What is AC Generator ? All About It

Electrical generator is that type of machine which converts mechanical energy to electrical energy.

The AC generator works on Faraday’s Law of Electromagnetic Induction principles. Which states that if a conductor is exposed to the changing magnetic fields. It will induce a current.

The generator works on the same principle. There are two different types of generators. The classification is based on the output they produce.

One is AC Generator and the other is a DC generator. Let’s discuss AC Generator in this article.

What is AC Generator?

AC Generator is the type of generator that converts mechanical energy into electrical AC energy.

The output electrical energy is in the form of alternating current (AC current). Therefore, AC generators are sometimes referred to as Alternators.

The mechanical energy to the generators is provided in the form of combustion engines, steam, or gas turbines.

Who invented Ac Generator?

Charles F. Brush is the inventor of ac generator. Charles Francis Brush was born on March 17, 1849, and passed away on June 15, 1929.

Inventors and industrialists who invented an electric alternating current generator with a changeable voltage regulated by the load and constant current. Brush founded the Brush Electric Company and was the founding president of Linde Air Products Company.

The American Academy of Arts and Sciences awarded him the Rumford Medal for his electrical works in 1899.

In 1876, he received funding from Cleveland’s Wetting Supply Company to develop his “dynamo” (an electrical generator) for lighting arc lights. Brush began with Zénobe Gramme’s dynamo design, but his final design was a significant departure, keeping the ring armature pioneered by Antonio Pacinotti.

In his U.S. Patent 189,997, Brush stated, “The finest kinds of magneto-electric apparatus before the market are excessively big, heavy, and costly, and are more or less wasteful of mechanical power.”

After testing in 1878, the Franklin Institute of Philadelphia deemed Brush’s dynamo better than the Gramme dynamo and other European competitors due to its simpler design and maintainability.

Working Principle of AC Generator

We discussed above that generators work on the principle of Faraday’s Law of Electromagnetic Induction. The law states that when a current-carrying conductor is moved in a uniform magnetic field, An EMF is induced.

We can achieve this process by two different methods. Either to place a stationary conductor in a rotating magnetic field or to rotate a current-carrying conductor in a static magnetic field. But the most preferred method is to keep the coil stationary.

AC generator follows the same process. The induced EMF depends upon the number of turns in the armature coil, Speed of the rotating field, and magnetic field strength.

What are the uses of Ac Generator?

An alternating current generator (AC generator) is used to power larger business facilities such as offices and commercial buildings, as well as smaller electrical goods in your home such as coffee makers, PCs, and all other household appliances.

AC generators are widely employed in a variety of applications. To generate alternating current, all power plants rely on AC generators.

These devices can be classified according to the many applications of AC generators. The following are some examples of AC generator where do we use it:

  • Maritime alternators – They are used to generate power in marine applications.
  • Brushless alternators- They are the primary source of power in electrical power plants.
  • Diesel-electric locomotive alternators – Locomotives require both electric and mechanical power, and the power source is AC generators.

How many types of AC generator?

AC generators are classified into two types:

  1. Asynchronous Generators
  2. Synchronous Generators

Asynchronous Generators

The asynchronous generator is also called an induction generator. It is an alternating current (AC) electrical generator that utilizes the fundamentals of induction motors to generate electricity. Induction generators work through mechanically rotating their rotors more quickly than synchronous speeds.

A normal AC induction motor can typically be utilized as a generator without internal modifications. Since they are able to recover energy using relatively easy control, induction generators can be effective in applications like miniature hydro power plants and wind turbines, as well as in reducing high-pressure gases to lower levels.

An asynchronous generator typically draws its power source via an electric grid. In this way, asynchronous generators are not able to begin black in the de-energized distribution system. However, sometimes they self-excite using capacitors.

Synchronous Generators

Synchronous generator is a machine that converts mechanical energy into AC electrical power via using electromagnetic induction. Synchronous generators are often known as alternators or AC generator.

