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Electric Motor Failure

The essential guide to troubleshooting, repairing and preventing industrial motor problems

Table of contents

    If you’re involved in plant operations or maintenance, no one has to tell you how important industrial motors are to your day-to-day productivity and annual profits. Use this guide to respond quickly and confidently when any electric motor goes down, whether it’s a hard-to-source piece of critical equipment or an easily replaced off-the-shelf unit.

    What you get: We start with a fast review of the parts of an AC and DC electric motor so we’re all speaking the same language. Then we identify the 5 common causes of industrial motor failure. Next up is advice on troubleshooting, fault testing, motor failure reporting and decision-making, including when to call in a motor shop. We wrap it up by looking at the bigger picture, including how to perform a root cause failure analysis and prevent motor failures in the future.

    Parts of an industrial electric motor

    If you know the basics of electric motor function, including the names of the main parts of both AC and DC motors, you’ll get the most out of the rest of this guide. Already an expert? Skip to the next section on common causes of electric motor failure.


    The stator is the stationary part of the motor. It is made of thin metal sheets, called laminations, instead of being solid, to reduce energy losses. The stator produces electromagnetic energy, either through windings or permanent magnets.


    Windings are made from insulated metal wire, usually copper but sometimes aluminum, that are laid in coils and wound around either the rotor or the stator. When the winding is energized, it forms magnetic poles.


    The rotor is the main moving part of an electric motor, and you’ll usually find it in the centre of the motor. Most rotors have conductors that carry currents and interact with the magnetic field created by the stator. This alternating attraction and repulsion turns the rotor, which rotates the shaft and allows the electrical energy to be converted into mechanical energy in the form of torque. The rotor is sometimes called an armature.


    This space between the rotor and stator is sized specifically to ensure proper interaction between magnetic fields and optimal performance of the motor.


    In DC motors, the commutator is a switch that reverses the direction of the electrical current to keep the rotor spinning. A brush assembly is often used to shift the current.


    Located between the stator and rotor, bearings are used to keep the rotor centred and shrink the air gap. Ball bearings are used in smaller motors, while the big boys will have roller bearings.



    Common causes of electric motor failure

    Old stuff breaks down. Right?

    Not so fast. It’s easy to blame motor failure on age, but the reality is that the unstoppable creep of time is responsible for fewer than 20% of motor failures. In fact, Cooper Bussmann says old age causes only 10% of electric motor failures.

    A. Bonnett and C. Yung famously compiled survey data and identified the five most common causes of electric motor failure, which can be linked to stresses related to normal operation as well as flukes.

    Bearings – 51%

    Bearings are the culprit in more than half of motor failure cases. Improper lubrication, inappropriate mechanical loads, heat, contamination and shaft currents are all the usual suspects when it comes to bearing problems. Get the bearing failure checklist.

    Windings – 16%

    A winding failure—which is often caused by a breakdown of the winding’s insulation—will cause a short in the motor. Common causes of winding failure include high temperature, too many starts, contamination, too much current, excessive voltage and physical damage. We've put together a post on winding failure, with awesome photos courtesy of EASA, if you're looking for examples.

    External causes -16%

    Tied with windings for almost 1 in 5 motor failures, external factors include temperature, contamination, poor maintenance, inappropriate mechanical loads and events such as flooding.

    Unknown – 10%

    1 in 10 failures are mysterious. Either no one bothered to determine why the failure occurred, so Bonnett and Yung didn’t have any data to work with, or the investigation failed to determine a cause.

    Rotor – 5%

    Starts (too many, not enough of a break between them or extended start times) are the culprit in some rotor failures. Other failures are caused by vibration, physical damage and situations that causes excessive heat.

    Shaft or coupling – 2%

    Shaft failure is the least common cause of motor failure. Physical damage, corrosion, improper installation and excessive loads are often the root cause.

    Repair or replace?

    Find out in less than 2 minutes. EASA-validated.

