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Common Motor Issues


Common Motor Winding Failures

Shorted to Ground

A shorted to ground winding failure is a common cause of motor winding failure. A shorted to ground winding failure occurs when there is unusually low resistance in the motor caused by a degradation of the insulation within the windings. This can happen from a variety of conditions such as physical damage, contaminants, abrasion, corrosion, and overheating. If the problem is not fixed, it can cause insufficient isolation in the motor windings and conductors. In turn, that may cause short circuits and leaks. Eventually, motor failure can result from a shorted to ground winding failure.

Shorted Phase to Phase

A shorted phase to phase motor winding failure is typically caused by contaminants. A shorted phase to phase winding failure is also called a “turn to turn” winding failure. When a phase to phase winding failure occurs, it is often due to a breakdown in insulation. Several contributing factors can cause a breakdown in the insulation, including contamination, vibration, abrasion, and voltage surges. A shorted phase to phase winding failure results in a shorted coil that is grounded in the slot or at the edge. It can also cause a shorted connection. Analyzing the insulation pinpoints the cause of deterioration.

Insulation Breakdown

Along with bearing failures, insulation breakdown is one of the most common types of winding failures in electric motors. Insulation breakdown can happen for a number of reasons. Environmental factors are a main cause of insulation breakdown, which includes exposure to contaminants and debris. Abrasion can also cause the insulation to break down. Problems within the motor can also cause the winding insulation to break down more quickly than it should, including voltage surges. Motors that are used outside of their recommended operating conditions and tolerance ranges may experience insulation breakdown more rapidly as well.


An overloaded motor winding failure happens when the motor is subjected to excessive loads beyond its recommended capacity. An overloaded failure may present with several telling symptoms, including insufficient torque, overheating, and excessive current draw. Of those symptoms, excessive motor heat is one of the main causes of complete motor failure. Sometimes, individual parts on the motor may still perform with no problem, but the motor itself will run hot. Up to 30% of winding motor failures are caused by overload. Therefore, an overloaded motor should be considered when a motor runs hot or continues to overheat.

Single phased

A single phased motor winding failure happens when there is an open in one phase of the motor’s power supply. There are several possible causes for a single phased motor winding failure, including an open contactor, poor connections, a broken power line, or a blown fuse. Using a motor outside of its recommended operating conditions can cause excess stress, which in turn increases the risk of a singled phased motor winding failure. Proper motor maintenance can help detect and fix problems that may cause a single phased motor winding failure and keep a motor running longer.


A DV/DT is another common type of winding failure in electric motors. This type of winding failure occurs when rise time in voltage suddenly increases beyond normal levels. The abbreviation “DV” indicates a change in voltage, while “DT” indicates a change in time. Along with a rise time in voltage, a larger DV/DT value can also indicate a winding failure. The larger DV/DT value indicates higher motor insulation stress levels. The rise rate in voltage is calculated by dividing DV by DT. If stress continues to affect the insulation unchecked, considerable damage and motor failure may result.


Overheated motor winding failures are one of the most common causes of electric motor problems. They are responsible for nearly 60% of all electric motor malfunctions and breakdowns. Therefore, an electric motor that runs hot or gets overheated quickly should be inspected for heat-related problems including overheating. Exposure to excessive levels of heat can quickly and easily cause a motor’s insulation to deteriorate and break down. Windings that are exposed to 10 C of additional heat may have their insulation life cut in half. If the overheated problem is not fixed, the electric motor’s lifespan may be drastically reduced.


Debris and contamination are main causes of dirty electric motor winding failures. Insulation that is impacted by contamination has a much shorter lifespan than insulation protected from exposure to contaminants. While insulation can be negatively affected fairly easily by contamination, the problem can also be addressed before it causes a complete breakdown. Regular service and maintenance is key to proper motor performance and to avoid problems with the dirty insulation before larger problems result, including a complete motor breakdown. Reduced motor performance, overheating, and surges in voltage are all indications that there is dirt and debris in the insulation.

