fastest, and cheapest way to check the quality and efficient operation of electrical circuits.

The test is performed using a digital voltmeter when the electric motor is under load.

The voltmeter measures the voltage drop in the circuits under load.

Since current takes the path of least resistance, excessive current will flow toward the voltmeter and allow readings.

If the circuit is interrupted, the voltmeter will create a temporary current flow in an attempt to isolate the area where the voltage drop occurred.

Voltage drop indication is often an early sign that the electric motor needs cleaning or repair.

Surge Current Test

The surge current test can reliably show whether the electric motor has burned out, determine whether a short circuit has occurred, and assess the insulation of the conductors.

Deposits on windings, factory defects in production, or errors during rewinding lead to wear of the insulation on the windings.

During the test, a voltage pulse (or surge current) is applied to each set of windings to check the condition of the insulation of the individual set and the condition of the set in a comparative comparison of sets.

Standards for testing are defined by IEEE 522, which specifies voltage values for a wide range of windings.

Core Loss Test

Every electric motor has certain energy losses; however, a sudden increase in losses indicates a much larger problem: damage has occurred, overheating, or the windings are not functioning as they should.

Core losses represent unnecessary energy consumption in electric motors. Testing core losses shows that there is a difference between input and output power.

Regular testing and recording results will give you a trend that immediately shows the condition and whether it is within the limits prescribed by the standard.

Some losses are common and acceptable, while significant losses indicate a problem that can be rectified before it leads to an electric motor breakdown.

It also indicates whether it is necessary to replace the electric motor because it has reached the end of its lifespan and is no longer cost-effective to service.

Hipot Test

The Hipot test is used to test dielectric resistance when we want to check the quality of the insulation of the electric motor's electrical cable. The cable should be visually inspected and its resistance tested.

Hipot Test Device
Image: Hipot Test Device (Source)

Then, by applying direct or alternating voltage, the cable is tested so that the current flows between electrical circuits.

The pre-voltages applied during this testing are unique to each electric motor and its specific voltage.

When evaluating the strength of new windings, the test is performed at a voltage of 1,000 V increased by twice the specific voltage of the tested electric motor for 60 seconds and at a frequency of 50 Hz to 60 Hz.

The Hipot test should be performed once at full power and then at 85% power during additional testing to avoid overloading the insulation.

In the case of installed renewed insulation, the test should be conducted at 60% of the normal test voltage to avoid overloading.

Megger Test

The Megohmmeter (or Megger according to the manufacturer) device for testing is used for periodic checking of insulation resistance for various types of electrical equipment.


Slika: Megohmmeter (Source)
High voltage is applied to the electrical circuit for a certain duration, and it is observed where the current has broken through the insulation.

The measurement result is displayed in the form of resistance, and when measured regularly, it can be graphically displayed to assess the overall condition of the electric motor's insulation over time.

The results show present wear and/or damage, allowing for preparations and repairs before a major failure occurs.

During testing, the electric motor must be disconnected from the power supply and appropriately isolated so that various winding conditions can be analyzed.

Keep in mind that regular monitoring and diagnostics of faults before repair or general service prevent unplanned downtime, sudden breakdowns, and unforeseen costs while ensuring reliable operation and service planning.

Using asset management software, you can easily and comprehensively keep track of the condition of electric motors, conducted tests, general services, bearing replacements, lubrication, and all repair costs.

When an industrial electric motor has such damage that it is uneconomical to repair or has reached the end of its lifespan, it should be replaced with a new one.

The choice of a new electric motor depends on its purpose and the power of the driven machine.

Purchasing a new electric motor should be planned in advance because the procurement process is complex and time-consuming, requiring significant investments.

To illustrate, small DC electric motors or mini motors with a power rating up to 180 W will cost up to 10 euros.

The price of an electric motor for starting an air compressor or a gas mixture, with a power rating of several MW, will be in the hundreds of thousands of euros.

Small Electric Motor for Modeling, Mini Locomotives, or Small Aircraft
Image: Small electric motor for modeling, mini locomotives, or small aircraft (Source)

Katarina Knafelj Jakovac
Katarina Knafelj Jakovac social media icon
December 7, 2023

Katarina Knafelj Jakovac holds Master degree in Mechanical engineering with long term work experience in Oil industry. She is Certified Reliability Leader specialized for mechanical equipment and operational excellence. Author of blog Strojarska Radionica(Mechanical Workshop) where she shares professional knowledge and personal experience in maintaining various rotating machines, machine systems and process equipment. Adores mechanics, thermal engineering and internal combustion engines. She is dedicated to the continuous improvement of machine maintenance and quality management of physical assets.