Alternator | Working Principle

What is Alternator?

The working rule of an alternator is extremely simple. It is a bit like the essential principle of DC generator. It also depends upon Faraday’s law of electromagnetic induction which says the present is induced within the conductor inside a magnetic flux when there’s a relative motion between that conductor and the magnetic field.


This single turn loop can rotate opposite the ABCD axis a-b. Suppose this loop starts rotating clockwise. After 90o rotation the side AB or conductor AB of the loop comes ahead of S-pole and conductor CD comes ahead of N-pole. At this position the tangential motion of the conductor AB is simply perpendicular to the magnetic flux lines from N to S pole.

For understanding working of alternator allow us to believe one rectangular turn placed in between two opposite magnetic poles as shown above.

Hence, the speed of flux cutting by the conductor AB is maximum here and for that flux cutting there’ll be an induced current within the conductor AB and the direction of the induced current can be determined by Fleming’s right-hand rule. As per this rule the direction of this current are going to be from A to B. At an equivalent time conductor CD comes under N pole and here also if we apply Fleming right-hand rule.

Main Parts of Alternator

we’ll get the direction of induced current and it will be from C to D.
Now after dextrorotation of another 90o the turn ABCD comes at the vertical position as shown below. In this case, the line speed of conductors AB and CD is parallel to the magnetic flux lines, so there will be no current in the flow, so there is no current in the conductor.
While the turn ABCD comes from a horizontal position to a vertical position, the angle between flux lines and direction of motion of conductor, reduces from 90o to 0o and consequently the induced current within the turn is reduced to zero from its maximum value.

Armature and Rotor Diagram

After a dextrotation of 90 digree, the bend again returns to the horizontal position. And it falls under the conductor AB N-pole and CD S-pole, and here if we apply the Fleming rule again. we’ll see that induced current in conductor AB, is from point B to A and induced current within the conductor CD is from D to C.

As at this position the turn comes at a horizontal position from its vertical position, the current in the conductors comes to its maximum value from zero. That means current is circulating within the close turn from point B to A, from A to D, from D to C and from C to B, provided the loop is closed although it’s not shown here. That means the present is in reverse of that of the previous horizontal position when the present was circulating as A → B → C → D → A.

While the turn further proceeds to its vertical position the present is again reduced to zero. So if the turn continues to rotate the present successively continually alternate its direction. During every full revolution of the turn, the present successively gradually reaches to its maximum value then reduces to zero then again it involves its maximum value but in other way and again it involves zero. In this way, the present completes one full wave cycle during each 360o revolution of the turn. So, we have seen how alternating current is produced in a turn is rotated inside a magnetic field. From this, we’ll now come to the particular working rule of an alternator.

Now we place one stationary coat each connection . If we connect two terminals of an external load with these two brushes, we’ll get an AC within the load. This is our elementary model of an alternator.

Cross section view of Alternator

Alternator Working Principle

Having understood the very fundamentals of an alternator, allow us to now have an insight into its basic operational principle of a practical alternator. During the discussion of the essential working rule of an alternator, we’ve considered that the magnetic flux is stationary and conductors (armature) is rotating. Whereas in the manufacture of alternators, armature conductors are stable. And field magnets rotate in the middle of it. The rotor of an alternator or a synchronous generator is mechanically connected to a shaft or turbine blade. which is formed to rotate at synchronous speed Ns under some mechanical force leads to magnetic flux cutting of the stationary armature conductors housed on the stator.

As an immediate consequence of this flux cutting an induced emf and current starts to flow through the armature conductors which first flow in one direction for the primary half cycle then within the other direction for the last half cycle for every winding with a particular delay of 120o thanks to the space displaced arrangement of 120o between them as shown in the figure below. This particular phenomenon leads to three-phase power effuse of the alternator which is then transmitted to the distribution stations for domestic and industrial uses.