    Troubleshooting motor failure – signs to look for

    It’s one thing to identify the component that’s failed and another thing to isolate the cause of that component failure. Yet determining the cause is a critical step in protecting your motors. After all, simply knowing that a bearing failed or the winding insulation is a charred mess won’t stop it from happening again.

    Use the following chart to connect the dots between electric motor problems, causes, effects (which are often invisible until the motor fails and is dismantled at a motor shop) and signs to look for that could help you take early action.

    Transient voltage (“surges” or “spikes”) Power factor correction capacitor banks

    Adjacent loads turning on and off

    Electrical storms

    Power quality issues
    Degrades windings


    “Micro-jogging” from motor timing interruptions
    There may be no physical damage to motor

    Excess heat Vibration
    Voltage imbalance Power quality issues

    Faulty circuits

    High resistance connections
    Stress on each phase circuit

    Insulation breakdown

    Core losses
    Excessive heat



    Increased operating costs
    Current imbalance Problems with leads, fuses, connections

    Caused by either power supply or motor itself
    Core losses

    Torque pulsations
    Excess heat

    Vibration Missing balance weights

    Uneven mass in motor windings

    Unbalanced magnetic field

    Uneven mounting (“soft foot”)

    Shaft looseness

    Shaft imbalance


    Insufficient load
    Bearing problems Insulation breakdown

    Broken shaft

    Broken feet

    Rotor damage
    Tips for motor vibration diagnosis
    High operating temperature Inadequate motor cooling

    Wrong voltage supply

    Buildup of dirt or debris in motor fins

    Poor power quality

    Voltage imbalance

    Improper connections

    Wrong motor for application
    Reduces effectiveness of lubricants

    Degrades windings
    Motor is hot to touch

    Hot spots in motor windings
    Harmonic distortion Faulty power circuit

    Power quality issues
    Reduces motor efficiency

    Insulation deterioration



    Reduced torque

    Bearing currents
    Motor overload Excessive current draw

    Insufficient torque
    Damaged windings

    Broken shaft

    Damaged bearings

    Reduced torque
    Misalignment Ineffective flexible coupling

    Misalignment between motor drive shaft and load

    Pipe strain

    Uneven air gap

    Bent or bowed shaft

    Out-of-round rotor
    Bearing problems

    Excessive motor wear

    Damage to shaft
    Vibration of shaft

    Vibration of load

    High housing temperature close to bearings

    High oil discharge temperature

    Unusual oil leakage at bearing seals
    Moisture Storing motor in a location without adequate climate control

    Not using a totally enclosed fan cooled motor in a damp environment

    Not positioning weep holes so water can drain from motor

    Not keeping the temperature of the motor warmer than ambient air temperature
    Corrosion of motor shaft, bearings and rotor

    Insulation failure

    Bearing current/shaft current Leakage current from the armature windings (DC motors)

    Non-symmetrical magnetic fields

    Induced voltage from a VFD
    Bearing destruction Noise


    Frosting of polished metal surfaces

    Premature grease darkening

    Repair or replace?

    Find out in less than 2 minutes. EASA-validated.

    Testing for motor faults

    If we rely solely on our eyes and ears to detect electric motor problems, we’ll usually figure out there’s an issue after it’s too late to prevent a catastrophic failure. Thankfully, computerized equipment and tools can detect issues at levels and in places that human senses aren’t able to.

    That doesn’t mean humans are off the hook, however. The machines provide the data. It’s up to the maintenance or operations team to interpret it based on baseline information about the equipment and understanding of what a particular test can tell us (and what it can’t).

    Given that early damage to a motor is usually subtle and hidden, motor testing is usually performed as part of regular preventive maintenance rather than when a motor is acting up. That way, action can be taken before it’s too late.

    Types of motor tests

    Energized testing takes place while the equipment is under a simulated load to reveal problems that occur during operation. With the motor running, you can perform “dynamic testing” to identify issues with temperature, balance and distortion.

