Three phase induction motors run more industrial equipment than any other single class of machinery. They are reliable, efficient, and well understood, but they do fail, and when they fail they take production with them. The cost of an unplanned motor failure on a critical drive routinely runs from low five figures for a small process pump up to six figures or more once you account for downtime, secondary damage to driven equipment, expedited replacement, and overtime labor. The cost of catching that same failure six weeks earlier, when the first symptoms appeared, is a small fraction of that.

This guide is built on more than seventy five years of motor service experience at Malloy Electric and conforms to the practices defined in the EASA AR100 standard for the repair of rotating electrical apparatus. It walks through the symptoms field technicians and maintenance teams actually encounter on motors in the field, the root causes those symptoms point toward, and the diagnostic steps that turn a guess into a confirmed finding.

The scope is constrained to low voltage motors (600 volts and below, occasionally up to 1000 volts) in NEMA standard frames (typically 42 through 449 frames) and IEC standard frames (typically 56 through 355). This covers the vast majority of industrial motors in service: fractional horsepower through approximately 500 horsepower, AC induction (squirrel cage and wound rotor), totally enclosed fan cooled (TEFC), open drip proof (ODP), washdown duty, severe duty, explosion proof, and inverter duty configurations. Medium voltage motors and above-NEMA frame machines have additional considerations not addressed here and warrant a separate guide.

Why Electric Motors Fail

Electric motors fail along a small set of well documented pathways. Industry studies, including the long running EPRI and IEEE motor reliability surveys, consistently identify the same failure distribution. Bearings account for roughly 40 to 50 percent of motor failures. Stator winding failures account for another 30 to 40 percent. Rotor failures, shaft and coupling issues, and external causes make up the remainder.

Beneath those statistics is a smaller list of underlying root causes. Bearings fail because of contamination, lubrication problems, electrical erosion from variable frequency drive applications, mechanical overload from misalignment or unbalance, or simply because they reached the end of their fatigue life. Stator windings fail because of thermal overload, voltage stress, contamination, vibration induced abrasion, or moisture ingress. Rotors fail because of broken rotor bars in induction machines, end ring failures, or rotor core problems.

Most motor failures announce themselves before they become catastrophic. The signals are quiet: a small temperature rise on one bearing housing, a slight increase in vibration at 1x running speed, a current imbalance that creeps from 2 percent to 5 percent, an insulation resistance reading that drops over a year from 500 megohms to 50 megohms. Catching those signals requires two things: a baseline to compare against, and a structured way to interpret what you observe.

This guide covers the structured interpretation. The five symptom categories that cover virtually every low voltage motor failure are starting problems, overheating, excessive vibration, unusual noise, and electrical anomalies. Each is addressed below.

Symptom 1: Motor Starting Problems

A motor that fails to start correctly is rarely the motor itself. The starting circuit, the power supply, the protective devices, and the driven load all participate in the start sequence, and the failure mode determines where to look first.

Motor Will Not Start at All

The motor remains dead when energized. No movement, no hum, no current draw.

The likely causes are upstream of the motor: open disconnect, blown fuses, tripped breaker, failed contactor, broken control circuit, open thermal overload, or open conductor between the starter and the motor. The first action is to verify voltage at the motor terminals with a multimeter. If voltage is present at all three terminals at nameplate value, the problem is the motor itself (open winding, broken connection inside the terminal box, locked rotor). If voltage is absent or unbalanced, the problem is upstream.

A motor that reads continuity across its windings but will not draw current when energized usually has a broken connection at the terminal block or at the lead connections inside the conduit box.

Motor Hums but Will Not Rotate

The motor draws current and produces a magnetic field, but the rotor does not turn.

Most likely causes: single phasing (one phase open with the other two still energized), seized bearings, jammed driven equipment, severe overload on starting, or a faulty starting capacitor on single phase motors. The diagnostic move is to measure current on all three phases simultaneously. If only two phases show current, single phasing is confirmed. If all three phases show current at locked rotor amperage (typically six to eight times nameplate full load amps), the motor is trying but cannot break loose mechanically; check the driven equipment first, then the motor bearings.

