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Electronic Soft Starters

Electronic Soft Starters. Fairford North America. How and Why they Work, and What they can do for You. The Starting Point. The performance of a soft starter is crucially dependent on the performance of the motor it controls.

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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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