    De-energized testing runs a motor through its paces while it’s turned off. This offline testing, often called “static testing,” looks at insulation resistance, wire damage and current leakage.

    What does a typical motor test measure?

    • Vibration of shaft and housing
    • Temperature of components
    • Torque 
    • Alignment
    • Condition of windings
    • Position and speed of components
    • Generation of current and voltage
    • Coast-down time

    Common motor testing tools

    • Digital multimeter – an all-in-one tool for measuring volts, ohms and amps
    • Megohmmeter (aka “megger”) – an ohmmeter used to assess the insulation on wires and windings
    • Clamp-on ammeter – measures the current in a circuit without having to de-energize the system
    • Spot thermometer – like a radar gun for taking a motor’s temperature
    • Power quality analyzer – takes the digital multimeter to the next level (including price tag) by adding the ability to assess power quality

    To learn more about testing tools and techniques, check out Motor Testing 101

    Repair or replace?

    Find out in less than 2 minutes. EASA-validated.

    Motor failure reporting

    When a motor goes down, someone has to be told. 

    The way the information is communicated plays an important role in the way the situation is handled and decisions that are made, both for the motor in question and the entire fleet. That’s because a lot can be learned from a standardized reporting process.

    We can see your eyes rolling. Who’s got time to write a motor failure report? 

    We get it. Everyone is busy. But if you’re honest, how much of your time is spent firefighting instead of focusing on your to-do list? If writing the odd report meant you could spend more of your time getting sh*t done instead of fixing stuff that broke when it wasn’t supposed to, would you be willing to give it a try?

    Here’s how a motor failure report can help with motor management.

    1. Better understanding of causes. The knee-jerk conclusion about the cause of the motor failure is often very different from the conclusion a tech will reach after writing a formalized report. The structured process of report writing gets you to think more and differently about what happened. In this case, different is better.
    2. Better communication with the motor shop. Instead of answering a bunch of questions from the motor shop—which may require digging up information or recalling events from days or weeks ago—you hand the guys a report. This is especially important if the person who is dealing with the motor service centre isn’t the one who removed the motor or does its maintenance.
    3. Better justification of recommendations. Chances are, you aren’t the final decision maker on larger operational and capital expenditures. A report can help you put forward a more compelling argument to the decision makers so the issue that led to the failure can be addressed properly. This report may come from your motor shop. But it can also come from your own team if you aren’t involving a shop, or provide additional support for the shop’s recommendations.

    How to write a motor failure report

    Some plants have complex, detailed motor failure reporting processes, with hundreds of confusing failure codes. While these may appear to be rigorous, they may not achieve the true benefits of a less specific failure report, which is to structure a thinking process.

    Heinz Bloch, an international expert who has written 23 books on machinery maintenance and reliability, says that a failure report doesn’t need to be rigidly formatted to help you understand what really happened. Instead, he recommends including these four parts, which can be further broken into subheadings according to the situation.

    Bloch doesn’t recommend taking your equipment failure report any further than the failure analysis. You’ll notice there are no “solutions” or “implementation” sections. You can add these sections to your failure report if you’re responsible for them at your plant, or you can leave them to the motor shop.


    Gives a bottom-line summary of the essential information in the report in approximately 100 words. It should concisely answer what, where, when, how and possibly why.


    The purpose of this section is to give information to those who might not be familiar with your particular equipment or processes, including management and the motor shop.


    This section is where you give the evidence for the cause of the failure. Include data and observations. Organize the content under subheadings that make sense.


    This is the logical outcome, given what you’ve written in the rest of the report. Re-state what you felt caused the failure. Identify causes you eliminated and why.

    Repair or replace?

    Find out in less than 2 minutes. EASA-validated.

    Repair or replace?
    That is the question.

    When an electric motor fails, there’s one question that must be answered.

    Should we repair the motor or replace it?

