An AC generator is an electrical machine that convert mechanical energy into alternating electrical energy using the principle of electromagnetic induction. It is also commonly called an alternator especially when referring to synchronous generator used in power system. When there is relative motion between a magnetic field and a conductor the magnetic flux linked with the conductor changes and an electromotive force (EMF) is induced. As the relative direction of motion reverse periodically the generated EMF also get reverse, producing alternating current. The main part of an AC generator include the stator, rotor, armature winding, field system, shaft and a prime mover. Depending on the design slip ring and brushes may also be used. AC generator are used in power station, wind turbine, hydroelectric plant, diesel generator set and other electricity generation system.

In this article, we will learn about the AC generator diagram, construction, working principle, EMF equation, frequency, types, advantages, applications and differences between AC and DC generator. The information in this article helps you extensively in your SSC JE Electrical and GATE Electrical preparation journey.

What is an AC Generator?

An AC generator is a machine that convert mechanical energy into electrical energy in the form of alternating voltage and current. It works according to Faraday's law of electromagnetic induction, which states that an EMF is induced whenever the magnetic flux linked with a conductor or coil changes. The required change in magnetic flux can be produced by rotating a coil in a stationary magnetic field or by rotating a magnetic field relative to a stationary armature winding. In a simple AC generator, a coil rotate between magnetic pole and the generated voltage is collected through slip ring and brushes. In practical large alternator the armature winding is generally placed on the stator while a DC excited magnetic field rotates on the rotor.

The basic energy conversion is: Mechanical Energy → Rotational Motion → Electromagnetic Induction → Alternating Electrical Energy

AC Generator Diagram

An AC generator diagram represent the arrangement used to produce alternating EMF through electromagnetic induction. In the elementary model, a rectangular armature coil rotate between the north and south pole of a magnet. The two end of the coil are connected to separate slip ring, while stationary carbon brushe connect the rotating coil to the external circuit. As the coil rotate, its orientation relative to the magnetic field continuously changes. Therefore, the magnetic flux linked with the coil also changes. An EMF is induced in the coil, and its direction reverses after every half rotation.

A labelled AC generator diagram shown above:

• North and south magnetic poles
• Armature coil
• Axis or shaft of rotation
• Slip rings
• Carbon brushes
• External load
• Direction of rotation
• Direction of magnetic field
• AC output waveform

The diagram usually represents the coil, magnetic field and slip ring, along with the sinusoidal wave of EMF produced over time. It helps to understand how mechanical motion is converted into alternating electrical energy. 

The construction of an AC generator consist mainly of a magnetic field system and an armature arranged so that relative motion exists between them. A prime mover provide the mechanical energy needed to rotate the machine. In a simple classroom model, the armature coil rotates in a fixed magnetic field. However practical large AC generator usually use a stationary armature and rotating magnetic field. This arrangement make it easier to insulate the high voltage armature winding and take the generated output directly from stationary terminal.

Stator

The stator is the stationary part of an AC generator. In most practical alternator it carries the armature winding in which the alternating EMF is induced. The stator core is generally laminated to reduce eddy current losses. Slots are provided on its inner surface to accommodate the armature conductor.

Rotor

The rotor is the rotating part of the generator and is mounted on the shaft. In a practical synchronous AC generator, the rotor normally carries the field winding. When DC excitation is supplied to the rotor field winding, it produces magnetic poles. Rotation of this magnetic field relative to the stator conductors produces alternating EMF in the stator winding.

Armature Winding

The armature winding is the winding in which the useful EMF is induced. In large AC generators, it is usually placed on the stator. A stationary armature makes it easier to insulate high voltage windings and connect the generated output to the external electrical system.

Field Winding

The field winding produces the main magnetic field required for electromagnetic induction. In a conventional synchronous generator, it is generally placed on the rotor and supplied with DC excitation. As the rotor rotates its magnetic field sweeps past the stationary armature conductors.

