IEC TS 60034-25:2004

TECHNICAL IEC
SPECIFICATION TS 60034-25
First edition
2004-04
Rotating electrical machines –
Part 25:
Guide for the design and performance
of cage induction motors specifically designed
for converter supply
Reference number
IEC/TS 60034-25:2004(E)
Publication numbering
As from 1 January 1997 all IEC publications are issued with a designation in the

60000 series. For example, IEC 34-1 is now referred to as IEC 60034-1.

Consolidated editions
The IEC is now publishing consolidated versions of its publications. For example,

edition numbers 1.0, 1.1 and 1.2 refer, respectively, to the base publication, the

base publication incorporating amendment 1 and the base publication incorporating

amendments 1 and 2.
Further information on IEC publications
The technical content of IEC publications is kept under constant review by the IEC,
thus ensuring that the content reflects current technology. Information relating to
this publication, including its validity, is available in the IEC Catalogue of
publications (see below) in addition to new editions, amendments and corrigenda.
Information on the subjects under consideration and work in progress undertaken
by the technical committee which has prepared this publication, as well as the list
of publications issued, is also available from the following:
• IEC Web Site (www.iec.ch)
• Catalogue of IEC publications
The on-line catalogue on the IEC web site (http://www.iec.ch/searchpub/cur_fut.htm)
enables you to search by a variety of criteria including text searches, technical
committees and date of publication. On-line information is also available on
recently issued publications, withdrawn and replaced publications, as well as
corrigenda.
• IEC Just Published
This summary of recently issued publications (http://www.iec.ch/online_news/
justpub/jp_entry.htm) is also available by email. Please contact the Customer
Service Centre (see below) for further information.
• Customer Service Centre
If you have any questions regarding this publication or need further assistance,
please contact the Customer Service Centre:
Email: custserv@iec.ch
Tel: +41 22 919 02 11
Fax: +41 22 919 03 00
TECHNICAL IEC
SPECIFICATION TS 60034-25
First edition
2004-04
Rotating electrical machines –
Part 25:
Guide for the design and performance
of cage induction motors specifically designed
for converter supply
 IEC 2004  Copyright - all rights reserved
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 the publisher.
International Electrotechnical Commission, 3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, Switzerland
Telephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch Web: www.iec.ch
PRICE CODE
Commission Electrotechnique Internationale X

International Electrotechnical Commission
МеждународнаяЭлектротехническаяКомиссия
For price, see current catalogue

– 2 – TS 60034-25  IEC:2004(E)

CONTENTS
FOREWORD.5

INTRODUCTION.7

1 Scope.8

2 Normative references .8

3 Terms and definitions .9

4 System characteristics.10

4.1 General .10
4.2 System information.10
4.3 Torque/speed considerations.10
4.3.1 General .10
4.3.2 Torque/speed capability.11
4.3.3 Voltage/speed characteristics .12
4.3.4 Limiting factors on torque/speed capability .12
4.3.5 Resonant speed bands .13
4.3.6 Duty cycles.13
4.4 Converter control types .13
4.4.1 General .13
4.4.2 Converter type considerations .14
4.5 Converter output voltage generation.15
4.5.1 Pulse Width Modulation (PWM) .15
4.5.2 Hysteresis (sliding mode) .15
4.5.3 Influence of switching frequency.16
4.5.4 Multi-level converters .17
4.6 Motor requirements .17
5 Losses and their effects.19
5.1 General .19
5.2 Losses in motors supplied from voltage-source converters .19
5.3 Location of the extra losses and ways to reduce them .20
5.4 Converter features to reduce the motor losses .20
5.5 Temperature and life expectancy.21
5.6 Determination of motor efficiency .21

6 Noise, vibration and oscillating torques .21
6.1 Noise and vibration in an induction motor supplied from a converter.21
6.1.1 General .21
6.1.2 Changes in noise emission due to changes in speed .22
6.1.3 Magnetically excited noise.23
6.1.4 Torsional oscillation.23
6.2 Sound power level determination and limits .24
6.2.1 Methods of measurement .24
6.2.2 Test conditions .24
6.2.3 Sound power level limits.24
6.3 Vibration level determination and limits .24
6.3.1 Method of measurement .24
6.3.2 Test conditions .25
6.3.3 Vibration level limits .25

