General Purpose Motor: What the Label Actually Guarantees
Quick Answer
A general purpose motor is built to standard output ratings, frame dimensions and operating characteristics, allowing motors from different makers to fit the same installation when their specifications match. The term describes a specification category, not a quality grade. It refers to usual service conditions, including 40 °C ambient, 1,000 m altitude and ±10 % voltage. It does not specify starting torque, efficiency or performance on an unbalanced supply.
- The Definition: A Specification Term, Not a Quality Grade
- Usual Service Conditions: The Fine Print That Defines the Category
- General, Definite and Special Purpose Side by Side
- The Four NEMA Design Letters and What They Cost You
- Enclosure, Frame and Mounting in One Table
- Efficiency: The Spec That Stopped Being Optional
- Where a General Purpose Motor Stops Being the Right Answer
- Supply Faults: Voltage Imbalance, Phase Loss and the 50/60 Hz Question
- Shaft Current: The Failure That Hides Until the Bearing Dies
- Symptom to Cause: A Diagnostic Table
- Insulation Resistance and the Idle-Motor Restart
- What to Put on the Enquiry
- References
- FAQ
The Definition: A Specification Term, Not a Quality Grade
“General purpose” describes a motor built to standard dimensions and ratings. The label can help identify interchangeable products, but the frame, mounting, shaft and electrical details still need to match. Motors designed for a defined application or unusual requirement fall into definite-purpose or special-purpose categories.
NEMA MG 1 defines the term using a continuous standard rating, standard operating characteristics and standard mechanical construction. It also describes a motor intended for usual service conditions without restriction to one application. In practice, the relevant MG 1 tables provide the ratings and dimensions behind that category.
IEC standards address related requirements separately. IEC 60034 covers ratings and tolerances, IEC 60034-30-1 covers efficiency classes, and frame standards specify dimensions and mounting. The term “general purpose” does not map exactly to a single IEC 60034 category, so buyers should compare the stated ratings and dimensions.
This difference can cause confusion during procurement. A US data sheet may specify “general purpose, Design B, 145T frame,” while another supplier may use “general purpose” descriptively and list the motor’s actual ratings. When comparing products, confirm the numbers and interfaces rather than relying on the label alone.
| Fixed by the category | Left to the order |
|---|---|
| Output steps from the standard table (0.75, 1.1, 1.5, 2.2 kW …) | Winding variant: voltage and frequency combination |
| Frame dimensions: foot hole spacing, shaft height, shaft end | Enclosure within the range the maker offers |
| Continuous S1 rating at 40 °C ambient | Efficiency class, where the regulation allows a choice |
| Usual service conditions as declared | Terminal box position, cable entry direction, accessories |
| Tolerances on output, torque and temperature rise | Paint system, nameplate language, test certificate scope |
The standard values support interchangeability; order-specific options explain why two motors with the same power rating may still differ.
Usual Service Conditions: The Fine Print That Defines the Category
General purpose motor ratings assume defined service conditions. IEC 60034-1 and NEMA MG 1 specify similar limits, including ambient temperatures from −15 °C to +40 °C, altitude up to 1,000 m, voltage within ±10 % of the nameplate, frequency within ±5 %, and a combined absolute voltage and frequency deviation within 10 %. Check the applicable standard and motor documentation for the specific rating.
These limits define the conditions behind the rating. At 55 °C ambient, 2,400 m altitude or a 15 % supply variation, the motor is operating outside those stated conditions. Its performance may then require derating or a different specification.
Altitude affects cooling because air density falls with elevation. Under the standard atmosphere, air density at 2,000 m is about 78.5 % of its sea-level value. This can reduce the cooling available from a shaft-mounted fan. Manufacturers usually provide derating guidance for operation above 1,000 m; the required adjustment depends on the motor and cooling method. Separately ventilated motors may allow fan speed to be adjusted.