Slip Rings and Brushes

Slip ring and brushes may be used to provide an electrical connection between stationary and rotating part. In the elementary rotating coil generator two continuous slip ring connect the rotating armature to the external circuit. In many practical synchronous generator, slip ring may instead carry the relatively small DC excitation current to the rotating field winding. Brushless excitation systems are also commonly used.

Shaft

The shaft supports the rotor and transfer mechanical energy from the prime mover to the rotating part of the generator.

Prime Mover

A prime mover supplies the mechanical input required to rotate the generator. Depending on the generating station the prime mover may be a steam turbine, water turbine, gas turbine, wind turbine or internal combustion engine.

Parts of an AC Generator and Their Function

Each component of an AC generator performs a specific function in converting mechanical energy into electrical energy.

The exact construction varies with generator size and application, but the basic requirement remains the same: relative motion between a magnetic field and electrical conductor must produce a changing flux linkage.

Working Principle of AC Generator

An AC generator work on the principle of Faraday's law of electromagnetic induction. When the magnetic flux linked with a conductor or coil changes, an EMF is induced in it. If the electrical circuit is closed, the induced EMF drives a current through the circuit.

According to Faraday's law:

\(e=-N\frac{d\Phi}{dt}\)

Where:

• e = induced EMF
• N = number of turns
• Φ= magnetic flux through one turn

The negative sign represents Lenz's law which indicates that the induced EMF acts in a direction that opposes the change responsible for producing it In the elementary AC generator a coil rotate inside a uniform magnetic field. During rotation the angle between the magnetic field and the normal to the coil continuously changes. This changes the magnetic flux linked with the coil and produces an alternating EMF. The direction of the induced current can be determined using Fleming's right hand rule.

How does an AC Generator produce Alternating Current?

An AC generator produce alternating current because the direction of the induced EMF reverses after every half rotation of the coil or magnetic field. During one half cycle the conductor move through the magnetic field in one relative direction. During the next half cycle their relative motion reverses, causing the polarity of the induced EMF to reverse.

Mechanical Rotation → Magnetic Flux Linkage Changes → EMF is Induced → Direction Reverses After Half Rotation → Alternating Voltage is Produced → AC Flow When the Circuit is Closed. This periodic reversal gives the generated voltage its alternating nature. For uniform rotation in an ideal sinusoidally distributed magnetic field, the generated EMF has a sinusoidal waveform.

AC Generator Derivation

Consider a coil having N turns and area A, rotating with angular velocity ω\omega in a uniform magnetic field of flux density B. If θ is the angle between the magnetic field and the normal to the plane of the coil, the magnetic flux linkage is:

\(N\Phi=NBA\cos\theta\)

For uniform rotation: θ=ωt

Therefore: \(N\Phi=NBA\cos\omega t\)

According to Faraday's law: \(e=-\frac{d(N\Phi)}{dt}\)

Hence: \(e=NBA\omega\sin\omega t\)

If: \(E_m=NBA\omega\)

Then: \(\boxed{e=E_m\sin\omega t}\)

This equation shows that the instantaneous EMF generated by an ideal rotating coil varies sinusoidally with time.

When is EMF Maximum in an AC Generator?

The induced EMF is maximum when: sin⁡ωt=1

Therefore: e=Em

At this instant the plane of the coil is parallel to the magnetic field, so the flux through the coil is zero but its rate of change is maximum.

When is EMF Zero in an AC Generator?

The induced EMF becomes zero when: sin⁡ωt=0. At this position, the plane of the coil is perpendicular to the magnetic field. Magnetic flux is maximum or minimum but its instantaneous rate of change is zero. Maximum magnetic flux does not mean maximum induced EMF. EMF depend on the rate of change of magnetic flux.

