How to read a motor nameplate before replacing it
Many guides list ten nameplate fields and define them one by one. That helps with terminology, but not much when you are standing beside a failed motor with a phone camera and a purchase order. Replacement errors often start when a buyer matches the largest figures and overlooks the smaller ratings. The mismatch may not become clear until commissioning.
Quick Answer
A motor nameplate gives the conditions tied to its ratings. Start with six fields that determine whether a replacement will fit and perform as required: output power, voltage and connection, rated speed, frame and mounting, duty, and enclosure. Then check four more that affect its operating limits: full-load current, service factor, insulation class and temperature rise, and efficiency. The examples below include IEC and NEMA plates plus a lower-speed four-pole motor. They also show why rated and synchronous speed must be kept separate: 2,780 rpm is a loaded speed below the 3,000 rpm synchronous speed of a two-pole motor on 50 Hz.
- What a nameplate tells you
- Reading an IEC nameplate
- Reading a NEMA nameplate
- Speed: rated and synchronous
- Voltage and connection: delta and star
- Current: FLA, SFA and measured readings
- Temperature: insulation class, rise, duty and ambient
- Frame and mounting dimensions
- Enclosure: IP and ODP ratings
- Other nameplate fields to check
- Replacement worksheet
- Six common misreadings
- References
- FAQ
What a nameplate tells you
A datasheet describes a motor family. The nameplate records the rating of a specific unit and identifies the standard to which it is declared. A test certificate adds supporting documentation, even when the motor itself is physically identical.
Nameplate ratings also depend on operating conditions. IEC 60034-1 specifies reference service conditions that include 40 °C ambient temperature and an altitude of 1,000 m; the applicable coolant condition depends on the cooling method. Operation at 50 °C or 2,000 m may require a rating adjustment or a motor designed for those conditions. Confirm the adjustment with the manufacturer. These conditions are relevant when identifying a general purpose motor.
IEC and NEMA labels
IEC and NEMA use different labels for many of the same motor ratings. Use the table to translate the main fields when comparing nameplates.
| What it tells you | IEC 60034 style | NEMA MG 1 style |
|---|---|---|
| Output | kW | hp |
| Speed | min⁻¹, rated full-load | RPM, rated full-load |
| Enclosure | IP code, for example IP55 | ENC code, for example ODP or TEFC |
| Mounting | IM code, for example B3, B5, B35, B34 | Foot, C-face, or both, with a frame number |
| Frame | Shaft height in mm, for example 63 or 80 | Frame number, for example 48, 56C, 145TC |
| Insulation | Class letter, often with a rise letter | Class letter only |
| Ambient | Implied by the standard, 40 °C | Printed, MAX AMB 40C |
| Overload margin | No service factor | Service factor, for example 1.15 |
| Efficiency | IE class where in scope | NEMA nominal efficiency and NEMA Premium |
Reading an IEC nameplate
This example shows the nameplate of a small three-phase motor. Each line gives a rating, limit or construction detail. Pay particular attention to the four-row voltage table, which is easy to misread.
| Printed | Means | What it decides at replacement |
|---|---|---|
| 3~Mot | Three-phase motor | Supply type; a single-phase replacement needs a capacitor and different wiring |
| Type GS 632-2 B34 | Series, frame 63, size 2, 2-pole, mounting B34 | Pole count and mounting are encoded in the type string, not only in the table |
| S1-100% | Duty S1, continuous, rated at 100 percent output | An S2 or S3 replacement is not equivalent at the same kW |
| IEC60034 | Built and rated to IEC 60034 | The reference conditions and tolerances follow that standard |
| IP55 | Dust protected, water jets | Not a washdown or outdoor-immersion rating |
| Ins.Cl.F | Class F insulation; thermal class limit 155 °C | The plate does not show temperature rise; request that rating separately |
| N.W. 4.2 kg | Net mass | Shipping, and a quick sanity check that the replacement is the same build |
| Kaltleiter | PTC thermistors embedded in the windings | The replacement must have them, or the existing protection relay is left blind |
| S/N 2209085108 | Serial number | Traceability; the only field that identifies this unit rather than its type |
The voltage table has four rows, each describing a complete operating point.
