guideto motors and starting - lgtrempart.fr · this arrangement produces a rotating magnetic field...
TRANSCRIPT
Guide to motors and starting
2
Introduction
Electric motors are deservedly the mostpopular prime movers for industry andcommerce. Compared with other sources ofmechanical power, they're inexpensive,compact, reliable and versatile. As a result ofthis popularity, millions of motors are installedeach year and, except for the very tiniest, everyone needs a starter. The manufacture of thesestarters is the basis on which the control gearindustry was built, and motor starters are stillat the core of almost every control gearsupplier's business.
Starters commonly used today, however, differ from
their predecessors. Some types, such as the faceplate
starter, have disappeared altogether. Other types,
such as primary resistance starters, are fast declining
in popularity. In addition, asynchronous induction
motors are now almost universal, virtually eliminating
the need for the specialised starters used by other
types of motor.
This supplement deals with a wide range of starting
techniques for asynchronous motors, an area where
Schneider's engineers have unrivalled expertise. The
benefits and limitations of various starting methods are
explained and, unlike some ostensibly similar
publications, the information presented is right up to
date - current, useful and practical data is presented in
a clear concise form.
Your comments on the contents of this supplement are
welcome, as are your suggestions for topics which you
would like to see covered in future issues.
Introduction
3
Scope
Motors – a few basics
The scope of this publication
Principle of operation
The rotating magnetic field
This supplement has been written to provide
engineers, designers and users of motor starters with
a brief overview of current techniques to assist in their
understanding, and to help them in the design of
equipment. It by no means covers all aspects of
motor starting but, nevertheless, it deals with the vast
majority of applications likely to be encountered in
industry and commerce. For those requiring further
information, a short list of sources is included at the
end of the supplement.
Three-phase asynchronous motors are, by far, the
most widely used type. The operation of this type of
motor relies upon the creation of an induced current in
a conductor which is, itself, under the influence of a
magnetic field. It is this principle of operation which
gives rise to the commonly used term "induction
motor."
A typical motor has three stator or field windings
which are arranged at an angle of 120° relative to each
other. These windings are fed from the three phases
of the mains supply which are, themselves, offset by
120° . This arrangement produces a rotating magnetic
field which, as it turns, tends to pull the motor's rotor
round with it.
The magnetic field rotates once during each complete cycle of thesupply current. Motor speed is, therefore, directly related to thesupply frequency (f in cycles per second or Hz), and the number ofpole pairs (p) which the motor uses. The motor's so-calledsynchronous speed is given by:
Ns (in revolutions per minute) = 60f / p
The majority of motors in use are four-pole machines (2 pairs), whichhave synchronous speeds of 1500 rpm at 50Hz and 1800 rpm at 60Hz.
Motors – a few basics
4
Motorconstruction
Slip
In practice, an induction motor can never run at its
synchronous speed, since it can only generate torque if
there is an induced current in the rotor conductors. This
can only be the case if there is relative movement
between the rotor and the rotating magnetic field. The
rotor must, therefore, rotate slightly more slowly than the
field which rotates at synchronous speed. This is why
the motor is described as asynchronous.
The difference between the synchronous speed (Ns) and
the actual nominal rotor speed (Nn) is called the slip. Slip is
always expressed as a percentage of the synchronous
speed:
slip = 100(Ns - Nn)/ Ns
Slip
Motor construction
A three-phase asynchronous motor comprises two maincomponents, the stator and the rotor.
As its name suggests, the stator is the stationary part of the motor, and
consists of a strong casing (usually manufactured from cast-iron or alloy)
into which is fixed a ring of laminated silicon steel sections. The
laminations are slotted so as to accommodate the stator windings which
create the rotating magnetic field. Each of the main windings, of which
there are three in a three-phase motor, comprises a number of coils. The
magnetic coupling of the windings is arranged to give the required
number of pole pairs (and thus synchronous speed) of the motor.
The rotor is the rotating part of the motor which drives the machine to
which it is coupled. It is similar to the stator, but is made up of a greater
number of laminated sections. Together, these form a cylinder which is
keyed to the motor shaft. There are two principle types of rotor - squirrel
cage and wound.
5
Controlling speed
Controllingspeed
Motorconstruction
Squirrel cage rotors are, by far, the most common. They have straight
conductors set into slots around the periphery of the rotor. These
conductors are connected together by rings at each end of the rotor, so
that their arrangement somewhat resembles a circular squirrel cage, from
which the assembly gets its name. A popular variation is the double-
cage, which has two concentric cages and offers a higher starting torque
than single-cage versions. No external electrical connections can be
made to any type of squirrel cage rotor.
