L-33(NKD)(ET) ((EE)NPTEL) - Module 8 Three-phase Induction...

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Module 8 Three-phase Induction Motor Version 2 EE IIT, Kharagpur
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Lesson 33 Different Types of Starters for Induction Motor (IM) Version 2 EE IIT, Kharagpur
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Instructional Objectives Need of using starters for Induction motor Two (Star-Delta and Auto-transformer) types of starters used for Squirrel cage Induction motor Starter using additional resistance in rotor circuit, for Wound rotor (Slip-ring) Induction motor Introduction In the previous, i.e. fourth, lesson of this module, the expression of gross torque developed, as a function of slip (speed), in IM has been derived first. The sketches of the different torque-slip (speed) characteristics, with the variations in input (stator) voltage and rotor resistance, are presented, along with the explanation of their features. Lastly, the expression of maximum torque developed and also the slip, where it occurs, have been derived. In this lesson, starting with the need for using starters in IM to reduce the starting current, first two (Star-Delta and Auto-transformer) types of starters used for Squirrel cage IM and then, the starter using additional resistance in rotor circuit, for Wound rotor (Slip-ring) IM, are presented along with the starting current drawn from the input (supply) voltage, and also the starting torque developed using the above starters. Keywords: Direct-on-Line (DOL) starter, Star-delta starter, auto-transformer starter, rotor resistance starter, starting current, starting torque, starters for squirrel cage and wound rotor induction motor, need for starters. Direct-on-Line (DOL) Starters Induction motors can be started Direct-on-Line (DOL), which means that the rated voltage is supplied to the stator, with the rotor terminals short-circuited in a wound rotor (slip-ring) motor. For the cage rotor, the rotor bars are short circuited via two end rings. Neglecting stator impedance, the starting current in the stator windings is (see lesson 32) is 2 2 2 2 1 ) ( ) ( ) ( + = x r E I r st where, Starting current in the motor (stator) = = = a I I I st st st / ) ( ) ( ) ( 2 2 1 Effective turns ratio between stator and rotor windings = = r T a / T s Input voltage per phase to the motor (stator) = = = r r s E a E E Induced emf per phase in the rotor winding = r E Rotor resistance in terms of stator winding = = 2 2 2 r a r Rotor reactance at standstill in terms of stator winding = = 2 2 2 x a x Version 2 EE IIT, Kharagpur
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The input voltage per phase to the stator is equal to the induced emf per phase in the stator winding, as the stator impedance is neglected (also shown in the last lesson (#32)). In the formula for starting current, no load current is neglected. It may be noted that the starting current is quite high, about 4-6 times the current at full load, may be higher, depending on the rating of IM, as compared to no load current. The starting torque is (
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L-33(NKD)(ET) ((EE)NPTEL) - Module 8 Three-phase Induction...

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