| V | Hz | kW | A | cos φ | min⁻¹ |
|---|---|---|---|---|---|
| 400 Δ | 50 | 0.25 | 0.65 | 0.79 | 2,780 |
| 690 Y | 50 | 0.25 | 0.38 | 0.79 | 2,780 |
| 480 Δ | 60 | 0.30 | 0.65 | 0.79 | 3,340 |
| 828 Y | 60 | 0.30 | 0.38 | 0.79 | 3,340 |
The rows form two pairs. In each pair, the motor is rated for one frequency and supply voltage with two possible winding connections. The 50 Hz and 60 Hz ratings keep the same volts-per-hertz ratio of 8.0, so rated flux and torque are similar. The 60 Hz ratings run faster, which raises output power; the listed current remains 0.65 A for delta and 0.38 A for star.
Reading a NEMA nameplate
A North American nameplate uses different labels and includes an enclosure code and service factor. This example is a half-horsepower capacitor motor on a 48 frame, with a separate set of ratings for 50 Hz operation.
| Printed | Means | What it decides at replacement |
|---|---|---|
| HP 1/2 | 0.37 kW of shaft output | Match output, not frame; the frame only follows |
| 115/208-230 V, 60 Hz | Dual voltage, reconnectable | The wiring diagram inside the terminal box decides which |
| FLA 6.9 / 3.5-3.4 | Full-load current at each voltage | Use with the motor instructions and applicable rules when selecting overload protection |
| SFA 9.5 / 5.1-4.7 | Service factor amps | The current at the service factor load, not at rated load |
| SF 1.60 (60 Hz) / 1.0 (50 Hz) | Overload allowance, per frequency | A 50 Hz replacement does not have 60 percent margin |
| RPM 3,450 / 2,850 | Rated speed at each frequency | Two-pole machine; the drop at 50 Hz is the frequency, not a fault |
| INS B | 130 °C insulation system | Replacing B with F is safe; replacing F with B is a downgrade |
| MAX AMB 40C | Rated ambient | Printed on NEMA plates, implied on IEC ones |
| FR 48, ENC ODP | 48 frame, open drip proof | ODP is not an IP rating; a wet location needs a different enclosure |
| KVA CODE P | Locked-rotor kVA band | Sizes the generator or breaker behind the motor |
| DUTY CONT | Continuous duty | The NEMA equivalent of S1 |
The service factor differs by frequency: 1.60 at 60 Hz and 1.0 at 50 Hz. Do not carry the 60 Hz value into a 50 Hz specification. State the supply frequency and confirm the corresponding service-factor and current ratings with the supplier.
Speed: rated and synchronous
The speed printed on an induction-motor nameplate is its rated speed under load. It is below synchronous speed; the difference is called slip. A four-pole motor on 50 Hz may therefore have a rated speed near 1,450 rpm rather than the 1,500 rpm synchronous speed. At synchronous speed, an induction motor develops no torque.
Synchronous speed, rated speed and slip
Calculate synchronous speed as 120 × frequency ÷ pole count. Compare that result with the nameplate speed to find the slip.
| Plate | Printed speed | Synchronous | Poles | Slip |
|---|---|---|---|---|
| GS 632-2 at 50 Hz | 2,780 min⁻¹ | 3,000 | 2 | 7.3% |
| GS 632-2 at 60 Hz | 3,340 min⁻¹ | 3,600 | 2 | 7.2% |
| NEMA 48 frame at 60 Hz | 3,450 rpm | 3,600 | 2 | 4.2% |
| NEMA 48 frame at 50 Hz | 2,850 rpm | 3,000 | 2 | 5.0% |
| GSA 632-4 at 50 Hz | 1,310 min⁻¹ | 1,500 | 4 | 12.7% |
Slip varies with motor design and load. Small motors often have a higher slip percentage than larger ones. A 12.7 percent value for a 0.18 kW four-pole example should be checked against that motor’s data; the same figure on a 200 kW motor would call for investigation.