A lower ambient temperature can partly offset the reduced cooling at altitude. The standard atmosphere has an average temperature lapse rate of roughly 1 K per 150 m in the lower troposphere, but actual site conditions vary. Whether the motor can retain its rated output at a given altitude and ambient temperature must be checked against the manufacturer’s guidance.
| Condition | Standard limit | What happens beyond it | Usual remedy |
|---|---|---|---|
| Ambient temperature | −15 °C to +40 °C | Winding runs hotter; insulation life shortens | Derate, or specify a higher class |
| Altitude | 1,000 m | Less cooling air mass; higher rise at the same load | Derate 0.5–1 % per 100 m, or uprate the fan |
| Supply voltage | ±10 % of nameplate | Over-voltage saturates; under-voltage raises current | Tap change, or correct the feeder |
| Frequency | ±5 % | Speed error, and a shift in the volts-per-hertz ratio | Do not run a 60 Hz motor at 50 Hz without derating voltage |
| Combined deviation | 10 % absolute sum | Torque margin disappears before the motor trips | Measure at the terminals, not at the panel |
| Duty | Continuous S1 | Short-time or intermittent duty needs a different rating basis | Specify S2/S3 with the cycle |
Below −15 °C, bearing lubricant selection and starting conditions need review. Grease consistency at low temperature can increase bearing drag, depending on the grease formulation and bearing design. Motors stored in unheated spaces may also collect condensation on the windings when the air warms faster than the motor. Space heaters can help manage moisture during storage.
General, Definite and Special Purpose Side by Side
These categories describe how closely a motor is designed around a particular application, rather than ranking quality. A definite-purpose motor is designed for a narrower duty than a general purpose motor, and its added specifications may affect its cost.
| Category | Bound to | Typical examples | What changes |
|---|---|---|---|
| General purpose | Nothing beyond the standard tables | Centrifugal pumps, fans, conveyors, compressors, machine tools, simple mixers | Nothing is optimised; everything is standardised |
| Definite purpose | One application class | Fire pump motors, crane and hoist duty, oil-well pump motors, brake motors, HVAC OEM motors, explosion-protected motors | Torque curve, duty cycle, enclosure or certification is fixed by a code |
| Special purpose | One unusual requirement | High-slip, low-inertia, submersible, low-magnetic-signature, wide-temperature, very low noise | A characteristic is pushed outside the standard range on purpose |
The category can also affect sourcing. Some definite-purpose motors have fewer suppliers because certification or application requirements limit who can provide them. If only a small number of suppliers can meet the stated requirements, review whether the specification describes a standard motor or a specific application class.
The classification runs across motor types, which is where confusion starts. A general purpose motor is usually a squirrel-cage induction machine, because that construction has the widest standard tables — the alternatives are compared in BLDC versus induction motor and induction motor versus AC motor. A DC motor can be general purpose too, and that term covers constructions that do not interchange, as set out in what a DC motor is and the brushed DC motor range. The squirrel-cage machine is the default because it has no wear parts in the power path; the trade-off is in applications of the squirrel-cage induction motor.
The Four NEMA Design Letters and What They Cost You
One specification to confirm is the motor design letter or its equivalent torque data. Two general purpose motors with the same power, frame and voltage can have different locked-rotor torque. NEMA identifies designs by letter, while IEC-style data sheets typically state torque ratios.
| Design | Locked-rotor torque | Breakdown torque | Slip at rated load | What it is for |
|---|---|---|---|---|
| A | Normal, about 150–200 % | High, above 200 % | Under 5 %, often under 2 % | Legacy general purpose; higher inrush than B |
| B | Normal, about 150–200 % | About 200 % | Under 5 % | The default general purpose motor |
| C | High, about 200–250 % | Normal | Under 5 % | Conveyors, positive-displacement pumps, crushers; lower starting current |
| D | Very high, about 275 % | Lower | 5–13 % | Punch presses, hoists, flywheel loads; energy stored in slip |
When a catalogue omits a design letter, buyers should confirm the starting characteristics rather than assume a particular design. A centrifugal pump or fan may have relatively low starting load, while a screw pump or loaded conveyor can require higher breakaway torque. Selecting only by shaft power may leave insufficient starting torque.
IEC motor data sheets generally state torque and current ratios rather than using NEMA design letters. For example, a sheet listing “Tst/TN = 2.2, Tmax/TN = 2.3, Ist/IN = 6.5” gives those ratios directly. Do not infer a NEMA design solely from a similar set of values; compare the applicable definitions and tolerances.
The slip column matters more than it looks. A four-pole 50 Hz Design B motor at 1,470 rpm is running 2 % slip; the same motor at 1,440 rpm would be a high-slip design. Pole count sets the synchronous base and the design letter sets the deviation, as worked through in number of poles in a motor and how to calculate motor torque.
