IEC 60034-26:2026

IEC 60034-26 ®
Edition 2.0 2026-01
INTERNATIONAL
STANDARD
REDLINE VERSION
Rotating electrical machines -
Part 26: Effects of unbalanced voltages on the performance of three-phase cage
induction motors
ICS 29.160.01 ISBN 978-2-8327-1009-8
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or
by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either
IEC or IEC's member National Committee in the country of the requester. If you have any questions about IEC copyright
or have an enquiry about obtaining additional rights to this publication, please contact the address below or your local
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3, rue de Varembé info@iec.ch
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About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

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CONTENTS
FOREWORD . 2
INTRODUCTION . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Effects of unbalanced voltages on performance . 5
4.1 General . 5
4.2 Currents . 5
4.3 Heating . 5
4.4 Torque . 5
4.5 Full-load speed . 6
4.6 Efficiency . 6
5 Derating of motor to prevent overheating . 6
Annex A (informative) Worked example .
Annex A (informative) Determination of the symmetrical components of the line-to-line
voltages U1, U2, U3 of a three-phase system . 10
A.1 Graphical determination . 10
A.2 Analytical determination . 11
Annex B (informative) Approximate determination of the unbalance factor . 12
Bibliography . 13

Figure 1 – Typical values of derating of design N or NE, three-phase cage induction
motors within the scope of IEC 60034-12 . 8
Figure A.1 – Phasor diagram . 10

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Rotating electrical machines -
Part 26: Effects of unbalanced voltages on the performance of three-
phase cage induction motors
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as "IEC Publication(s)"). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
This redline version of the official IEC Standard allows the user to identify the changes made
to the previous edition IEC 60034-26:2006. A vertical bar appears in the margin wherever a
change has been made. Additions are in green text, deletions are in strikethrough red text.

IEC 60034-26 has been prepared by IEC technical committee 2: Rotating machinery. It is an
International Standard.
This second edition cancels and replaces the first edition published in 2006. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) clarification that voltages are line-to-line voltages in Clause 4, Annex A and Annex B;
b) addition of design NE according to IEC 60034-12 in Clause 4.
The text of this International Standard is based on the following documents:
Draft Report on voting
2/2224/CDV 2/2251/RVC
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts of IEC 60034 series, under the general title Rotating electrical machines can
be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
INTRODUCTION
When the line voltages applied to a three-phase cage induction motor are not equal a balanced,
symmetric three-phase system, the currents in the phases of the stator winding will also be
unequal. A small percentage voltage unbalance will result in a much larger percentage current
unbalance.
The application of unbalanced voltages to a three-phase induction motor introduces a negative
sequence voltage, and this produces a flux in the air-gap, a flux rotating against the rotation of
the rotor, thus tending to produce high currents having a slip of almost 200 %. For high values
of the slip, the motor impedance is low, thus tending to produce a high negative sequence
current in the stator winding and high currents in the rotor cage. A small negative sequence
voltage may can produce currents in the windings winding phases considerably in excess of
those present under balanced voltage conditions. Consequently, the temperature rise of the
motor operating at a particular load and a particular percentage of voltage unbalance will be
greater higher than for the motor operating under the same conditions with balanced voltages.
The analytical and graphical methods used to calculate the symmetrical components from the
voltage readings of the three phases are well-known and can be taken from textbooks. Thus,
these calculation schemes are not incorporated in this document but shown in the informative
Annex A. Besides, the evaluation of the symmetrical components can be done automatically by
modern instrumentation.
An approximate evaluation of imbalance is given in the informative Annex B.