Wound rotors have windings similar to those used in the stator. One end
of each winding is connected to a common (star) point, and the other
ends are connected to slip rings. External connections to the rotor
windings are made via these slip rings, allowing additional resistance to
be added to the rotor circuit during starting. This enables the motor's
starting current and torque to be controlled.
Varying the supply voltage alone is
a comparatively ineffective way of
controlling the speed of an
induction motor. Voltage increases
raise speed somewhat, but this
effect is limited by magnetic
saturation in the windings.
Conversely, voltage reductions
decrease speed but, again, the
range of control is very limited, and
torque is adversely affected.
Today, the most popular method of
speed control is the use of a
variable frequency (inverter) drive.
These vary both the voltage and
frequency of the supply to the
motor, giving a wide range of
control over speed, without loss of
torque. With inverter drives,
standard 50Hz motors can be
operated successfully over at least
the range of supply frequencies
from 5 to 50Hz with only a slight
loss of operating torque. If
operated continuously at
frequencies of 25Hz or less, the
cooling provided by the motor's
built-in fan is likely to be
insufficient, and additional forced
cooling should be considered.
Startingcurrent
6
Starting current
If a stationary squirrel cage motor
is connected directly to the supply,
it will typically draw a starting
current of 5 to 8 times its normal
full-load current (FLC). For smaller
motors, this is often acceptable,
but for large machines, or where
supply capacity is limited, some
means of reducing the starting
current becomes necessary. This
is usually done by reducing the
voltage applied to the motor during
starting. Most of the starters
described in the remainder of this
publication have been developed
specifically to limit motor starting
current.
With conventional contactor-based
starters, however, there is a
problem - reduced starting current
means reduced starting torque
which may, in some applications,
be unacceptable. This limitation is
examined in more detail in later
sections which describe particular
starter types. It is worth noting,
however, that inverters, which
control both supply frequency and
voltage, allow starting currents of
1.5 x FLC or less, while still
providing high starting torque.
When using soft starters, starting
currents are generally between 2
and 5 x FLC.
Summary
Three-phase asynchronous induction motors are the mostcommonly used type in industry. Their speed is largely determinedby supply frequency, with voltage variations having comparativelylittle effect. Connected directly to the supply, these motors havetypical starting currents of 5 to 8 x FLC. Often, starting currentsneed to be reduced, and various forms of starter have beendeveloped to make this possible.
Starters
7
Direct-on-line (DOL) starters
With this type of starter, the stator
windings of the motor are
connected directly to the three-
phase mains supply. The motor
starts and accelerates in a way
determined by its own
characteristics. Typically, the peak
starting current is between 5 and 8
times normal full-load current, and
the peak starting torque is between
0.5 and 1.5 times the motor?s
nominal operating torque.
Overloads designed to
BS EN 60947-4-1 are based on a
starting current of 7.2 times normal
full-load current.
21
43
65
21
43
65
2 4 6
1/L
1
3/L
2
5/L
3
U V W
-F1
-KM1
-Q1
M3
Motorcurrent
Speed
1
2
3
4
5
6
7
0 0.25 0.50 0.75 1
Instantaneous motor current
Instantaneous motor current
Motor Torque
Motor Torque
Load Torque
Load Torque
Torque
2.5
2
1.5
1
0.5
0 0.25 0.50 0.75 1
Speed
Direct-on-linestarter diagram
Direct-on-line current/speed characteristics Direct-on-line torque/speed curve
Starters
Starters
8
Although DOL starters offer a number of advantages,
including simplicity, low cost and high starting torque,
their use is limited to applications where:
• low-power motors are being used,
and the supply capacity is high, so
that the starting current surge
does not adversely affect other
equipment using the same supply
• the equipment driven by the motor
is fitted with a gearbox or some
other device which will soften the
mechanical shock produced by
the high starting torque
• a high starting torque is needed -
for example, the equipment starts
against its full mechanical load.
When the limitations of DOL
starting are not acceptable, it is
necessary to use alternative
starting techniques which reduce
the peak starting current and,
therefore, the peak starting torque.
The normal approach is to arrange
for the motor to be started at
reduced voltages, and a number of
methods have been developed for
doing this.