Enclosure, Frame and Mounting in One Table
The second thing a general purpose label fixes is the mechanical interface. If the frame is standard, the motor is a bolt-in replacement. If it is not, the purchase becomes a modification project, which is the most common reason a “cheap” motor turns out expensive.
| Item | NEMA vocabulary | IEC vocabulary | The trap |
|---|---|---|---|
| Enclosure | ODP, TEFC, TENV, TEAO | IP23, IP55, IP65 per IEC 60529 | IP55 and TEFC are close but not identical in test basis |
| Frame | 48, 56, 143T, 145T, 182T, 213T, 254T … | 63, 71, 80, 90, 100, 112, 132, 160, 180, 200, 225, 250, 280, 315, 355 | A 1.5 kW 4-pole IEC 90 and a 2 hp 1800 rpm NEMA 145T are not drop-in |
| Foot mounting | F1 / F2 foot position | B3 | F1 versus F2 moves the feet to the other side of the frame |
| Flange mounting | C-face, D-flange | B5, B14 | Pilot diameter and bolt circle are both fixed; neither is optional |
| Foot plus flange | — | B35 | Rarely available off the shelf in small frames |
| Shaft end | Keyway per NEMA; length varies by frame | Keyway per IEC 60072 | Same diameter, different key width is a real failure |
NEMA and IEC frame designations use different dimensional systems. NEMA frames specify critical dimensions such as shaft height, foot-hole spacing and shaft dimensions; the T suffix identifies a modern NEMA frame series. IEC frame numbers refer to shaft height in millimetres, so an IEC 132 frame has a nominal shaft-centre height of 132 mm. Check the full mounting and shaft dimensions before selecting a replacement.
Enclosure is another point to check. An open drip-proof motor and a totally enclosed fan-cooled motor may not suit the same wash-down conditions, even at the same power. IEC 60529 defines IP ratings; compare the specific protection each rating provides and the test conditions.
Efficiency: The Spec That Stopped Being Optional
Motor selection has traditionally focused on power, frame and price, with efficiency varying by design. In many markets, regulations now set minimum efficiency requirements for some motor types and ratings.
IEC 60034-30-1 defines the IE1 to IE4 classes for single-speed, three-phase, squirrel-cage induction motors from 0.12 kW to 1,000 kW, with 2, 4, 6 and 8 poles, rated up to 1,000 V, rated for continuous duty and capable of operating directly on line. That scope is almost exactly the scope of the general purpose category — which is the point. If a motor falls inside it, its efficiency is a classified quantity, not an engineering choice.
In the European Union, Regulation 2019/1781 sets ecodesign requirements for covered electric motors and variable-speed drives. The applicable minimum depends on product scope, rating and implementation dates; check the regulation and amendments for the motor being specified.
Two details belong in a specification. The IE class is defined at the rated point only; part-load efficiency is not part of it, and part load is where most machines run. And “Class F insulation” is not a temperature rise: most general purpose motors use a Class F system rated at Class B rise, which leaves about 25 K of margin. By the ten-degree rule that margin is worth several times the thermal life.
The measurement basis for both numbers is in IEC 60034-1, and the test methods are in IEC 60034-2-1. If the comparison is between an induction machine and an electronically commutated one, the class boundaries stop applying entirely, as explained in motor efficiency class and in the high-efficiency brushless DC motor range.
Where a General Purpose Motor Stops Being the Right Answer
Knowing when a standard general purpose motor may not suit the application can help avoid a mismatch. The following situations are common reasons to review the specification.
Altitude and ambient
Above 1,000 m or above 40 °C the rating has to be corrected, not just noted. A motor selected at its nameplate power for a site at 2,000 m and 45 °C is running outside the contract on two axes at once, and the two effects multiply rather than add.
Sustained light load
At light loads, magnetising current remains a significant part of the total current, so power factor and efficiency may fall. The actual values depend on the motor. Oversizing can contribute to prolonged light-load operation; select a motor using the duty point and expected load range rather than adding capacity without checking.
Variable speed
When a general purpose motor is used with a variable-frequency drive, check its insulation, bearing-current risk and cooling at low speed. Fast voltage rise times and common-mode voltage can affect the motor, while a shaft-mounted fan provides less cooling as speed falls. Inverter-duty features may address some of these conditions. See how a variable speed motor works and IEC 61800-9-2 for drive-system considerations.