Estimating pole count from speed
If the type string does not identify the pole count, estimate it from speed. Divide 120 times the frequency by the printed speed, then choose the conventional even pole count just below the result. Because rated speed is below synchronous speed, this quotient is slightly higher than the actual pole count.
For the 2,780 min⁻¹ rating at 50 Hz, 6,000 ÷ 2,780 is 2.16, which points to a two-pole motor. For 1,310 min⁻¹, 6,000 ÷ 1,310 is 4.58, which points to four poles. The full method, including calculations in both directions, is set out in number of poles in a motor.
Why the speed figure decides the replacement
Motors rated at 2,780 and 1,310 min⁻¹ are not interchangeable just because their power ratings are similar. A pump or fan with the same impeller can require roughly eight times as much power when its speed nearly doubles. A gearbox or belt drive designed for one speed may also be unsuitable for the other. Torque moves in the opposite direction: 0.25 kW at 2,780 min⁻¹ is 0.86 N·m, while 0.18 kW at 1,310 min⁻¹ is 1.31 N·m.
For a quick torque estimate, use T = 9,550 × kW ÷ rpm, with torque in N·m. The derivation is in how to calculate motor torque. The torque-speed plane shows the same relationship: at equal output power, a lower-speed motor produces more shaft torque.
Voltage and connection: delta and star
On many dual-voltage three-phase nameplates, a slash separates two connections of the same winding rather than a continuous voltage range. Typical nominal pairs include 220/380, 230/400, 400/690 and 480/828 V; the higher value is about √3 times the lower. The lower voltage is usually delta and the higher star. Confirm the connection diagram for the specific motor.
Why both connections give similar winding conditions
In delta at 400 V, each winding receives 400 V. In star at 690 V, each winding receives 690 ÷ √3, or about 398 V. The winding voltages are therefore close, so flux and torque capability are similar. For the same winding current, line current in star is lower by √3 than in delta.
If a 400 Δ / 690 Y motor is connected in delta to a 690 V supply, each winding receives 690 V instead of its 400 V rating. At constant frequency, the excess voltage can saturate the core, raise magnetising current sharply and overheat the winding. Check the link bars against the nameplate before energising the motor.
Reading dual-frequency ratings
The four-row table also shows the volts-per-hertz relationship. Both 400 V at 50 Hz and 480 V at 60 Hz give a ratio of 8.0. Rated torque is similar, while the 60 Hz speed is 20 percent higher; the output rating rises from 0.25 kW to 0.30 kW and the listed current remains 0.65 A. Use the row that matches the actual supply frequency and voltage.
Capacitors on single-phase nameplates
A single-phase nameplate may list a capacitor value, such as 20 µF 450 V. The capacitor is part of the motor’s starting or running circuit, so a replacement with a different specified value may affect starting torque or operating temperature. Use the single-phase input-power formula when checking current; the three-phase √3 factor does not apply.
Current: FLA, SFA and measured readings
Full-load current (FLA) is the current at rated output, voltage and frequency. It is not a maximum current or an overload trip setting on its own. Some setups use a value around 1.0 to 1.15 times FLA; the correct multiplier depends on service factor and applicable rules. Follow the motor instructions, electrical code and protection-device requirements.
A NEMA plate may also list service-factor amps (SFA), the current associated with operation at the service-factor load. FLA and SFA describe different operating points. Do not use SFA as though it were FLA when setting or checking motor protection.
| Reading | What it usually means |
|---|---|
| Well below FLA | Light load, or a pump running out on its curve; not a fault by itself |
| At FLA with low output | Voltage imbalance, worn bearings, or a partially blocked impeller |
| Above FLA on all phases | Overload, or a 50 Hz motor on 60 Hz without a voltage change |
| One phase high, two normal | Supply imbalance; a 3 percent voltage unbalance produces 18 to 30 percent current unbalance |
| High at start, normal running | Normal for a direct start; a soft start or drive changes the profile, not the rating |
Measure the supply before assuming the motor is at fault; voltage imbalance may originate upstream. The checks that distinguish a supply problem from a winding problem are described in maintenance of three-phase asynchronous motors.