EMF Equation of AC Generator

For the elementary rotating coil AC generator, the instantaneous generated EMF is:

\(e=E_m\sin\omega t\)

Where: \(E_m=NBA\omega\)

Thus:\( \boxed{e=NBA\omega\sin\omega t}\)

Where:

• N = number of turns
• B = magnetic flux density in tesla
• A = area of each turn in m2m^2
• ω = angular velocity in rad/s
• t = time in seconds
• Em= maximum induced EMF

For a sinusoidal waveform, the RMS value is: \(E_{rms}=\frac{E_m}{\sqrt{2}}\)

The equation show that generated EMF can be increased by increasing the number of turn, magnetic field strength, coil area or rotational speed subject to the practical design limits of the machine.

EMF Equation of a Practical Alternator

For a practical sinusoidal alternator, the RMS generated EMF per phase is commonly written as:

\(\boxed{E_{ph}=4.44K_pK_d f\Phi T}\)

or:\(E_{ph}=4.44K_wf\Phi T\)

Where: \(K_w=K_pK_d\)

Here:

• Eph = generated RMS EMF per phase
• Kp = pitch factor
• Kd = distribution factor
• Kw= winding factor
• f = generated frequency
• Φ= flux per pole in weber
• T = series turns per phase

The simple NBAω equation explains the fundamental generator action, while the 4.44KwfΦT equation is more useful for practical alternator winding calculation.

Frequency of an AC Generator

The frequency of an AC generator depend on the rotor speed and the number of magnetic pole. In a synchronous generator, the generated electrical frequency is:

\(\boxed{f=\frac{PN}{120}}\)

Where:

• f = frequency in Hz
• P = number of poles
• N = rotor speed in rpm

Rearranging: \(\boxed{N_s=\frac{120f}{P}}\)

Where Ns is the synchronous speed in rpm.

For example consider a 4 pole generator rotating at 1500 rpm: \(f=\frac{4\times1500}{120} , f=50\text{ Hz}\)

Thus, a 4 pole synchronous generator operating at 1500 rpm produce a frequency of 50 Hz.

Why are Slip Rings used in an AC Generator?

Slip rings are used to maintain continuous electrical contact between a rotating winding and a stationary external circuit without reversing the physical connection after each half rotation.In the elementary rotating armature AC generator each end of the rotating coil is connected to a separate continuous slip ring. Stationary carbon brushes remain in contact with these ring and transfer the generated electrical output to the external circuit. Unlike a split ring commutator, slip ring do not mechanically rectify the alternating EMF. Therefore the natural reversal of the induced voltage appear at the external terminal as AC. In practical alternator with a stationary armature, slip ring may instead be used to supply DC excitation to the rotating field winding. Some modern generator use brushless excitation and do not require conventional brush and slip ring excitation.

Types of AC Generator

AC generator can be classified according to the number of output phases, rotor construction and excitation method. The most common classifications are discussed below.

Single Phase AC Generator

A single phase AC generator produces a single alternating voltage waveform. It may be used where the required electrical load and power rating are relatively small. Single phase generation is commonly associated with smaller generator system and certain portable or specialised application.

Three Phase AC Generator

A three phase AC generator produce three alternating EMFs of the same frequency and magnitude displaced from one another by 120 electrical degree. Three phase generator are widely used for large scale power generation because three phase system are well suited to efficient transmission, distribution and industrial load.

Salient Pole Alternator

A salient pole rotor has projecting magnetic pole mounted on the rotor. It is generally suited to relatively low speed prime mover and can have a large rotor diameter with a shorter axial length. Salient pole machine are commonly associated with hydroelectric generating station.

Cylindrical Rotor Alternator

A cylindrical or non-salient rotor has a smooth cylindrical construction with field winding placed in rotor slot. It is mechanically well suited to high speed operation and is commonly used with steam or gas turbines in turbo alternator.

AC Generator vs Alternator

An alternator is an AC generator; in electrical engineering the term commonly refer to a synchronous generator that produce alternating electrical power. Therefore AC generator and alternator are often used interchangeably. However “AC generator” is the broader descriptive term for a machine that generate alternating electrical energy while “alternator” is commonly used for practical synchronous AC generating machine.