Term “Alternator” is used since it generates AC power and it’s also known as a “synchronous generator” because it needs to be driven at asynchronous speed to generate AC power at the desired frequency.

The three-phase synchronous generators provide numerous advantages when it comes to transmission, distribution, and generation. Large synchronous generators utilize in the thermal, nuclear, and hydropower system to generate the voltages.

Read my detailed article about synchronous generator for more information.

Which is better synchronous or asynchronous generator?

Synchronous generators are better than asynchronous, induction, generators because, they have higher efficiency than Asynchronous generators. Synchronous generators are more able to adapt to changes in the power factor of the load.

Synchronous generators can be activated by supplying the rotor’s the field excitation of batteries. A synchronous generator’s output frequency may be more readily adjusted to maintain a consistent value.

In the meanwhile, Permanent magnet synchronous generators require no rotor field excitation.

Read also my article about electric generator battery for beginners

Why we use synchronous generator In Power Plant?

Within the GSS (Grid SubStation), all power plant components are linked to one another. For successful parallel operation “Synchronous generators” are in use. In power plants, it operates on asynchronous speed and has an output of synchronous frequency. Synchronous generators have an efficiency of 99%.

If induction generators were being used in parallel there would be a speed differences problem which will cause unbalance because the  voltage produced will not be the same.

In order to operate in parallel with two generators, they must be operating with identical voltage, frequency, and the phase. If any of the three parameters are distinct, it can result in an increase in the loss.

Let’s take two generators, for example, connected in parallel.

  1. The current will move from high voltage to lower voltage generators if the voltage is not the same.
  2. If phase and frequency become different, then the output current and voltages have distortions (not absolute sine waves).

For more information read my detailed article about electric generator synchronization

Is AC generator rated in kVA or kW?

Generator power delivered to the load depends on the load power factor. Manufacturers don’t have any idea about the load type which will be connected to the generator, so its wise to rate the generator by KVA.

Note that AC generators produce both active and reactive power and deliver it to the load depending on its type. Active and reactive power are the apparent power that is measured in volt-amperes (VA) rather than watts (W) or kilowatt (KW).

The output of the generator will depend on the load power factor.

For example,

In case of a pure resistive load with unity power factor. A 200kVA generator would provide 200kW to the load.

P (KW) = S (KVA) x Cos Φ = 200 x 1 = 200 kW

In case of inductive load, with power factor is 0.85, the same generator would provide power of, S x Cos Φ = 200 x 0.85= 170kW  to the load.

Different Parts of AC Generator

AC generator consists of different parts such as armature, field excitation, slip rings, stator, and rotor. The detailed description of each part is below.

  • Field Excitation

The field includes copper conductor coils that receive a voltage from an external source for producing magnetic flux.

The magnetic flux cuts the armature flux in the field to produce voltage. Which the output of a generator.

Brushless synchronous generators have excitation system independent on external sources.

  • Armature

The armature is the part of the Ac generator in which voltage is produced. It consists of an armature coil.

  • Prime Mover

It is the mechanical part of the generator used for driving the generator. The prime mover could be a steam turbine, diesel engine or either wind energy.

  • Stator

The static part in the generator is called a stator. It consists of a field or an armature. If we provides excitation to the stator  and will act as a field.

  • Rotor

The rotating part of the Ac generator is called a rotor. The prime mover is used for driving the rotor.

Depending upon the type of generator if we provide excitation it will act as a field. and if the voltage is generated here the rotor will act as a field.

  • Slip Rings

The electrical connections are used for power transfer from the rotor of an AC generator.

The basic function of slip rings is to conduct the flow of current from a stationary part to a rotating part using carbon brushes.

In which part of the AC generator is the output generated?

electrical generator winding varnishing
generator stator

An armature is the part of an AC generator that produces electricity, armature is also well known as stator, as it doesn’t rotate.

Armature is generally made up of wire coils large enough to handle the generator’s full-load current.


Because the output power is derived from stationary windings, it may be connected through fixed terminals. Because there are no sliding contacts in this style of architecture, greater currents may be handled, and the whole output circuit is continually insulated.

