Malcolm Barnes. Practical Variable Speed Drives and Power Electronics
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Speed measurement transducers
259
C.2.3 Output interface connections
To cater for the various types of electronic circuits available, rotary digital encoders are
manufactured with several output options. The maximum output frequency is typically
100 kHz, but can be higher with some units for particular applications.
The type of connection between a rotary digital encoder on a machine, and the
electronic decoder in the control equipment some distance away, is determined mainly by
the distance between the two. Series volt drop, shunt capacitance and induced electrical
noise all affect the quality of the signals.
Several output interface connections are possible between encoders and decoders:
• NPN current sink interface (unbalanced – single wire) are used mainly for
inputs to control equipment such as PLC counters
• PNP current source interface (unbalanced – single wire) are used mainly for
inputs to control equipment such as PLC counters
• Push-Pull – NPN or PNP interface are used mainly for inputs to control
equipment such as PLC counters
• Line driver interface (balanced differential – two wire EIA-422) are used
mainly for speed feedback from digital encoders to variable speed drive
controllers
For short distances between the encoder and electronic decoder, up to approximately
10 m, and where externally induced noise is not a major problem, a simple type of
encoder line driver will provide adequate results. In these cases, an unbalanced (single-
wire) type of connection is adequate for each Channel. Power for the encoder is normally
provided from the decoder end and the output is connected in an unbalanced
configuration.
Voltage and current levels depend on the type of decoder, which can be 5 V (TTL-
standard), 12 V or 24 V. The latter two voltages are often used with PLCs.
Typical circuit configurations are as follows:
Figure C.10:
NPN current sink interface (unbalanced – single wire) with a pull-up resistor to the +volt supply line and
switching to 0 volt line
260
Practical Variable Speed Drives and Power Electronics
Figure C.11:
PNP current source interface (unbalanced – single wire) with a pull-down resistor to the 0 volt line and
switching to +volt line
Figure C.12:
Push-pull unbalanced line driver can be used
For longer distances between the encoder and electronic decoder, of up to ±100 m, and
where externally induced noise is more of a problem, a balanced differential line driver is
necessary to achieve acceptable reliability. With rotary encoders this is the most
commonly used circuit. In these cases, a two-wire interface (per channel), based on the
well-known EIA-422 (RS-422) interface standard is commonly used for all channels.
Each channel produces two voltages, being the signal voltages (A, B & O) and their
complement (A, B & O). Power for the encoder is usually provided from the decoder end.
For this type of connection, terminating resistors are necessary at the decoder end to
avoid reflections.
Figure C.13:
EIA-422 balanced differential line driver with remote power supply
Speed measurement transducers
261
For even longer distances between the encoder and electronic decoder, of up to
±1000 m, and where externally induced noise and power supply voltage drop are a
problem, a balanced differential line driver (EIA-422) with a local power supply source
is necessary to achieve acceptable reliability. In this case, terminating resistors are also
necessary at the decoder end to avoid reflections.
On modern absolute encoders, to reduce the number of connecting wires and to
improve the performance in areas of high electrical noise, additional internal electronics
is used to convert the parallel code (up to 24 bits) to an equivalent serial data code for
transmission, usually using synchronous communication over 2 or 4 wires.
Appendix D
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International trade, and the acceptability of imported and exported products, is dependent
on the existence of internationally acceptable standards and norms. This appendix
summarizes the various international and national standards, which are applicable to
rotating electric machines and variable speed drives.