Single phasing on a three phase motor is destructive. Within seconds the two energized windings carry the full load current of three, and winding temperature rises rapidly. Most modern overload relays detect single phasing, but older installations or motors fed through worn contactors may not be protected.

Motor Starts but Trips Immediately

The motor begins to rotate but the protective device opens within seconds.

Likely causes: overload from a stuck or jammed load, short circuit in the windings, ground fault, severe voltage imbalance, motor not properly sized for the starting duty, or incorrectly set overload relay. Verify the overload relay setting matches the motor nameplate. Confirm the load is free to rotate by hand. Measure insulation resistance from each phase to ground after lockout to rule out a winding fault.

Motor Starts Slowly or Will Not Reach Full Speed

The motor begins to rotate but accelerates slowly, runs at reduced speed under load, or fails to reach synchronous speed.

Likely causes: low voltage at the motor terminals, single phasing under load (running on two phases after one phase opens during start), broken rotor bars in the squirrel cage, severe overload, or wrong voltage tap connection on dual voltage motors. Measure the actual voltage at the motor terminals under load. Compare phase to phase to detect imbalance. NEMA MG1 specifies that voltage imbalance should not exceed 1 percent for continuous duty operation, because each 1 percent of voltage imbalance produces approximately 6 to 10 percent current imbalance and significantly elevated winding temperature.

Motor Reverses Direction on Start

A three phase motor starts in the wrong direction.

This is virtually always a phase rotation issue. Two phases have been swapped at the motor leads, at the starter, or at the disconnect. The fix is to swap any two leads at the motor terminal box. Single phase motors with reverse rotation are a different issue, usually a wrong connection at the start winding.

Symptom 2: Motor Overheating

Motor temperature is the most underused diagnostic instrument in most plants. Stator winding temperature, bearing temperature, and frame surface temperature each tell a different story, and a temperature that drifts over weeks or months is one of the earliest and most reliable warnings a motor can give.

Common Heat Related Symptoms and Responses

• Frame temperature significantly above baseline: overload, blocked ventilation, ambient temperature too high, blocked or fouled fan, internal winding problem, or under voltage causing high current.

• One bearing housing significantly hotter than the other: bearing distress, lubrication problem, misalignment loading one bearing, or coupling driving thrust into one end.

• Stator temperature alarm or trip: overload, single phasing, voltage imbalance, high ambient, blocked cooling, or insulation degradation.

• Heat concentrated at the conduit box: loose or oxidized terminal connection, undersized lead conductors, or arcing connection.

How Hot Is Too Hot

NEMA MG1 defines temperature rise classes based on the insulation system. Class B insulation is rated for an 80 C rise over a 40 C ambient (total 120 C). Class F insulation is rated for a 105 C rise (total 145 C). Class H insulation is rated for a 125 C rise (total 165 C). Most modern industrial motors use Class F insulation but operate at Class B rise to extend insulation life. The general rule is that every 10 C of sustained temperature rise above the design point cuts insulation life roughly in half.

Frame surface temperature is not the same as winding temperature. The winding runs significantly hotter than the frame. A frame at 90 C (too hot to touch comfortably) corresponds to a winding well above 100 C. Use embedded RTDs or thermistors for winding temperature, not the external frame.

Diagnostic First Steps

When a motor runs hot, work the obvious causes first. Verify load current against nameplate. Check the cooling path: fan condition, air inlet and outlet clearance, frame surface clean of dust and debris that insulates the heat dissipation surface. Measure phase voltage and current balance. Check ambient temperature. On TEFC motors in dusty environments, a 1 inch dust coating on the fins of a 100 horsepower motor can raise winding temperature by 20 C or more.

If the obvious causes check out, the problem is internal: bearing distress increasing friction, winding insulation degradation, or rotor bar problems on induction motors.