1 Scope
This part of IEC 60034 describes the effects of unbalanced voltages on the performance of
three-phase cage induction motors.
2 Normative references
The following referenced documents are indispensable for the application of this document. For
dated references, only the edition cited applies. For undated references, the latest edition of
the referenced document (including any amendments) applies.
IEC 60034-12, Rotating electrical machines – Part 12: Starting performance of single-speed
three-phase cage induction motors
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
4 Effects of unbalanced voltages on performance
4.1 General
The effects of unbalanced voltages on motor performance are as described in 4.2 to 4.6.
4.2 Currents
The negative sequence component of the voltage produces a flux in the air-gap rotating against
the rotation of the rotor. A small negative-sequence component of the voltage can produce
currents in the winding phases considerably in excess of those present under balanced voltage
conditions. The frequency of the current in the cage is almost twice the rated frequency. Thus,
in the case of cages with current displacement, the increase of the rotor winding losses is
substantially higher than the increase of the stator winding losses.
The currents at normal operating speed will be greatly unbalanced in the order of approximately
6 to 10 times the voltage unbalance.
The locked-rotor current will be unbalanced to the same degree that the voltages are
unbalanced, but the locked-rotor apparent power will increase only slightly.
4.3 Heating
The temperature rise of the stator winding is always higher than in operation at balanced supply
voltages due to the increase of the losses produced by the negative-sequence components of
the currents and voltages.
The increase of the rotor losses is amplified by the current displacement (see 4.2).
In addition, unbalance of the voltages often is associated with a reduction of the positive-
sequence component of the voltage, which causes, for a given load, an increase of the positive-
sequence components of the currents in stator and rotor.
4.4 Torque
The locked-rotor, pull-up and breakdown torques are decreased when the voltages are
unbalanced. If Should the voltage unbalance should be extremely severe, the torques might will
perhaps not be adequate sufficient for the application.
Voltage unbalance is associated with the generation of an oscillating torque of twice the line
frequency. Its amplitude increases linearly with the product of the negative and the positive
sequence component of the voltages; at an unbalance factor of f = 0,05 (see Clause 5), its
u
peak value is in the range of 25 % of the rated torque. Impermissible torsional vibrations of the
complete shaft system can be excited, when its critical torsional speed is close to twice the line
frequency.
4.5 Full-load speed
The full-load speed is reduced slightly when the motor operates with unbalanced voltages
because of the higher slip associated with the additional rotor losses.
4.6 Efficiency
Due to higher losses and temperatures, there is also influence on the efficiency and a motor
can perhaps not reach the declared energy efficiency classification IEx.
5 Derating of motor to prevent overheating
When an AC motor for use on a power supply of rated frequency is connected to a three-phase
voltage system having a negative-sequence component exceeding 1 % of the positive-
sequence component of the voltages over a long period, i.e. at least the thermal time constant
of the machine, the permissible power of the motor is less than the rated power to reduce the
possibility of damage to the motor. A typical derating factor for motors of designs N and NE
within the scope of IEC 60034-12 is given in Figure 1 on the supposition that the positive-
sequence component of the supply voltage is close to the rated voltage. Operation of the motor
above a 5 % voltage unbalance condition is not recommended. Ways to determine U and U
p n
are provided in Annex A.
The negative sequence components of the voltage produce in the air gap a flux rotating against
the rotation of the rotor. A small negative-sequence component of the voltage may produce
currents in the winding considerably in excess of those present under balanced voltage
conditions. In addition, in the case of cages with current displacement, the increase of the rotor
winding losses is substantially higher than the increase of the stator winding losses due to rotor
currents of nearly twice rated frequency.

The unbalance factor f in Figure 1 is defined as
u
f = U /U
u 2 1
where
U is the r.m.s. value of the negative-sequence component of the supply voltage;
U is the r.m.s. value of the positive-sequence component of the supply voltage.
1,0
0,9
Y
0,8
0,7
0         0,01        0,02         0,03        0,04        0,05
X
IEC  1202/02
Key
X axis is voltage unbalance (f )
u
Y axis is the derating factor
Figure 1 – Typical values of derating of design N, three-phase
cage induction motors within the scope of IEC 60034-12

f =U /U
(1)
u np
where
U is the RMS value of the negative-sequence component of the supply voltage;
n
U is the RMS value of the positive-sequence component of the supply voltage;
p
U and U can be determined graphically (see Figure A.1) or analytically as shown in Annex A.
n p
An approximation of the unbalance factor is given in Annex B.