DOL starters are not suitable when:
• the peak starting current would
result in a serious voltage drop on
the supply system
• the equipment being driven cannot
tolerate the effects of very high
peak torque loadings
• the safety or comfort of those
using the equipment may be
compromised by sudden starting
as, for example, with escalators
and lifts
This type of starter may only be used where access is
possible to both ends of all three stator windings. In
addition, the windings must be rated to withstand the
full supply voltage when delta-connected. With star-
delta starting, the
peak starting
current is typically
between 1.5 and
2.6 times the
normal full-load
current, and the
peak starting
torque is between
0.2 and 0.5 times
the motor's
nominal operating
torque.
9
Star-deltastarters
Star-delta starters
Star-delta starting torque/speedcharacteristics
On starting, the supply is first
applied to the motor with its stator
windings star-connected. As the
motor accelerates, its speed
stabilises when its developed
torque become equal to its load
torque. This usually happens at
about 75% - 80% of nominal
speed. The star contactor is then
de-energised, and the delta
contactor energised to delta-
connect the stator windings. Each
winding is now
fed with the full
supply voltage,
and the motor
adopts its normal
operating
characteristics.
The run-up time with the windings
star-connected is controlled by a
timer which, typically, can be
adjusted from 0 to 30 seconds.
This timer is adjusted during
commissioning to ensure that the
star-delta changeover occurs, as
closely as possible, at the point of
torque equilibrium. The transition
time from star to delta is also
important, and a special timer is
normally used to ensure that there
is a period of between 30ms and
50ms between the opening of the
star contactor and the closing of
the delta contactor. This allows
time for any switching arcs to be
extinguished.
Star-delta starter
Star-delta startingcurrent/speedcharacteristics
current
Speed
1
2
3
4
5
6
7
0 0.25 0.50 0.75 1
Current in star connection
Current in star connection
Current in delta connection(direct)
Current in delta connection(direct)
Torque
Speed
2.5
2
1.5
1
0.5
0 0.25 0.50 0.75 1
Torque in delta
(dire
ct)
Torque insta
r
Load torque
Torque in delta
(dire
ct)
Torque insta
r
Load torque
21
43
65 1 3 5
1 3 5
1 3 5
2 4 6
2 4 6 2 4 6 2 4 6
2 4 6
1/L
1
3/L
2
5/L
3
U1
V1
W1
U2
V2
W2
-KM2 -KM3 -KM1
-F2
-Q1
M13
Star-delta starters
Primary resistance
starters
10 Star-delta starters are particularly suited to machines
which do not present a high load torque at start-up, or
which normally start off-load. It is also important to
note that, during the star-to-delta transition, a high
transient current is generated. If a magnetic short-
circuit protective device is to be used in the starter,
this transient must be taken into account in the
selection of the device, in order to prevent nuisance
tripping.
Although the transient produced at the star-delta
transition is very brief, the current can be quite large
and, particularly for larger motors, some form of
current limiting may be necessary. One solution is to
introduce a delay of 1 to 2 seconds during the star-to-
delta transition. To avoid too large a speed drop
during the transition, however, this method can only be
used with low-inertia loads.
Primary resistance starters
Starters of this type start the motor at reduced voltage
by connecting a resistance bank in series with the
motor windings. Once the motor has run up and its
speed has stabilised, the resistance bank is shorted
out, and the motor becomes direct-connected. This
changeover is normally controlled by an adjustable
timer within the starter. Unlike star-delta starters,
primary resistance starters do not require access to
both ends of the stator windings.
Values of starting current and torque are determined
by the values of the resistors used. Typically, however,
the peak starting current will be around 4.5 times
nominal full-load current, and peak starting torque will
be around 0.75 times nominal operating torque.
Primaryresistance
starters
11
Torque7
6
5
4
3
2
1
0 0.25 0.50 0.75 1
Speed
Current on 2nd. step without resistance(dire
Current on 2nd. step without resistance(dire
Current on 1st. step with resistance
Current on 1st. step with resistance
21
43
65
2 4 6
1 3 5
2U
4V
2W
-F1
-KM1
-KM11
R2
R4
R6
RU
RV
R6
R1
R3
R5
M3
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65
1L
1
3L
2
5L
3
-Q1
Torque
2.5
2
1.5
1
0.5
0 0.25 0.50 0.75 1
Speed
Torque on 1st. step with resistance
Torque on 1st. step with resistance
Torque on 2nd. step
without resistance (direct)
Torque on 2nd. step
without resistance (direct)
Load TorqueLoad Torque
Primary resistance starters are
especially suitable for applications,
such as ventilator fans, where the
load torque increases with speed.