Symptom 3: Motor Vibration

Motor vibration is the most measurable mechanical symptom and the most diagnostic. ISO 10816 and the newer ISO 20816 series define acceptance bands for industrial machinery. The most reliable practical reference is the baseline taken when the motor was new or freshly rebuilt.

What the Vibration Spectrum Tells You

Overall vibration level identifies that something is wrong. Spectral analysis identifies what. The dominant frequencies and their relationship to running speed point directly at the source.

• High 1x running speed: rotor unbalance, bent shaft, broken or eccentric rotor, or coupling unbalance.

• High 2x running speed: angular misalignment, mechanical looseness, soft foot, or cracked shaft.

• High at line frequency (60 Hz in North America) or twice line frequency (120 Hz): electrical issue including stator winding fault, rotor bar problems, eccentric air gap, or unbalanced phase voltage.

• Bearing defect frequencies present: outer race, inner race, ball, or cage fault on a specific bearing.

• Broadband high frequency energy above 1 kHz: lubrication problems, advanced bearing wear, or VFD related electrical erosion of bearings.

Mechanical vs Electrical Vibration

The simplest field test to separate mechanical from electrical vibration: cut power to the motor and observe whether the vibration disappears instantly or coasts down with the motor. Mechanical vibration coasts down. Electrical vibration disappears the instant the field collapses. A motor vibrating at 120 Hz that disappears at power removal is almost always an electrical problem in the stator or rotor, not a mechanical problem.

ISO 10816 Reference Bands

For Class III machinery (motors above 75 kW on rigid foundations), the zone boundaries fall at approximately 1.8 mm/s RMS (acceptable for new commissioned equipment to acceptable for long term operation), 4.5 mm/s (acceptable to unsatisfactory), and 11.2 mm/s (unsatisfactory to severe). For smaller motors on flexible mounts (Class II machinery), the bands shift accordingly.

These numbers are starting points. A specific motor running at 3.0 mm/s might be perfectly healthy if that has been its baseline for years. A different motor at 1.5 mm/s might be in early distress if its baseline was 0.5 mm/s. Trend matters more than absolute value.

Symptom 4: Motor Noise

A healthy electric motor produces a consistent, low level hum. Any change in character is a signal, and the character of the noise often identifies the source.

Common Motor Noises and What They Mean

• Steady hum, normal in pitch: normal operation, no action required.

• Loud growling or humming, especially at start: single phasing, severe voltage imbalance, or broken rotor bars producing a beat frequency.

• High pitched whine that varies with load: bearing in early wear or VFD related noise on inverter duty motors.

• Grinding or rough rumble: bearing in advanced wear, contamination in the bearing, or foreign material in the air gap.

• Knocking or scraping at running speed: rotor rubbing the stator from severe bearing wear, bent shaft, or misalignment severe enough to bring the rotor into contact with the stator bore.

• Clicking or popping at random intervals: loose internal connection, bearing element with intermittent contact, or arcing inside the motor.

• Whistling or siren sound: cooling fan damage or air path obstruction creating turbulence.

The Stator Hum Specifically

The normal 120 Hz hum from a three phase motor on 60 Hz supply is the magnetostriction of the stator core under cyclic magnetic flux. It is unavoidable and harmless at normal levels. Significantly increased hum without other symptoms can indicate a loosening of the stator core lamination stack, which is a serious finding and should be inspected at the next available opportunity.

Diagnostic Approach to Motor Noise

Use a mechanic stethoscope or a contact probe at the bearing housings, the frame, and the conduit box to isolate the source. Compare against a known healthy motor of the same type if one is available on site. Record the noise with a smartphone or vibration analyzer audio function and compare the recording against the same motor a month later; trend in noise character is diagnostic.

Symptom 5: Electrical Anomalies

Electrical symptoms point past the mechanical condition of the motor into the windings, the rotor, the power supply, or the control circuit. They are the most diagnostic symptoms when interpreted correctly and the most commonly misdiagnosed when interpreted in isolation.