What is an AC generator exciter used for?

The exciter is an AC generator (a small one) component that is part of the generator or alternator’s excitation system. It’s responsible for the production of the necessary excitation to feed the rotor to produce electrical power.

The brushless synchronous generators have an excitation system, a small AC generator with its rotor rotates with the main rotor. The produced AC current is to converted in to DC and then feeds the main rotor.

Because the magnetic field of the exciter stator cuts through the copper windings of the exciter rotor, producing a voltage, the exciter rotor creates electrical energy as it spins.

The induced power in the exciter rotor changes with the amount of power provided to the exciter stator by the AVR (automatic voltage regulator), which monitors the voltage at the generator terminals and attempts to adjust it to a predetermined value.

Brush-less AC synchronous generator

To feed the rotor with excitation current, slip rings and carbon brushes are being used. The brush system needs periodic maintenance and replacement. To overcome this issue brush- less excitation system is used instead.

In very simple form this brush-less excitation is a small generator consists of two parts, One is fixed and called the exciter which is energized from the residual magnetism of the synchronous generator.

The second part rotates with the rotor, fixed on rotor shaft and feds the rotor with DC current to produce its field. By this way the brushes and slip rings are no more needed, that’s why it’s called brushless.

What is residual magnetism?

If current pass through a conductor it produces magnetic field around it, When current stops there is magnetic field remains in the coil. This is called residual magnetism.

Read my detailed article about synchronous generator for more information about residual magnetism.

AC generator control

Generators, like all electrical equipment need a control and sensing system. Speed, voltage, current, frequency, oil pressure, cooling water temperature and level, all these quantities need to be measured and analyzed to protect and control the output and performance of the generator.

Based on the generator output power the sensing and protection system gets more complicated.

Generator common faults

Generators are machines, and even the largest power plant with the most advanced control and protection system it may has faults as any other small portable generators.

The main  difference is the chance of the fault is greater in small generators. On the other hand one major fault in large generator costs a lot.

hereunder some faults which are common

  • Batteries: battery failure is one of the most common failures of generators, Always make sure that battery charger, all cables and connectors in good conditions. Make periodic inspection for batteries.
  • Internal winding short circuit: due to insulation failure. This failure may happen due to humidity.
  • Loss of excitation: Excitation is necessary for power production, excitation loss will cause output power loss.
  • Coolant water level loss: Water is used for cooling cycle and should be in a certain level to work properly.
  • Oil pressure loss: this may lead to shutting down the engine as oil loss is destructive.
  • Running out of fuel: Sometimes fuel indicators gives false level and then the fuel run out.
  • High Fuel Level: This alarm is designed to prevent any overfill of the fuel tank.
  • Circuit breaker trip: due to a reason in the load side like short circuit or overload the CB trips to protect the generator.
  • Frozen fuel:  in very cold areas if no special fuel is used then it may get frozen and stop the engine, specialized fuel is in cold areas.
  • Dirty Air Filters: like your car generators need air filter and these filters should be kept clean for longer life time and effectively.

AC generator protection

Protecting generators should be against inside and outside faults. As we know that a generators are connected to power system (OHTL and transformers) or to a direct load. Any fault in the power system or load side should be cleared fast otherwise it may case the generator damage.

  • Protection against internal short circuit due to insulation failure. Protecting generator using differential rely can help in case of insulation failure or internal short circuit.
  • Stator overheat protection: Many causes can lead stator to overheat, Such as insulation failure, overloading and cooling failure. To detect the excessive temperature rise temperature sensors are used in different point of the stator winding.
  • Cooling water level protection: Loss of cooling liquid may should be detected to prevent any temperature rise.
  • Oil pressure protection: Oil is used as a lubrication liquid, so its pressure is detected and checked for any loss or leakage.
  • Over current protection to protect against load increasing.
  • Over speed protection to keep frequency steady.
  • Fuel level protection.
  • Protection against vibration.