Any gearbox, brake or special flange
The moment the motor is delivered as a geared unit, the gearbox ratio, the output bearing and the mounting interface are all application decisions, and the assembly is a definite purpose product. The distinction matters commercially because the price is set by the gearbox, not the motor.
Hazardous area, marine or fire service
Each of these is defined by a code that also defines the certification, the test house and the documentation package. A general purpose motor cannot be converted by adding a certificate.
Very low noise or vibration, as a contractual number
A general purpose motor is balanced and tested to a general standard, not to a measured limit at a specified point. Where the sound power is contractual, the motor must be specified against it; the levers are in how to reduce electric motor noise.
Supply Faults: Voltage Imbalance, Phase Loss and the 50/60 Hz Question
Supply problems can cause motor overheating and failure. Voltage imbalance, phase loss and frequency mismatch are measurable conditions that should be checked during diagnosis.
Voltage imbalance
Voltage imbalance can produce a larger current imbalance. A commonly cited range is six to ten times the voltage imbalance, though the actual relationship depends on the motor and supply. NEMA MG 1 provides derating guidance; consult the applicable curve when setting the motor’s operating load.
| Voltage unbalance | Derating factor | Approximate current unbalance | Effect on a motor at full load |
|---|---|---|---|
| 1 % | 0.98 | 6–10 % | Negligible in practice |
| 2 % | 0.95 | 12–20 % | Rise climbs a few kelvin |
| 3 % | 0.88 | 18–30 % | One phase overheats; life drops sharply |
| 4 % | 0.82 | 24–40 % | Thermal protection may not catch it in time |
| 5 % | 0.75 | 30–50 % | Do not run at nameplate load |
Measure the three line voltages at the motor terminals with the motor running, not at the board with it stopped. A 3 % unbalance means 88 % of nameplate output, which on a motor sized without margin means overload. An overload relay set to nameplate current will not trip: the highest phase is only slightly above the setting while the other two sit below it.
Phase loss
If one phase is lost while a three-phase motor is running, the motor may continue operating while the remaining phases carry excessive current. A motor generally cannot start normally with a phase missing and may draw locked-rotor current until protection opens the circuit. Possible causes include a blown fuse, failed contactor pole, damaged supply conductor, loose terminal or broken winding lead.
The 50/60 Hz direction
Motor flux is approximately proportional to the voltage-to-frequency ratio. Raising frequency from 50 to 60 Hz at constant voltage reduces that ratio by about 17 %; synchronous speed rises by 20 %, while available torque and power depend on the motor and load. A 50 Hz motor may operate on 60 Hz if its ratings and mechanical limits permit it, but the application should be checked.
Running a 60 Hz motor at 50 Hz without reducing voltage increases the volts-per-hertz ratio by 20 %, which can drive the magnetic circuit toward saturation and increase magnetising current. The motor may overheat. Maintaining the same ratio requires reducing voltage proportionally; for example, 480 V at 60 Hz and 400 V at 50 Hz are both 8.0 V/Hz. Confirm the motor and load ratings before changing supply frequency.
Tolerances for both directions are set in IEC 60034-1, and the mechanical consequences of the 20 % speed change — bearing limits, coupling balance, fan load — are the buyer’s to check.
Shaft Current: The Failure That Hides Until the Bearing Dies
Shaft current is current that flows through the path shaft–bearing–end shield–frame–base instead of through the windings. It does no useful work. What it does is erode the raceway and the balls, and the damage is progressive, silent and usually blamed on the grease.
The classical causes are all asymmetries in the magnetic circuit: an uneven air gap from rotor eccentricity, a split or segmented stator core with gaps between the halves, a poor choice of core segment count, supply harmonics, and any asymmetry in the winding. Those causes produce a voltage between shaft ends at line frequency, and they are still the explanation for shaft current in a directly supplied machine.
In a drive-fed machine the dominant cause is different and much larger. The inverter creates a common-mode voltage on all three phases together, and it couples capacitively through the winding-to-rotor capacitance onto the shaft, which then discharges through the bearings in short bursts. This is the mechanism behind the fluting pattern — a regular washboard marking across the raceway — on motors that have run a year on a drive and were never specified for one.