A possible disadvantage is the high
peak current at the instant of
starting, but this can be reduced by
increasing the resistor values. Care
must be taken, however, since this
also reduces starting torque.
Primary resistance starter
Primary resistance startingcurrent/speed characteristics
Primary resistancestarting torque/speed
characteristics
Auto-transformerstarters
12
In auto-transformer starters, the motor is startedat reduced voltage which is supplied from anauto-transformer. The starting sequence hasthree stages.
Auto-transformer starters
Auto-transformer starting is particularly used for largemotors (above 100kW), but tends to be an expensivesolution, largely because of the cost of the auto-transformer itself. These starters may also produce a
current peak atthe instant when the motor is switched directly to thesupply. This peak can be minimised by careful designof the auto-transformer, but only at the expense ofincreasing the peak current at the commencement ofthe first stage of the starting sequence.
Torque
2.5
2
1.5
1
0.5
0 0.25 0.50 0.75 1
Load Torque
Torque on 1st. step
Torque on 2nd. step
Load Torque
Torque on 1st. step
Torque on 2nd. step
During the first stage, the auto-transformer is star-connected, and the line contactor is closed. Thisstarts the motor with a reduced voltage, the value ofwhich depends upon the ratio selected for thetransformer. Auto-transformers are normallyprovided with taps to allow the best ratio to bechosen during commissioning.
In the second stage, the star connection is opened,and the auto-transformer acts as an inductorconnected in series with the motor. This transitionis normally timed to occur when the motor speedhas stabilised at the end of the run-up period. Thethird stage then follows almost immediately, andinvolves shunting the transformer completely, sothat the motor is direct-connected to the supply.
The starting current and torque are reduced as afunction of the reduced starting and run-up voltages(Usupply/Ustarting)2. Typical values for peak startingcurrent are 1.7 to 4 times nominal full-load currentand, for peak starting torque, 0.5 to 0.85 timesnominal operating torque.
21
43
65
2 4 6
11
/L1
3/L
2
5/L
3
3 5 1 3 5
2U
4V
2 22
U1
U2W
-KM3
-F1
-Q1
-KM2
-T1
-KM1M3
24
V1
V2
26
1 3 5W
1W
2
U3
V3
W3
Current
0
1
2
3
4
5
6
7
0.25 0.50 0.75 1
Speed
Current without auto-transformer (direct)
Current on 1st. step
Current on 1st. step
Current without auto-transformer (direct)
Current on 2nd. step
Current on 1st. step
Auto-transformer starting current/speedcharacteristics
Auto-transformer startingtorque/speed characteristics
Auto-transformer starter
13
Electronic soft starters
Electronic soft starters
This relatively recently introduced
form of starter is rapidly growing in
popularity. Soft starters operate by
gradually increasing the voltage
applied to the motor, so as to
produce steady, smooth
acceleration. This technique
eliminates sudden changes in
voltage which could produce peaks
in both starting current and torque.
The steadily increasing supply
voltage for the motor during
starting is produced by a thyristor
bridge which, in each phase, has
two thyristors connected back-to-
back. By varying the firing angle of
each set of thyristors, it is possible
to control the starting voltage and,
hence, the starting current. Note
that, unlike inverter drives, soft
starters do not vary the frequency
of the supply to the motor.
The detailed design of soft starters
varies from manufacturer to
manufacturer, but a representative
unit is the Telemecanique Altistart 46.
This is fitted with a six-thyristor
power-switching bridge which
allows complete control over the
starting and stopping of a three-
phase squirrel-cage motor. It
provides:
• control of the acceleration and
deceleration ramps of the motor in
such a way as to keep within all
required limits on current and torque
• thermal overload protection for itself,
and for the motor which it is
controlling
• mechanical protection for the
machine being driven, by eliminating
sudden changes in current - and,
therefore, torque - during starting
and stopping.