High Current Draw

The motor draws current significantly above nameplate full load amps under normal load.

Likely causes: mechanical overload from process change, increased friction in the driven equipment, low voltage at the motor terminals (current rises as voltage falls to maintain power), single phasing, broken rotor bars (current pulsates around an elevated average), or stator winding fault (turn to turn short). Verify the load condition first. Measure voltage and current on all three phases. Measure the average voltage and compare against nameplate. A motor running at 95 percent of rated voltage draws approximately 10 percent more current to produce the same shaft power.

Current Imbalance Between Phases

Phase currents differ by more than a few percent.

NEMA MG1 specifies that current imbalance under normal operation should not exceed about 10 percent, with most well designed installations running below 5 percent. Higher imbalance indicates voltage imbalance at the supply, internal winding problems, or a connection issue at the terminals. The general relationship: 1 percent voltage imbalance produces 6 to 10 percent current imbalance. Measure voltage imbalance first; if voltage is balanced, the imbalance is internal to the motor.

Low Insulation Resistance

A megohm meter (megger) reads insulation resistance below the acceptable threshold.

IEEE 43 provides the standard reference. The general rule for motors at 600 volts or below is that insulation resistance corrected to 40 C should be at least the rated voltage in kilovolts plus 1, expressed in megohms. For a 480 volt motor, that means at least 1.48 megohms minimum. Modern motors in good condition typically read hundreds of megohms or higher. A reading below 5 to 10 megohms on a previously healthy motor indicates moisture, contamination, or insulation degradation and warrants investigation before energizing.

The polarization index (PI) is a more diagnostic test. Apply DC test voltage for 10 minutes and divide the 10 minute reading by the 1 minute reading. PI above 2.0 indicates good clean dry insulation. PI between 1.0 and 2.0 indicates contamination or moisture. PI below 1.0 indicates degraded insulation that should not be returned to service without further investigation.

Insulation Failure to Ground or Between Phases

The motor fails a megger test outright, trips ground fault protection, or trips short circuit protection.

This is a winding failure. Causes include moisture ingress, contamination, thermal overload that broke down the insulation, mechanical damage from vibration, voltage surge from lightning or switching, or insulation degradation from age. A motor with a confirmed winding fault should not be returned to service without rewinding or replacement, depending on the economics.

VFD Related Electrical Issues

Variable frequency drive applications introduce additional electrical considerations:

• Common mode voltage and shaft voltage: VFDs produce common mode voltage that capacitively couples to the motor shaft. The result is voltage potential across the bearings, which can discharge through the bearing in an electrical discharge machining (EDM) action that erodes the bearing surface (often called bearing fluting). Insulated bearings, shaft grounding rings, or both are required on motors above approximately 100 horsepower on VFDs.

• Reflected wave overvoltage: long cables between VFD and motor can produce voltage doubling at the motor terminals, stressing the winding insulation. Motors fed from VFDs through cables longer than about 50 feet typically require inverter duty rated insulation systems per NEMA MG1 Part 31, and may require dV/dt filters or sine wave filters.

• Harmonic heating: VFD output is not a pure sine wave. The harmonic content produces additional heating in the motor windings. Inverter duty motors are designed for this. Standard induction motors run on VFDs experience accelerated insulation aging from the harmonic heating.

• Reduced cooling at low speed: TEFC motors rely on a shaft mounted fan for cooling. At reduced speed from a VFD, fan cooling drops as the square of speed, while torque output remains constant for constant torque loads. Motors operating below approximately 50 percent of base speed under load typically require auxiliary cooling (separately powered blower) to prevent overheating.

Root Causes: The Underlying Failure Modes

The symptoms above are how motor problems present. The root causes are why those problems develop in the first place.

Bearing Failures

Bearing failures account for the largest share of motor failures across the industry. The underlying causes break down into several categories:

• Lubrication problems: too much grease, too little grease, wrong grease, contaminated grease, incompatible grease mixed with existing grease, or extended re-greasing intervals.

