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Introduction When a motor is switched on, there is a high inrush current from the mains which may, especially if the power line section is inadequate, cause a drop in voltage likely to affect receptor operation. This drop may be severe enough to be noticeable in lighting equipment. To overcome this, some sector rules prohibit the use of motors with direct on-line starting systems beyond a given power. See pages K34 and K39 of the Distribution BT 1999/2000 catalogue and the tables of voltage drop
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  Introduction When a motor is switched on, there is a high inrush current from the mains which may,especially if the power line section is inadequate, cause a drop in voltage likely to affectreceptor operation. This drop may be severe enough to be noticeable in lighting equipment. Toovercome this, some sector rules prohibit the use of motors with direct on-line starting systemsbeyond a given power. See pages K34 and K39 of the Distribution BT 1999/2000 catalogue andthe tables of voltage drops permitted by standard NF C 15-100.There are several starting systems which differ according to the motor and load specifications.The choice is governed by electrical, mechanical and economic factors.The kind of load driven isalso important in the choice of starting system. Main starting modes Direct on-line startingThis is the simplest mode, where the stator is directly connected to the mains supply (C Fig.1).The motor starts with its own characteristics.When it is switched on, the motor behaves like a transformer with its secondary, formed by thevery low resistance rotor cage, in short circuit.There is a high induced current in the rotor which results in a current peakin the mains supply:Current on starting = 5 to 8 rated Current.The average starting torque is:T on starting = 0.5 to 1.5 rated T.In spite of its advantages (simple equipment, high starting torque, faststart, low cost), direct on-line starting is only suitable when:- the power of the motor is low compared to that of the mains, whichlimits interference from inrush current,  - the machine to drive does not need to speed up gradually or has adamping device to limit the shock of starting,- the starting torque can be high without affecting machine operation orthe load that is driven.This starting system (C Fig.2) can only be used with a motor where bothends of its three stator windings are fitted to a terminal board.Furthermore, the winding must be done so that the delta connection matchesthe mains voltage: e.g. a 380V 3-phase supply will need a motor with 380Vdelta and 660V star coiling.The principle is to start the motor by connecting the star windings at mains voltage, which divides the motor’s rated star voltage by Ö3 (in the example  above, the mains voltage at 380V = 660V / Ö3).The starting current peak (SC) is divided by 3:- SC = 1.5 to 2.6 RC (RC rated Current).A 380V / 660V motor star-connected at its rated voltage of 660V absorbsa current Ö3 times less than a delta connection at 380V. With the starconnection at 380V, the current is divided by Ö3 again, so by a total of 3.As the starting torque (ST) is proportional to the square of the supplyvoltage, it is also divided by 3:ST = 0.2 to 0.5 RT (RT Rated Torque)  The motor speed stabilises when the motor and resistive torques balanceout, usually at 75-85% of the rated speed. The windings are then deltaconnectedand the motor recovers its own characteristics. The change fromstar connection to delta connection is controlled by a timer. The deltacontactor closes 30 to 50 milliseconds after the star contactor opens, whichprevents short-circuiting between phases as the two contactors cannotclose simultaneously.The current through the windings is broken when the star contactor opensand is restored when the delta contactor closes. There is a brief but strongtransient current peak during the shift to delta, due to the counterelectromotiveforce of the motor.Star-delta starting is suitable for machines with a low resistive torque or whichstart with no load (e.g. wood-cutting machines). Variants may be required tolimit the transient phenomena above a certain power level. One of these isa 1-2 second delay in the shift from star to delta.Such a delay weakens the counter-electromotive force and hence the transientcurrent peak.This can only be used if the machine has enough inertia to prevent too muchspeed reduction during the time delay.Another system is 3-step starting: star-delta + resistance-delta.  There is still a break, but the resistor in series with the delta-connectedwindings for about three seconds lowers the transient current. This stopsthe current from breaking and so prevents the occurrence of transientphenomena.Use of these variants implies additional equipment, which may result in asignificant rise in the cost of the installation.Autotransformer startingThe motor is powered at reduced voltage via an autotransformer which isbypassed when the starting process is completed (C Fig.5).The starting process is in three steps:- in the first place, the autotransformer is star-connected, then the motoris connected to the mains via part of the autotransformer windings.The process is run at a reduced voltage which depends on thetransformation ratio. The autotransformer is usually tapped to selectthis ratio to find the most suitable voltage reduction value,- the star connection is opened before going onto full voltage. The fractionof coil connected to the mains then acts as an inductance in series withthe motor. This operation takes place when the speed balances out atthe end of the first step,- full voltage connection is made after the second step which usually only
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