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Enerco Energy Solutions LLP www.econserve.in
AN INTRODUCTIONTO THE FULLY AUTOMATIC LOW STARTING CURRENT, CONTINUOUS TORQUE
STARTERS WITH KORNDORFER CLOSED TRANSITION FOR SQUIRREL - CAGE
INDUCTION MOTORS
THE AUTO-STRESTERAUTO-STRESTER is a fully automatic low starting current starter, which is having the followingadvantages over &
above the other starters :
The Auto-Strester does not carry any handle. To start your Squirrel cage induction motor
just one push button is required to be pressed - that is all.
The greatest advantage of the Auto-Strester is that it draws less starting current from the
MAINS and
delivers simultaneously more current to the motor for the proper operation & quicker
acceleration of a Squirrel cage motor.
The Auto-Strester motor starter can limit the starting current of a Squirrel cage induction
motor to any desired value- depending upon the rules & regulations of the individual Supply
Authorities.
The life of the contacts of the Auto-Strester (contactors type starter) is much more as it
incorporates only automatic contactors.
The Auto-Strester can be Switched ON / Switched OFF by any nos. of Remote-control
stations.
The main advantage of the Auto-Strester is that because of its low starting current, one can
adjust the over load relay too close to the normal load of the motor & as such it will trip-off
itself whenever theres overloading / single-phasing, etc.
In our Auto-Strester we use Siemens / L&T / Schneider / C-H / Jyoti make Contactors, Over
load relay, & Time delay relays etc.
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Auto-Strester is Guaranteed for two years against any Technical- defect / bad
workmanship.
SALIENT FEATURES: Easy operation.
Time tested.
Complying with the requirements specified as per IEC : 947, IS : 3914 & IS : 8544 (Part 4).
Comprehensive range.
Maintenance of Motor starters is very easy. Everything is front Operated.
Wall Mounting Design upto 75 HP & Floor Mounting Design 80 HP& above.
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STARTING METHODS OF SQUIRREL CAGE INDUCTION MOTORS :
The cheapest drive in the world is a drive operated Electrically i.e. an Electric - Motor. There are
different types of Electric-motors available in the market, but the cheapest, simple & robust out ofthe whole lot is a Squirrel cage induction motor. This type of motor has got very many advantages if
compared with that of a similar electric motor having wound rotor.
The initial cost of the installation of a motor having wound rotor (slip- ring ) is practically double if
compared with that of an ordinary Squirrel cage induction motor & moreover the running
maintenance of a motor having wound rotor is too high i.e. it requires the cleaning / replacement
of wornout carbon brushes , slip rings , etc. alongwith its other associated gears & as such a
motor having wound rotor has not become much popular in the world.
Squirrel cage motors are having a slight draw back because these motors take higher Starting
current from the MAINS at the time of starting. Direct On Line is the simplest starting procedure
& in this method high starting torque is associated with very high starting currentand as such theSupply Authorities in different countries / states have formulated various regulations to limit the
Starting- current of these Squirrel cage induction motors. In some countries / states, the Starting
current of these motors is brought to a considerable limit by incorporating a Star Delta starter.
The Star Delta method of motor starting gives a fixed starting torque, as the applied voltage in
Star is a fixed ratio of its line voltage. By Star Delta starting the starting current and starting
torque will reduce to 33% of the direct on line values. Furthermore, because of considerable
Current Transients produced by the Open Switching process ( when changing over from Star to
Delta connections ) the Star Delta method of starting the large Squirrel cage motors is not
considered to be a safer method ( ISS : 3914 - 7.1.6 ).
As per code of practice for selection of ac induction motor starters (voltage not exceeding 1000volts ) in different countries / states -The Star Delta starting may only be employed for such
drives which permit bringing up the motor, even when connected in Star ( start ) , almost close (
about 90 percent ) to the full speed.
An earlier changeover from Star ( start ) to Delta ( run ) may lead to heavy inrush currents & not
too smooth a starting.