One common mitigation is to insulate one bearing rather than both, but the correct arrangement depends on the motor and drive system. Insulating both ends can leave current to find another path through a coupling or instrumentation. A mitigation package may include an insulated non-drive-end bearing, a shaft-grounding ring at the drive end, and suitable common-mode filtering or shielded cabling with proper termination.
Symptom to Cause: A Diagnostic Table
Most general purpose motor complaints arrive as one of five symptoms. The order of measurement matters more than the list of possible causes, because the first reading eliminates half of them.
| Symptom | First measurement | If the reading is abnormal | If the reading is normal |
|---|---|---|---|
| Runs hot | Line current against nameplate current, at the actual terminal voltage | Load, supply imbalance or phase loss | Cooling path, ambient, harmonics, or a blocked fan cowl |
| Vibration | Spectrum: 1×, 2×, and high frequency | 1× balance or alignment, 2× alignment or looseness, high frequency bearing | Electromagnetic: check current balance and air gap |
| Bearing hot | Bearing temperature and grease condition | Grease quantity, mixing of incompatible thickeners, over-greasing | Alignment, belt tension, shaft current |
| Will not start | Three line voltages at the terminals, then winding resistance | Phase loss, low voltage, open winding | Load torque above the design letter’s locked-rotor capability |
| Trips after minutes | Current trend over time, not a single reading | Thermal overload from a genuine overload | Protection set below nameplate, or a class mismatch |
Two rows deserve a comment. In the “runs hot” row a reading at the panel misleads: a long undersized cable reads normal at the panel and low at the terminals, and the low voltage is what drives the high current. In the “will not start” row the design letter decides — a motor that turns a fan easily may refuse to break a loaded screw pump away, and no electrical check will find a fault, because there is none.
For noise specifically, the boundary between acceptable and contractual is a measurement standard rather than an opinion, and the comparison basis is set out in electric motor testing standards. Scheduled inspection items — insulation, grease, alignment, cooling — are listed in maintaining a three-phase asynchronous motor.
Insulation Resistance and the Idle-Motor Restart
A motor that has stood idle for months is not the motor that was commissioned. Moisture migrates into the winding, dust and oil collect on the end turns, and the first start is when the damage becomes visible. Insulation resistance is the check that costs two minutes and prevents most of it.
Insulation-resistance criteria depend on the applicable standard, motor voltage, temperature and test procedure. The simple formula sometimes cited in textbooks should not be treated as a universal acceptance threshold. NEMA MG 1 and IEEE 43 provide guidance that must be applied to the specific motor and test conditions; correct readings for temperature and follow the manufacturer’s instructions. The 1 MΩ and 5 MΩ values in the table below should be verified against the applicable standard before use.
| Reading at 500 V DC, corrected to 40 °C | Interpretation | Action |
|---|---|---|
| Above 100 MΩ | Dry, healthy winding | Record as baseline; re-check at the next outage |
| 5–100 MΩ | Acceptable, but the trend decides | Compare with the previous reading before deciding |
| 1–5 MΩ | Marginal; moisture or contamination present | Dry before running, then re-measure |
| Below 1 MΩ | Do not energise | Dry, then re-measure; investigate the ingress path |
The reading is meaningless without the temperature it was taken at, because insulation resistance roughly halves for every 10 K rise. A motor measured hot and a motor measured cold are not comparable, which is why the standard corrects to 40 °C. It is also why the trend across successive readings at the same ambient carries more information than any single pass or fail.
If the reading is low, drying may be required, but the method should be selected for the motor and the cause of the low resistance. Options can include running unloaded, applying a controlled low-voltage supply, or circulating warm air. Electrical drying methods require appropriate limits and qualified personnel. After drying, allow the motor to cool, repeat the measurement and record the result.
What to Put on the Enquiry
Six basic details can help suppliers prepare a useful quotation. If an enquiry omits them, the supplier may need to ask follow-up questions before quoting.
- Output and speed, or the duty point: power, rated speed, and whether the load is centrifugal or constant torque.
- Frame or mounting: shaft height, foot or flange, and the existing baseplate if it is a replacement.
- Voltage and frequency, including the actual supply tolerance rather than the nominal figure.
- Enclosure and environment: IP class, ambient range, altitude, wash-down, outdoor, or corrosive atmosphere.
- Duty and starts: continuous or intermittent, starts per hour, and whether the load breaks away under torque.
- Efficiency class and certification required by the destination market, with the standard named.
