Working Principle of Alternator

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Key learnings:
  • Alternator Definition: An alternator is a machine that converts mechanical energy into alternating electrical energy using electromagnetic induction.
  • Working Principle: The alternator working principle is based on Faraday’s law where motion between a conductor and a magnetic field induces an electrical current.
  • Induction Process: Maximum current induction in an alternator occurs when the conductor’s motion is perpendicular to the magnetic flux lines.
  • Current Alternation: In an alternator, the electrical current reverses direction with each half-turn of the rotor, simulating a complete sine wave during each rotation.
  • Practical Configuration: Modern alternators typically feature a stationary armature and a rotating magnetic field, enhancing efficiency in generating three-phase AC for widespread electrical distribution.

The working principle of an alternator is straightforward, mirroring the basic principle of DC generator. It relies on Faraday’s law of electromagnetic induction, which states that curren is induced in a conductor moving relative to a magnetic field.Alternator Working Principle
For understanding working of alternator let us think about a single rectangular turn placed in between two opposite magnetic poles as shown above.

Say this single turn loop ABCD can rotate against axis a-b. Suppose this loop starts rotating clockwise. After 90o rotation the side AB or conductor AB of the loop comes in front of S-pole and conductor CD comes in front of N-pole. At this position the tangential motion of the conductor AB is just perpendicular to the magnetic flux lines from N to S pole. Hence, the rate of flux cutting by the conductor AB is maximum here and for that flux cutting there will be an induced current in 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 will be from A to B. At the same time conductor CD comes under N pole and here also if we apply Fleming right-hand rule we will get the direction of induced current and it will be from C to D.

After another 90-degree clockwise rotation, the loop ABCD reaches a vertical position. Here, the motion of conductor AB and CD aligns parallel to the magnetic flux lines, resulting in no flux cutting and, consequently, no current generation.

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 in the turn is reduced to zero from its maximum value.

After another clockwise rotation of 90o the turn again comes to horizontal position, and here conductor AB comes under N-pole and CD comes under S-pole, and here if we again apply Fleming right-hand rule, we will see that induced current in conductor AB, is from point B to A and induced current in the conductor CD is from D to C.

As the loop moves from vertical to horizontal, the current in the conductors increases from zero to its maximum. The current flows in a closed loop from B to A, A to D, D to C, and C to B, reversing the earlier direction from A to B, B to C, C to D, and D to A.

As the loop approaches a vertical position again, the current drops to zero. With continued rotation, the current changes direction. Each full turn results in the current peaking, dropping to zero, peaking in the opposite direction, and returning to zero, completing a sine wave cycle every 360-degree rotation. This process illustrates how alternating current is generated by rotating a conductor within a magnetic field.

Now we place one stationary brush on each slip ring. If we connect two terminals of an external load with these two brushes, we will get an alternating current in the load. This is our elementary model of an alternator.

single loop ac generator
Having understood the very basic principle of an alternator, let us now have an insight into its basic operational principle of a practical alternator. During the discussion of the basic working principle of an alternator, we have considered that the magnetic field is stationary and conductors (armature) is rotating. But generally in practical construction of alternator, armature conductors are stationary and field magnets rotate between them. The rotor of an alternator or a synchronous generator is mechanically coupled to the shaft or the turbine blades, which is made to rotate at synchronous speed Ns under some mechanical force results in magnetic flux cutting of the stationary armature conductors housed on the stator.

As a direct 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 first half cycle and then in the other direction for the second half cycle for each winding with a definite time lag of 120o due to the space displaced arrangement of 120o between them as shown in the figure below. This particular phenomenon results in three-phase power flow out of the alternator which is then transmitted to the distribution stations for domestic and industrial uses.
three-phase generated voltage

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