Figure 1 – Typical values of derating of design N or NE, three-phase cage induction
motors within the scope of IEC 60034-12

Annex A
(informative)
Worked example
L3
U
i
U
L3L1
U = √3U
i 2
U
L2L3
U
L1L2
L1
L2
U = −j(U /√3)
2 i
U
IEC  1203/02
Figure A.1 – Phasor diagram
Graphical method to determine the positive-sequence component U and the negative-
sequence component U from the phasor diagram of the unbalanced voltages U , U ,
2 L1L2 L2L3
U .
L3L1
Example: U = 400 V
L1L2
U = 484 V
L2L3
U = 387 V
L3L1
The graphical method results in:
U = 64 V
U = 419 V
f = U /U = 15,2 %
u 2 1
Annex A
(informative)
Determination of the symmetrical components of the line-to-line voltages
U1, U2, U3 of a three-phase system
A.1 Graphical determination
Figure A.1 – Phasor diagram
Steps of the graphical determination of the symmetrical components U and U as displayed in
p n
Figure A.1:
– draw the phasor diagram of the three line-to-line voltages U , U , U , the sum of which is
1 2 3
always zero;
– erect an equilateral triangle upon one of the phasors (U is chosen in Figure A.1);
– the connection of the upper corners of the two triangles results in the auxiliary phasor U ;
i
– draw a straight line, perpendicular to the phasor U and meeting with the opposite corner of
i
the triangle built up by U , U , U ;
1 2 3
– the elongation of this line results in the negative-sequence component U , if its length is
n
made U 3 ;
i
– the connection of the tip of the phasor U and the tip of the phasor U is the phasor of the
n 1
positive sequence component U .
p
A.2 Analytical determination
The symmetrical components are defined by Equations (A.1) and (A.2):
– positive-sequence component
( )
U = U + aU + a U (A.1)
p 1 2 3
– negative-sequence component
U = (U + a U + aU ) (A.2)
n 1 2 3
where
j π
a= e 1+ a+ a = 0
and .
When the values and the phase-angles of the three line-to-line voltages are known, the
symmetrical components can be calculated directly from the complex Equations (A.1) and (A.2).
If only the RMS values of the line-to-line voltages are known, the symmetrical components can
be computed from Equations (A.3) and (A.4):
1 π

U U+ U− 2UU cos φ− (A.3)
,
n 1 2 12 1
3 
U U+ U− 2UU⋅⋅cosφ (A.4)
p n 1 n1 n
where
3
UU− sinφ
1 21 π

φ arc sin −
n

3U
n

2 22
 
U + UU−
1 2 3
φ = arc cos 
 
2UU
 
Equations (A.3) and (A.4) contain real quantities only.
=
=
=
Annex B
(informative)
Approximate determination of the unbalance factor
For practical purposes, the simplified formula
ΔU
' max
f =
u (B.1)
U
average
'
can be used for the determination of the magnitude of unbalance factor and applied to
f
u
Figure 1, where:
U is the average value of the RMS value of the three line-to-line voltages U , U ,
average 1 2
U ,
∆U is the maximum of the differences between U and the RMS values U , U ,
max average 1 2
U .
'
NOTE A mathematically correct interrelation between the factors f and f determined according to Annexes A and
u
u
B does not exist. The factor according to Annex B gives only an approximation of the physical reality. Although
normally below 3 %, in unusual circumstances, the factor according to Annex A can be up to 14 % higher than the
approximate value from Annex B resulting in a maximum deviation of 7 % of the obtained derating factor.
Bibliography
IEC 60034-12:2024, Rotating electrical machines - Part 12: Starting performance of single-
speed three-phase cage induction motors

___________
IEC 60034-26 ®
Edition 2.0 2026-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Rotating electrical machines -
Part 26: Effects of unbalanced voltages on the performance of three-phase cage
induction motors
Machines électriques tournantes -
Partie 26: Effets d'un système de tensions déséquilibrées sur les
caractéristiques de fonctionnement des moteurs à induction à cage triphasés
ICS 29.160.01  ISBN 978-2-8327-0985-6

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or
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About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigendum or an amendment might have been published.

