Motor nameplate data and efficiency classes
Three Phase Systems · Lesson 20 · Advanced
Purpose
Read a motor nameplate as a coordinated set of ratings and correctly interpret current, connection, duty, enclosure and efficiency information.
Learning objectives
- Identify rated operating point
- Interpret dual-voltage connection
- Distinguish output from input
- Use IE class within scope
A nameplate describes declared operation at stated voltage, frequency, connection, duty and environmental conditions. No single value can be selected safely in isolation.
Core theory
Rated kW on an ordinary motor plate is typically mechanical shaft output; electrical input is higher because efficiency is below unity. Rated current, PF and efficiency apply at declared conditions.
A marking such as 230/400 V Δ/Y means each winding is designed for about 230 V: use delta on a 230 V line supply or star on a 400 V line supply, subject to exact manufacturer data.
IE classes are standardised efficiency bands within defined motor scope; IEC 60034-30-1:2025 includes IE5. IE class does not state starting current, duty suitability, converter compatibility, frame fit, or whole-drive-system efficiency.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| Rated output | Declared mechanical shaft power at the rated point | PN | kW |
| Duty type | Declared loading/time pattern such as continuous S1 | Not applicable | Not applicable |
| IE class | International nominal motor-efficiency class within standard scope | Not applicable | Not applicable |
Assumptions: Efficiency is 0.91 at the stated operating point.
- Relation: η = Pout/Pin.
- Rearrange: Pin = Pout/η.
- Calculate: Pin = 15/0.91 ≈ 16.48 kW.
Estimated active input is approximately 16.48 kW.
Reasonableness check: Input exceeds output by about 1.48 kW of losses.
- Treating shaft kW as electrical input
- Reversing Δ/Y voltage order
- Assuming IE class proves application suitability
Where this appears in practice
Nameplate interpretation supports replacement, starter settings, supply checks, efficiency studies, and maintenance baselines.
Knowledge check
Does a higher IE class by itself prove a motor will start its load?
No. Starting torque/current, inertia, duty, supply and control method remain separate checks.
Answer: No. Starting torque/current, inertia, duty, supply and control method remain separate checks.
Practical exercise
Annotate an anonymised plate with electrical input data, mechanical output data, connection, duty, insulation and enclosure fields.
Summary
- Ratings form one operating point
- Output kW differs from input
- IE scope has limits
Sources and review
- IEC 60034-30-1: Efficiency classes of line-operated AC motors: IEC; 2025; International
- IEC 60076-3: Power transformers: insulation and dielectric tests: IEC; 2013+A1:2018; International
- The safe isolation of plant and equipment (HSG253): Health and Safety Executive; Second edition; Great Britain
Editorial review date: 2026-08-22. Professional electrical review is pending.