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 *I*F = const and *n* = *n*1.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 *K*C and power
factor.

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