The same figures can provide a rough current estimate: I = output ÷ (√3 × U × η × cos φ). For the 0.25 kW example, using 70 percent efficiency and a 0.79 power factor gives about 0.65 A, close to the nameplate value. If measured current differs by more than about 10 percent, check the voltage, load and efficiency assumptions before drawing a conclusion.
Temperature: insulation class, rise, duty and ambient
A nameplate may list three temperature-related items that describe different limits. Insulation class sets a thermal limit for the winding insulation, temperature rise describes how much hotter the winding gets than ambient, and duty describes the operating cycle.
Insulation class and temperature rise
Class F insulation has a thermal class limit of 155 °C, a limit tied to the insulation system’s thermal endurance. It is not the motor’s normal operating temperature. A motor with Class F insulation and Class B temperature rise (80 K) would have an average winding temperature near 120 °C at a 40 °C ambient, leaving a nominal 35 K below the Class F limit. Actual hot-spot temperature and allowable rise depend on motor design and measurement method, so ask for the rise rating as well as the insulation class.
Duty, ambient temperature and altitude
S1 denotes continuous duty at rated output. S2 denotes short-time duty and is normally followed by a duration in minutes. S3 denotes periodic duty and includes a cyclic duration factor as a percentage. A motor marked S1-100% is rated for continuous operation at its stated output; an S2 60 min rating at the same kW does not indicate the same continuous capability.
Ambient temperature, altitude and coolant conditions can affect the available rating. IEC reference conditions commonly include 40 °C ambient and 1,000 m altitude; the coolant reference also depends on cooling method, so verify the applicable value for the motor. There is no single derating percentage that applies to every design. Rules of thumb such as about 1 percent per kelvin above 40 °C or per 100 m above 1,000 m are preliminary estimates only; confirm the adjustment with the manufacturer. A NEMA plate may state MAX AMB 40C directly.
NEMA service factor
A service factor of 1.15 indicates that the motor can deliver up to 15 percent above rated output under specified conditions, with a corresponding service-factor current. It is a conditional allowance. Choose a motor whose rated output meets the normal load; operating above rated load can affect temperature and service life. IEC nameplates generally do not use the NEMA service-factor notation, so compare the stated ratings and conditions as well as the kW figures.
Frame and mounting dimensions
Matching the frame number alone does not guarantee a mechanical fit. Check shaft height, foot-hole spacing and shaft diameter; two motors that appear similar can differ in these dimensions.
| IEC 63 | NEMA 48 | NEMA 56C | |
|---|---|---|---|
| Basis | Shaft height, 63 mm | Frame series, not a dimension | Frame series with C-face flange |
| Mounting | Feet, flange, or both, by IM code | Feet, resilient base, or face | Face and feet, flange register locates it |
| Typical use | Small industrial machines | Small fans, pumps, appliances | Pumps, gear reducers, close-coupled loads |
| Interchangeable with the other? | No. Shaft height, hole spacing and shaft diameter are all set by different tables. |
IM codes and the B34 mounting
IEC mounting arrangements use combined codes. B3 is foot-mounted, B5 is flange-mounted, B35 combines feet with a flange using clearance holes, and B34 combines feet with a flange using tapped holes. Check the flange drilling before ordering; the two arrangements can look alike until installation.
For a close-coupled load, the flange register and shaft fit locate the motor and determine alignment. A foot-mounted motor with the same power and speed may not fit. An adapter plate can cost more than the correct motor and adds another alignment interface, so confirm the dimensions before choosing one.
Gearmotor nameplates
A gearmotor nameplate lists the motor ratings along with a reduction ratio and output speed. Do not confuse output speed with motor speed; with a large reduction ratio, the error can approach a factor of 100, and the torque estimate will also be wrong. The conventions are covered in selection of speed reducer motors.