TS 60034-25  IEC:2004(E) – 3 –

7 Motor insulation electrical stresses .25

7.1 General .25

7.2 Causes.25

7.3 Winding electrical stress.27

7.4 Insulation stress limitation .29

7.5 Responsibilities .29

7.6 Converter characteristics.30

7.7 Methods of reduction of voltage stress .30

7.8 Motor choice .31

8 Bearing currents .31
8.1 Sources of bearing currents in converter-fed motors.31
8.1.1 General .31
8.1.2 Magnetic asymmetry.31
8.1.3 Electrostatic buildup .32
8.1.4 High frequency voltages .32
8.2 Generation of high frequency bearing currents .32
8.2.1 General .32
8.2.2 Circulating current .33
8.2.3 Shaft grounding current .33
8.2.4 Capacitive discharge current .33
8.3 Common-mode circuit.33
8.3.1 General .33
8.3.2 System common-mode current flow .33
8.4 Stray capacitances .34
8.4.1 General .34
8.4.2 Major component of capacitance .34
8.4.3 Other capacitances.35
8.5 Consequences of excessive bearing currents .35
8.6 Preventing high frequency bearing current damage .36
8.6.1 Basic approaches .36
8.6.2 Other preventive measures.36
9 Installation.38
9.1 Grounding, bonding and cabling .38
9.1.1 General .38
9.1.2 Grounding .38

9.1.3 Bonding of motors .39
9.1.4 Motor power cables .39
9.2 Reactors and filters .44
9.2.1 General .44
9.2.2 Output reactors.44
9.2.3 Voltage limiting filter (dv/dt filter) .44
9.2.4 Sinusoidal filter.44
9.2.5 Motor termination unit.44

Annex A (informative) Converter output spectra.46

Bibliography.48

– 4 – TS 60034-25  IEC:2004(E)

Figure 1 – Component parts of a PDS.7

Figure 2 – Torque/speed capability .11

Figure 3 – Converter output current .11

Figure 4 – Converter output voltage .12

Figure 5 – Effects of switching frequency on motor and converter losses .16

Figure 6 – Effects of switching frequency on acoustic noise.16

Figure 7 – Effects of switching frequency on torque ripple .17

Figure 8 – Example of measured losses W, as a function of frequency f and supply type .19

Figure 9 – Additional losses ΔW of a motor (same motor as Figure 8) due to converter
supply, as a function of pulse frequency fp, at 50 Hz rotational frequency.20
Figure 10 – Fan noise as a function of fan speed.22
Figure 11 – Typical surges at the terminals of a motor fed from a PWM converter .26
Figure 12 – Typical voltage surges on one phase at the converter and at the motor
terminals (2 ms/division) .26
Figure 13 – Individual short rise time surge from Figure 12 (1 μs/division) .27
Figure 14 – Definition of the peak rise time t of the voltage at the motor terminals .28
r
Figure 15 – First turn voltage as a function of the surge rise time .28
Figure 16 – Discharge pulse occurring as a result of converter generated voltage surge
at motor terminals (100 ns/division) .29
Figure 17 – Limiting curves of impulse voltage V , measured between two motor
pk
phase terminals, as a function of the impulse rise time t .30
r
Figure 18 – Possible bearing currents.32
Figure 19 – Motor capacitances .35
Figure 20 – Bearing pitting due to electrical discharge (pit diameter 30 μm to 50 μm) .36
Figure 21 – Fluting due to excessive bearing current .36
Figure 22 – Bonding strap from motor terminal box to motor frame .39
Figure 23 – Examples of shielded motor cables and connections .40
Figure 24 – Parallel symmetrical cabling of high-power converter and motor.41
Figure 25 – Converter connections with 360º HF cable glands, showing the ‘Faraday Cage’ 42
Figure 26 – Motor end termination with 360º connection .42
Figure 27 – Cable shield connection .43
Figure 28 – Characteristics of preventative measures .45

Figure A.1 – Typical frequency spectra of converter output voltage of a) constant
frequency PWM control and b) hysteresis control.46
Figure A.2 – Typical frequency spectra of converter output voltage of a) random PWM
control and b) hysteresis control .47
Figure A.3 – Typical time characteristics of motor current of a) constant frequency

PWM control and b) hysteresis control .47

Table 1 – Significant factors affecting torque/speed capability .12
Table 2 – Motor design considerations.17
Table 3 – Motor parameters .18
Table 4 – Sound power level as a function of output power.24
Table 5 – Effectiveness of bearing current countermeasures .37

