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STARTING-METHODOLOGIES.pdf - UNIT 4: Electrical Motor...

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UNIT 4: Electrical Motor Starting Methodologies4.1 starting methods for D.C. motors and its characteristics:At the instant of staring as the machine isnotrotating, there is no back e.m.f. therefore, the need forstarter to is inevitable. The current is thus limited by armature resistance alone, unless anarrangement is made to add further/extra resistancein armature circuit. Thus for all ‘fractional outputpower’ motors, appreciable resistance, a resistor is inserted into the armature circuit and thenremoved in steps, as the motor accelerate up to its correct running speed. The arrangement isincorporated in a unit, called a ‘motor starter’ or more simply a ‘starter’4.1.1 Starting rheostat or Voltage ControlThis is achieved in various ways for different kinds of D.C. motor but the key requirement is to reducethe voltage applied to the machine armature. Thus a large variable rheostat is connected in series withthe armature or the latter supplied from a variable voltage supply. The method is used to lower andcontrol is in adownwarddirection only.4.1.2 Automatic starter or Field Control.This is the most common control type. When a motor is loaded, its speed varies with the load. It maybe desired to adjust the speed for any load condition, i.e. keep it constant over the working range orto raise it above normal running speed. Field control is used because its addition to the field circuit iseasily achieved, control is smooth and little energy wasted as heat.It must be remember that this type of control gives speed variation in anupwarddirection only. It isused for raising speed above normal and as flux is weakened, for the same driving torque, armaturecurrent rises.4.2 Starting methods for A.C. motors and its characteristics:Most induction motors are Direct-on-Line (DOL) switch-started because such starters are inexpensiveand simple to operate and maintain. The high starting current surge will not cause serious heatingdamage to the motor unless the motor is repeatedly started and stopped in a short time period.When very large motors are started DOL they cause a significant disturbance of voltage (voltage dip)on the supply lines due to the large starting current surge.This voltage disturbance may result in the malfunction of other electrical equipment connected to thesupply e.g. lighting dip and flickering effects.To limit the starting current some large induction motors are started at reduced voltage and then havethe full supply voltage reconnected when they have accelerated close to their rated speeds.Reduced voltage starting is used for large motors driving loads like cargo pumps and bow thrusters.Two methods of reduced voltage starting by switching are calledstar-delta startingandautotransformer startingbut an electronic"soft" startingoption is also used.

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Term
Winter
Professor
;jm/;ljm. ;/l
Tags
Electric motor, DOL, a c motors

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