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# Machine Riddle No.31 – Air gap flux in short circuited machine

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• #644
Abhishek Kumar

Why airgap flux can't change instantaneously in synchronous machine when short circuited?
Why airgap flux can't change instantaneously in synchronous machine when short circuited?

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• #2307
Glavizh

At first , control system should detect the short circuit situation . At short circuit situation , system doesn’t require excitation current same as what it needed in the past . As soon as , short circuit is detected by control system , the excitation current and consequently , air gap flux will change .

#2308
Hamid

In the case of the unloaded generator, the air gap flux is equal to the rotor flux. On a loaded generator, the air gap flux is equal to the vector sum of the rotor and armature reaction flux. When the generator is supplying lagging current, the armature reaction flux detracts from the rotor flux, reducing the voltage induced in the stator windings and hence the terminal voltage. Leading current produces an armature reaction flux that is additive to the rotor flux, boosting the induced stator voltage and the generator terminal voltage. When the generator is under load, the stator current will lead or lag Eq by angle B, depending on whether the load is capacitive or inductive, as shown in Figure below. Note that angle B is not the power factor angle. The power factor angle is measured between the current and the generator terminal voltage; under load, Eq is an internal generator voltage. Load current produces a second flux within the generator. This flux, sometimes referred to as the armature reaction flux, is formed in the air gap by the vector summation of the alternating flux in each stator phase winding. The intuitive conclusion would be that the summation of the phase flux would be zero, because the balanced load currents that produce the individual phase flux sum to zero. This conclusion would be correct if the phase windings were physically aligned with each other. The individual phase flux would then be displaced by 120

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