    Sounds simple. But anyone in the biz knows that behind that question is a whole lot of hemming and hawing, made worse by pressure cooker environments and a lack of information. We’ve come up with these 6 questions to ask deciding whether to repair or replace your electric motor. The questions are based on these four criteria.

    Repair or replace criteria

    Motor design. Design is critical to the repair-replace decision. How efficient is the motor that failed? Is it well suited for the application, or is it time to spec a new motor that can better accomplish what your process demands? How unique is it? Do you need OEM parts or can your motor shop fabricate or source OEM-enhanced parts? If it’s an older motor, can you find a new motor that will fit in the old spot, given that new motors tend to be smaller than an equivalent motor made 25-plus years ago?

    Extent of failure. If the failure was catastrophic, repair may not be a good idea. Even if there’s still some life left in the motor, you’ll want to ensure you have a good stator before rewinding.

    Cost. This is another biggie. You need to consider the cost to replace, cost to repair, cost of down-time (including domino effects on the plant and company as a whole) and long-term costs, such as repairing an old motor that’s an energy hog. Know the breakpoints at which you’re willing to repair vs. replace. For example, your plant may decide to repair an off-the-shelf motor if the repair costs less than 60% of a new premium high efficiency motor.

    Timing. How long will it take to get the motor repaired? What’s the lead time on a new motor? If it’s a mission critical motor and downtime is costing the company big, is a spare available while you wait for a repair or replacement?

    Motor failure policy

    Having a motor failure policy will help your team make proactive decisions, including procuring services and equipment ahead of time. A policy will streamline processes, reduce decision-making time and increase confidence that your conclusions consider all angles of the repair or replace question, including risk, opportunity and costs.

    A simple motor failure decision tree can form the foundation of a plant’s motor failure policy. Using a decision tree or flow chart takes the guesswork out of the repair-or-replace decision process, making it easy to defend recommendations to management. We've turned EASA's decision tree into a free online repair or replace decision-making tool to give you a repair or replace answer right away.

    How to choose a motor repair shop

    Some plants prefer to perform their own motor repairs. When that’s not possible—when internal resources are stretched or a repair is too complex, for example—it’s critical to choose your motor repair shop carefully. 

    But don’t take our word for it.

    Instead, rely on the U.S. Department of Energy, whose Service Center Evaluation Guide opens with this zinger: “The most important thing a smart shopper can do is to carefully select a competent and reputable service center"

    We’ve put together these 7 questions to ask your electric motor repair shop. If they don’t pass the test, keep shopping.

    Who will repair the motor?

    Will the shop do the work themselves, or will they outsource it? How long have the staff been there? How many years of experience do they have?

    Does the shop have experience with your motor?

    Consider the size, age, type and application of your motor. Does the shop have a significant amount of experience with your motor? If not, you might not get the most qualified team or best price.

    What’s the shop’s inventory of parts and materials for repairs?

    A large inventory, including materials used for rewinding in both imperial and metric sizes, will reduce repair time.

    What testing does the shop perform?

    Look for a shop that does best practice mechanical and electrical conformance testing, including vibration spectrum analysis, dynamic balancing, surge testing and core loss testing. (Here are 7 ways to screw up vibration analysis.)

    What standards does the shop meet?

    Are the techs CSA-certified? Do they follow the ANSI/EASA AR100 best practices? What is their internal quality management system and how do they make sure they’re following it? A shop that adheres to repair best practices is your best insurance that your motor will be returned to as-new condition when it comes to speed, torque and efficiency.

    How does the shop maintain records?

    Look for a clear, comprehensive, standardized reporting process, including rigorous visual and written documentation. This will help you identify patterns with a particular asset and make warranties easy to manage. Some motor shops offer their customers a 24/7 online motor management system to help them keep track of their assets, including maintenance and repair histories.

    What does the shop look like?

    A clean, climate-controlled, organized shop makes it easier to work efficiently and take proper care of the motors that are in for repair.

