11

Electronic Soft Starters

How and Why they Work, and How and Why they Work, and What they can do for You.What they can do for You.

Fairford North America Fairford North America

22

The Starting Point

The performance of a soft starter is The performance of a soft starter is crucially dependent on the performance crucially dependent on the performance of the motor it controls.of the motor it controls.

Therefore, it is essential to understand Therefore, it is essential to understand motor characteristics in order to motor characteristics in order to understand soft starter applications.understand soft starter applications.

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Statements of Fact - 1

Energy cannot be created or destroyed Energy cannot be created or destroyed - but it can be converted from one form - but it can be converted from one form to another.to another.

In a conversion system, an event such In a conversion system, an event such as a surge in demand, experienced in as a surge in demand, experienced in one form of energy is immediately one form of energy is immediately reflected in the other form of energy.reflected in the other form of energy.

44

Statements of Fact - 2

An induction motor is an energy An induction motor is an energy conversion device, converting electrical conversion device, converting electrical energy into mechanical energy and some energy into mechanical energy and some heat energy.heat energy.

For an induction motor, torque is For an induction motor, torque is proportional to the motor terminal voltage proportional to the motor terminal voltage squared. (Vsquared. (V22). So, for example, half the ). So, for example, half the voltage produces a quarter of the torque.voltage produces a quarter of the torque.

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Induction Motors - 1

Were invented about 100 years ago.Were invented about 100 years ago. Because of their simplicity and Because of their simplicity and

efficiency, they are the workhorses of efficiency, they are the workhorses of industry throughout the world.industry throughout the world.

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Induction Motors - 2

The induction motor has two basic The induction motor has two basic functions to fulfil :functions to fulfil :

11 To accelerate itself and its load to To accelerate itself and its load to full speed.full speed.

22 To maintain the load at full speed To maintain the load at full speed efficiently and effectively over efficiently and effectively over

the the full range of loading.full range of loading.

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The motor speed-torque curve

Locked Rotor Torque (LRT, MA)

0 SSpeed

Pull Up Torque

Pull Out (Peak) Torque (MK)

Full Load Torque(FLT, MN)

Torque

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The load speed-torque curve

0 SSpeed

Full Load Torque(FLT)

Torque

0 SSpeed

Load

Torque

Motor

99

Putting the two curves together we get

Torque

Load

Torque

MotorLocked Rotor

Torque

Peak Torque

Full Load Torque(FLT)

S’

The accelerating torqueat any speed S’ is

(T2 - T1)

0 SSpeed

T2

T1

1010

Because the load starting torque is Because the load starting torque is usually near zero, an induction motor usually near zero, an induction motor will develop far too much torque when will develop far too much torque when connected directly to the supply. So, connected directly to the supply. So,

At the instant of start-up, there is an un-At the instant of start-up, there is an un-necessary heavy power surge on necessary heavy power surge on bothboth the electrical supply the electrical supply andand the the mechanical drive components.mechanical drive components.

What does this mean in practice?

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The mechanical effects

The sudden impact at start up on the The sudden impact at start up on the load, followed by the rapid acceleration load, followed by the rapid acceleration to full speed causes excessive wear on :-to full speed causes excessive wear on :-

Belts and pulleysBelts and pulleys Gears and chainsGears and chains Couplings and bearings and :-Couplings and bearings and :- Cavitation in pumps etc........Cavitation in pumps etc........

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The electrical effects

A heavy current surge on the electrical A heavy current surge on the electrical supply which can be severe enough to supply which can be severe enough to cause voltage dips and flickering lights.cause voltage dips and flickering lights.

Pitting and burning of contacts due to Pitting and burning of contacts due to high currents which are many times the high currents which are many times the motor full-load current.motor full-load current.

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The user experiences:-

Continuous maintenance.Continuous maintenance. Greater chance of unscheduled Greater chance of unscheduled

shutdownsshutdowns Oversized mechanical and electrical Oversized mechanical and electrical

components to cater for the power components to cater for the power surge on start up.surge on start up.

Short component life.Short component life.

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Primitive, but familiar - Just a simple Primitive, but familiar - Just a simple contactor in circuit between the motor and contactor in circuit between the motor and supply giving only unsynchronised on-off supply giving only unsynchronised on-off controlcontrol

No control over inrush currents or torque No control over inrush currents or torque surge at start upsurge at start up

Very rapid acceleration of load with large Very rapid acceleration of load with large transient forces placed on drive train and transient forces placed on drive train and electrical systemelectrical system

Direct-On-Line Starters

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Direct-On-Line Starting

CURRENT

TIME

600%

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Star-Delta Starters

Uses three contactors and a timing Uses three contactors and a timing mechanism to switch between them.mechanism to switch between them.

Installation costs are greater because Installation costs are greater because of the need for 6 connections between of the need for 6 connections between the motor and starter.the motor and starter.

If the transfer occurs at less than 80% If the transfer occurs at less than 80% of normal speed, large current and of normal speed, large current and torque surges can arise.torque surges can arise.