M3
Note: = firing angle ofthymistors
5
2
2.5
4
ATS
2
0 0.25 0.50 0.75 VN 1.25
0
2
1TdB
2
3
2
1
1
1
0.25 0.50 0.75 VN 1.25
TC
TdCTdB
TBTB
TdA
TN
TA
1A
IN
lB
lC
Electronic “soft starter”
Electronic “soft starter”current/speedandtorque/speedcharacteristics
Electronicsoft starters
14
This type of starter may be used with any
asynchronous motor. The Telemecanique Altistart may
be bypassed by a contactor at the end of the
acceleration ramp, the contactor being controlled by a
contact provided for this purpose. This will avoid
thyristor heating and losses which occur during normal
running. Even with the starter bypassed, however, the
protective devices of units rated at 18.5kW/415V and
above remain operational, thus protecting both the
starter and the motor. For smaller units, a separate
thermal overload is required. Other features which can
be provided by soft starters include controlled
deceleration, and braking to a complete stop.
The peak starting current may be adjustedbetween 2 and 5 times nominal full-load current,corresponding to a range of starting torquesfrom 0.1 to 0.7 times the starting torque whichwould be produced if the motor were startedwith a DOL starter.
Rotor resistancestarters
15
Starters of this type can only be used with motors
having a wound rotor to which external connections
can be made, usually via slip rings. This type of motor
cannot be started direct on line because the peak
starting current at the instant that the supply is
connected would be far too high. Instead, the motor
is started with a resistance bank connected in series
with the rotor windings (NOT the stator windings, as in
primary resistance starters).
The starter is designed so that, at start-up, there is
maximum resistance in the rotor circuit. Various
sections of the resistance bank are then shorted out
progressively until, during normal running, no
resistance remains and the rotor windings are simply
star-connected.
Rotor resistance starters
21
43
65
2 4 6
U V W
K L M
-F1
-KM1
-R2A
M3
21
43
65
1/L
1
3/L
2
5/L
3
-Q1
-R2B
R2C
-R1A
-R1B
-R1C
A2B2
C2
-KM12
1 3 5
2 4 6
A1B1
C1
-KM11
1 3 5
2 4 6
Rotor resistance starter
Rotor resistancestarting
current/speedcharacteristics
Rotor resistancestartingtorque/speedcharacteristics
Current
Speed
1
2
3
4
5
6
7
0 0.25 0.50 0.75 1
Current on 3rd. step (no
resistors, direct)
Current on 2nd. step (some resistors)Current on 1st. step (all resistors)
Current on 3rd. step (no
resistors, direct)
Current on 2nd. step (some resistors)Current on 1st. step (all resistors)
Torque
2.5
2
1.5
1
0.5
0 0.25 0.50 0.75 1
Speed
Torque on 1st. step (all resistors)
Torque on 2nd. step(som
eresistors)
Torque without resisors)
Torque on 1st. step (all resistors)
Torque on 2nd. step(som
eresistors)
Torque without resisors)
Rotor resistance
starters
16
SummaryThe principle objective of all methods of motor starting is to matchthe torque characteristics to those of the mechanical load, whileensuring that the peak current requirements do not exceed thecapacity of the supply. Many starting methods are available, each of which has slightly different characteristics. The following tablesummarises the main characteristics for the most popular forms of starter.
For this type of motor, the torque is
virtually proportional to motor
current. A starting torque of twice
normal full-load current, therefore,
produces a starting torque which is
twice the nominal operating torque.
This is much better than a DOL
starter, where 6 x full-load current
produces only 1.5 x nominal torque
during starting. Slip-ring motors
with rotor resistance starters are,
therefore, ideal for high-inertia
loads which need to be started on-
load, but where the peak current
taken from the supply must be
limited. Further, the values of the
resistances and the number of
stages can be calculated so as to
match the motor characteristics to
those of the application.
Selecting a starter
17Selecting a starter
• The power for the machine
installation will normally be
supplied by the Regional Electricity
Company, and the user will need
to comply with any local
regulations. The Regional
Electricity Company will normally
limit DOL starting to a maximum
motor rating. If the motor is below
the DOL starting limit, determine
the peak starting current which
it would draw if started
direct-on-line.
• Check that this peak starting
current is within the capacity of
the supply.
• The installation will normally be fed
from a stepdown power
transformer. Check that the peak
starting current will not initiate a
circuit breaker trip on the high-
voltage (primary) side of the
transformer.
• Check that the supply line will not
introduce unacceptable voltage
drops when the peak current is
taken. If this is a problem, the
choice lies between installing
larger cables or selecting a
starting method other than DOL.
• If the above conditions are all
satisfied, DOL starting will provide
an economical solution, provided
that the mechanical load can
handle the peak starting torque
produced.