• Contamination: water ingress through seals or breathers, particulate ingress in dusty environments, or process chemicals in food and chemical applications.

• Electrical erosion: bearing currents from VFD common mode voltage, ground fault current paths through the bearing, or stray current from inadequate motor grounding.

• Mechanical overload: misalignment, coupling unbalance, belt tension excessive on belt driven applications, or vibration from the driven equipment.

• End of life fatigue: bearings have a calculated fatigue life (L10) that depends on load, speed, and operating conditions. A correctly sized and well maintained bearing will eventually reach end of life through normal rolling contact fatigue.

Stator Winding Failures

Stator winding failures break down into several patterns, each with a distinctive failure mode and an underlying cause:

• Phase to phase short: insulation failure between two phases, usually in the end turns where phases are close together. Often caused by contamination, mechanical damage from vibration, or surge from switching.

• Turn to turn short: insulation failure between adjacent turns within a single phase. Often caused by surge stress from VFD reflected wave, switching, or lightning.

• Phase to ground fault: insulation failure from winding to the grounded stator core. Often caused by moisture, contamination, or insulation degradation.

• Single phasing: one phase opens during operation, leaving the other two to carry the full load. Caused by upstream contactor failure, blown fuse, or open conductor.

• Thermal failure: insulation degraded across the entire winding from sustained overheating. Caused by overload, blocked cooling, voltage imbalance, or high ambient.

Rotor Failures

Rotor failures are less common than stator failures but more difficult to diagnose:

• Broken rotor bars: cracks in the rotor bars or end rings, common on motors with frequent across the line starts. Produces a characteristic current modulation at slip frequency.

• Rotor core damage: insulation breakdown between rotor laminations, usually from severe overheating.

• Air gap eccentricity: the rotor is not concentric with the stator bore. Static eccentricity is from bearing wear or assembly errors. Dynamic eccentricity is from a bent shaft or thermal bowing.

Field Diagnostic Procedures

The diagnostic methods used to confirm a motor problem are mature and well documented.

Insulation Resistance and Polarization Index

A megohm test takes one to two minutes; a PI test takes ten minutes plus setup. Both are non destructive and should be performed on any motor that has been out of service for an extended period, after any water exposure, and as part of regular preventive maintenance on critical motors. Test voltage is typically 500 volts DC for motors rated 600 volts and below, applied phase to ground with all three leads connected together. Record the value, correct for temperature per IEEE 43, and trend over time.

Vibration Measurement

Measure at every accessible bearing location, in horizontal, vertical, and axial directions. Record overall velocity in mm/s RMS. Capture a spectrum from 10 Hz to at least 1 kHz, with envelope or demodulated spectrum for bearing diagnostics. Record load condition and temperature with every reading. Maintain a baseline for comparison.

Thermography

Image the motor at steady operating condition, not immediately after start. Capture frame surface temperature distribution, bearing housing temperatures (drive end and opposite drive end), conduit box and terminal block, and motor leads. Compare bearings against each other on the same motor. Differences of more than 10 to 15 C between drive end and opposite drive end bearings usually indicate a problem.

Motor Current Signature Analysis (MCSA)

Spectral analysis of the motor current waveform during operation reveals rotor problems that are invisible on vibration analysis. Broken rotor bars produce sidebands at twice slip frequency around the line frequency peak. Eccentric air gap produces sidebands at rotor speed. MCSA requires a specialized analyzer or a current transducer feeding a spectrum analyzer.

Surge Testing and Winding Diagnostics

For comprehensive winding evaluation, surge comparison testing identifies turn to turn weaknesses that megger tests cannot detect. Surge testing should be performed by trained personnel on motors with suspected winding issues, motors being returned to service after extended storage, and as part of acceptance testing on rewound or new motors.

Repair, Rebuild, or Replace

The decision between repair, rewind, and replacement depends on the motor type, the failure mode, the age of the unit, and the application criticality.