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The Star-Delta Trauma

1818

Star/Delta Start

1919

Inserts a resistance in one or more of Inserts a resistance in one or more of the stator phase connections. The the stator phase connections. The resistance is progressively reduced as resistance is progressively reduced as acceleration occurs.acceleration occurs.

Starts are limited Starts are limited Installation and running costs are high.Installation and running costs are high. Smoothest and least stressful method Smoothest and least stressful method

of electro - mechanical startingof electro - mechanical starting

Primary Resistance Starter

2020

Other Means Of Starting

Auto transformerAuto transformer Primary ReactancePrimary Reactance

2121

Semiconductor Motor Controller

Is based upon the silicon controlled rectifier Is based upon the silicon controlled rectifier or ‘thyristor’.or ‘thyristor’.

By pulsing a thyristor, it switches from ‘off’ By pulsing a thyristor, it switches from ‘off’ to ‘on’ until the current stops flowing though to ‘on’ until the current stops flowing though it - which occurs every half cycle in an AC. it - which occurs every half cycle in an AC. supply.supply.

By controlling electronically the thyristor By controlling electronically the thyristor turn on point, it is possible to regulate the turn on point, it is possible to regulate the energy passing through it.energy passing through it.

2222

Thyristor Control

ONE MAINS CYCLE

TIME OR DEGREES

VO

LTA

GE

OR

EN

ER

GY

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Semiconductor Motor Controller (cont.....)

By starting with a large delay angle and By starting with a large delay angle and gradually reducing it , the motor gradually reducing it , the motor terminal voltage is increased from a low terminal voltage is increased from a low value to full voltage, giving a smooth, value to full voltage, giving a smooth, stepless, start.stepless, start.

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A Soft Start compared

CURRENT

6-700%

DIRECT-ON-LINE

STAR-DELTA

SOFT START

100%

TIME

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The Result is :-

In a simple, reliable and cost In a simple, reliable and cost effective piece of equipment, all the effective piece of equipment, all the problems of electro-mechanical problems of electro-mechanical starting have been banished to the starting have been banished to the past!past!

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Running Induction Motors

LOAD0 1/2 1/1

EFFICIENCY

100%

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The Action of the Optimising Mode - 1

Optimising continually monitors the Optimising continually monitors the motor to improve the part-load motor to improve the part-load efficiency by reducing the degree of efficiency by reducing the degree of over-fluxing of the stator.over-fluxing of the stator.

The part load pF of the motor is The part load pF of the motor is improved.improved.

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The Action of the Optimising Mode - 2

This produces a significant reduction in This produces a significant reduction in kVAr and kVA, as well as a useful kVAr and kVA, as well as a useful reduction in kW.reduction in kW.

Electrical losses in the stator and feed Electrical losses in the stator and feed cables are reduced.cables are reduced.

Motor performance is unimpaired.Motor performance is unimpaired.

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Applications

There is virtually no fixed-speed There is virtually no fixed-speed application of an induction motor which application of an induction motor which cannot benefit from the fitting of a QFE cannot benefit from the fitting of a QFE soft-starter.soft-starter.

In particular, pumps and pumping In particular, pumps and pumping systems benefit from the soft-stop systems benefit from the soft-stop feature and the reduction in cavitation feature and the reduction in cavitation effects.effects.

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Applications

Fans, particularly high inertia ones, Fans, particularly high inertia ones, enjoy smooth, stepless acceleration at enjoy smooth, stepless acceleration at controlled current levels without the controlled current levels without the damaging effects of the star-to-delta damaging effects of the star-to-delta transfer.transfer.

The life of frequently-started drives and The life of frequently-started drives and of reversing drives is considerably of reversing drives is considerably extended by the soft start action.extended by the soft start action.

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Applications

Compressors, Compressors, Chillers, Escalators, Chillers, Escalators, Conveyors, Conveyors, Mixers,Mixers, Rollers,Rollers,

Screw feedersScrew feeders.. MG sets (elevators)MG sets (elevators)

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Applications

Ball mills, Crushers,Ball mills, Crushers, Blowers, PelletisersBlowers, Pelletisers Grinders, Presses, Grinders, Presses, Saws, Washers, Saws, Washers, Moulders, Sifters, Moulders, Sifters, Screens, Tumblers, etc..,etc...,Screens, Tumblers, etc..,etc...,

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Applications

All these and many other All these and many other applications have been applications have been successfully softstarted by the successfully softstarted by the QFE.QFE.

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Reliability

IEC 60947-4-2, EN 60947-4-2IEC 60947-4-2, EN 60947-4-2AC Semi-Conductor Motor Controllers AC Semi-Conductor Motor Controllers and Starters’ defines the soft starter in and Starters’ defines the soft starter in every important aspect. Purchasing a every important aspect. Purchasing a controller conforming to this standard controller conforming to this standard will safeguard the user from poor will safeguard the user from poor equipment.equipment.

UL listed for the United States and UL listed for the United States and CanadaCanada

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The End