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Machine Question No.12 – Starting of synchronising motor

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  • #224

      Whether can we start a synchronizing motor similar an induction motor?
      What points shall be considered?

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    • Author
    • #1452

        Yes, of course suitable facility shall be considered by motor manufacturer. During starting, the motor field winding must be disconnected from the exciter and loaded with a resistor, to limit the high induced voltage, to prevent damage to the rectifier elements of the brushless type, and to prevent the circulation of ac current through a slip-ring-type dc exciter. The switching is done with a contactor for the slip-ring type, and with thyristors on the rotating rectifier assembly for the brushless type. Except for the disconnection for starting, the synchronous-motor excitation system is practically the same as for an ac generator of the same rating. Brushless-type exciters are now used on all new high-speed synchronous motors (two to eight poles) that formerly were built with direct-drive dc exciters and slip rings. The brushless-type exciters require minimum maintenance and can be used in explosive-atmospheres. The semi controlled bridge with three diodes and three thyristors rectifies the output of the ac exciter generator and supplies the motor field winding. The thyristors act as a switch to open the rectifier during starting and to close it during running, whereas the ac exciter generator is excited with its own field current. The resistor is permanently connected across the motor field winding during starting and running. It improves the torque characteristics during starting, and protects the bridge elements against transient over voltages during running. The capacitor protects the diodes and thyristors against commutation Over voltages.

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