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Date post: 08-Dec-2021
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For internal circulation of BSNL only E2-E3 ELECTRICAL TECHNICAL SOFT STARTERS
PowerPoint PresentationE2-E3
Module for the Topic: Soft Starter.
• Eligibility: Those who have got the Upgradation from E2
to E3.
• This presentation is last updated on 15-3-2011.
• You can also visit the Digital library of BSNL to see this
Induction Motors
effectively over the full range of loading.
For internal circulation of BSNL only
The motor speed-torque curve
The load speed-torque curve
Putting the two curves together we get
(T2 - T1)
0 SSpeed
• Because the load starting torque is usually near zero, an
induction motor will develop far too much torque when
connected directly to the supply.
• At the instant of start-up, there is an un-necessary heavy
power surge on both the electrical supply and the
mechanical drive components.
For internal circulation of BSNLonly
For internal circulation of BSNL only
The mechanical effects
• The sudden impact at start up on the load, followed by
the rapid acceleration to full speed causes excessive
wear on :
The electrical effects
• A heavy current surge on the electrical supply which can be severe
enough to cause voltage dips and flickering lights.
For internal circulation of BSNL only
The user experiences:
• Oversized mechanical and electrical components to cater
for the power surge on start up.
• Short component life.
• Primitive, but familiar - Just a simple contactor in circuit
between the motor and supply giving only
unsynchronised on-off control
• No control over inrush currents or torque surge at start
placed on drive and electrical system
Direct-On-Line Starters
Direct-On-Line Starting
Star-Delta Starters
between them.
• Installation costs are greater because of the need for 6
connections between the motor and starter.
• If the transfer occurs at less than 80% of normal speed,
large current and torque surges can arise.
For internal circulation of BSNL only
• Although an apparent improvement over the direct system,
significant disadvantages still remain.
• The transfer from star to delta momentarily removes the
motor from the supply. During this time the motor is under
the mechanical influence of the rotating load and, at the
instant of disconnection, current will still flow in the rotor
bars due to the time delay necessary for the magnetic flux
to die away.
For internal circulation of BSNL only
• Because of this, there is a residual flux "frozen" on the
surface of the rotating rotor, which cuts the stator
windings, generating a voltage in the stator winding at a
frequency depending on the rotor speed which has to be
lower than mains frequency.
• Although these effects are present for only a very short
period of time, about one fifth of a second, they are
sources of great stress and damage to the whole drive
system, and when frequent starting is necessary, they
bring increased risk of catastrophic failure and high
maintenance costs.
The Star-Delta Trauma
Star/Delta Start
Semiconductor Motor Controller
• By pulsing a Thyristor, it switches from ‘off’ to ‘on’
until the current stops flowing though it - which
occurs every half cycle in an AC supply.
• By controlling electronically the thyristor turn on
point, it is possible to regulate the energy passing
through it.
Thyristor Control
• Since the average voltage is the root-mean square of
each individual voltage level at any instant during a
sinusoidal wave, and since the motor torque is
proportional to the square of the voltage applied to the
motor, it follows that the longer the thyristor is switched
off, the less the torque generated by the motor during
that wave.
Semiconductor Motor Controller (cont.....)
a low value to full voltage, giving a smooth, stepless,
A Soft Start compared
Soft Motor Starter
Soft starter provides a controlled release of power to the
motor, thereby providing smooth, stepless acceleration
and deceleration.
Soft Starter
Description of the softstarters
- Design, settings and signals
analogue type or a combination of
For internal circulation of BSNL only
• Printed circuit board is used to control the firing of the
thyristors based on the current and voltage references,
and also for the calculation of different values, for
example the power factor, active power, etc. It can also
be used for storing historical data, the event
log,indicating trends and much more.
For internal circulation of BSNL only
• Heat sink is used to get rid of the heat in the softstarter
generated by the current during the start and the
continuous run.
The capacity of the heat sink very much reflects the
starting capacity and the operational current of the
• Fans are used to increase the cooling capacity of the
heat sink. One, two or several fans can be used
depending on size and design.
• Some smaller softstarters don’t have fans at all and the
number of starts may be limited.
For internal circulation of BSNL only
• Housing can be made of plastic material,metal or a
combination of these, and its function is to protect the
inside components from mechanical and electrical
damage. It is also used to protect the components from
dust and dirt.
• For total outside protection from dust and dirt a separate
enclosure is often required since the degree of protection
(IP class) of the unit itself is too low.
For internal circulation of BSNL only
• Thyristors are semi-conducting components connected
in an anti-parallel fashion and placed in two or three
phases of the main circuit. They regulate (by increasing
or decreasing) the level of voltage during start and the
stop ramp, as described in the picture below.
• During a continuous run the thyristors are conducting
For internal circulation of BSNL only
For internal circulation of BSNL only
• Start: The thyristors let part of the voltage through at the
beginning and then increase it, according to the set ramp
time for the start.
ramp time for stop.
• When starting, AC Induction motor develops more
torque than is required at full speed. This stress is
transferred to the mechanical transmission system
resulting in excessive wear and premature failure of
chains, belts, gears, mechanical seals, etc.
• Additionally, rapid acceleration also has a massive
impact on electricity supply charges with high inrush
currents drawing +600% of the normal run current.
For internal circulation of BSNL only
• Soft starter provides a reliable and economical solution
to these problems by delivering a controlled release of
power to the motor, thereby providing smooth, stepless
acceleration and deceleration.
• Motor life will be extended as damage to windings and
bearings is reduced.
• Soft Start & Soft Stop is built into 3 phase units,
providing controlled starting and stopping with a
selection of ramp times and current limit settings to suit
all applications.
Figure - Soft Starter: Starting current, Stress profile during starting
For internal circulation of BSNL only
Advantages of Soft Starter:
• Another feature of the softstarter is the softstop
function, which is very useful when stopping pumps
where the problem is water hammering in the pipe
system at direct stop as for star-delta starter and direct-
on-line starter.
conveyor belts to prevent material from damage when
the belts stop too quickly.
For internal circulation of BSNL only