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ABB drives

Technical guide No
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4

Technical guide No
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All rights reserved
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3AFE61389211 REV C 12
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2011

Technical guide No
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4

Contents
Chapter 1 - Introduction
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7
Chapter 2 - Processes and their requirements
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8
Industrial segments with VSD processes
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10
Machines are used to alter materials’ properties
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11
Indefinite shape
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and to transport materials
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12
Liquid materials
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12
Chapter 3 - The workhorse of industry: the electric motor
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13
Motors convert electrical energy into mechanical energy
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15
The efficiency of the drive system
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17
The load, friction and inertia resist rotation
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19
The drive torque and load torque are equal at nominal speed
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21
Variable material flow and input/output requirements
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22
The best control method is VSD
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24
Hydraulic coupling
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24
AC drive
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25
Maintenance costs
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25
Energy saving
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25
The AC drives market is growing fast
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27
The basic functions of an AC drive
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28

Technical guide No
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29
Reversing
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30
Eliminating mechanical vibrations
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31
Stall function
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32
Flying start
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33
EMC
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34
Technical differences between other systems and AC drives
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36
Factors affecting cost
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38
The motor
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38
Installation costs: throttling compared to AC drive
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40
Total cost comparison
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42

6

Guide to variable speed drives | Technical guide No
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Special attention has been given to electrical VSDs and especially to AC Drives
...
No special knowledge of VSDs is required, although basic technical know-how
is required to fully understand the terms and descriptions used
...
4 | Guide to variable speed drives 7

Chapter 2 - Processes and their
requirements

Why variable speed control?
To understand why variable speed control is necessary, we first
need to understand the requirements of different processes
...

Common to both main categories, however, is the need to be
able to adjust the process
...

This chapter describes the main industrial and non-industrial
processes using VSDs
...
4

Processes and their requirements

Examples
Industrial:
Chemical industry
Pulp, paper, printing
Food & beverage
Power plants
Mining
Metal industry
Machine shops
Non-industrial:
Plastics
HVAC
Textiles
Water treatment

Industrial segments with VSD processes
Industrial processes are numerous, and the list above mentions
just some of the industrial segments with VSD processes
...

For example, in air conditioning applications (part of HVAC), air
flow requirements change according to the humidity and temperature in the room
...
These adjustments are carried out with VSDs
...

In both cases, the fans need to be adjusted according to the
main process
...
Likewise, the need for VSDs differs according to the
process
...
4 | Guide to variable speed drives 9

Processes and their requirements

Variables in processing systems
This diagram shows what kinds of variables affect the processing
system
...
In the processing system itself, material or energy
is processed by means of mechanical power, electromagnetic
influence, thermal influence, chemical and biological reactions
or even nuclear power
...
The product or final material state is
the output of the process, but in every process, waste, in the
form of energy and/or material, is also produced
...

Material treatment can also be controlled by VSDs
...
The process is controlled by controlling the speed of
the hot air fans using VSDs
...
4

Processes and their requirements

Machines are used to alter materials’ properties
...
The first category is material
treatment, which is accomplished using various types of processing apparatus to alter a material’s properties into another form
...
The shape
can be either well defined or indefinite
...
Examples are paper machines, rolling mills and
saw mill lines
...
, are processed with plant equipment
...


Technical guide No
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and to transport materials
The second category consists of machines which transport
material to a desired location
...
These machines can
be divided into three different sub-groups according to whether
the type of material being treated is a solid, liquid or gas
...
Such apparatus includes cranes, conveyors and elevators
...

Gaseous materials
Gaseous materials such as air are transported using fans, compressors or blowers
...

In the diagram above, five different types of machines are presented
...


12

Guide to variable speed drives | Technical guide No
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It can be said that the electric
motor is the workhorse of industrial processes
...


Electric motors drive most machines
Every machine consists of four different components, shown in
the diagram
...
Together, the first three
components comprise the so called “drive system”
...
Energy is supplied to the drive system from the power
supply
...
Variable speed control can be accomplished,
for example, using a frequency converter as the energy control
component, a two speed motor as the motor component and
gears as the transmission component
...
Electric motors can be divided into AC and DC motors
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Technical guide No
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The voltage
in stator windings forms the current and magnetic flux
...