Power quality

A power quality motor winding failure can result from several causes. This includes transient voltage, harmonic distortion, and voltage imbalance. Transient voltages can come from a variety of internal and external sources. Transient voltages are simply voltages that vary in frequency and amplitude. They can cause power quality problems including insulation breakdown and erosion. Harmonic distortion refers to unwanted sources of currents or AC voltages that deliver power to the motor windings. A voltage imbalance refers to an imbalance in load distribution or impedance in all three phases of a distribution system that creates stress on the system.


Overheating is a common cause of motor winding failure that is caused by start or stop. A motor that is exposed to excessive amounts of heat may encounter a rapid deterioration in the winding insulation. For instance, a motor that has a predicted lifespan of 20 years may have its life reduced to just one year if it is continually subjected to high levels of heat, which is about 40 C above the recommended temperature. Overheating can be caused by other sources besides start and stop including overload, environmental factors, poor power conditions, and high effective service factors.


Environmental factors are a major cause of electric motor winding failures. Environmental factors primarily include dirt, debris, and contaminants. Improper cooling and chemical abrasion are other hazardous environmental conditions. Exposure to these elements can reduce a motor’s performance immediately and over a longer period of time. Sometimes, a motor’s longevity and performance can be restored by checking the insulation periodically for signs of damage caused by environmental conditions. Preventative maintenance is a primary resource for optimal electric motor health. If an environmental motor winding failure is not discovered and fixed, the motor can overheat or suffer other damage.


Altitude is another catalyst for electric motor winding failures. Altitude belongs to the broader category of environmental hazards that an electric motor may face. Electric motors are designed to perform within a specific set of conditions. That includes a precise range of temperatures and specific operating conditions. If a motor is exposed to conditions outside of those optimal operating conditions, significant damage can result. High altitude operation is a factor that can cause undue wear and tear on an electric motor. At higher altitudes, motors are subject to pressures and often temperatures that are outside of their recommended operating conditions.

Worn brushes

Worn brushes are another potential cause of electric motor winding failure. Brushes should ride on the commutator smoothly and seamlessly. They should have no sparking or brush noise, which is also called chatter. Worn brush condition can be checked when the motor is stopped. The brushes should move freely in the holder, and there should be equal spring tension on each brush. All brushes should have an even, polished surface. The commutator should appear clean and smooth. It should have a polished and even surface where the brushes hit. Brushes should be returned to their original container after removal.

Wrong brush

Sometimes, using the wrong brush can also cause electric motor winding failure. One common cause is failing to put the correct brushes back in their original holder. Interchanging brushes will ultimately decrease commutation ability. Brushes that are the wrong size or installed incorrectly can cause excessive friction. If the problem is not addressed, the motor will start to perform inadequately. Before wear is apparent on the wrong brush, you may notice sparking on the commutator. This is a good indication that it’s time to check the brushes to ensure the correct size or look for signs of a problem.

Insulation class

Incorrect insulation class also causes electric motor winding failure. Insulation classes are important because they determine a motor’s ability to withstand heat. There are four general classes of insulation, which are A, B, F, and H. Classes B and F are the most common. These classes can withstand temperatures up to 104ºF. Insulation is designed to perform up to the maximum rated winding temperature, which is the combination of allowed temperature and ambient temperature. Motors that are operated at a temperature higher than the recommended insulation class will suffer from reduced performance quality and a shorter service life.

Cracked leads

Cracked leads are a type of winding motor failure caused by excessive stress and strain. Cracked leads can appear where stress raisers (also called stress concentrations) occur. Shaft damage or corrosion can cause stress raisers, which in turn may result in cracked leads. Cracked leads, which are also called fatigue cracks, can occur from repeated stress. Corrosion is another possible cause of cracked lead motor winding failures. Corrosion can be caused by exposure to environmental stresses such as heat and humidity. Ultimately, the leads may fail, and eventually full motor failure may result if the problem is not fixed.


Common Motor Bearing Failures

Excessive belt load

A belt that is too tight in a system can cause considerable stress and strain on the bearings. This stress, which is called excessive belt load, can cause over-amperage of the motor. If the problem is not corrected, complete electric motor failure may eventually result. Excessive belt load can also cause premature bearing failure. Preventative measures can stop excessive belt load motor bearing failure from happening in the first place. This includes performing precise maintenance on belts and other parts of the motor’s power transmission drives. Preventing excessive belt load also increases the motor’s reliability.