• If any of the conditions are not
satisfied, use the table to choose
an alternative method of starting.
Be particularly careful to ensure
that the starting torque produced
by the method of starting chosen
is adequate for the application.
When choosing a starter for a particular application,
the following procedure should be used:
Speedregulation ofasynchronous
motors
18 Speed regulation ofasynchronous motors
While speed regulation, strictly, goes a little beyond
motor starting, the two subjects are so closely related
that a brief discussion of speed regulation is included
here for the sake of completeness.
For many years, the scope for varying the running
speed of asynchronous motors was rather restricted.
Only motors with pole-changing facilities, and those
with separate windings, were popular for applications
requiring multi-speed operation, but even these types
could only operate at one of a number of fixed speeds.
This situation changed dramatically with the
introduction of frequency inverters which allow the
running speeds of standard motors to be accurately
controlled over a wide range. Inverter technology is so
successful that AC inverter drives are now being
adopted for many applications where, in the past, only
DC machines, with their inherent ease of speed
control, would have been suitable.
While various methods of speed control are possible,
which use only conventional components such as
contactors and resistors, these methods are fast
becoming obsolete as they are replaced by inverter
systems. This supplement will, therefore, deal
principally with speed control by inverter.
Speedregulation ofasynchronous
motors
19The frequency inverter drive
Inverter drive operation
This type of drive is intended mainly for use with three-phase squirrel
cage motors. It operates by using a technique called pulse-width
modulation (PWM) to synthesise a sinusoidal waveform, the frequency
of which can be varied, that is used to supply the motor. By varying the
frequency of the supply to the motor, the stepless motor speed
variation is possible over a wide range. Since the synthesised supply
waveform is very close to sinusoidal, smooth motor rotation is achieved
even at low speeds.
The AC supply (single or three phase) to the inverter is rectified by a
full-wave diode bridge, and is used to charge the main reservoir capacitors.
This provides the system with a high-voltage DC source which is then
switched by the output power bridge to produce a pulse train made up
of precisely controlled long and short pulses. The train of pulses
produces a sinusoidal current in the motor, the voltage and frequency
of which can be accurately controlled. By retaining the correct
voltage/frequency ratio in the supply to the motor, its torque can be
maintained over a wide speed range.
W
V
U
M3
rectifierBridge
Reservoircapacitor
Transistor outputpower bridge
Main circuit of afrequency inverter
Summary of characteristics of various starting methodsSquirrel cage motors
Advantages • Simple starter• Low cost• High starting
torque
• Simple, economicstarter
• Good startingtorque/currentperformance
• Possibility ofadjusting startingparameters
• No break in supplyto motor duringstarting
• Good reduction inpeak transientcurrents
Disadvantages • Very high startingcurrent and torque
• Supply mustwithstand peakcurrent
• Mechanically harshstarting sequence
• Low startingtorque
• Non-adjustablestartingparameters
• Break in supply tomotor leads tosevere transientpeak current
• Small reduction inpeak current
• Resistance bankrequired
Typicalapplications
• Small machinesmay often bestarted on full-load
• Machines startingon no-load (smallcentrifugal pumps,fans, etc.)
• High inertiamachines withnormal startingcurrent/torquecharacteristics
Run-up time 2 to 3 seconds 3 to 7 seconds 7 to 12 seconds
Direct-on-line Star-delta Primary resistance starting starting starting
Peak starting 4 to 8 In 1.3 to 2.6 In 4.5 InCurrent
Peak starting 0.6 to 1.5 Tn 0.2 to 0.5 Tn 0.6 to 0.85 Tntorque
Control On or off On or off 1 fixed step
Economic and rugged squirrel cage motor
20
Slip-ring motors
7 to 12 seconds Adjustable, 1 to 60seconds
0.1 to 999 seconds • 3-step : 2.5s• 4 and 5 step : 5s
• Good startingtorque/currentperformance
• Possibility ofadjusting startingparameters
• No break in supplyto motor duringstarting
• Parameters arefully adjustedduringcommissioning
• Compact• Solid state• Easily adapted to
the application
• Parameters arefully adjustedduringcommissioning
• Compact• Solid state• Easily adapted to
the application• Infinitely variable
speed• In-built motor
protection• Low starting
current
• Good startingtorque/currentperformance
• Possibility ofadjusting startingparameters
• No break in supplyto motor duringstarting
• Expensive auto-transformerrequired
• Not tolerant tosupply linetransients
• Can causeinterference on thesupply duringstarting andstopping
• Can causeinterference on thesupply
• Relativelyexpensivecompared todirect-on-line
• Expensive slip-ringmotor required
• Resistance bankrequired
• High inertiamachines where areduction ofstartingcurrent/torque isrequired
• Machines requiringvery smoothstarting (centrifugalpumps and fans,conveyors, etc.)