By changing the direction of the voltage in stator windings, the
direction of the flux can also be changed
...
The motor’s
rotor will then follow this flux with a certain slip
...

This control can be achieved using a frequency converter
...
A frequency converter consists of three parts
...
The DC voltage
is fed into the DC bus circuit, which filters the pulsating voltage
...

To receive the flux direction shown in the diagram, switches V1,
V4 and V5 should be closed
...

If switch V5 is not opened, the circuit will short circuit
...


14

Guide to variable speed drives | Technical guide No
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In
two positions, the voltage is zero, ie, when all the phases are
connected to the same DC bus, either negative or positive
...

The diagram shows these six switching positions and the flux
directions, which the voltage in the windings generates in each
case
...

In practice, control is not quite as simple as presented here
...
These rotor currents complicate the situation
...
Nevertheless, with today’s technology and know-how,
it is possible to effectively deal with interference
...
Furthermore, control is better than with
conventional methods, because electrical VSDs also provide the
possibility for stepless control
...
4 | Guide to variable speed drives 15

The workhorse of industry: the electric motor

The efficiency of the drive system
The total efficiency of the drive system depends on the losses in
the motor and its control
...
Input power to the drive system is
electrical in form, while output power is mechanical
...

Electrical input power Pin depends on voltage (U), current (I) and
the power factor (cosϕ)
...
To produce the required mechanical
power, active power is required
...

Mechanical output power P out depends on the required torque
(T) and rotating speed (n)
...
This has a direct effect
on how much power the drive system draws from the electrical
supply
...

Electrical switching with transistors is very efficient, so the efficiency of the frequency converter is very high, from 0
...
99
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82 and 0
...
So it can be said that
the total efficiency of the drive system is always above 0
...


16

Guide to variable speed drives | Technical guide No
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In addition, torque direction requirements might change
...
The
name comes from the four different quadrants (I to IV) shown
in the diagram
...

Because the torque is in the same direction as the speed, the
drive is accelerating
...

III & IV quadrants: In the third and fourth quadrant, the motor
is rotating counterclockwise and the drive is again accelerating
or decelerating, depending on the torque direction
...
To produce an
efficient four quadrant drive, some kind of braking arrangement
is required
...


Technical guide No
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Load torque consists of friction, inertia of the moving
parts and the load itself, which depends on the application
...

Load factors change according to the application
...
In
fans and blowers, air pressure changes affect the load torque,
and so on
...
4

The workhorse of industry: the electric motor

The motor has to overcome the loading torque
In any case, the loading torque has to be known before selecting the motor for the application
...
Only then can a suitable motor be selected for
the application
...
For example, in crane applications, a motor that is too small may not be able to lift the
required load quickly enough to the desired height
...
This could
be disastrous for people working at the harbour or site where
this crane would be used
...
4 | Guide to variable speed drives 19

The workhorse of industry: the electric motor

The drive torque and load torque are equal at nominal speed
A motor’s torque/speed curve is unique and has to be calculated
for every motor type separately
...
As can be seen, the maximum load
torque is reached just below nominal speed
...
Depending on the
application it can be linear or quadratic
...
This point is shown on the graph as the intersection of
Tm and Tl
...

These are the principles that govern how an ordinary squirrel
cage motor works
...
This will be introduced later in this guide
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4

Chapter 4 - Variable volumes require some
form of control
In most processes there is at least one variable
...
Therefore variable
processes and material volumes need some form of control
...
We
will also examine different control methods
...
These
parameters may need to be constant or they may need to be
changed according to a preset pattern
...

In some processes there is no interference and the input is
constant
...
However, if the output parameters need to be changed,
the input is variable or there is interference present, then variable speed control might be the solution to fulfilling the process
requirements
...
It also shows the reasons for the control;
input, interference or output
...
4 | Guide to variable speed drives 21

Variable volumes require some form of control

Simpler control methods
There are many simpler control methods in existence such as
throttling or bypass control
...