    Repair or replace?

    Find out in less than 2 minutes. EASA-validated.

    Motor failure root cause analysis

    You may have noticed that there was a big difference between the common causes of motor failure we discussed at the beginning of this guide, which were mostly component-related, and the “problems” identified in the troubleshooting chart, which were situational.

    Let’s take bearing failure, for example. It’s responsible for 51% of motor failures. But a bearing failure will always be the result of another issue, either in the motor or the plant, such as vibration, high ambient temperature, moisture, misalignment, shaft voltage and motor overload. And those issues will have another cause. And so on.

    That’s why it’s so important to get to the “root cause” of a failure. Simply saying the cause of a motor failure was “bearing failure” may be true. But it won’t help you prevent it in the future. The key is to dive deeper into understanding why the component failed. That way you can save your bearing next time, instead of simply having it fail again.

    Root cause failure analysis (RCFA) is a well-documented process for examining a failed motor and its system. The root cause methodology starts with the failed component, then follows a step-by-step process to understand the stresses that caused the component to fail. That way your chosen solution doesn’t just treat the symptom. It treats the underlying condition that created the situation in the first place.

    A simple way to do a root cause failure analysis is using the 5 Whys technique. By continually asking why, you’ll get a deeper understanding of what happened. (If you’ve had small children in your life, you’ll also have flashbacks.) Some motor failures will require asking fewer than five whys. Others will need more than five.

    Here’s an example of the 5 Whys in action.

    Motor Failure


    Bearing failed


    Pitting and fluting of bearing race


    Bearing current


    Inadequate grounding in VFD application


    Shaft grounding not done during installation


    Installer wasn't aware of bearing current as a potential problem.

    6 steps of a root cause failure analysis

    • 1 Pick your team.
      Who knows about the motor? Who likes solving a good mystery? Get those folks together to figure out what happened and what to do about it.
    • 2 Collect data.
      Do this as soon as possible. Data should come from before the failure (e.g. maintenance history, information about the application), during the failure (readings if available) and after the failure (appearance. Document actions taken at the time of the failure.
    • 3 Make an assessment.
      Use the data to work backwards to the root cause—the real reason the proverbial poop hit the fan. The 5 Whys work here, but there are other assessment methods that are maybe more rigorous.
    • 4 Do something about it.
      Make sure to put each of your ideas for corrective action under the microscope. Will it prevent recurrence? Will it introduce new risks? Is it actually do-able? What are the downstream impacts of taking the action? Note that deciding not to do something is still doing something. If the team decides not to take action, make sure the data and assessment support this. If there are big potential costs or other impacts, get buy-in from the company before moving ahead.
    • 5 Tell people about it.
      Communicate the outcome of your root cause failure analysis and any action taken to higher ups, employees, suppliers and contractors.
    • 6 Circle back.
      Did the process make a difference? Is the action the team took having the effect you expected?

    A root cause failure analysis can be relatively quick and informal. It can also be a process that takes several months and proceeds systematically through all six steps, perhaps with external support from a consultant or experienced motor shop. Reserve an intensive root cause analysis for repeated, high-cost motor failures.

    Repair or replace?

    Find out in less than 2 minutes. EASA-validated.

    Motor failure prevention

    We’ve spent most of this guide talking about what to do when a motor fails. But wouldn’t it be better to avoid failure in the first place? Here’s how to set your motors up for success, not failure.

    Ongoing motor monitoring

    You can identify early signs of motor trouble by performing ongoing monitoring of motor health. Monitoring has two components: inspection and testing.

    Things to look for during an inspection of your motor include:

    • Signs of corrosion, dirt or debris on components, including fins, windings, contacts and relay
    • A burning smell that may indicate overheating
    • Signs of wear on the commutator and brush assembly
    • Anything unusual with the motor mount, stator, rotor and belts
    • Unusual noise or vibration
    • Dirt buildup, inadequate lubrication and signs of wear on bearings
    • A bearing housing that’s hot to the touch
    • Oil and grease coming from bearings
    • Oil level in bearings
    • Loose connections, including in the starter switch and fuses\

    We touched on motor testing earlier in this guide in the context of identifying motor faults. 