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CONTENTS
FOREWORD . 2
INTRODUCTION . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Effects of unbalanced voltages on performance . 5
4.1 General . 5
4.2 Currents . 5
4.3 Heating . 5
4.4 Torque . 5
4.5 Full-load speed . 6
4.6 Efficiency . 6
5 Derating of motor to prevent overheating . 6
Annex A (informative) Determination of the symmetrical components of the line-to-line
voltages U , U , U of a three-phase system . 7
1 2 3
A.1 Graphical determination . 7
A.2 Analytical determination . 8
Annex B (informative) Approximate determination of the unbalance factor . 9
Bibliography . 10

Figure 1 – Typical values of derating of design N or NE, three-phase cage induction
motors within the scope of IEC 60034-12 . 6
Figure A.1 – Phasor diagram . 7

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Rotating electrical machines -
Part 26: Effects of unbalanced voltages on the performance of three-
phase cage induction motors
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as "IEC Publication(s)"). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC 60034-26 has been prepared by IEC technical committee 2: Rotating machinery. It is an
International Standard.
This second edition cancels and replaces the first edition published in 2006. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) clarification that voltages are line-to-line voltages in Clause 4, Annex A and Annex B;
b) addition of design NE according to IEC 60034-12 in Clause 4.
The text of this International Standard is based on the following documents:
Draft Report on voting
2/2224/CDV 2/2251/RVC
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts of IEC 60034 series, under the general title Rotating electrical machines can
be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
INTRODUCTION
When the line voltages applied to a three-phase cage induction motor are not a balanced,
symmetric three-phase system, the currents in the phases of the stator winding will also be
unequal. A small percentage voltage unbalance will result in a much larger percentage current
unbalance.
The application of unbalanced voltages to a three-phase induction motor introduces a negative
sequence voltage, and this produces a flux in the air-gap rotating against the rotation of the
rotor, thus having a slip of almost 200 %. For high values of the slip, the motor impedance is
low, thus tending to produce a high negative sequence current in the stator winding and high
currents in the rotor cage. A small negative sequence voltage can produce currents in the
winding phases considerably in excess of those present under balanced voltage conditions.
Consequently, the temperature rise of the motor operating at a particular load and a particular
percentage of voltage unbalance will be higher than for the motor operating under the same
conditions with balanced voltages.
The analytical and graphical methods used to calculate the symmetrical components from the
voltage readings of the three phases are well-known and can be taken from textbooks. Thus,
these calculation schemes are not incorporated in this document but shown in the informative
Annex A. Besides, the evaluation of the symmetrical components can be done automatically by
modern instrumentation.
An approximate evaluation of imbalance is given in the informative Annex B.

1 Scope
This part of IEC 60034 describes the effects of unbalanced voltages on the performance of
three-phase cage induction motors.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
4 Effects of unbalanced voltages on performance
4.1 General
The effects of unbalanced voltages on motor performance are as described in 4.2 to 4.6.
4.2 Currents
The negative sequence component of the voltage produces a flux in the air-gap rotating against
the rotation of the rotor. A small negative-sequence component of the voltage can produce
currents in the winding phases considerably in excess of those present under balanced voltage
conditions. The frequency of the current in the cage is almost twice the rated frequency. Thus,
in the case of cages with current displacement, the increase of the rotor winding losses is
substantially higher than the increase of the stator winding losses.
The currents at normal operating speed will be greatly unbalanced in the order of approximately
6 to 10 times the voltage unbalance.
The locked-rotor current will be unbalanced to the same degree that the voltages are
unbalanced, but the locked-rotor apparent power will increase only slightly.
4.3 Heating
The temperature rise of the stator winding is always higher than in operation at balanced supply
voltages due to the increase of the losses produced by the negative-sequence components of
the currents and voltages.
The increase of the rotor losses is amplified by the current displacement (see 4.2).
In addition, unbalance of the voltages often is associated with a reduction of the positive-
sequence component of the voltage, which causes, for a given load, an increase of the positive-
sequence components of the currents in stator and rotor.
4.4 Torque
The locked-rotor, pull-up and breakdown torques are decreased when the voltages are
unbalanced. Should the voltage unbalance be extremely severe, the torques will perhaps not
be sufficient for the application.
Voltage unbalance is associated with the generation of an oscillating torque of twice the line
frequency. Its amplitude increases linearly with the product of the negative and the positive
sequence component of the voltages; at an unbalance factor of f = 0,05 (see Clause 5), its
u
peak value is in the range of 25 % of the rated torque. Impermissible torsional vibrations of the
complete shaft system can be excited, when its critical torsional speed is close to twice the line
frequency.
4.5 Full-load speed
The full-load speed is reduced slightly when the motor operates with unbalanced voltages
because of the higher slip associated with the additional rotor losses.
4.6 Efficiency
Due to higher losses and temperatures, there is also influence on the efficiency and a motor
can perhaps not reach the declared energy efficiency classification IEx.
5 Derating of motor to prevent overheating
When an AC motor for use on a power supply of rated frequency is connected to a three-phase
voltage system having a negative-sequence component exceeding 1 % of the positive-
sequence component of the voltages over a long period, i.e. at least the thermal time constant
of the machine, the permissible power of the motor is less than the rated power to reduce the
possibility of damage to the motor. A typical derating factor for motors of designs N and NE
within the scope of IEC 60034-12 is given in Figure 1 on the supposition that the positive-
sequence component of the supply voltage is close to the rated voltage. Operation of the motor
above a 5 % voltage unbalance condition is not recommended. Ways to determine U and U
p n
are provided in Annex A.
The unbalance factor f in Figure 1 is defined as
u
f =U /U
u np (1)
where
U is the RMS value of the negative-sequence component of the supply voltage;
n
U is the RMS value of the positive-sequence component of the supply voltage;
p
U and U can be determined graphically (see Figure A.1) or analytically as shown in Annex A.
n p
An approximation of the unbalance factor is given in Annex B.

