There are basically three different types of flux in a rotating electrical machine: Stator flux, Rotor flux and Resultant air gap flux. Resultant air gap flux is the net flux in the air gap of machine due to combined action of stator as well as rotor flux. In Synchronous machine, we are well aware of the term Armature Reaction which affects the field (field winding is wound on the rotor for synchronous machine) generated flux.
Load angle is also defined as the angle between the no load excitation voltage Ef and terminal voltage Vt. Note that this definition is having no difference with the previous definition. How? This is because; under no load condition the emf will be induced in the armature or stator winding just because of the field flux or rotor flux Øf. This emf will be lagging by 90 degree with the field flux.
Always remember that the generated emf always lags by 90 degree the flux that generates it. Here emf is induced by the field flux; hence it lags the field flux by 90 degree.
Terminal voltage is the voltage across the armature terminal which is induced due to the resultant air gap flux. Thus terminal voltage Vt will lag behind the resultant air gap flux Ør by 90 degree. Thus the angle between Ef and Vt will be equal to the angle between field flux and resultant air gap flux. A simple phasor diagram showing the relation between the Ef, Vt, Øf and Ør.
Power angle can also be defined in terms of armature or stator mmf and resultant air gap mmf. In a synchronous generator, the stator mmf lags behind the resultant air gap mmf. This angle of lag is called load or power angle.
Significance of Load Angle
In synchronous machine, load angle is a very important parameter. Let us first understand the physical significance of this parameter before going into some sort of mathematical formula or expression. As discussed earlier in this post, load angle is the angle between the field flux and the resultant air gap flux. This means, as the mechanical power input to the synchronous generator is increased through prime mover, the field poles will be dragged ahead of the stator pole or flux. This in turn will increase the load angle. An increased load angle will cause electromagnetic torque to increase. Since the electromagnetic torque opposes the prime mover torque in synchronous machine, as soon as this torque balances the prime mover torque, the mechanical input will be converted to electrical output. Thus increasing the load angle increases the power output.
The power output of a synchronous generator is given as
P = (EfVtSinδ) / Xs
From the above expression, it is clear that increase in angle δ increases the power output provided field excitation and generator terminal voltage is kept constant. This is the reason, load angle is also known as power angle. Now you might think, can we keep on increasing the load angle to increase the generator output?
No, we cannot. Why? This is because if we increase the angle δ beyond 90 degree, the generator output will fall below its maximum output of (EfVt)/ Xs. Since the electrical output of generator has decreased while the mechanical input is still more, the generator will lose synchronism. Thus the steady state stability is affected by the load angle.