TS 60034-25  IEC:2004(E) – 5 –

INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
ROTATING ELECTRICAL MACHINES –

Part 25: Guide for the design and performance of cage induction motors

specifically designed for converter supply

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 provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
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) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
The main task of IEC technical committees is to prepare International Standards. In
exceptional circumstances, a technical committee may propose the publication of a technical
specification when
• the required support cannot be obtained for the publication of an International Standard,
despite repeated efforts, or
• The subject is still under technical development or where, for any other reason, there is
the future but no immediate possibility of an agreement on an International Standard.
Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC 60034-25, which is a technical specification, has been prepared by IEC technical
committee 2: Rotating machinery.

– 6 – TS 60034-25  IEC:2004(E)

The text of this technical specification is based on the following documents:

Enquiry draft Report on voting

2/1271/DTR 2/1288/RVC
Full information on the voting for the approval of this technical specification can be found in

the report on voting indicated in the above table.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

The committee has decided that the contents of this publication will remain unchanged until
2007. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
A bilingual edition of this Technical Specification may be issued at a later date.

TS 60034-25  IEC:2004(E) – 7 –

INTRODUCTION
This introduction is intended to explain the aim of this part of IEC 60034.

Motor categories
There are 2 categories of cage induction motors, which can be applied in variable speed
electric drive systems.
• Standard cage induction motors, designed for general purpose application. The design

and performance of these motors are optimized for operation on a fixed-frequency

sinusoidal supply. Nevertheless they are generally also appropriate for use in variable
speed drive systems.
Guidance on this field of application is given in IEC 60034-17.
• Cage induction motors specifically designed for converter operation. The design and
construction of such motors may be based on standard motors with standardized frame
sizes and dimensions, but with modifications for converter operation.
This category is covered by this part of IEC 60034, and it is recommended that the motor
be marked with a reference to this part of IEC 60034.
Motors for converter supplies greater than 1 000 V, or for converters other than voltage
source, will be considered in later editions of this part of IEC 60034.
Incorporation of the motor into the power drive system
Figure 1 illustrates the Power Drive System (PDS). A PDS consists of a motor and a
Complete Drive Module (CDM). It does not include the equipment driven by the motor. The
CDM consists of a Basic Drive Module (BDM) and its possible extensions such as the feeding
section or some auxiliaries (for example ventilation). The BDM contains converter, control and
self-protection functions. The rating and performance of the complete PDS is covered in
general by IEC 61800-2.
NOTE Figure 1 of IEC 61800-2 provides further details of the structure of a PDS.
The motor itself and additional specific requirements for its proper incorporation into the PDS
are covered by the IEC 60034 series.
Power supply
V-Power supply
System Standard
Power Drive
System (PDS)
IEC 61800-2
IEC 61800
- rating specification and performance of the complete
(all parts)
not
PDS – for individual subsystem units
Power
- converter characteristics and their relationship with
conversion
the PDS
IEC 60146
Control,
- application guideline (control strategies, diagnostics,
(all parts)
protection
topologies)
and
auxiliaries
Guides for converter supplied
Motor
cage induction motors
IEC 60034 M
(all parts)
IEC 60034-17 general purpose motors
IEC 60034-25 motors specifically designed for
converter operation
Driven equipment
IEC  445/04
Figure 1 – Component parts of a PDS

Environment, installation
– 8 – TS 60034-25  IEC:2004(E)

ROTATING ELECTRICAL MACHINES –

Part 25: Guide for the design and performance of cage induction motors

specifically designed for converter supply

1 Scope
This part of IEC 60034 describes the design features and performance characteristics of
polyphase cage induction motors specifically designed for use on voltage source converter
supplies up to 1 000 V. It also specifies the interface parameters and interactions between the
motor and the converter including installation guidance as part of a power drive system.
NOTE 1 For motors operating in potentially explosive atmospheres, additional requirements as described in the
IEC 60079 series apply.
NOTE 2 This technical report is not primarily concerned with safety. However, some of its recommendations may
have implications for safety, which should be considered as necessary.
NOTE 3 Where a converter manufacturer provides specific installation recommendations, they should take
precedence over the recommendations of this technical report.
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 60034-1, Rotating electrical machines – Part 1: Rating and performance
IEC 60034-2:1972, Rotating electrical machines – Part 2: Methods for determining losses and
efficiency of rotating electrical machinery from tests (excluding machines for traction vehicles)
Amendment 1 (1995)
Amendment 2 (1996)
IEC 60034-6, Rotating electrical machines – Part 6: Methods of cooling (IC Code)
IEC 60034-9, Rotating electrical machines – Part 9: Noise limits
IEC 60034-14, Rotating electrical machines – Part 14: Mechanical vibration of certain
machines with shaft heights 56 mm and higher – Measurement, evaluation and limits of