In place repair is appropriate when the failure is external (terminal block connections, leads, fan cover, conduit box, external bearing for some configurations), when bearings can be replaced without removing the motor from the application, or when the lubrication system is the source and the windings are confirmed healthy.

Shop rebuild is appropriate when bearings need replacement that requires motor disassembly, when the windings test acceptable but the unit needs cleaning, dipping, and bearing replacement, or when minor mechanical work (shaft repair, key replacement, fan replacement) is needed.

Rewind is appropriate when the winding has failed but the core and frame are in serviceable condition, when the motor is in a frame size and rating that justifies the rewind cost against new replacement, and when the rewind can be performed to EASA AR100 standards that maintain or improve the original efficiency.

Replacement is appropriate when the failure is catastrophic, when the motor is older than approximately 15 to 20 years and the technology has advanced significantly (NEMA Premium efficiency motors offer meaningful energy savings over older designs), when the application has changed and the existing motor is no longer correctly sized, or when total repair cost approaches 60 to 70 percent of new motor installed cost.

The Efficiency Consideration

Modern NEMA Premium efficiency motors run 1 to 4 percentage points more efficient than the standard efficiency motors common in installations more than 15 years old. On a 100 horsepower motor running 8000 hours per year, a 2 percentage point efficiency improvement saves approximately 12,000 kWh per year, or roughly 1,000 to 1,500 dollars at typical industrial electric rates. The payback on replacement versus rewind often runs three to five years on motors in continuous duty applications, which is significantly faster than the remaining service life of the rewound motor.

Preventive Practices That Actually Work

The plants that experience the fewest unplanned motor failures are not the plants with the newest equipment. They are the plants with the most disciplined practices.

Lubrication discipline starts with a written grease standard for every motor in the plant: brand, type, quantity, and interval. Color coded grease guns prevent cross contamination between incompatible greases. Over greasing is as destructive as under greasing; excess grease churns and overheats, and on shielded bearings it can force the shield into the rotating elements.

Electrical maintenance discipline means insulation resistance testing on a regular interval, with results trended over time. Connections in conduit boxes and at motor leads should be torqued to specification and re-torqued after the first thermal cycle. Loose connections produce arcing, overheating, and eventual conductor failure.

Alignment discipline means laser aligning every coupling after installation and after any drive train work. Soft foot checks at every hold down point are part of every alignment. Belt drives need correct tension and parallel pulley alignment.

Monitoring discipline means installing permanent monitoring on critical motors at minimum (bearing temperature, vibration), and using portable measurement on a regular schedule for the rest. Set alarm thresholds based on the established baseline, not on absolute manufacturer maximums.

VFD specific discipline means specifying inverter duty motors for VFD applications, installing insulated bearings or shaft grounding rings on motors above approximately 100 horsepower on VFDs, and managing cable length and dV/dt filtering on long cable runs.

When to Call a Specialist

Some calls should not wait. Pick up the phone the moment any of the following occur on a critical motor:

• Insulation resistance reading drops below 5 megohms or polarization index drops below 1.5.

• Sudden vibration step change above 50 percent of established baseline.

• Bearing housing temperature 15 C or more above its sister bearing.

• Current imbalance above 10 percent with balanced supply voltage.

• Any smoke, burning insulation odor, or visible damage at the conduit box or motor leads.

• Trips that recur after reset without clear external cause.

• Any uncertainty about whether the motor is safe to continue operating.

When you call, have the nameplate photo ready (manufacturer, frame, horsepower, voltage, full load amps, service factor, RPM, insulation class, enclosure type, design letter, serial number), a description of the application and driven equipment, a specific symptom description, recent insulation and vibration data if available, and photos of any visible damage. The more information available at the start of the conversation, the faster the right response gets dispatched.