Even a small degree of misalignment can have significant negative impacts on the bearings’ operational life. Misalignment problems should be addressed as soon as possible. Otherwise, they cause a decrease in motor efficiency. Eventually, machinery with alignment problems will fail more quickly and easily from increased loads on major components including the seals, couplings, and bearings. Regular inspection and preventative maintenance can prevent alignment problems from happening in the first place or from causing major motor damage. Operating motors within their recommended operational range can also prevent electric motor bearing failures from misalignment and preserve the motor’s longevity.

Worn housing

Worn housing is another cause of electric motor bearing failures. Worn housings are a bearing-related problem that is responsible for up to 50 percent of all electric motor failures. Therefore, it is a problem that should be considered in the event of a malfunctioning electric motor. The housings on electric motors can wear out quickly due to excessive loads, environmental stresses, excessive levels of heat, water, or moisture, and using the motor outside of its recommended operating conditions. The shafts, bearings, and bearing houses should all be evaluated when checking motor bearing failure due to worn housing.

Bent shaft

A bent shaft is a common cause of bearing failures and should be considered when an electric motor’s performance starts to suffer. A shaft often becomes bent when it is subjected to excessive amounts of pressure and stress. Sometimes, exposure to environmental conditions such as heat, cold temperatures, and moisture can cause strain on the motor and cause the shafts to become damaged. A shaft that is worn or damaged in any way should be repaired as soon as possible. Repairs entail applying new metal to the surface of the shaft by applying a metallizing flame spray.

Bad bearing journal

A failure in the bearing journal can wreak havoc on a motor. As with other bearing-related problems, it is among the most common causes of failure in electric motors. Usually, the bearing surfaces and shaft are separated by a layer of oil. This oil keeps the engine’s components adequately lubricated so that they can rotate and perform without a problem. If the bearing journal loses its protective layer of oil from damage, corrosion, overheating, too much stress or overloading, and other problems, the motor will stop functioning properly. Vibrations and running hot indicate a bad bearing journal.


Contamination is a leadinc cause of motor bearing failures in electric motors. Regularly cleaning and maintaining the electric motor is the best way to avoid contamination, which can occur from many sources. Contamination happens when foreign substances work their way into cleaning solutions or bearing lubricants. Dust, dirt, abrasive materials, and small particles of metal, such as steel, can find their way into the electric motor from other machinery, contaminated tools, or if the motor is handled by technicians with dirty hands. Denting in the raceways and rolling elements and vibrations indicate motor bearing failure due to contamination.

Wrong lubrication

Like the oil in a car, electric motors need the right kind of lubricant to function properly. However, problems with the wrong lubrication happen quite frequently. Improper lubrication is responsible for up to 80% of all motor bearing failures. The right lubricant should be carefully selected for the motor based on the motor’s age and operating conditions. Along with using the wrong type of lubricant, subjecting the motor to temperatures and conditions outside the suggested operating range can cause the lubricant to degrade quickly than normal. Discolored rolling elements, excessive bearing wear, and overheating indicate lubrication problems.

Over greased

While electric motors require enough lubricant to work properly, too much lubrication can be equally problematic. Over greased bearing components can impact a motor’s function. Too much grease volume in the bearing cavity causes the rotating bearing elements to churn the excess grease. The grease is then pushed through other parts of the motor. Excessive grease can cause the rolling elements to wear out quickly. Eventually, this can lead to failure in the individual components or total motor failure. Rising internal temperatures, overheating, and energy loss are signs that the bearing components may be over greased.

Under greased

Lubricant is the “life blood” of an electric motor. However, the lubrication in the motor must be applied precisely as directed for the motor to function properly. Having too little lubrication in the motor can cause the components to overheat and degrade, which can cause motor damage or failure. Premature bearing failure due to under greased components is responsible for up to 60% of motor failures. An under greased motor can wear out in just a few years. The shaft, bearing assembly, and motor should be kept aligned, dry, clean, and properly greased to reduce shock load and vibrations.

