A generator running at synchronous speed and generating three-phase voltage is called a three-phase synchronous generator or alternator. Similar to the dc generator an ac generator or alternator works on the same principle i.e., Faraday's Law of Electromagnetic Induction.
When a rotating armature placed in a stationary magnetic field, the armature conductors are cut by the magnetic flux. Thus, there is a change in flux linkage by the armature conductors due to its motion, hence emf is induced in the armature conductors.
Let us consider a coil A1 A2 placed on a rotor slot. The rotor is placed in the magnetic field produced by stationary poles and is free to rotate in the field. When the rotor is driven by the prime mover, there will be a change in flux linkage with the coil ( flux passing through the coil ).
The magnitude and direction of induced emf depend upon the rate of change of flux linkage and direction of rotation of the rotor ( coil ) respectively. Let us see the amount of induced emf at different positions of the coil.
Consider that the coil is at a position perpendicular to the magnetic flux lines as shown in figure (a). At this position, the maximum flux passes through the coil. The rate of change of flux linkage due to change in its position will be minimum. Therefore, emf induced in the coil conductors will be zero.
Suppose the coil moved to the position as shown in figure (b). There will be an increase in the rate of change of flux linkage. This makes the coil to induce emf and it increases accordingly with the rotation. When the coil reaches a position as shown in figure (c) i.e., parallel to the flux lines. There will be a maximum change in the rate of flux linkage and induced emf will also be maximum.
Again there will be a reduction in induced emf as the coil keeps on rotating and becomes zero at a position shown in figure (d). This completes the half rotation of the coil. Beyond this position, the coil will now face towards the south pole. Hence the induced emf in the coil is in the reverse direction compared to previous positions.
At a position shown in figure (e). There will be a maximum induced emf in the reverse direction and again becomes zero at a position shown in figure (f). From the above, we can conclude that after every half rotation the induced emf reverses in the coil. Hence an alternating current is produced. The below shows the waveform of induced emf for one revolution of the coil.
The induced emf is collected from the rotating armature through brushes and slip-rings and supplied to the load. This is all about a single-phase ac generator.
Generation of Three-Phase EMF :
The generation of electrical energy at power plants is at high voltage in the three-phase. In order to generate 3-phase voltage, 3-phase alternators are used. To generate high voltages, a large number of coils connected in series are placed in the armature slots on the rotor.
For a generation of three-phase emf RYB three separate winding (each for phase), either connected in star or in the delta are used. The three windings are placed in such a way that they are displaced at an angle of 120° from each other. The below shows the diagram of three-phase winding one coil per-phase (R1 R2, Y1 Y2, B1 B2) placed on the rotor.
From the above diagram, the maximum amount of emf induced in the coil Y1 Y2 as that of R1 R2 will be after a time-phase difference of 120° i.e., after the rotor has rotated by 120° electrical. Similarly, the maximum emf in the coil B1 B2 is at another time-phase difference of 120° after the coil Y1 Y2.
Practical Alternator or AC Generator :
Till now we have seen the alternator with rotating armature and stationary field poles. But in practice due to the number of advantages, the armature windings mounted on a stationary part called stator and a field winding on a rotating part called rotor as shown below.
When the rotor rotates with the help of a prime mover the stationary armature conductors (stator winding) are cut by the magnetic flux produced by field winding. Since the magnetic poles are alternately N and S, they induce an alternating emf in the stator winding.
Hence, an alternating voltage is produced in the stator conductors, whose frequency depends on the number of poles moving past conductors in one second and the direction of the current is one second is given by Fleming's Right-hand rule.