About Malloy Electric Motor Services

Malloy Electric has provided motor and power transmission services to industrial customers since 1945. Our motor service line spans field troubleshooting, in shop repair and rewind to EASA AR100 standards, predictive maintenance programs, replacement sourcing, and engineered upgrades. We serve customers across the northern plains and mountain west from eight Centers of Excellence in Sioux Falls, Dakota Dunes, Fargo, Mandan, Omaha, Cedar Rapids, Gillette, and Billings. Authorized partner brands include ABB, Siemens, WEG, and Regal Beloit.

We Service What We Sell. We Solve Problems.

Frequently Asked Questions About Electric Motor Troubleshooting

What insulation resistance reading is acceptable for a low voltage motor?

For motors rated 600 volts and below, IEEE 43 recommends a minimum insulation resistance of rated voltage in kilovolts plus 1, expressed in megohms, corrected to 40 C. For a 480 volt motor, that is approximately 1.48 megohms minimum. Modern motors in good condition typically read hundreds of megohms or higher. A reading below 5 to 10 megohms on a previously healthy motor indicates moisture, contamination, or insulation degradation and warrants further investigation before energizing.

What does a polarization index tell me?

The polarization index (PI) is the ratio of the 10 minute insulation resistance reading to the 1 minute reading. PI above 2.0 indicates good clean dry insulation. PI between 1.0 and 2.0 indicates contamination or moisture. PI below 1.0 indicates degraded insulation. PI is more diagnostic than a single megohm reading because it reveals the absorption characteristics of the insulation, which change with moisture and contamination.

Why does my motor trip on overload but the load seems normal?

The most common causes are voltage imbalance at the supply (which produces disproportionately large current imbalance), single phasing during operation, broken rotor bars producing pulsating current, or an incorrectly set overload relay. Measure voltage on all three phases at the motor terminals under load. Check the overload relay setting against the nameplate full load amps. If voltage is balanced and the relay is set correctly, the problem is likely inside the motor.

How often should I grease a motor?

Follow the motor nameplate or manufacturer specification. Typical intervals for industrial TEFC motors are 6 to 12 months for continuous duty applications, longer for intermittent duty. Quantity matters as much as interval; over greasing is destructive. Most modern motors have specific grease quantity recommendations stated in cubic centimeters or grams per service interval. Use only the grease type specified by the OEM, and never mix incompatible grease types.

Can I run a standard motor on a variable frequency drive?

Standard motors can run on VFDs within limits, but several considerations apply. Motors above approximately 50 horsepower on VFDs typically benefit from inverter duty rated insulation per NEMA MG1 Part 31. Motors with long cable runs to the VFD (over 50 feet) may need inverter duty insulation or filtering. Motors above approximately 100 horsepower on VFDs should have insulated bearings or shaft grounding rings to prevent bearing fluting from common mode voltage. Constant torque loads operating below 50 percent of base speed usually require auxiliary cooling. New installations should specify inverter duty motors; retrofits of existing standard motors require evaluation of the specific application.

Is it cheaper to rewind or replace a failed motor?

The general guideline is that rewinding makes economic sense up to about 60 to 70 percent of new motor installed cost. Above that threshold, replacement usually wins on total cost of ownership, especially when factoring in the efficiency improvements of modern NEMA Premium motors. The decision depends on the motor frame size (smaller motors are often cheaper to replace), the application criticality, the availability of replacement, and the efficiency rating of the existing motor versus current standards.

How long should an industrial motor last?

A correctly specified, properly installed, and well maintained industrial motor should provide 15 to 20 years of service life in standard duty, longer in light duty. Severe service environments, frequent starts and stops, high ambient temperatures, and chronic lubrication or alignment neglect shorten that. The plants that get the longest service life from their motors are the plants with disciplined specification practices on the front end and disciplined maintenance practices on the back end.

This guide was prepared by the application engineering team at Malloy Electric. For specific motor troubleshooting support, rewind quotation, or new motor specification, contact your local Malloy Center of Excellence. Visit malloyelectric.com for service line information across motor repair, gearbox and power transmission, VFDs, custom control panels, field services, and predictive maintenance.