Bearing fluting

Bearing fluting is a type of motor bearing damage that causes the motor to stop working properly. Fluting damage appears as washboard-like ridges that develop on the bearing raceway. The first indication of a problem is usually loud noise that develops when the bearing balls travel over frosted and pitted surfaces. By the time noise develops, however, the motor has typically sustained extensive amounts of damage. Using insulated bearings, shielded cables, and grounding the motor shaft can prevent damaging fluting. Properly maintaining the motor and regularly inspecting the bearings for signs of damage also prevent bearing fluting.

Bearing insulation failure

Bearing insulation failure is a leading cause of motor bearing failures in electric motors. Sufficient amounts of bearing insulation, and having the right kind of insulation, is important for preventing electrical damage. Insulated bearings can help avoid premature bearing failure that can be caused by stray electrical currents. Insulating just one bearing can stop unwanted electrical currents from traveling through the bearing and ground. Seized bearing and mechanical binding are signs of bearing insulation failure. Hearing rasping or scraping noises and feeling resistance when the shaft rotates are other indications that the electric motor’s bearing insulation may be failing.


Common Insulation components

Inverter rated wire

The inverter rated wire is a common insulation component in electric inverter-duty motors. The inverter rated wire is important for helping to control the motor’s speed. Inverter-duty motors are designed to operate at relatively low speeds. They are also designed to withstand considerable amounts of heat, which in turn prevents them from overheating. The inverter rated wire helps the motor have a wider speed range with constant torque than a standard motor. As with other insulation components, the inverter rated wire is grouped in several classes. The wire’s class determines the maximum temperature it can safely withstand.

Insulation system

The insulation system includes a series of wires in electrical equipment. The insulation system includes electrical components that are divided into classes based on their temperature ratings. It is important to avoid subjecting the insulation system components to temperatures above their recommended operating levels. Otherwise, the motor’s insulation can start to deteriorate. This can cause the surrounding components to wear out more quickly than normal. It also subjects the motor to electrical stresses caused by partial discharge. Motors used at every 10ºC rise over the recommended rating will have their expected lifetime significantly reduced, sometimes up to 50%.

NanoMax wire

NanoMax wire is a standard insulating wire on most electric motors. NanoMax wire is a type of inverter duty wire. It is a magnetic wire normally made of copper. The advantage of a NanoMax wire is that it is exceptionally durable and can withstand high levels of heat that the electric motor may be exposed to. The NanoMax won’t break down easily with repeated exposure to high levels of heat, which in turn protects other parts in the motor from premature damage and wear and tear as well. NanoMax wires are valued for their corona resistant properties.

Class N vs. Class F

Insulation components are grouped into several classes according to NEMA classifications. They are primarily distinguishable by maximum temperature ratings. Two common insulation classes are Class N and Class F. Class N has a maximum temperature rating of 392ºF. Class F insulation has a maximum temperature rating of 311ºF. NEMA performance ratings assume an ambient temperature of 40ºC. However, the ambient temperature used in an actual application may be slightly lower or higher. The actual rating affects the degree of temperature rise that an insulation system may experience without exceeding the tolerance limitations of its prescribed class.

Vacuum Pressure Impregnation (VPI)

Vacuum Pressure Impregnation, or “VPI,” is a process where porous materials are sealed together using resin or varnish. They are connected using pressure and a vacuum. The VPI process is designed primarily for use in high-voltage generators and motors. It is used to create void-free insulation in a variety of different devices. Vacuum Pressure Impregnation follows a multi-step process. The process begins with preheating, followed by dry vacuuming, filling, wet vacuuming, pressure, draining, and curing. This process prevents varnishes from losing their volume. If the process is performed correctly, VPI can prevent volume loss by 50 percent.


Common DC Motor Failures

DC-Motor inspection

A DC motor inspection is critical to ensure the motor’s proper performance and longevity. The motor inspection can act as a preventive and corrective maintenance tool that evaluates the testing, replacement, cleaning, and lubrication tasks associated with a motor’s health. The inspection process begins by evaluating a motor’s history of service. This can help identify any problems with the motor or its components. The service history will also detail any problematic external conditions that negatively affect the motor. A thorough visual inspection of the DC motor helps identify current problems as well including broken leads, burnt windings, and failed components.

