AC synchronous generator (alternator) - construction and working

The term 'AC generator' is often used interchangeably with 'alternator' (AC synchronous generator). However, it more broadly includes all devices that produce alternating current — such as induction generators, which operate without external excitation and are commonly used in wind turbines and regenerative braking systems. Read more about :
'Induction Generators' and 'Difference between AC generator and Alternator'.

Basically, an AC generator is an electrical machine that converts mechanical energy into electrical energy in the form of Alternating Current (AC). Basic working principle of an AC synchronous generator is based on Faraday's law of electromagnetic induction.

How does an AC synchronous generator or alternator work?

The above figure illustrates how an alternator or AC synchronous generator works. According to Faraday's law of electromagnetic induction, whenever a conductor moves in a magnetic field, an EMF gets induced across the conductor. If a closed path is provided to that conductor, the induced emf causes current to flow in the circuit.

Now, in the above figure, see how the conductor coil A-B-C-D is placed in a magnetic field. Direction of the magnetic flux will be from N pole to S pole. The coil is connected to slip rings, and the load is connected through brushes that are resting on the slip rings.

Now, consider the case 1 from the above figure. The coil is rotating clockwise, in this case, the direction of induced current can be given by Fleming's right-hand rule, and it will be along A-B-C-D.

As the coil is rotating clockwise, the position of the coil will be changed after half of the rotational period, as shown in the second case of the above figure. In this case, the direction of the induced current, according to Fleming's right-hand rule, will be along D-C-B-A. It shows that the direction of the current changes after every half of the rotational time period, that means we get an alternating current.

Construction of AC synchronous generator (alternator)

Salient pole type alternator

The main parts of an alternator, obviously, consists of a stator and a rotor. But, unlike other machines, in most of the alternators, field exciters are rotating and the armature coil is stationary.

Stator: Unlike in construction of a DC machine, the stator of an alternator is not meant to serve a path for magnetic flux. Instead, the stator is used for holding armature winding. The stator core is made up of lamination of steel alloys or magnetic iron, to minimize the eddy current losses

Why is the armature winding stationary in alternators?

• At high voltages, it is easier to insulate the stationary armature winding, which may be as high as 11 kV or even more in some cases.
• The generated high voltage output can be directly taken out from the stationary armature. Whereas for a rotary armature, there will be large brush contact drop at higher voltages, also the sparking at the brush surface will be a problem to look after.
• If the field exciter winding is placed in the rotor, low voltage DC can be transferred safely to the exciter winding via slip-rings.
• The armature winding can be braced well, to prevent deformation caused by high centrifugal force if it was in the rotor.

Rotor: There are two types of rotor used in an AC synchronous generator/alternator: (i) Salient and (ii) Cylindrical type

1. Salient pole type: Salient pole type rotor is used in low and medium speed alternators. The construction of an AC synchronous generator of a salient pole type rotor is shown in the figure above. This type of rotor consists of a large number of projected poles (called salient poles), bolted on a magnetic wheel. These poles are also laminated to minimize the eddy current losses. Alternators featuring this type of rotor are large in diameters and short in axial length.
2. Cylindrical type: Cylindrical type rotors are used in high-speed alternators, especially in turbo-alternators. This type of rotor consists of a smooth and solid steel cylinder having slots along its outer periphery. Field windings are placed in these slots.

Salient pole rotors are typically used in hydropower stations where generators run at lower RPM. In contrast, cylindrical rotors are used in thermal and nuclear power stations (turbo-alternators) where high-speed operation is required for efficiency.

A DC supply is given to the rotor winding through the 'slip-rings and brushes' arrangement.

Connecting an alternator in the grid is called as synchronization of alternator, read more about it at the link.

Also Read: Synchronous generator vs. Induction generator

Key Takeaways

• Alternators (AC synchronous generators) convert mechanical energy into alternating current using electromagnetic induction.
• They work based on Faraday’s Law and Fleming’s Right-Hand Rule.
• The armature winding is placed on the stator (stationary part) to handle high voltage safely and efficiently.
• Rotors can be of salient pole type (for low-speed) or cylindrical type (for high-speed) depending on the application.
• Alternating current is produced because the induced EMF changes direction with the coil’s rotation.
• Before connecting to the grid, alternators must undergo synchronization to match frequency, voltage, and phase.

Frequently Asked Questions (FAQs)

What is an alternator in electrical engineering?

An alternator, or AC synchronous generator, is an electrical machine that converts mechanical energy into alternating current using electromagnetic induction.

How does an alternator work?

It works by rotating a coil within a magnetic field. This movement induces an EMF, and as the coil rotates, the current direction reverses every half turn, producing AC.

What is the difference between salient pole and cylindrical rotor?

Salient pole rotors have projecting poles and are used in low-speed generators, such as hydroelectric plants. Cylindrical rotors are smooth and used in high-speed generators like those in thermal or nuclear power plants.

Why is the armature winding stationary in an alternator?

Keeping the armature stationary allows easier insulation, better mechanical stability, and safer extraction of high-voltage output.

What is synchronization of an alternator?

Synchronization is the process of matching an alternator's output with the grid’s voltage, frequency, and phase before connecting it to ensure safe and efficient operation.