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Standardization is coordinated internationally by the ISO – International Standards
Organization, which draws its members from all countries of the world. ISO
concentrates on coordinating standards internationally. ISO has published almost 9000
standards, covering a broad range of subjects. To facilitate international trade, there is a
general movement in most countries towards bringing their own standards into harmony
with the International standards. Standardization is achieved through the participation of
interested parties, usually from national standards organizations, interested academics,
scientists and manufacturers. Reaching consensus on standards internationally is a tedious
process and it is coordinated through:
• TC: Technical committees
• SC: Technical sub-committees
• WG: Working groups
Standardization of electrotechnical matters is coordinated internationally by the
International Electrotechnical Commission (IEC). In a similar way to ISO,
standardization is achieved through the participation and consensus of interested parties,
usually from national standards organizations, interested academics, scientists and
manufacturers. As in ISO, standardization is coordinated through:
• TC: Technical committees
• SC: Technical sub-committees
International and national standards
• WG: Working groups
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To a greater or lesser degree, all European countries have developed their own national
standards. Since 1945, the major countries of Europe have been actively working towards
a common market and, during this period, there has been a great expansion of inter-
European trade. Initially this common market was called the European Economic
Community (EEC), then the European Community (EC) and most recently the European
union (EU). To overcome the technical differences between the products manufactured in
European countries, several committees have been established to harmonize standards. In
some cases, new standards known as European norms (EN) have been introduced.
The following committees have been established to coordinate European
standardization:
• CEN: Comité European de Normalisation (Committee for European
standardisation)
• CENELEC: Comité European de Normalisation – Electrotechnique
(Committee for European Electrotechnical Standardization)
• CCITT: Comité Consultatief International Telegraphique et Telephonique
(consultative committee for international telegraph and telephone)
• ECISS: European Committee for Iron and Steel Standards
Because of the economic power of Europe, these ‘Norms’ will inevitably have
relevance to non-European countries, which seek to trade with Europe. Many of these
ENs will become de facto international standards or be embodied in ISO or IEC
standards.
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Economically and militarily, the USA has been the most powerful and influential country
in the world for at least the past 50 years. The sheer size of its economy and the large
number of high caliber technical experts working there has provided the USA with an
unusually high capacity for technical innovation. Many well-known standards, which
have become de facto international standards, have been developed in the USA. Most
electrical engineers are familiar with the USA standards, developed by organizations such
as ANSI, FCC, IEEE, NEMA, MIL, EIA/TIA and UL. Many of these have been formally
embodied in ISO and IEC standards. Also, because of the large defense industry in USA,
a wide range of military (MIL) standards have been developed for difficult environments.
These are usually significantly tighter than their civilian counterparts.
In spite of this economic and technical power, standardization in the USA is largely out
of step with the rest of the world. This is mainly a result of the different units of measure
used in the USA compared to the rest of the world. Almost all countries outside the USA,
such as Western Europe, Eastern Europe, Japan, Canada, Australia & New Zealand,
Africa, South America, China, Taiwan and the fast growing SE Asian countries have
based their industries on the metric system of measurement and have actively tried to
harmonize their standards along the lines of ISO and IEC. On the other hand, the USA is
only partially metricized and has largely kept the old English system of measurement,
which is based on feet, inches, pounds, gallons, etc. In general, the USA has largely been
Practical Variable Speed Drives and Power
reluctant to move in the direction of harmonizing their standards with ISO and IEC. This
continues to create difficulties with international free trade.
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All industrialized countries have their own national standards bodies, which establish and
coordinate national standards. These standards organizations are usually members of ISO
and IEC.
To facilitate international trade, national standards are increasingly ‘harmonized’ with
the relevant ISO and IEC standards. Any differences that still remain between the
national and international standards are usually associated with some special local
conditions and established local practice.
The national standards bodies of the larger industrialized countries of the world are
listed in the table shown in Figure D.1.
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In order to harmonize with the rest of the world, SA Bureau of Standards has resolved to
adopt international standards wherever possible, provided that they are appropriate. This
has changed the emphasis away from writing our own standards to looking critically at
those from other organizations, such as ISO, IEC and CENELEC and participating in the
continual review and updating of those standards.
