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Chapter: Electrical machines - Electromechanical Energy Conversion and Concepts in Rotating Machines

Torque in Ac And Dc Machines

As fulfilled previous considerations, only the fundamental waves of the effects caused by the air gap field are taken into account. Rotating mmf, caused in stator windings, is revolving.

Torque In Ac And Dc Machines

 

As fulfilled previous considerations, only the fundamental waves of the effects caused by the air gap field are taken into account. Rotating mmf, caused in stator windings, is revolving. An according rotating mmf is evoked in the rotor windings. Initially no assumptions are made for the number of pole pairs, angular frequency and phase angle of rotating magneto-motive forces of stator- and rotor. With appliance of Ampere’s law, the resulting air gap field calculates from superimposing of both rotating magneto-motive forces of stator and rotor



Effective torque exerted on the shaft derives from transmitted air-gap power divided by synchronous speed. Neglecting stator copper losses, the absorbed active power is equal to the air-gap power.

 


The torque equation (8.28) solely applies for stationary operation with IF = const and n = n1.If the load increases slowly, torque and angular displacement increases also, until break down torque is reached at V, and the machine falls out of step – means standstill in motor operation and running away in generator mode. High pulsating torques and current peaksoccur as a consequence of this. In this case machines need to be disconnected from the mainsimmediately. Overload capability, the ratio of breakdown torque and nominal torque, only depends on no load-short-circuit-ratio KC and power factor.





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