Page 104 - Electrician - TT (Volume 2)
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ELECTRICIAN - CITS



           connected to the same line. The initial current is controlled by applying a reduced voltage to the stator winding
           during the starting winding during starting period and then the full normal voltage is applied when the motor has
           run up to speed. For small capacity motors up to 3 hp full normal voltage can be applied at the start.
           Compare the characteristics of a squirrel cage induction motor

            S.No    Characteristics   Squirrel cage induction motor          Slip ring induction motor
            1      Speed             Nearly constant but slightly falls   More speed falls with load than  Squirrel
                                     with load                       cage induction motor
            2      Torque            Starting torque is not so good  Starting torque higher than Squirrel cage
                                     Running torque is good          induction motor and is three times full load
                                                                     torque. Running torque is good
            3      Current           Starting current is high, about five  Starting current is low, about two times of full
                                     times that full load current    load current
            4      Speed control     Speed can be varied by pole     It can be slightly changing the rotor
                                     changing method                 resistance
            5      Power factor      Low power factor 0.7-.08        High power factor 0.8-.09

            6      Cost              Low                             High
            7      Uses              Factories for lathe , drill,    Used for high starting torque is wanted e.g. :
                                     blower,etc.                     lift, crane,etc.
            8      Rotor construction  Bars are used in rotor. Squirrel   Winding wire is used. Wound rotor requires
                                     cage rotor is very simple, rugged   attention.  Slip ring and brush fear need
                                     and long lasting. No slipring.  frequent maintenance.
            9      Starting          Cam ne started by DOL ,star     Rotor resistance starter is required.
                                     delta, auto transformer starters.
            10     Starting torque   Low                             Very high
            11     Starting current  High                            Low

            12     Speed variation   Not easy but could be varied in   Easy to vary speed but speed change
                                     larger steps by pole changing   through pole changing is not possible.
                                     or smaller incremental steps    Speed change possible by insertion of rotor
                                     through thyristors or by frequency  resistance –using thyristors –using frequency
                                     variation.                      variation –injecting emf in the rotor circuit
                                                                     -cascading

            13     Acceleration on   Just satisfactory               Very good
                   load

            14     Maintenance       Almost nil                      Requires frequent maintenance

           Types of starters
           a  Direct on line starter
           b  Star delta starter
           c  Step down transformer starter

           d  Auto transformer
           Selection of starter
           Many factors must be considered when selecting starting equipment. These factors include starting current the
           full load current voltage rating of motor, voltage drop, cycle of operation, type of load, motor protection and safety
           of operation
           Types of over load relay
           a  Magnetic overload relay

           b  Thermal (bimetallic) overload relay

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 CITS : Power - Electrician & Wireman - Lesson 70-75  CITS : Power - Electrician & Wireman - Lesson 70-75
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