Brush Grade

Brush grade is important for a DC motor’s proper performance and maximum lifespan. The brush grade refers to the electrical and mechanical characteristics of brushes in the motor. The brushes are attached to the shaft and the stator magnets. They have certain characteristics such as hardness, current density, and maximum pressure that play a role in the motor’s overall performance. As with other components, brush grade can be negatively affected when a motor is used outside of its normal operating conditions. In the case of brush grade, usage outside the normal range can accelerate brush wear.


Arching is a type of electrical failure that causes the bearings to wear out. Arching, along with other electrical problems, commonly results from a motor’s contamination. Therefore, operators should make sure to thoroughly clean the motor at least once each year to get rid of debris and contaminants. Arching occurs from loose carbon dust distributed from worn brushes. Some dust may accumulate on the commutator, but the dust can also travel into the motor. When this happens, grounding is more likely to occur. The insulation can eventually break down and fail as well, causing total motor failure.

Burning bars

Burning bars happen primarily from commutator contamination. Contamination is caused by chemicals, dust, and brush wear. Preventative maintenance is key to finding and stopping burning bars before they cause major problems. Changing brushes that are worn can stop burning bars, as can periodically inspecting in the condition of winding insulation. Certain factors may be present as well that indicate burning on slot bars along with unusual markings. This includes incorrect neutral setting, improper interpole strength, contamination, and electrical overload. If the trailing edges on the dark bars are burned or etched, the commutator should be resurfaced.

Brush bounce

Brush bounce is an indication of an electric motor brush failure. The brushes bounce when soft blocks composed of carbon, which are essential for completing electrical contact with the commutator, wear out. The blocks wearing out ultimately causes the brushes to wear out over time. The motor can spark when this happens, which interrupts the electrical contact. Intermittent electrical failures and shortages can result from brush bounces as well. The commutator may also become damaged by the uneven force from the worn brushes. Major motor damage can also result if brush bounce is not corrected.

Flash over

Flash over is a type of failure in DC motor bearings. Flash over occurs with damage to the brush holders and commutators. If the underlying problem is not addressed, flash over damage can affect multiple components within a motor, including the brush holders, commutator, and commutator banding. A flash over essentially occurs with contamination that reduces the dielectric strength of air in the motor. Contamination can occur from a number of factors within an operating DC motor, including carbon dust, heat, and humidity. Proper maintenance and adhering to the correct compounding recommendations can prevent triggers for a flash over.

Setting neutral

Setting neutral on a DC motor is one way to extend the motor’s brush life and in turn prolong the motor’s lifespan. Motor longevity is largely affected by brush film. Maintaining brush film relies on a number of factors, including setting neutral. Setting neutral is important to prevent sparking. It also regulates motor speed and loading. Without a neutral brush setting, erosion can impact the motor and prevent it from functioning properly. Erosion can be caused by a wear condition like threading or an improper commutator film. Setting neutral involves isolating the brushes and the commutator by installing insulation paper.

Environment and contamination

Environment conditions and contamination are a main cause of motor failure. Environmental conditions refer to a number of factors, including the ambient temperature at which the motor operates. Every motor has its own distinct recommended operating range. The operating range is determined by the viscosity of the bearings’ lubricant and the motor’s ability to withstand heat. If the ambient temperature is outside of the motor’s operating environment, contamination can result. Contamination happens when the motor is exposed to temperatures that are lower or higher than the ambient temperature. The lubricant, for instance, may freeze at colder temperatures.

Brush springs

Brush springs help maintain the correct current distribution in a DC motor. The brush springs must all have equal amounts of pressure to evenly distribute current. When the springs become worn, however, the current distribution can become erratic and uneven. To check for proper spring pressure in the brush springs, operators should check the spring tension, which can be done using a force gage. Spring pressure can also be maintained by placing brushes in individual cartridges with a torsion spring. This limits the brushes’ ability to travel and maintains the right amount of force between the commutator and brush.