In case ofWauchope closed transition star delta starter,one can not vary the starting torque,
though it is a closed transition type star delta starter. In case of drives where starting torque
required by the machine is more than what is developed by the motor, there would be heavy
current-transients while changing from star to delta connections, as the applied voltage in Star is
fixed.
While considering the Primary Resistor Starters this method consists of connecting the motor to the
line through a series resistor in each phase. The main disadvantage of this type of starter is that for
the same starting torque, the line current & thus power taken from the MAINS is much higheras
compared with the star delta or an autotransformer starter.
The other method of reduced voltage starting is an Auto Transformer Starter (conventional). This
starter gives reduced voltage to the motor by means of an auto-transformer in the motor circuit at
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the time of starting ; after a definite time interval and after the motor accelerates, it is transferred
from the reduced voltage to the full voltage.
The auto-transformers are generally provided with voltage taps. It is possible to vary the starting
current and torque within the limits of the tappings provided. UNLESS CLOSED TRANSITION
KORNDORFER METHOD IS ADOPTED THIS STARTER HAS THE SAME DISADVANTAGE OF OPENING
THE CIRCUIT BEFORE THE MOTOR IS CONNECTED TO THE LINE AND PRODUCING SIMILAR
TRANSIENT CURRENT AND STRESSES.
FROM THE ABOVE EXPLANATIONS ONE MAY CONCLUDE THAT THE BEST ELECTRICAL METHOD
OF STARTING A SQUIRREL CAGE INDUCTION MOTOR IS A FULLY AUTOMATIC AUTO-
TRANSFORMER STARTER WITH KORNDORFER CLOSED TRANSTION PRINCIPLE.
HERE WE MAY MENTION THAT WE ARE GLAD THAT WE HAD DEVELOPED THE SAME
IN THE NAME - AUTO - STRESTER
THE MAJOR ADVANTAGE OF THE AUTO - STRESTER IS THAT IN MOST OF THE CASES IT REPLACES THE INSTALLATION OF
SLIP-RING MOTOR WITH SQUIRREL CAGE MOTOR.
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Direct OnLine StarterAdvantages It is the cheapest starter available
It provides 100% torque at the time of starting
Motor is connected in delta at motor terminals
Only one set of cable is required from starter to motor
Disadvantages
It draws 6 times current of the Full load ampere at the time ofstarting resulting in possible burnout of motor and / or starter.
There is a big voltage dip in the electrical installation because of
high in-rush current and therefore not suitable for higher size
squirrel cage motors.
Significant energy wastage and reduced motor life.
Star Delta Starter
Advantage It draws 2 times starting current of the full load ampere of the
motor connected
It is an open transition principle method while changing from star
to delta connections
Disadvantage It provides only 33% starting torque and if the load connected tothe subject motor requires higher starting torque at the time of
starting than very heavy transients and stresses are produced while
changing from star to delta connections, and because of these
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transients and stresses many electrical and mechanical break-down
occurs In this method of starting initially motor is connected in star and
than after change over the motor is connected in delta. The delta of
motor is formed in starter and not on motor terminals
It requires 2 set of cables from starter to motor
ATS (Conventional)
Advantages It draws less current from the mains and simultaneously delivers
more current to the motor
It can limit the starting current to any desired value
One can vary the starting torque of the motor as per the tappings
provided
Only one set of cable is required from starter to motor
Motor is connected in delta at motor terminals
Disadvantages It is an open transition principle method while changing from ATS to
delta connections
Heavy transients and stresses are produced while changing from
ATS to delta connections because of open transition principle and
these results in electrical break-down
It is costlier than Star Delta Starter
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THE AUTO-STRESTERFully Automatic Autotransformer Starter with
Korndorfer Closed Transition Principle with
Primary Off Logic
Advantages It draws less current from the mains and
simultaneously delivers more current to the
motor
It can limit the starting current to any desired
value
One can vary the starting torque of the motoras per the tappings provided very conveniently
It is a Korndorfer closed transition principle
method while changing from ATS to delta
connections
Practically no transients and stresses are
produced while changing from ATS to deltaconnections and thus increases the electrical
and mechanical life of the installation
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Only one set of cable is required from starter
to motor
Motor is connected in delta at motor terminals
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