oSIST prEN IEC 60034-26:2025

Rotating electrical machines - Part 26: Effects of unbalanced voltages on the performance of three-phase cage induction motors

Drehende elektrische Maschinen - Teil 26: Auswirkungen von Spannungsunsymmetrien auf das Betriebsverhalten von Drehstrom-Induktionsmotoren

Machines électriques tournantes - Partie 26: Effets d'un système de tensions déséquilibrées sur les caractéristiques de fonctionnement des moteurs à induction à cage triphasés

Električni rotacijski stroji - 26. del: Učinki napetostnih neravnovesij na delovanje trifaznih indukcijskih motorjev s kletko

General Information

• Status: Not Published
• Public Enquiry End Date: 20-Apr-2025
• ICS: 29.160.01 - Rotating machinery in general
• ICS: 29.160.30 - Motors
• Technical Committee: ERS - Rotating machinery
• Current Stage: 4020 - Public enquire (PE) (Adopted Project)
• Start Date: 04-Feb-2025
• Due Date: 24-Jun-2025
• Completion Date: 16-Jun-2025
• Ref Project: prEN IEC 60034-26:2025 - Rotating electrical machines - Part 26: Effects of unbalanced voltages on the performance of three-phase cage induction motors

Relations

• Revises: SIST EN 60034-26:2007 - Rotating electrical machines - Part 26: Effects of unbalanced voltages on the performance of three-phase cage induction motors (IEC 60034-26:2006)
• Effective Date: 09-Jul-2024
• Revises: SIST EN 60034-26:2007/AC:2015 - Rotating electrical machines - Part 26: Effects of unbalanced voltages on the performance of three-phase cage induction motors (IEC 60034-26:2006/COR1:2014)
• Effective Date: 09-Jul-2024

Overview

prEN IEC 60034-26:2025 (CLC) - Rotating electrical machines - Part 26: Effects of unbalanced voltages on the performance of three-phase cage induction motors - provides guidance on how voltage unbalance affects currents, heating, torque, speed and efficiency of three‑phase cage induction motors. This Committee Draft for Vote updates the 2006 edition and clarifies that the voltages considered are line‑to‑line voltages. It also adds guidance for design NE motors per IEC 60034‑12.

Keywords: IEC 60034‑26, rotating electrical machines, voltage unbalance, three‑phase cage induction motors, negative‑sequence voltage, derating.

Key topics and technical requirements

• Voltage unbalance definition and measurement
  • Unbalance factor f = U_n / U_p, where U_n is the RMS negative‑sequence component and U_p the RMS positive‑sequence component of the line‑to‑line voltages.
  • Annex A (informative) provides graphical and analytical methods to determine symmetrical components; Annex B gives an approximate evaluation method.
• Effects on currents
  • Negative‑sequence voltages induce a rotating flux opposing rotor rotation, producing high negative‑sequence currents. Phase current unbalance is typically about 6–10× the voltage unbalance.
  • Rotor currents can have frequency close to twice the rated frequency, amplifying rotor losses especially in cages with current displacement.
• Heating and losses
  • Stator and rotor temperature rise increases due to extra losses from negative‑sequence currents; rotor loss amplification is significant.
• Torque and dynamic effects
  • Locked‑rotor, pull‑up and breakdown torques are reduced by unbalance. An oscillating torque at twice line frequency appears; at f = 0.05 its peak can be ~25% of rated torque, possibly exciting torsional resonances.
• Performance and derating
  • Full‑load speed is slightly reduced; efficiency can fall and declared energy class may not be met.
  • When U_n exceeds 1% of U_p for durations comparable to the motor thermal time constant, motor power must be derated to avoid overheating. Operation above 5% voltage unbalance is not recommended.
• Normative reference
  • Cross‑references IEC 60034‑12 (starting performance for single‑speed three‑phase cage induction motors).

Applications and users

Who benefits from this standard:

• Motor designers and manufacturers - to specify thermal limits, derating curves and acceptable unbalance thresholds.
• Electrical engineers and system designers - to assess supply quality, select motors and design protection.
• Maintenance teams and test laboratories - for diagnostics, balancing checks, and interpreting measurements using symmetrical components.
• Utilities and facilities managers - to set site power quality criteria and avoid motor failures caused by long‑term unbalance.

Related standards

• IEC 60034 series (Rotating electrical machines) - see IEC 60034‑12 for motor starting performance and design NE guidance referenced in this part.

This standard is practical for anyone managing three‑phase cage induction motors in industrial power systems where voltage unbalance may occur.