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The names and initials of the major standards organizations
Appendix E
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AC alternating current, an electrical transmission system where the voltage
and the associated current alternatively adopt a positive and negative
polarity, typically at 50 cycles per sec
A/D converter device used to convert analog signals into the digital format
Algorithm a procedure or set of steps which are used, usually in a computer program
or processor, to solve a problem
Analog a continuously varying waveform that may represent a voltage, current or
any other continuously changing value
ANSI American National Standards Institute, which is the main standards
organization in the USA
ASCII American standard code for information interchange, a 7-bit code,
commonly used in data communications, for encoding alphabetic and
numeric characters – originally specified by ANSI, it is now also
specified as International Standards ISO-646 and CCITT alphabet #5
ASIC application specific integrated circuit
Asynchronous a data communications technique where the group of bits, representing a
character, can be transmitted at an arbitrary time with a variable ‘idle’
time between characters – when a character is transmitted, the group of
bits is preceded by a start-bit, which synchronizes the receiver, and is
terminated by a stop-bit – close synchronization is necessary during
transmission of the bit-group
Glossary of common terms used with AC variable speed drives
AS Australian standards of the Standards Association of Australia (SAA), the
main standards organization in Australia
Attenuation the decrease in the magnitude of a signal, sound or voltage down a
transmission path – usually measured in decibels or dB
(
Bandwidth range of frequencies in Hz, being the difference between the lowest and
highest frequency, which can be transferred along a transmission medium
without significant attenuation
Band pass filter a filter that permits a limited range of frequencies to pass through. The
frequencies outside this range are sharply attenuated
BCD binary coded decimal, a code that represents each character in a decimal
number with a 4-bit code
Bit binary digit, represented by either a logic condition 1 or 0
BJT bipolar junction transistor, (also see GTR), 3-terminal power transistor
rated for high currents between the emitter and collector terminals,
usually comprising a double or triple Darlington connection, which can
be turned on and off by means of a current applied to the base terminal
Baud unit of measurement for signal speed, which is based on events per
second (usually bits per second) – if each event has more than one bit
associated with it, then the baud rate and bits per second are not equal –
Baud is derived from the name of the famous French telegrapher,
Maurice Emile Baudot
Bps bits per second, the speed at which bits are transferred
BRC block redundancy check, an error checking technique in data
communications
Bridge an interface device, used to connect two similar networks, which enables
the network to be extended – it is often called a ‘repeater’
BS British standards of the British Standards Institution (BSI), the main
standards organization in the United Kingdom
Buffer a temporary storage location for data (FIFO) in the receiving end device –
compensates for the difference in the data flow rates between the
transmitting device and the receiving device
Byte a group of 8 bits, which are usually the code for a character
)
Capacitance the property of an AC electrical circuit to store electrical energy when a
voltage is applied –it is normally associated with two plates, where the
capacitance is proportional to the area of the plates and inversely
proportional to the distance between them – when an alternating voltage
is applied, the resulting current will be such that the current peak will
Practical Variable Speed Drives and Power Electronics
lead the voltage peak by 90
o
– units of capacitance are measured in
Farads
CAD computer aided design, a computer based design program.
CAE computer aided engineering, a computer based engineering program.
CAM computer aided manufacturing, a computer based manufacturing
program.
CENELEC Comité European de Normalisation Electrotechnique, whose standards
are often applied by European countries
Character refers to any alphabetic letter, numeral, punctuation mark or other
symbol, which may be used in the transfer of data
Checksum the result of binary addition of bits, representing characters in a data
message, used for error detection
CNC computer numerical control, a computer controlled machine
CPU central processing unit, the intelligent core of a digital device
CRC cyclic redundancy check, a very effective error checking technique in
data communications, using an error message 2 bytes long (16 bits) – two
versions are commonly used, being CRC-16 and CRC-CCITT
CSA Canadian Standards Association, the main standards organization in
Canada
CSI current source inverter, a configuration of frequency converter where rate
of change of current (di/dt), behind the inverter, is limited by a large
choke connected in series in the DC link
*
Darlington
connection a ‘cascade’ connection of transistors which increases the amplification
factor of a power transistor module, in order to reduce the base current
required from the control circuit – double or triple Darlington
connections are commonly used in power converters
D/A device used to convert digital signals into the analog format
dB decibel – a measure of attenuation in a signal, based on the logarithmic
ratio of the signal magnitude V
1
and V
2
at the two ends – the ratio is
expressed as, dB = 20log
10
V
1
/V
2
DC direct current, an electrical transmission system where voltage and
current retain a fixed polarity (positive or negative)
DCE data circuit-terminating equipment, a name from the EIA/TIA standards
that applies to a particular configuration of comms port
DCS distributed control system, an industrial control computer usually used for
process control applications
DEC decimal