• All machineswhere speedneeds to be variedto improveproduction andreduce mechanicalwear
• Machines whereenergy can besaved by reducingspeed (centrifugalpumps, fans, etc.)
• Machines startingon-load, wheresmooth run-up isrequired, etc.
Auto-transformer Electronic Variable speed Rotor resistancestarting “soft-starting” drives starting
1.7 to 4 In Adjustable, 2 In 1.8 In for 200 ms <2.5 Into 5 In
0.4 to 0.85 Tn Adjustable, 0.1 1.7 Tn <2.5 Tnto 0.7 Tn
3 fixed step Gradual Variable 1 to 5 fixed steps
21
22
Inverter driveapplications
Frequency inverter drives are very easy to use with
standard squirrel cage motors. Their torque
capabilities allow their use with all types of load,
including those requiring very high torques. For
applications where overhauling loads may be
encountered (hoists, mechanical handling, etc.) drives
are available for four-quadrant operation. These can
control both forward and reverse (hoist and lower)
operations, and they often include a braking facility.
Inverter drives almost invariably incorporate electronic
protection against thermal overloads and short
circuits. This protects both the motor and the drive.
Many drives also incorporate communications
capabilities which facilitate their integration into
automated systems.
Inverter drive applications
Variable voltagecontrollers
Variable-voltage controllers
An alternative to inverter drives, these units offer
another method of achieving motor speed control
electronically. As they are much less versatile than
frequency inverters, however, they are now declining in
popularity.
The principle of operation in this type of controller is to
vary only the voltage applied to the motor. The torque
produced by an asynchronous motor is proportional to
the square of the supply voltage. This type of drive
operates by regulating the voltage such that the torque
produced just balances the load torque at the speed
required. The motor supply voltage is usually
controlled by varying the firing angle of a pair of back-
to-back thyristors in each phase of the supply.
The use of variable-voltage controllers is limited by the
high losses in the rotor, which occur when
asynchronous motors are operated under high-
slip/low-speed conditions. These drives are most
suitable for motors with ratings of 3kW or less.
Summary
The availability of inverter drives has made
variable speed operation for asynchronous
motors increasingly popular. While other
methods of speed control are available, none
offers the versatility and performance of
frequency inverters.
23
24
Starters by design
Starters by design
This supplement has dealt with the general principles
of motor starting, and it is intended as an aid to
choosing the best starting technique for a particular
application. With the starter type decided, the next
step is either to select an off-the-shelf starter, if it is a
simple standard type or, for more complex
applications, to design a suitable starter.
Design guidelines for popular starter types are readily
found in the literature available from control gear
suppliers, but designers are also encouraged to talk to
their suppliers. Products and methods are constantly
evolving and, perhaps even more important, new
standards and regulations are imposing new duties
and responsibilities on designers. There is no better
way to keep up-to-date than to talk to an expert
supplier which has a strong focus in the control gear
market.
Furtherinformation
Further information
In this short supplement, is has not been possible to
do more than discuss briefly the most popular
methods of starting and motor speed control. Further
information is, however, readily available.
Telemecanique, a brand of the Schneider group, offers
two invaluable publications which are particularly
relevant.
Power control and protection
components
This contains technical details and
characteristics of motor starting components
necessary for the starting methods
described in this supplement.
Practical Aspects of Industrial
Control Technology
This comprehensive and up-to-date 290-page
hardback publication is available for purchase
from Telemecanique. It provides proven
design and application information covering
both electric and electronic products for
industry, and it has substantial sections
dealing with motor starting and control.
These publications are available from:
Telemecanique
University of Warwick Science Park
Sir William Lyons Road
Coventry CV4 7EZ
Tel: (01203) 416255
25
GSUK 0244 MAR 98
Schneider Limited University of Warwick Science Park Sir William Lyons Road Coventry CV4 7EZ Tel: 01203 416255 Fax: 01203 690209
Internet address: http://www.schneider.co.uk