Standards Content (Sample)

SLOVENSKI STANDARD
01-april-2025
Električni rotacijski stroji - 26. del: Učinki napetostnih neravnovesij na delovanje
trifaznih indukcijskih motorjev s kletko
Rotating electrical machines - Part 26: Effects of unbalanced voltages on the
performance of three-phase cage induction motors
Drehende elektrische Maschinen - Teil 26: Auswirkungen von Spannungsunsymmetrien
auf das Betriebsverhalten von Drehstrom-Induktionsmotoren
Machines électriques tournantes - Partie 26: Effets d'un système de tensions
déséquilibrées sur les caractéristiques de fonctionnement des moteurs à induction à
cage triphasés
Ta slovenski standard je istoveten z: prEN IEC 60034-26:2025
ICS:
29.160.30 Motorji Motors
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

2/2224/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 60034-26 ED2
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2025-01-31 2025-04-25
SUPERSEDES DOCUMENTS:
2/2203/CD, 2/2220/CC
IEC TC 2 : ROTATING MACHINERY
SECRETARIAT: SECRETARY:
United Kingdom Mr Charles Whitlock
OF INTEREST TO THE FOLLOWING COMMITTEES: HORIZONTAL FUNCTION(S):
SC 121A
ASPECTS CONCERNED:
SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING
Attention IEC-CENELEC parallel voting
The attention of IEC National Committees, members of
CENELEC, is drawn to the fact that this Committee Draft
for Vote (CDV) is submitted for parallel voting.
The CENELEC members are invited to vote through the
CENELEC online voting system.
This document is still under study and subject to change. It should not be used for reference purposes.
Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of
which they are aware and to provide supporting documentation.
Recipients of this document are invited to submit, with their comments, notification of any relevant “In Some
Countries” clauses to be included should this proposal proceed. Recipients are reminded that the CDV stage is the
final stage for submitting ISC clauses. (SEE AC/22/2007 OR NEW GUIDANCE DOC).

TITLE:
Rotating electrical machines - Part 26: Effects of unbalanced voltages on the performance of
three-phase cage induction motors

PROPOSED STABILITY DATE: 2027
NOTE FROM TC/SC OFFICERS:
electronic file, to make a copy and to print out the content for the sole purpose of preparing National Committee positions.
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IEC CDV 60034-26 ED2 © IEC 2024 2 2/2224/CDV
1 CONTENTS
2 FOREWORD . 3
3 INTRODUCTION . 5
4 1 Scope . 6
5 2 Normative references . 6
6 3 Effects of unbalanced voltages on performance . 6
7 3.1 Currents . 6
8 3.2 Heating . 6
9 3.3 Torque . 7
10 3.4 Full-load speed . 7
11 4 Derating of motor to prevent overheating . 7
12 Annex A (informative) Determination of the symmetrical components of the line-to-line
13 voltages U , U , U of a three-phase system . 9
1 2 3
14 A.1 Graphical determination . 9
15 A.2 Analytical determination . 10
16 Annex B (informative) Approximate determination . 11
18 Figure 1 – Typical values of derating of design N or NE, three-phase cage induction
19 motors within the scope of IEC 60034-12 . 8
20 Figure A.1 – Phasor diagram . 9
IEC CDV 60034-26 ED2 © IEC 2024 3 2/2224/CDV
23 INTERNATIONAL ELECTROTECHNICAL COMMISSION
24 ____________
26 ROTATING ELECTRICAL MACHINES –
28 Part 26: Effects of unbalanced voltages on the performance
29 of three-phase cage induction motors
32 FOREWORD
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66 International Standard IEC 60034-26 has been prepared by IEC technical committee 2:
67 Rotating machinery.
68 This standard cancels and replaces IEC 60034-26 ed. 1 published in 2006. This edition
69 constitutes a technical revision.
70 This edition includes the following significant technical changes with respect to the previous
71 edition:
Clause or
Change
subclause
4, Annexes Clarification that voltages are line-to-line voltages
4 Design NE according to IEC 60034-12 added
73 The text of this standard is based on the following documents:

IEC CDV 60034-26 ED2 © IEC 2024 4 2/2224/CDV
FDIS Report on voting
75 Full information on the voting for the approval of this standard can be found in the report on
76 voting indicated in the above table.
77 This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
78 A list of all parts of IEC 60034 series, under the general title Rotating electrical machines can be found
79 on the IEC website.
80 The committee has decided that the contents of this publication will remain unchanged until
81 the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
82 the data related to the specific publication. At this date, the publication will be
83 • reconfirmed,
84 • withdrawn,
85 • replaced by a revised edition, or
86 • amended.
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