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Whenever conductor cuts magnetic field an e.m.f is induced in it. b.Whenever current carrying conductor placed in magnetic field it experiences a force Why motor draws heavy current at start? : 25-Aug-10 4 KPCL (ACD R & E) Why motor draws heavy current at start? Need of Starters:- : 25-Aug-10 5 KPCL (ACD R & E) Need of Starters:- Starting a AC Induction motor can be a stressful experience for both the user’s power distribution system as well as the mechanical system that is being powered by the motor. Typical symptoms of “across the line” starting can include: • dimming of lights • wear on belts, couplings, motors • damage to end products • tripping of overload protection Types of starters:- : 25-Aug-10 6 KPCL (ACD R & E) Types of starters:- Direct online starters (DOL) Star-delta starters Auto transformer starters VFD (Variable frequency drives) Direct online starters:- : 25-Aug-10 7 KPCL (ACD R & E) Direct online starters:- Direct online starters(DOL):- : 25-Aug-10 8 KPCL (ACD R & E) Direct online starters(DOL):- In electrical engineering, a direct on line (DOL) or across the line starter starts electric motors by applying the full line voltage to the motor terminals. This is the simplest type of motor starter. A DOL motor starter also contain protection devices, and in some cases, condition monitoring. Direct online starters(DOL):- : 25-Aug-10 9 KPCL (ACD R & E) A direct on line starter can be used if the high inrush current of the motor does not cause excessive voltage drop in the supply circuit. The maximum size of a motor allowed on a direct on line starter may be limited by the supply utility for this reason. For example, a utility may require rural customers to use reduced-voltage starters for motors larger than 10 kW. Direct online starters(DOL):- Advantages of DOL starter:- : 25-Aug-10 10 KPCL (ACD R & E) Advantages of DOL starter:- Generally used for motors below 10kW as the current in the system is not so high for these motors. They are small in size & cheap as compared to other starters for such lower motor range. Autotransformer starter:- : 25-Aug-10 11 KPCL (ACD R & E) Autotransformer starter:- Autotransformer starters:- : 25-Aug-10 12 KPCL (ACD R & E) Autotransformer starters:- In this system, the first step is connected to one of the reduced voltage taps, which results in a reduced starting current and torque. After a preset time delay, the second step applies full rated voltage to the motor for normal operation. For the same value of line current drawn during the starting period, motors with auto-transformer type starters/panels deliver the maximum starting torque when compared to other type of starters. Autotransformer starters:- : 25-Aug-10 13 KPCL (ACD R & E) Autotransformer starters:- 1.The motor terminal voltage is not a function of load current and remains constant during the acceleration time. 2. Due to the turn ratio advantages, the primary line current is less than the secondary motor currents. A three-coil autotransformer is connected in a wye configuration and connected to the motor in such a way as to supply reduced voltage to the motor when the line voltage is applied to the Autotransformer. taps are provided different values of reduced voltage (NEMA standards are 80%, 65% and 50% of the full line voltage). Advantages:- : 25-Aug-10 14 KPCL (ACD R & E) The major advantage of this method of starting motors are:- 1. For hard to start loads, adjustable starting torque, can be used with any standard motors and less strain on the motor. 2. Auto-transformer starting uses an motor starting auto-transformer to reduce the line voltage during the starting period. the auto-transformer voltage taps provide a choice of three different starting currents and torques, depending on the load requirements. Advantages:- Disadvantages:- : 25-Aug-10 15 KPCL (ACD R & E) Disadvantages:- These starters are very bulky. These starters suffer the problem of oen transition. Costly. Need a lot of maintenance. Star-Delta Starters:- : 25-Aug-10 16 KPCL (ACD R & E) Star-Delta Starters:- Star-delta starters : 25-Aug-10 17 KPCL (ACD R & E) Star-delta starters What is Star Delta starting? Star Delta starting is when the motor is connected (normally externally from the motor) in STAR during the starting sequence. When the motor has accelerated to close to the normal running speed, the motor is connected in DELTA. Star-Delta starters:- : 25-Aug-10 18 KPCL (ACD R & E) Star-Delta starters:- Star connection:- Vline=((sqrt)3) x Vphase ILine=I phase Delta connection:- Vline=Vphase ILine=((sqrt)3) x Iphase What are the advantages of using Star Delta starting? : 25-Aug-10 19 KPCL (ACD R & E) What are the advantages of using Star Delta starting? The most significant advantage is the reduction in starting current. The starting current will to a large extent determine the size of the cables used, the size of the circuit breakers, the size of the fuses, as well as the transformers. What are the disadvantages of using Star Delta starting? : 25-Aug-10 20 KPCL (ACD R & E) What are the disadvantages of using Star Delta starting? The disadvantage of using star delta starting is the reduction in starting torque. hence proper matching of load torque & motor torque has to be done. Variable Frequency drives (VFD):- : 25-Aug-10 21 KPCL (ACD R & E) Variable Frequency drives (VFD):- Variable Frequency Drive:- : 25-Aug-10 22 KPCL (ACD R & E) Variable Frequency Drive:- The principle of operation of VFD is Ns=(120*F)/P Where:- F=Frequency of system in Hz. P=Number of poles of motor. Working Principle:- : 25-Aug-10 23 KPCL (ACD R & E) Working Principle:- Variable Frequency Drive:- : 25-Aug-10 24 KPCL (ACD R & E) Solid State Reduced Voltage Soft Starters use power semiconductors called silicon controlled rectifiers (SCR) that block voltage in both directions in the “off” stage and conduct voltage in one direction only in the “on” state. By using a Current Controlled (Closed Loop) Soft Starter, we can achieve a much smoother starting action because current is directly proportional to torque, thus eliminating many problems associated with using almost any other method of starting AC motors. Variable Frequency Drive:- Comparison between Soft Starter & VFD:- : 25-Aug-10 25 KPCL (ACD R & E) Comparison between Soft Starter & VFD:- Soft Starters are usually current or voltage controlled, not frequency. Soft Starters do not have the ability to control speed. Soft Starters will have significant price benefits. Soft Starters will have a smaller footprint on larger HP application. Advantages of Micro Processor in systems:- : 25-Aug-10 26 KPCL (ACD R & E) Advantages of Micro Processor in systems:- Micro Processor Based Control Multiple Communication Networks Smaller Foot-Print Makes Micro OPAL Ideal for Retrofit or MCC Applications Cost Competitive Simple User Setup Options Included on Basic Unit All Parts Needed for Panel Building Included Custom Programmability Comparison : 25-Aug-10 27 KPCL (ACD R & E) Comparison Conclusion:- : 25-Aug-10 28 KPCL (ACD R & E) Conclusion:- Depending on the motor kW rating & application the drive selections should be done. Before selecting any motor for application the motor torque curve & load torque requirement should be matched. Slide 29: 25-Aug-10 29 KPCL (ACD R & E) Thank You You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.