Figure 1 – Typical values of derating of design N or NE, three-phase cage induction
motors within the scope of IEC 60034-12
Annex A
(informative)
Determination of the symmetrical components of the line-to-line voltages
U , U , U of a three-phase system
1 2 3
A.1 Graphical determination
Figure A.1 – Phasor diagram
Steps of the graphical determination of the symmetrical components U and U as displayed in
p n
Figure A.1:
– draw the phasor diagram of the three line-to-line voltages U , U , U , the sum of which is
1 2 3
always zero;
– erect an equilateral triangle upon one of the phasors (U is chosen in Figure A.1);
– the connection of the upper corners of the two triangles results in the auxiliary phasor U ;
i
– draw a straight line, perpendicular to the phasor U and meeting with the opposite corner of
i
the triangle built up by U , U , U ;
1 2 3
– the elongation of this line results in the negative-sequence component U , if its length is
n
made
...

IEC 60034-26 ®
Edition 2.0 2026-01
NORME
INTERNATIONALE
Machines électriques tournantes -
Partie 26: Effets d'un système de tensions déséquilibrées sur les
caractéristiques de fonctionnement des moteurs à induction à cage triphasés
ICS 29.160.01  ISBN 978-2-8327-0985-6

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SOMMAIRE
AVANT-PROPOS . 2
INTRODUCTION . 4
1 Domaine d’application . 5
2 Références normatives . 5
3 Termes et définitions . 5
4 Effets des tensions déséquilibrées sur les caractéristiques de fonctionnement . 5
4.1 Généralités . 5
4.2 Courants . 5
4.3 Échauffements . 5
4.4 Couple . 5
4.5 Vitesse à pleine charge . 6
4.6 Rendement . 6
5 Réduction de la charge du moteur pour prévenir la surchauffe . 6
Annexe A (informative) Détermination des composantes symétriques des tensions
entre phases U1, U2, U3 d'un système triphasé . 8
A.1 Détermination graphique . 8
A.2 Détermination analytique . 9
Annexe B (informative) Détermination approximative du facteur de déséquilibre . 10
Bibliographie . 11

Figure 1 – Valeurs types de réduction de la charge des moteurs à induction à cage
triphasés de conception N ou NE couverts par le domaine d'application de
l'IEC 60034-12 . 7
Figure A.1 – Schéma de phaseur . 8

COMMISSION ÉLECTROTECHNIQUE INTERNATIONALE
____________
Machines électriques tournantes -
Partie 26: Effets d'un système de tensions déséquilibrées sur les
caractéristiques de fonctionnement des moteurs à induction à cage
triphasés
AVANT-PROPOS
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L'IEC 60034-26 a été établie par le comité d'études 2 de l'IEC: Machines tournantes. Il s'agit
d'une Norme internationale.
Cette deuxième édition annule et remplace la première édition parue en 2006. Cette édition
constitue une révision technique.
Cette édition inclut les modifications techniques majeures suivantes par rapport à l'édition
précédente:
a) clarification indiquant que ces tensions sont des tensions entre phases à l’Article 4, à
l’Annexe A et à l’Annexe B;
b) ajout de la conception NE conformément à l'IEC 60034-12 à l’Article 4.