    Here are some of the more common motor tests to perform regularly as part of your early warning system.

    Multi-purpose tests

    Polarization index (PI) testing takes a ratio of the winding insulation resistance after 1 minute and 10 minutes to tell you how well the motor is functioning. It can also detect the presence of dirt or moisture. Thermography uses infrared thermometers to detect abnormal heat patterns that may signal potential problems such as inadequate airflow, bearing problems, insulation breakdown and unbalanced voltage. 

    Insulation tests

    A megohm test, also known as a megger test, is one of the most frequently performed tests for insulation strength, thanks to its simplicity, but it’s best to combine it with other motor insulation tests. Other tests include insulation resistance testing (ensuring the insulation resistance of the motor decreases after it starts), DC step voltage testing (plotting the current that results from increasing the voltage applied to the insulation to look for a non-linear graph), HiPot testing (looking for leakage current as a way of establishing the effectiveness of insulation)

    Winding resistance tests

    You can identify dead shorts, loose connections and open circuits in coiled windings using a digital multimeter. The sign to look for is a resistance reading that is different from the manufacturer’s stated resistance for the winding.

    Bearing tests

    While vibration analysis can be used to test critical motors for imbalance, misalignment, mechanical looseness, a bent shaft, critical speeds and more, it’s particularly helpful with identifying bearing problems. But don’t screw it up. Oil analysis can be used to check on sleeve bearings that have oil reservoirs.

    Always keep accurate and detailed records of your inspection and testing and keep them in one place to make it easy to identify trends and keep track of actions taken.

    Motor maintenance

    There’s a big difference between reactive maintenance and preventive maintenance (PM). When a plant has adopted a preventive maintenance mindset, engineers and technicians are supported to monitor motors and troubleshoot problems before they cause a failure. PM has been shown to reduce downtime by as much as 50%.

    Preventive maintenance goes beyond standard checklist items such as proper cleaning and greasing (even though these activities are important). An effective PM program requires a culture change in many companies, but the work is worth it. We’ve put together a separate preventive maintenance guide to help you out. Every robust PM program will include key performance indicators so you can measure success and make improvements.

    Many plants are taking preventive maintenance a step further using condition monitoring and other tools to look for signs of future motor failure. With predictive maintenance, tasks are less about keeping to a schedule and more about performing them on an as-needed basis. Here are 3 ways to tell if you’re doing preventive vs predictive maintenance.

    Total motor management

    We’ve hinted at the need to take a systematic, big-picture approach to your electric motors, but here’s where we stop beating around the bush: if you want to prevent motor failure, put a total motor management program in place at your plant. Yes, it’ll require some upfront work. But the down-the-road benefits will pay off big.

    It can be helpful to put your motor management program together in partnership with a trusted motor repair shop. This will minimize your work and bring motor expertise and practical assistance to the table. Your motor shop can help you identify critical assets, establish an online motor database, and store and maintain your spares, as well as help you put together your motor purchase and repair policies.


    Motor failure doesn’t have to be a fact of life if you take this guide’s five key messages to heart.

    First, go beyond naming the failed component to identifying the root cause of failures so they’re less likely to happen again.

    Second, formalize your documentation and reporting so it’s easy to get all the information about a particular motor to the people that need it.

    Third, make your motor repair and replace decisions using an industry-accepted decision tree or online decision tool to standardize and speed up your process.

    Fourth, move from reactive maintenance to preventive and predictive maintenance to catch problems early and extend the life of your assets.

    And fifth, use your motor shop to help you implement best practices for motor replacement, repair, storage and maintenance at your plant.

    Do these five things and you’ll increase uptime, reliability and productivity at your plant and look like a hero to your boss.

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