However, there are many drawbacks
...
An increase in
production capacity usually requires reconstruction of the whole
process and with each direct on-line startup there is a risk of
electrical and/or mechanical damage
...
Therefore, the total life-cycle cost of investment in simple control methods is much higher than with VSDs
...
4

Variable volumes require some form of control

The best control method is VSD
The best control method for most systems is VSD
...
If you are driving on a highway and
entering a populated area, you need to reduce speed so that
you don’t risk your own and other peoples’ lives
...
Another possibility would be
to use the same gear, keep your foot on the gas and reduce
speed simply by braking
...
Furthermore, the original goal of
reducing speed without risking your own and other peoples’ lives
would not have been achieved
...
4 | Guide to variable speed drives 23

Variable volumes require some form of control

Mechanical, hydraulic and electrical VSDs
Above are the four most common VSDs in the industrial sector
...

Hydraulic coupling
In hydraulic coupling, the turbine principle is used
...
The oil amount is controlled
with pumps and valves
...
In the motor, a mechanical inverter, a
commutator, changes direct current to alternating current
...
The
speed of the motor is regulated by a frequency converter that
changes the frequency of the motor voltage, as presented earlier
in this guide
...

The diagram shows the location of the control equipment for
each type of VSD
...

In electrical VSDs, all control systems are situated in an electrical equipment room and only the driving motor is in the process
area
...
Other benefits
are presented on the following page
...
4

Variable volumes require some form of control

Electrical VSDs dominate the market
Here are the four most important arguments for using electrical
VSDs, presented along with estimated VSD market shares in
Europe in 2000
...

Maintenance costs
Direct on-line starting stresses the motor and also the electrical
equipment
...

Productivity
Process equipment is usually designed to cater for future productivity increases
...

With the AC drive, speed increases of 5 to 20 percent are not a
problem, and the production increase can be achieved without
any extra investment
...
Changing
production volumes by mechanical means is usually very inefficient
...
This saves a lot
of energy particularly in pump and fan applications, because the
shaft power is proportional to the flow rate to the power of three
...
The optimal process control leads
to the best quality end product, which means the best profit for
the customer
...
4 | Guide to variable speed drives 25

Variable volumes require some form of control

Due to these benefits, electrical VSDs are dominating the market,
as can be seen from the table above
...


The AC drives market is growing fast
This diagram shows the projected development of the electrical
VSDs market to the year 2000
...
The market
share of DC drives is diminishing, and the total DC market size
remains approximately constant
...

As presented earlier in this guide, the AC drive has many benefits
over other process control methods
...
These brushes need
regular maintenance and the commutator itself complicates the
motor structure and consumes energy
...


26

Guide to variable speed drives | Technical guide No
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In
the following chapter we will take a closer look at the different
features of the AC drive, and the levels of performance the drive
can offer
...

There are four different components in AC drive motor control
...

An electrical supply feeds the required electricity to the drive;
one selection criteria for the drive is the supply voltage and its
frequency
...
This conversion process is controlled by signals
from the process or user via the process and user interfaces
...
This makes
the drive easy to integrate with other process control equipment
and overriding process control systems
...
4 | Guide to variable speed drives 27

AC drive: the leading control method

A motor’s load capacity curves with an AC drive
If the motor is driven without a frequency converter, its load
capacity curves cannot be modified
...

With a frequency converter drive, there are different loading options
...
Other curves can only be used for certain periods
of time, because the motor’s cooling system is not designed for
this kind of heavy use
...
In certain applications, as much as twice the
amount of torque is required when starting
...
This reduces the investment cost
...
Otherwise the motor
or the converter will overheat and be damaged
...
4

AC drive: the leading control method

Important features:
- inputs and outputs
- reversing function
- ramp times acceleration/deceleration
- variable torque V/Hz settings
- torque boosting
- eliminating mechanical vibrations
- load limits to prevent nuisance faults
- power loss ride-through
- stall function
- slip compensation
- flying start

AC drive features for better process control
AC drives also have other internal features and functions which
are sometimes required for better process control
...
With inputs and outputs
for example, different kinds of process information can be fed to
the drive and it will control the motor accordingly
...