vibration severity
IEC 60034-17, Rotating electrical machines – Part 17: Cage induction motors when fed from
converters – Application guide
IEC 61800-2, Adjustable speed electrical power drive systems – Part 2: General requirements
– Rating specifications for low voltage adjustable frequency a.c. power drive systems
IEC 61800-3, Adjustable speed electrical power drive systems – Part 3: EMC product
standard including specific test methods
IEC 61800-5-1, Adjustable speed electrical power drive systems – Part 5-1: Safety
requirements – Electrical, thermal and energy

TS 60034-25  IEC:2004(E) – 9 –

3 Terms and definitions
For the purposes of this part of IEC 60034, the following terms and definitions apply.

3.1
bonding
electrical connection of metallic parts of an installation together and to ground (earth)

NOTE For the purposes of this part of IEC 60034, this definition combines elements of IEV 195-01-10
(equipotential bonding) and IEV 195-01-16 (functional equipotential bonding).

3.2
converter
operating unit for electronic power conversion, changing one or more electrical characteristics
and comprising one or more electronic switching devices and associated components, such
as transformers, filters, commutation aids, controls, protections and auxiliaries, if any
[IEC 61800-2, 2.2.1]
NOTE This definition is taken from IEC 61800-2, and for the purposes of this part of IEC 60034 embraces the
terms Complete Drive Module (CDM) and Basic Drive Module (BDM) as used in the IEC 61800 series.
3.3
EMC (electromagnetic compatibility)
ability of an equipment or system to function satisfactorily in its electromagnetic environment
without introducing intolerable electromagnetic disturbances to anything in that environment
[IEV 161-01-07]
3.4
field weakening
motor operating mode where motor flux is less than the flux corresponding to the motor rating
3.5
peak rise time
time interval between the 10 % and 90 % points of the zero to peak voltage (see Figure 14)
3.6
Power Drive System
PDS
system consisting of power equipment (composed of converter section, AC motor and other
equipment such as, but not limited to, the feeding section), and control equipment (composed
of switching control – on/off for example –, voltage, frequency, or current control, firing
system, protection, status monitoring, communication, tests, diagnostics, process interface/

port, etc.)
3.7
protective earthing
PE
earthing a point or points in a system or in an installation or in equipment for the purposes of
electrical safety
[IEV 195-01-11]
3.8
skip band
small band of operating frequencies where steady-state operation of the PDS is inhibited

– 10 – TS 60034-25  IEC:2004(E)

3.9
surface transfer impedance
quotient of the voltage induced in the centre conductor of a coaxial line per unit length by the

current on the external surface of the coaxial line

[IEV 161-04-15]
4 System characteristics
4.1 General
Although the steps in specifying motor and converter features are similar for any application,
the final selections are greatly influenced by the type of application. In this Clause, these
steps are described and the effects of various application load types are discussed.
4.2 System information
Complete application information, that considers the driven load, motor, converter, and utility
power supply, is the best way to achieve the required performance of the motor in the system.
In general this information should include:
• The power or torque requirements at various speeds.
• The desired speed range of the load and motor.
• The acceleration and deceleration rate requirements of the process being controlled.
• Starting requirements including the frequency of starts and a description of the load (the
inertia reflected at the motor, load torque during starting).
• Whether the application is a continuous process or a duty cycle of starts, stops, and speed
changes.
• A general description of the type of application including the environment in which the
drive system components will operate.
• A description of additional functionality that may not be met with the motor and converter
only (for example: motor temperature monitoring, ability to bypass the converter if
necessary, special sequencing circuits or speed reference signals to control the drive
system).
• A description of the available electrical supply power and wiring. The final configuration
may be affected by the requirements of the system selected.
4.3 Torque/speed considerations

4.3.1 General
The typical torque/speed characteristics of converter-fed cage induction motors, the
significant influencing factors and their consequences are shown in Figure 2, Figure 3 and
Figure 4. Depending on the performance requirements of the power drive systems, different
motor designs are possible for an adaptation of the individual limiting values.
NOTE Figure 2 to Figure 4 do not show the possible skip bands (see 4.3.5).

