Transcript Electronic Soft Starters

Fairford North America
Electronic Soft Starters
How and Why they Work, and
What they can do for You.
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The Starting Point
The performance of a soft starter is
crucially dependent on the performance
of the motor it controls.
 Therefore, it is essential to understand
motor characteristics in order to
understand soft starter applications.

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Statements of Fact - 1
Energy cannot be created or destroyed
- but it can be converted from one form
to another.
 In a conversion system, an event such
as a surge in demand, experienced in
one form of energy is immediately
reflected in the other form of energy.

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Statements of Fact - 2
An induction motor is an energy
conversion device, converting electrical
energy into mechanical energy and
some heat energy.
 For an induction motor, torque is
proportional to the motor terminal
voltage squared. (V2). So, for example,
half the voltage produces a quarter of
the torque.

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Induction Motors - 1
Were invented about 100 years ago.
 Because of their simplicity and
efficiency, they are the workhorses of
industry throughout the world.

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Induction Motors - 2
The induction motor has two basic
functions to fulfil :
 1 To accelerate itself and its load to
full speed.
 2 To maintain the load at full speed
efficiently and effectively over the
full range of loading.

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The motor speed-torque
curve
Pull Out (Peak) Torque
(MK)
Torque
Locked Rotor
Torque (LRT, MA)
Pull Up Torque
Full Load Torque
(FLT, MN)
0
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Speed
S
The load speed-torque curve
Torque
Motor
Full Load Torque
(FLT)
Load
0
8
Speed
S
Putting the two curves
together we get
Peak Torque
Torque
Locked Rotor
Torque
Motor
T2
Full Load Torque
(FLT)
The accelerating torque
at any speed S’ is
(T2 - T1)
Load
T1
0
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S’
Speed
S
What does this mean in
practice?
Because the load starting torque is
usually near zero, an induction motor
will develop far too much torque when
connected directly to the supply. So,
 At the instant of start-up, there is an unnecessary heavy power surge on both
the electrical supply and the
mechanical drive components.

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The mechanical effects
The sudden impact at start up on the
load, followed by the rapid acceleration
to full speed causes excessive wear on
: Belts and pulleys
 Gears and chains
 Couplings and bearings and : Cavitation in pumps etc........

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The electrical effects
A heavy current surge on the electrical
supply which can be severe enough to
cause voltage dips and flickering lights.
 Pitting and burning of contacts due to
high currents which are many times the
motor full-load current.

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The user experiences:Continuous maintenance.
 Greater chance of unscheduled
shutdowns
 Oversized mechanical and electrical
components to cater for the power
surge on start up.
 Short component life.

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Direct-On-Line Starters
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 up
 Very rapid acceleration of load with
large transient forces placed on drive
train and electrical system

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Direct-On-Line Starting
600%
CURRENT
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TIME
Star-Delta Starters
Uses three contactors and a timing
mechanism to switch 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.

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The Star-Delta Trauma
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Star/Delta Start
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Primary Resistance Starter
Inserts a resistance in one or more of
the stator phase connections. The
resistance is progressively reduced as
acceleration occurs.
 Starts are limited
 Installation and running costs are high.
 Smoothest and least stressful method
of electro - mechanical starting

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Other Means Of Starting
Auto transformer
 Primary Reactance

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Semiconductor Motor
Controller
Is based upon the silicon controlled
rectifier or ‘thyristor’.
 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.

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Thyristor Control
VOLTAGE OR ENERGY
ONE MAINS CYCLE
TIME OR DEGREES
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Semiconductor Motor
Controller (cont.....)

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By starting with a large delay angle and
gradually reducing it , the motor
terminal voltage is increased from a low
value to full voltage, giving a smooth,
stepless, start.
A Soft Start compared
DIRECT-ON-LINE
6-700%
STAR-DELTA
CURRENT
SOFT START
100%
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TIME
The Result is :-

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In a simple, reliable and cost
effective piece of equipment, all the
problems of electro-mechanical
starting have been banished to the
past!
Running Induction Motors
100%
EFFICIENCY
0
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1/2
LOAD
1/1
The Action of the Optimising
Mode - 1
Optimising continually monitors the
motor to improve the part-load
efficiency by reducing the degree of
over-fluxing of the stator.
 The part load pF of the motor is
improved.

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The Action of the Optimising
Mode - 2
This produces a significant reduction in
kVAr and kVA, as well as a useful
reduction in kW.
 Electrical losses in the stator and feed
cables are reduced.
 Motor performance is unimpaired.

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Applications
There is virtually no fixed-speed
application of an induction motor which
cannot benefit from the fitting of a QFE
soft-starter.
 In particular, pumps and pumping
systems benefit from the soft-stop
feature and the reduction in cavitation
effects.

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Applications
Fans, particularly high inertia ones,
enjoy smooth, stepless acceleration at
controlled current levels without the
damaging effects of the star-to-delta
transfer.
 The life of frequently-started drives and
of reversing drives is considerably
extended by the soft start action.

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Applications
 Compressors,
 Chillers,
Escalators,
 Conveyors,
 Mixers,
 Rollers,
 Screw
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 MG
feeders.
sets (elevators)
Applications
Ball mills,
 Blowers,
 Grinders,
 Saws,
 Moulders,
 Screens,

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Crushers,
Pelletisers
Presses,
Washers,
Sifters,
Tumblers, etc..,etc...,
Applications

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All these and many other
applications have been
successfully softstarted by the
QFE.
Reliability


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IEC 60947-4-2, EN 60947-4-2
AC Semi-Conductor Motor Controllers
and Starters’ defines the soft starter in
every important aspect. Purchasing a
controller conforming to this standard
will safeguard the user from poor
equipment.
UL listed for the United States and
Canada
The End
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