Enclosure: IP and ODP ratings
The two digits in an IP code describe separate protections. The first covers access and solid objects, including dust; the second covers water. Read each digit separately, since a high water-protection digit does not imply the same level of protection against solids.
| Marking | Solids | Water | Suits |
|---|---|---|---|
| IP23 | Fingers, tools above 12 mm | Spraying water, 60° from vertical | Dry indoor plant rooms only |
| IP54 | Dust protected, not dust tight | Splashing from any direction | General indoor, light washdown |
| IP55 | Dust protected | Water jets, 6.3 mm nozzle | Hose-down areas, covered outdoor |
| IP65 | Dust tight | Water jets, 6.3 mm nozzle | Dusty and wet, food and outdoor equipment |
| ODP, NEMA | Open drip proof | Dripping water at defined angles | Clean, dry, indoor; not an IP equivalent |
An IP code does not specify corrosion resistance, UV resistance, internal condensation control or enclosure surface temperature. A motor with the same IP rating may still be unsuitable if its material is incompatible with chemicals or if condensation can form in a cold environment.
Other nameplate fields to check
| Field | Why it matters at replacement |
|---|---|
| PTC thermistors, “Kaltleiter” | The protection relay expects them; a motor without them leaves the circuit blind |
| Temperature rise letter, if printed | Distinguishes a Class F motor limited to B rise from one run at its class limit |
| Serial number | Identifies the unit for warranty, and separates a factory build from a rewind |
| Net mass | A 30 percent lighter motor of the same rating is a different build |
| Bearing designation, if printed | Determines grease type and re-lubrication interval |
| Direction of rotation, if marked | A unidirectional fan or pump will run backwards and still look fine |
| KVA code, NEMA | Sizes the generator or breaker behind a direct-started motor |
| Absence of an IE mark | Covered three-phase motors from 0.12 kW to below 0.75 kW may require IE2; check scope and product documents |
Thermistors are easy to overlook during a replacement. If the protection relay depends on embedded PTC thermistors, a motor without them will not provide the same protection. When the original plate lists Kaltleiter or PTC, confirm the replacement includes compatible sensors and connections.
Replacement worksheet
Use these nine checks when specifying a replacement. Photograph the plate before the motor is removed; a faded plate can still provide useful details. For definitions of the terms, see electric motor basics.
- Output and speed together: kW or hp, and the rated speed at the supply frequency you actually have.
- Voltage, frequency and connection: the exact row of the table, including delta or star.
- Frame and mounting code: shaft height, foot spacing, flange register, shaft diameter and key.
- Duty: S1, S2 with minutes, or S3 with a percentage; and starts per hour if the machine cycles.
- Enclosure: IP code or NEMA enclosure, plus the environment the code does not describe.
- Insulation class and rise, and the site ambient and altitude.
- Protection devices: PTC thermistors, thermal contacts, or none.
- Current: FLA, and SFA where a service factor is quoted.
- Serial and year, for traceability and to identify a rewind.
Photograph the plate, terminal-box wiring and mounting interface before removal. The wiring details are often lost between removal and reinstallation. If the motor is still running, record its current and voltage as well. The nameplate describes its ratings; measurements show how it is operating in the actual installation.
If the replacement uses a different technology, use the old motor’s output, speed and mechanical envelope as starting requirements, then check the new motor’s parameters separately. See BLDC motor selection and parameters for DC motor selection for brushless and other DC motors. The motor discussed here is a squirrel-cage induction motor; related applications and routine checks are covered in squirrel cage induction motor applications and electric motor testing standards.
Six common misreadings
| Misreading | What actually happens | Correct reading |
|---|---|---|
| “1,450 rpm means a 1,500 rpm motor with a small error” | The 50 rpm is slip, and it is what produces torque | Rated speed is below synchronous by design |
| “230/400 V is a voltage range” | It is delta at 230 and star at 400 | Wire to the supply, not to the lower number |
| “Same kW and same frame means interchangeable” | Mounting code, shaft and flange still differ | Match shaft height, holes, register and shaft |
| “SF 1.15 means 15 percent more power available always” | It is a conditional overload capability, not a substitute for selecting rated output | Size for the normal load and confirm the SF conditions |
| “IP55 means waterproof” | It means dust protected and jet resistant | Immersion needs IP67 or better, and seals |
| “No IE mark means low efficiency” | A missing class marking may need checking against regulation scope and product documents | Confirm the rating, exclusions and declared efficiency |
There is also a data-handling issue. Copying plate details into a spreadsheet can introduce hidden spaces, replace superscripts with question marks, or drop the Greek delta symbol. If a part number fails in a cross-reference tool, check it manually and retype it if needed.