The sixth line is the one that changes the price most and is the one most often left to the supplier’s assumption. A machine destined for the EU and one destined for a market with no minimum efficiency requirement are different products at the same nameplate, and the difference is not visible from outside. The class definitions are in IEC 60034-30-1; the market requirements in Regulation 2019/1781; the safety requirements that apply when the motor is built into a machine in Regulation 2023/1230.
If a standard induction machine cannot meet the efficiency target, the alternative route is in BLDC versus induction motor and the high-torque BLDC technical guide; if it needs a geared output, start from high-torque AC gear motors or AC gear motors with speed control.
“General purpose” describes a standard motor category; it does not by itself confirm suitability for a particular installation. Check the torque requirements, ambient conditions, altitude and supply quality before ordering.
References
- International Electrotechnical Commission. (2022). IEC 60034-1:2022 — Rotating electrical machines, Part 1: Rating and performance. IEC. https://webstore.iec.ch/en/publication/65446
- International Electrotechnical Commission. (2025). IEC 60034-30-1:2025 — Efficiency classes of line operated AC motors (IE code). IEC. https://webstore.iec.ch/en/publication/91195
- International Electrotechnical Commission. (2014). IEC 60034-2-1:2014 — Standard methods for determining losses and efficiency from tests. IEC. https://webstore.iec.ch/en/publication/121
- International Electrotechnical Commission. (2013). IEC 60529:1989+AMD2:2013 — Degrees of protection provided by enclosures (IP code). IEC. https://webstore.iec.ch/en/publication/2452
- International Electrotechnical Commission. (2016). IEC TS 60034-30-2:2016 — Efficiency classes of variable speed AC motors (IE code). IEC. https://webstore.iec.ch/en/publication/30830
- International Electrotechnical Commission. (2017). IEC 61800-9-2:2017 — Ecodesign for power drive systems, motor starters and power electronics. IEC. https://webstore.iec.ch/en/publication/111276
- National Electrical Manufacturers Association. (2021). NEMA MG 1 — Motors and Generators. NEMA. https://www.nema.org/standards
- European Parliament and Council. (2019). Commission Regulation (EU) 2019/1781 laying down ecodesign requirements for electric motors and variable speed drives. Official Journal of the EU. https://eur-lex.europa.eu/eli/reg/2019/1781/oj
- European Parliament and Council. (2023). Regulation (EU) 2023/1230 on machinery and repealing Directive 2006/42/EC. Official Journal of the EU. https://eur-lex.europa.eu/eli/reg/2023/1230/oj
- U.S. Department of Energy. (2024). Motor systems — energy efficiency and system optimisation. Advanced Manufacturing Office. https://www.energy.gov/eere/amo/motor-systems
FAQ
Q: Is a general purpose motor lower quality than a definite purpose one?
No. The categories describe application scope rather than quality. A general purpose motor follows standard tables; a definite-purpose motor is designed for a defined class of duty. It may cost more, but that does not automatically make it more reliable.
Q: Why does the same 1.5 kW motor weigh different amounts from two suppliers?
Frame material, cooling method and efficiency class can all affect weight. For example, a 1.5 kW motor in an aluminium IEC 90 frame may weigh less than a cast-iron motor with the same output and a higher service factor. Compare the full specifications, not only nameplate power.
Q: Can I replace a NEMA 145T motor with an IEC 90 frame motor?
Usually not as a direct replacement. NEMA 145T and IEC 90 frames use different dimensional tables, so shaft height, foot-hole spacing and shaft dimensions may not match. The baseplate or coupling may need modification even when the electrical ratings are similar.
Q: What is the difference between IE3 and NEMA Premium?
They are efficiency designations based on different standards and scopes. A motor’s eligibility for one does not establish that it meets the other. Specify the required standard and rating rather than relying on a marketing label.
Q: Does a general purpose motor need a derating for altitude if the ambient is cold?
Possibly. Lower air density can reduce cooling, while a lower ambient temperature may offset some of that effect. The result depends on site conditions and the motor’s cooling design, so use the manufacturer’s derating guidance for the installation.
Q: Why does a motor overheat when it is only lightly loaded?
Light load alone does not explain every overheating problem. Check the three phase currents and supply voltage for imbalance, then review cooling, ambient temperature and the motor’s operating conditions. A voltage imbalance can produce a larger current imbalance, depending on the motor and supply.