In the following sections the listed features are presented in
more detail
...
4 | Guide to variable speed drives 29

AC drive: the leading control method

Reversing
Reversing the motor rotation is simple to accomplish with an AC
drive
...
Furthermore, it is possible to set different
acceleration and deceleration ramp times
...
In the diagram
(above, left) an S-ramp has been presented
...

Torque control
Torque control is relatively simple with an AC drive
...
Variable torque U/f settings
mean that maximum torque can be achieved at a lower speed
of rotation than normal
...
This means that when a motor is accelerated close to
its critical speed, the drive will not allow the actual speed of
the motor to follow the reference speed
...


30

Guide to variable speed drives | Technical guide No
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In such a situation, the AC drive will
continue to operate using the kinetic energy of the rotating motor
...

Stall function
With an AC drive, the motor can be protected in a stall situation
with the stall function
...
Protection is activated if three conditions are met at the same time
...
The drive frequency has to be below the preset stall frequency
...
The motor torque has to rise to a certain limit, calculated by
the drive software
...
The final condition is that the motor has been in the stall limit
for longer than the time period set by the user
...
4 | Guide to variable speed drives 31

AC drive: the leading control method

Slip compensation
If the motor load torque is increased, the speed of the motor will
decrease as shown in the diagram (above, left)
...

Flying start
The flying start feature is used when a motor is connected to a
flywheel or a high inertia load
...
In case of rotating motor, the inverter is first
started with a reduced voltage and then synchronised to the
rotating rotor
...


32

Guide to variable speed drives | Technical guide No
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The squirrel cage
motor is very compact and can be used in very hostile conditions
...

The frequency converter usually has an IP21 degree of protection
...
If a higher
degree of protection is required, it can be obtained, for example,
by installing the drive inside a cabinet with the required degree
of protection
...

EMC
Another important environmental feature is electromagnetic compatibility (EMC)
...
This means that the drive
system can bear conductive and radiative disturbances, and that
it does not send any conductive or radiative disturbances itself
either to the electrical supply or the surrounding environment
...

3, EMC Compliant Installation and Configuration for a Power
Drive System
...
4 | Guide to variable speed drives 33

Chapter 6 - Cost benefits of AC drives
In addition to their technical advantages, AC drives also provide
many cost benefits
...


At the moment there are still plenty of motors sold without variable speed AC drives
...
2 kW are sold with frequency converters, and how many
without
...

This is astonishing considering what we have seen so far in
this guide
...
But first let’s
review AC drive technology compared to other control methods
...
4

Cost benefits of AC drives

Technical differences between other systems and AC drives
AC drive technology is completely different from other, simpler
control methods
...

We could also compare AC drive technology to the development
from a floppy disk to a CD-ROM
...

The benefits of both these innovations are generally well known
...
In this guide, we have
presented the benefits of the AC drive compared to simpler
control methods
...
4 | Guide to variable speed drives 35

Cost benefits of AC drives

No mechanical control parts needed
To make a proper cost comparison, we need to study the configurations of different control methods
...
In traditional methods, there is always
a mechanical part and an electrical part
...
In On/Off
control, the same electrical components are needed, as well as
a pressure tank on the mechanical side
...
No mechanics are needed, because all control
is already on the electrical side
...

We can still use 220 V single phase supply, when speaking of
power below 2
...


36

Guide to variable speed drives | Technical guide No
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In
conventional methods there are both electrical and mechanical
components, which usually have to be purchased separately
...

Furthermore, mechanical parts wear out quickly
...
In conventional methods there are
also many electrical components
...

And last but not least, mechanical control is very energy consuming, while AC drives practically save energy
...