Le texte de cette Norme internationale est issu des documents suivants:
Projet Rapport de vote
2/2224/FDIS 2/2251/RVD
Le rapport de vote indiqué dans le tableau ci-dessus donne toute information sur le vote ayant
abouti à son approbation.
La langue employée pour l'élaboration de cette Norme internationale est l'anglais.
Ce document a été rédigé selon les Directives ISO/IEC, Partie 2, il a été développé selon les
Directives ISO/IEC, Partie 1 et les Directives ISO/IEC, Supplément IEC, disponibles
...

IEC 60034-26 ®
Edition 2.0 2026-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Rotating electrical machines -
Part 26: Effects of unbalanced voltages on the performance of three-phase cage
induction motors
Machines électriques tournantes -
Partie 26: Effets d'un système de tensions déséquilibrées sur les
caractéristiques de fonctionnement des moteurs à induction à cage triphasés
ICS 29.160.01  ISBN 978-2-8327-0985-6

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CONTENTS
FOREWORD . 2
INTRODUCTION . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Effects of unbalanced voltages on performance . 5
4.1 General . 5
4.2 Currents . 5
4.3 Heating . 5
4.4 Torque . 5
4.5 Full-load speed . 6
4.6 Efficiency . 6
5 Derating of motor to prevent overheating . 6
Annex A (informative) Determination of the symmetrical components of the line-to-line
voltages U , U , U of a three-phase system . 7
1 2 3
A.1 Graphical determination . 7
A.2 Analytical determination . 8
Annex B (informative) Approximate determination of the unbalance factor . 9
Bibliography . 10

Figure 1 – Typical values of derating of design N or NE, three-phase cage induction
motors within the scope of IEC 60034-12 . 6
Figure A.1 – Phasor diagram . 7

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Rotating electrical machines -
Part 26: Effects of unbalanced voltages on the performance of three-
phase cage induction motors
FOREWORD
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IEC 60034-26 has been prepared by IEC technical committee 2: Rotating machinery. It is an
International Standard.
This second edition cancels and replaces the first edition published in 2006. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) clarification that voltages are line-to-line voltages in Clause 4, Annex A and Annex B;
b) addition of design NE according to IEC 60034-12 in Clause 4.
The text of this International Standard is based on the following documents:
Draft Report on voting
2/2224/CDV 2/2251/RVC
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts of IEC 60034 series, under the general title Rotating electrical machines can
be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
INTRODUCTION
When the line voltages applied to a three-phase cage induction motor are not a balanced,
symmetric three-phase system, the currents in the phases of the stator winding will also be
unequal. A small percentage voltage unbalance will result in a much larger percentage current
unbalance.
The application of unbalanced voltages to a three-phase induction motor introduces a negative
sequence voltage, and this produces a flux in the air-gap rotating against the rotation of the
rotor, thus having a slip of almost 200 %. For high values of the slip, the motor impedance is
low, thus tending to produce a high negative sequence current in the stator winding and high
currents in the rotor cage. A small negative sequence voltage can produce currents in the
winding phases considerably in excess of those present under balanced voltage conditions.
Consequently, the temperature rise of the motor operating at a particular load and a particular
percentage of voltage unbalance will be higher than for the motor operating under the same
conditions with balanced voltages.
The analytical and graphical methods used to calculate the symmetrical components from the
voltage readings of the three phases are well-known and can be taken from textbooks. Thus,
these calculation schemes are not incorporated in this document but shown in the informative
Annex A. Besides, the evaluation of the symmetrical components can be done automatically by
modern instrumentation.
An approximate evaluation of imbalance is given in the informative Annex B.

