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Electronic Soft Starters

Learn why soft starters are vital for motor applications, how they prevent power surges, and reduce mechanical stress and energy wastage. Discover various starting methods and their impact on motor performance and longevity.

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Electronic Soft Starters

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  1. Electronic Soft Starters Fairford North America How and Why they Work, and What they can do for You.

  2. 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.

  3. 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.

  4. 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.

  5. Induction Motors - 1 • Were invented about 100 years ago. • Because of their simplicity and efficiency, they are the workhorses of industry throughout the world.

  6. 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.

  7. 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 Speed S

  8. The load speed-torque curve Torque Torque Motor Full Load Torque (FLT) Load 0 0 Speed Speed S S

  9. Putting the two curves together we get Peak Torque Torque Torque Locked Rotor Torque Motor T2 Full Load Torque (FLT) The accelerating torque at any speed S’ is (T2 - T1) Load T1 S’ 0 Speed S

  10. 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 un-necessary heavy power surge on both the electrical supply and the mechanical drive components.

  11. 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........

  12. 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.

  13. 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.

  14. 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

  15. Direct-On-Line Starting 600% CURRENT TIME

  16. 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.

  17. The Star-Delta Trauma

  18. Star/Delta Start

  19. 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

  20. Other Means Of Starting • Auto transformer • Primary Reactance

  21. 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.

  22. Thyristor Control

  23. Semiconductor Motor Controller (cont.....) • 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.

  24. A Soft Start compared DIRECT-ON-LINE • 6-700% STAR-DELTA CURRENT SOFT START 100% TIME

  25. The Result is :- • In a simple, reliable and cost effective piece of equipment, all the problems of electro-mechanical starting have been banished to the past!

  26. Running Induction Motors 100% EFFICIENCY 0 1/2 1/1 LOAD

  27. 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.

  28. 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.

  29. 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.

  30. 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.

  31. Applications • Compressors, • Chillers, Escalators, • Conveyors, • Mixers, • Rollers, • Screw feeders. • MG sets (elevators)

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

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

  34. Reliability • IEC 60947-4-2, EN 60947-4-2AC 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

  35. The End

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