Therefore it is not accurate to treat an alternator and an AC generator as two completely unrelated machine.

Difference between AC and DC Generator

Both AC and DC generators convert mechanical energy into electrical energy through electromagnetic induction. The main difference lies in the form of electrical output and the method used to connect or rectify the generated EMF.

In the elementary machine model, replacing continuous slip ring with a split ring commutator mechanically rectifies the internally alternating EMF so that the current delivered at the brushes is unidirectional. Practical AC and DC machine however differ significantly in their winding arrangements, construction and operating requirement.

AC Generator vs AC Motor

An AC generator and AC motor perform opposite energy conversion. A generator produces electrical energy from mechanical input while a motor produces mechanical output from electrical input.

In simple terms a generator produce electricity from motion, while a motor produce motion from electricity.

Why does Current reverse Every Half Rotation in an AC Generator?

The current reverses every half rotation because the direction in which the conductor cuts the magnetic field reverses relative to the previous half cycle. As a result the polarity of the induced EMF also reverses. During the first half rotation one side of the rotating coil move through the magnetic field in a particular direction and an EMF of one polarity is generated. After the coil turn through 180° the same conductor moves through the field in the opposite relative direction. According to Fleming's right hand rule the direction of induced EMF therefore reverses. This repeated reversal after each half rotation is what produces an alternating voltage and alternating current.

What Factors affect the EMF Generated in an AC Generator?

The magnitude of the EMF generated in an AC generator depend mainly on the magnetic field strength, number of turn effective conductor area and speed of relative motion.

From the elementary generator equation: \(E_m=NBA\omega\)

the following relationships can be observed:

• Increasing the number of turns N increases the induced EMF.
• Increasing the magnetic flux density B increases the induced EMF.
• Increasing the effective coil area A increases the maximum EMF in the simple loop model.
• Increasing the angular speed ω increases the rate of change of magnetic flux and therefore increases EMF.

In practical alternator the generated RMS voltage also depend on factors such as flux per pole, frequency, number of series turns and winding factor.

Losses in an AC Generator

An actual AC generator cannot convert all mechanical input power into electrical output because some energy is lost inside the machine.

The main losses include:

Copper Losses: Current flowing through the armature and field windings produces \(I^2R\) losses.

Core or Iron Losses: Alternating magnetic flux in the iron core produce hysteresis and eddy current losses. Laminated cores are used to reduce eddy current loss.

Mechanical Losses: Friction in bearing and air resistance or windage consume some mechanical input power.

Stray Load Losses: Leakage flux harmonic field and non ideal current distribution can produce additional small losses.

Generator efficiency is: \(\eta=\frac{\text{Electrical Output Power}}{\text{Mechanical Input Power}}\times100\)

• Higher efficiency is achieved by reducing electrical, magnetic and mechanical losses through proper machine design.

Applications of AC Generator

AC generator are used wherever mechanical energy need to be converted into alternating electrical power. They range from small generator sets to large machine supplying national electricity grid.

Major applications include:

• Thermal power plant: Steam turbine drive large three phase alternator.
• Hydroelectric power plant: Water turbine provide mechanical input to generator.
• Wind turbine: Rotor motion is converted into electrical power through generator system.
• Gas turbine plant: High speed turbine drive AC generator.
• Diesel generator set: Internal combustion engine provide mechanical input for backup or standalone generation.
• Automotive system: Alternator generate electrical energy for vehicle electrical system and battery charging, with rectification used for the vehicle's DC system.
• Marine system: Alternator supply electrical power aboard ship.
• Industrial plant: Generator set provide standby and emergency electrical power.

Thus AC generator form a fundamental part of both large scale electricity generation and local backup power system.

Understanding the AC generator diagram, construction, working principle, EMF equation, frequency and types provides the foundation for studying alternators, synchronous machine and electrical power generation.