1 Scope
This part of IEC 60034 describes the effects of unbalanced voltages on the performance of
three-phase cage induction motors.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
4 Effects of unbalanced voltages on performance
4.1 General
The effects of unbalanced voltages on motor performance are as described in 4.2 to 4.6.
4.2 Currents
The negative sequence component of the voltage produces a flux in the air-gap rotating against
the rotation of the rotor. A small negative-sequence component of the voltage can produce
currents in the winding phases considerably in excess of those present under balanced voltage
conditions. The frequency of the current in the cage is almost twice the rated frequency. Thus,
in the case of cages with current displacement, the increase of the rotor winding losses is
substantially higher than the increase of the stator winding losses.
The currents at normal operating speed will be greatly unbalanced in the order of approximately
6 to 10 times the voltage unbalance.
The locked-rotor current will be unbalanced to the same degree that the voltages are
unbalanced, but the locked-rotor apparent power will increase only slightly.
4.3 Heating
The temperature rise of the stator winding is always higher than in operation at balanced supply
voltages due to the increase of the losses produced by the negative-sequence components of
the currents and voltages.
The increase of the rotor losses is amplified by the current displacement (see 4.2).
In addition, unbalance of the voltages often is associated with a reduction of the positive-
sequence component of the voltage, which causes, for a given load, an increase of the positive-
sequence components of the currents in stator and rotor.
4.4 Torque
The locked-rotor, pull-up and breakdown torques are decreased when the voltages are
unbalanced. Should the voltage unbalance be extremely severe, the torques will perhaps not
be sufficient for the application.
Voltage unbalance is associated with the generation of an oscillating torque of twice the line
frequency. Its amplitude increases linearly with the product of the negative and the positive
sequence component of the voltages; at an unbalance factor of f = 0,05 (see Clause 5), its
u
peak value is in the range of 25 % of the rated torque. Impermissible torsional vibrations of the
complete shaft system can be excited, when its critical torsional speed is close to twice the line
frequency.
4.5 Full-load speed
The full-load speed is reduced slightly when the motor operates with unbalanced voltages
because of the higher slip associated with the additional rotor losses.
4.6 Efficiency
Due to higher losses and temperatures, there is also influence on the efficiency and a motor
can perhaps not reach the declared energy efficiency classification IEx.
5 Derating of motor to prevent overheating
When an AC motor for use on a power supply of rated frequency is connected to a three-phase
voltage system having a negative-sequence component exceeding 1 % of the positive-
sequence component of the voltages over a long period, i.e. at least the thermal time constant
of the machine, the permissible power of the motor is less than the rated power to reduce the
possibility of damage to the motor. A typical derating factor for motors of designs N and NE
within the scope of IEC 60034-12 is given in Figure 1 on the supposition that the positive-
sequence component of the supply voltage is close to the rated voltage. Operation of the motor
above a 5 % voltage unbalance condition is not recommended. Ways to determine U and U
p n
are provided in Annex A.
The unbalance factor f in Figure 1 is defined as
u
f =U /U
u np (1)
where
U is the RMS value of the negative-sequence component of the supply voltage;
n
U is the RMS value of the positive-sequence component of the supply voltage;
p
U and U can be determined graphically (see Figure A.1) or analytically as shown in Annex A.
n p
An approximation of the unbalance factor is given in Annex B.

Figure 1 – Typical values of derating of design N or NE, three-phase cage induction
motors within the scope of IEC 60034-12
Annex A
(informative)
Determination of the symmetrical components of the line-to-line voltages
U , U , U of a three-phase system
1 2 3
A.1 Graphical determination
Figure A.1 – Phasor diagram
Steps of the graphical determination of the symmetrical components U and U as displayed in
p n
Figure A.1:
– draw the phasor diagram of the three line-to-line voltages U , U , U , the sum of which is
1 2 3
always zero;
– erect an equilateral triangle upon one of the phasors (U is chosen in Figure A.1);
– the connection of the upper corners of the two triangles results in the auxiliary phasor U ;
i
– draw a straight line, perpendicular to the phasor U and meeting with the opposite corner of
i
the triangle built up by U , U , U ;
1 2 3
– the elongation of this line results in the negative-sequence component U , if its length is
n
made U 3 ;
i
– the connection of the tip of the phasor U and the tip of the phasor U is the phasor of the
n 1
positive sequence component U .
p
A.2 Analytical determination
The symmetrical components are defined by Equations (A.1) and (A.2):
– positive-sequence component
U = (U + aU + a U ) (A.1)
p 1 2 3
– negative-sequence component
U = (U + a U + aU )
(A.2)
n 1 2 3
where
j π
1+ a+ a = 0
a= e
and .
When the values and the phase-angles of the three line-to-line voltages are known, the
symmetrical components can be calculated directly from the complex Equations (A.1) and (A.
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