TS 60034-25  IEC:2004(E) – 11 –

4.3.2 Torque/speed capability
Figure 2 shows the torque/speed capability of converter-fed cage induction motors. The

maximum available torque is limited by the rating of the motor and by the current limitation of

the converter. Above the field-weakening frequency f and speed n the motor can operate
0 0
with constant power with a torque proportional to 1/n. If the minimum breakdown torque

(which is proportional to 1/n ) is reached, the power has to be further reduced proportional to
1/n, resulting in torque proportional to 1/n (extended range). The maximum speed n is
max
limited by the mechanical strength and stability of the rotor, by the speed capability of the

bearing system, and by other mechanical parameters.

C
x
C s
~ 1/n
n
~ 1/
T c Pc E x
n
n n
max
IEC  446/04
Key
——— continuous operation Tc – constant torque range Cx – separate cooling
-------- short-time operation Pc – constant power range Cs – self-cooling
—--—-- starting boost Ex – extended range
Figure 2 – Torque/speed capability
Figure 3 shows the corresponding converter output current (I) capability.

T P E
c c x
f f f
0 max
IEC  447/04
Figure 3 – Converter output current

I T
– 12 – TS 60034-25  IEC:2004(E)

4.3.3 Voltage/speed characteristics

The converter output voltage (V) can be varied with speed in several ways, as shown in

Figure 4.
V
max
A
C
D
B
f
max
f f
IEC  448/04
Key
A The voltage increases with speed, and the maximum converter output voltage V is achieved at the field-
max
weakening frequency f .
B The voltage increases with speed, and the maximum converter output voltage V is achieved above f .
max 0
This provides an extended speed range at constant flux (constant torque), but the available torque is
reduced in this speed range.
The voltage increases with speed up to f , and then increases at a lower rate, the maximum converter
C
output voltage V being achieved at f . This avoids excessive torque reduction in the constant flux
max max
range.
D A voltage boost is applied at very low speeds to improve starting performance.
In all of these cases, the voltage-speed dependence may be linear or non-linear, according to the torque-speed
requirements of the load.
Figure 4 – Converter output voltage

4.3.4 Limiting factors on torque/speed capability
The significant factors which influence the torque/speed capability are shown in Table 1.

Table 1 – Significant factors affecting torque/speed capability
Condition Motor Converter and motor
Breakaway
Maximum flux capability Maximum current
Constant flux
Cooling (I R losses due to current variations) Maximum current
Field weakening
Maximum speed (mechanical strength and stability) Maximum voltage
(reduced flux)
Maximum torque (breakdown torque)
Dynamic response Equivalent circuit parameters (determined by modelling) Control capability

V
TS 60034-25  IEC:2004(E) – 13 –

4.3.5 Resonant speed bands
The speed range of a converter-fed motor may include speeds that can excite resonances in

parts of the motor stator, in the motor/load shaft system or in the driven equipment.

Depending on the converter, it may be possible to skip the resonant frequencies. However,

even when resonant frequencies are skipped, the load will be accelerated through that speed

if the motor is set to run at any speed above this resonant speed. Decreasing the acceleration

time can help minimize the time spent in resonance.

4.3.6 Duty cycles
4.3.6.1 General
Duty cycle applications are those in which transitions between speeds or loads are common
(see IEC 60034-1). Several aspects of this type of application affect the motor and the
converter.
• Motor heat dissipation is variable, depending on rotation speed and cooling method.
• Torque demands above motor full-load torque may be required. Operation above motor full
load may be required to accelerate, handle peak loads, and even decelerate the load.
Operation above motor rated current will increase motor heating. This may require a
higher thermal class of insulation, a motor rated for the overload, or evaluation of the duty
cycle to determine if the motor has enough cooling for the application (see IEC 60034-1,
Duty type S10).
• DC injection, dynamic, or regenerative braking may be required to reduce the motor
speed. Regardless of whether the motor is generating torque to drive the application,
generating power back to the converter due to the motor being driven by the load, or
supplying braking torque during deceleration by applying DC current to the windings,
motor heating takes place approximately proportionally to the square of the current while
applied. This heating should be included in the duty cycle analysis. Furthermore, the
transient torques imposed on the shaft by braking should be controlled to a level that will
not cause damage.
NOTE IEC 61800-6 provides information on load duty and current determination for the entire PDS.
4.3.6.2 High impact loads
High impact loads are a special case of duty and are encountered in certain intermittent
torque applications (for example IEC 60034-1, Duty type S6). In these applications, the load is
applied or removed from the motor very quickly. It is also possible for this load torque to be
positive (against the direction of rotation of the motor) or negative (in the same direction as
motor rotation).
The impact load will result in a rapid increase or decrease in current demand (from the
converter). If the torque is negative, the motor may generate current back into the converter.
These transient currents create stresses in the stator winding. The magnitude of these
transient currents is a function of the size of the converter and of the motor.
4.4 Converter control types
4.4.1 General
There are various converter control types: scalar, vector (sensorless or feedback), direct flux
and motor torque control, etc. Each type has different characteristics, which are described in
4. 4. 1.1 to 4 . 4 . 1. 3.
– 14 – TS 60034-25  IEC:2004(E)