Technical guide No
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Only the pump itself is
not added to the costs because its price is the same regardless
of whether it’s used with an AC drive or valves
...
In an industrial environment there are
stricter requirements for valves and this increases costs
...
This is due to the 3-phase
motor used with the AC drive and the single phase motor used
in other control methods
...
Mechanical parts themselves are almost always less costly than a frequency converter, but electrical parts
also need to be added to the total investment cost
...
Only throttling in domestic use is as low
cost as the AC drive
...

Together with investment costs we need to look at installation
and operational costs
...
4

Cost benefits of AC drives

Throttling

AC drive

Installation material

20 USD

10 USD

Installation work

5h x 65 USD =
325 USD

1h x 65 USD =
65 USD

Commissioning
work

1h x 65 USD =
65 USD

1h x 65 USD =
65 USD

Total

410 USD

140 USD

Savings in installation: 270 USD!

Installation costs: throttling compared to AC drive
Because throttling is the second lowest investment after the AC
drive, we will compare its installation and operating costs to the
cost of the AC drive
...
This means twice
the amount of installation material is needed
...
To install a mechanical valve into a pipe is not that
simple and this increases installation time
...
Multiply this by the hourly rate charged by
a skilled installer to get the total installation cost
...
One hour is usually the time required in both cases
...
As you can
see, the AC drive saves up to USD 270 per installation
...


Technical guide No
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75 kW

0
...
1

300 USD

150 USD

Maintenance/year

40 USD

5 USD

Total cost/year

340 USD

155 USD

USD/kWh

Savings in one year: 185 USD!

Operational costs: maintenance and drive energy
In many surveys and experiments it has been proved that a 50%
energy saving is easily achieved with an AC drive
...
75
kW, with the AC drive it would be 0
...
If a pump is used
4000 hours per year, throttling would need 3000 kWh and the
AC drive 1500 kWh of energy per year
...
Here USD 0
...

As mentioned earlier, mechanical parts wear a lot and this is
why they need regular maintenance
...
In many
cases however, there is no maintenance required for a frequency
converter
...
This means that the payback time of
the frequency converter is two years
...
To retrofit an existing throttling system the pay-back
time is two years
...
4

Cost benefits of AC drives

Total cost comparison
In the above figure, all the costs have been summarised
...
Here the operational costs are rated to
the present value with a 10% interest rate
...
Most of the savings with
the AC drive come from the operational costs, and especially
from the energy savings
...

Taking the total cost figure into account, it is very difficult to
understand why only 3% of motors sold have a frequency converter
...


Technical guide No
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4

H
harbour 19
hydraulic coupling 24
I
industrial processes 13
inertia 18, 32
input power 16
interference 15, 21
inverter 14, 15, 24
IP21 33
IP54 33
L
linear ramp 30
load capacity curves 28
M
machine 11, 13, 24, 29
magnetic flux 14, 15
maintenance 24, 25, 26, 37, 40
mechanical power 16
mechanical vibrations 29, 30
motor load 32
motor losses 16
motor phase 14
motor size 16
motor stall condition 31
motor windings 14, 15
N
nuisance faults 29
O
output power 16
P
power factor 16
power loss ride-through 29, 31
power plants 22, 37
process control 25, 26, 27, 29
processing system 10
pump 24, 25, 36, 40
R
rated speed 16
Reactive power 16
reactive power 16
reactors 36
rectifier 14
reference speed 30
reversing function 29
right hand rule 14

Index

S
S-ramp 30
slip 14, 29, 32
squirrel cage motor 20, 24, 33
stall frequency 31
stall function 29, 31
stator 14
stepless control 15
T
temperature 15, 33
throttling 22, 36, 39, 40
torque 16, 17, 18, 19, 20, 28, 29,
30, 31, 32
transistors 16
V
valve 40
valves 24, 36
variable speed control 8, 21, 24
Variable Speed Drives 1, 3
voltage 14, 15, 16, 24, 27, 31
VSD 9, 15, 22, 23, 24, 25, 26

Technical guide No
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abb
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abb
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All rights reserved
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3AFE61389211 REV C EN 12
...
2011 #15677

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