References
- International Electrotechnical Commission. (2026). IEC 60034-1:2026 — Rotating electrical machines, Part 1: Rating and performance. IEC. https://webstore.iec.ch/en/publication/89961
- International Electrotechnical Commission. (2020). IEC 60034-7:2020 — Classification of types of construction, mounting arrangements and terminal box position (IM Code). IEC. https://webstore.iec.ch/en/publication/64225
- 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
- National Electrical Manufacturers Association. (2021). NEMA MG 1 — Motors and Generators. NEMA. https://www.nema.org/standards
- European Parliament and Council. (2019). Regulation (EU) 2019/1781 laying down ecodesign requirements for electric motors and variable speed drives. EUR-Lex. https://eur-lex.europa.eu/eli/reg/2019/1781/oj
- European Parliament and Council. (2014). Directive 2014/35/EU on the harmonisation of the laws relating to electrical equipment designed for use within certain voltage limits. EUR-Lex. https://eur-lex.europa.eu/eli/dir/2014/35/oj
- U.S. Department of Energy. (2024). Motor Systems — Advanced Manufacturing Office. DOE. https://www.energy.gov/eere/amo/motor-systems
FAQ
Q: What does 1,450 rpm on a motor nameplate mean?
It is the rated full-load speed. A four-pole motor on 50 Hz has a synchronous speed of 1,500 rpm; at rated load, its rotor typically runs about 3 to 5 percent slower. That difference is slip. At synchronous speed there is no relative motion between the rotating field and rotor, so an induction motor develops no torque.
Q: What does 400/690 V mean on a nameplate?
It usually identifies one winding with two connection options: delta at the lower voltage and star at the higher, in a ratio close to √3. At 400 V delta, each winding receives 400 V; at 690 V star, each receives about 398 V. Connecting the motor in delta to 690 V would overvoltage the windings and could cause core saturation, high magnetising current and overheating. Check the wiring diagram before energising it.
Q: Can I replace a 50 Hz motor with a 60 Hz motor of the same power?
Matching kilowatts alone is not enough. The example plate is rated 0.25 kW at 400 V and 50 Hz, and 0.30 kW at 480 V and 60 Hz. The volts-per-hertz ratio is the same, while the 60 Hz rating runs 20 percent faster and lists 20 percent more power at similar torque. Check the voltage and frequency, rated speed and frame for the actual supply and installation.
Q: Why is there no IE efficiency class on my nameplate?
The answer depends on the motor type and whether it falls within the regulation’s scope. Since 1 July 2021, covered three-phase motors from 0.12 kW to below 0.75 kW with 2, 4, 6 or 8 poles must meet at least IE2; requirements differ for other motor types and exclusions. For a motor of 0.75 kW or more, an absent IE class should also be checked against its scope and documentation. For a 0.25 kW motor, check the model details before deciding whether a missing class marking is expected.
Q: What is the difference between insulation class F and temperature rise B?
Class F identifies the insulation thermal class, with a 155 °C limit; it does not state the motor’s normal running temperature. Temperature rise describes the winding temperature above ambient. A Class F motor with an 80 K Class B rise would have an average winding temperature near 120 °C at 40 °C ambient, a nominal 35 K below the Class F limit. Confirm the rise rating and actual hot-spot temperature with the motor manufacturer, especially for a hot plant room.
Q: What does service factor 1.15 actually allow?
A 1.15 service factor indicates capacity up to 15 percent above rated output under specified voltage, frequency and ambient conditions, and the plate may list a corresponding service-factor current. It is a conditional capability, not a reason to select a motor below the normal load rating; operation above rated load can affect temperature and service life. IEC motors generally do not use the NEMA service-factor notation, so compare the stated ratings and conditions as well as the kW values.
Greensky Power manufactures three-phase induction, single-phase capacitor and brushless DC motors to IEC 60034, with Class F insulation, PTC protection and multi-row voltage tables where applicable. Send a nameplate photo and motor-duty details to the engineering team to check a replacement against the relevant ratings.