4.4.1.1 Scalar control
Scalar control is the original concept in a V/Hz converter. In such a converter, the output

voltage is controlled according to the output frequency. Figure 4 shows examples of the ways

in which this may be done.
With converter output voltage proportional to frequency, the motor is operating with

approximately constant flux even without speed feedback signals.

Voltage boost (a fixed voltage which is added to the converter output voltage), conventional

IR (stator winding resistance voltage drop) compensation, or advanced dynamic voltage

compensation are commonly used options to improve starting and operating performance in
the low speed region.
Voltage boost has more effect at low speeds when the motor voltage is low, and care should
be taken to ensure that the boost voltage is not so high that the motor saturates.
IR compensation, where at light loads the amount of boost voltage is proportional to the
amount of current in the motor, is an improvement. Many scalar controls use special
algorithms to dynamically compensate for the voltage drop caused by motor stator resistance
and inductance. This provides even better starting and operating performance in the low
speed region, and, by using additional motor voltage and current feedback signals, such
controls can generate torque values close to vector control even at lower frequency regions.
Scalar control is generally applied where fast response to torque or speed commands is not
required (for example, with centrifugal pumps and fans) and it is particularly useful if multiple
motors are to be supplied from a single converter.
4.4.1.2 Vector control
An AC vector controlled converter essentially decouples the components of the motor current
producing the magnetising flux and the torque, in order to control them separately.
This decoupling is achieved by calculation of the motor characteristics using an equivalent
circuit (mathematical model) with or without speed feedback signals.
According to the level of performance required, different approaches may be taken for this
equivalent circuit calculation. In addition, a speed feedback (sensor) signal may further
improve the performance.
4.4.1.3 Direct flux and motor torque control

A direct flux and motor torque controlled converter has a hysteresis (also known as ‘sliding
mode’) control type, which adjusts the flux and the torque of the motor by mathematical model
calculation of the motor, with or without speed feedback signals.
In this control type, there is no modulator, every switching transition of each converter power
semiconductor being considered separately. In addition, a speed feedback (sensor) signal
may further improve the performance.
The philosophy is to reach the required motor torque and speed as quickly as possible.
4.4.2 Converter type considerations
All three types of control can be used for constant torque applications, as well as for
applications where the torque increases with speed (for example, centrifugal pumps or fans).
However, when selecting a converter, each aspect of the performance requirement should be
considered to ensure optimal operation.

TS 60034-25  IEC:2004(E) – 15 –

In general, the following aspects should be noted.

• Using scalar control, it is possible to operate motors of different ratings in parallel with one

converter (multi-motor operation).

• Scalar control is typically insufficient for dedicated low speed load requirements (below

approximately 10 % of base speed), although the low speed performance can be improved

by applying dynamic voltage compensation.

• The steady state torque capability of scalar control can be made equivalent to the

sensorless vector control by applying dynamic voltage compensation.

• The most significant difference between scalar control and vector or direct flux and motor
torque control is the dynamic response.
• Vector or direct flux and motor torque control may be required if one or more of the
following characteristics are needed:
 operation around zero speed;
 precise torque control;
 high peak torque at low speed.
• Using vector control or direct flux and motor torque control, multi-motor operation can be
realized with or without speed feedback, provided that motors of the same rating are used.
• The characteristics of vector control and those of direct flux and motor torque control are
almost equivalent, because both use mathematical model calculations of the motor with or
without flux or speed sensors.
Further deta
...