Generator name plate important data

Like any electrical equipment generators have nameplate, on this nameplate the main electrical data like :

  1. generator power.
  2. output voltage.
  3. Rated current.
  4. frequency.
  5. rotation speed.
  6. number of poles of winding.
  7. manufacturing date.
  8. excitation data, current and voltage.

In fact the larger the generator the larger the nameplate and its data. this name plate is important and should be on the generator body all the time.

Be safe with AC generators

Electricity kills! No matter your expertise is, You always should be careful when dealing with generators. Unfortunately generators not only has electricity danger but also has mechanical danger as well.

As an electrical engineer I will focused on electrical hazard and how to keep it away.

  • Only authorized persons to get near the generator.
  • Make sure to read all manufacturer instructions before operating the generator.
  • Don’t expose to life parts of the generator.
  • Periodic electrical inspection for the generator parts and cables.
  • All current carrying parts, cables and connections should be isolated.
  • Electrical connections should tight and properly connected.
  • A good preventive maintenance program will keep the generator safe.
  • Don’t overload the generator as this may overheat the generator and may cause damage and fire.
  • Large generators are loud. This loud may case serious damage or rupture eardrums. Make sure to use earmuffs when working near the generator.
  • Always use proper caution signs in the AC generator location.

For more details about electrical hazard, read my article on my other safety frenzy site here.

What Factors do the generated EMF depend upon?

The electromotive force (emf) generated by an alternator depends upon the number of turns in the coil, the area encompassed by the coil, the angular speed of the coil, and the magnetic field.

A current is induced in a coil when it is turned into a magnetic field. The magnitude of this induced current is determined by magnetic flux. When the plane of the coil is parallel to the magnetic field, magnetic flux is greatest.

When the plane of the coil is perpendicular to the magnetic field, magnetic flux is at its lowest. While a result, as a coil spins in a magnetic field, the induced current in it continually varies from maximum to minimum, minimum to maximum, and so on.

The armature is designed to revolve freely in the magnetic field. When the armature rotates, a change in the magnetic flux produces an induced e.m.f.

The strength of e.m.f. can be improved by increasing the number of coil turns. Increasing the coil’s velocity. Increasing the magnetic field strength. Increasing the coil’s length. Increasing the angle formed by “V” and “B.” When the angle is 90°, the e.m.f. is at its maximum.

advantages and disadvantages of AC generators


  •  AC generator is easier and more efficient in transferring AC Voltage across long distances.
  • The design of an AC generator is simpler than that of other types of generators.
  • There is no requirement to match a voltage.
  • The total cost of ownership is cheaper.
  • They run quiet operations.
  • This also simplifies the generator’s maintenance.
  • Using transformers, an AC generator may convert its current to another voltage.
  • AC generators are only compatible with AC generators and not with DC generators.


  • Because of the higher voltages required to produce a set level of power, this presents a handling difficulty.
  • Because of the higher voltages required to produce a set level of power, these systems require more insulation.
  • Aside from these dangers, an AC generator is not as long-lasting as a DC generator.
  • Working with an alternating current system has several specific risks and problems compared to working with DC motors.

Why does AC generator need force to run?

To produce electricity we need magnetic field, a conductor and a relative movement between them. The movement is achieved by the mechanical engine, prime mover, to provide a  suitable force to rotate the rotor. This makes the generator converts mechanical energy to electrical energy.

This mechanical force is needed to run the AC generator because, two sets of magnets forces are generated in generators while  generating power. The generator’s magnets attract and repel each other, making it difficult to rotate.

The magnets get stronger as the generator produces more current as more load attached to the output, making it more difficult to crank the generator.

An alternator (AC generator) works on the idea of converting mechanical energy into electrical energy via EMF (electromotive force) that changes polarity as it passes through the magnetic field.

The frequency of the alternator is determined by the number of poles in the magnetic field and the synchronous speed of the prime mover (described by the equation N = 120f/P, where N is the synchronous speed, f is the frequency, and P is the number of poles).

The prim mover could be any mechanical energy source like:

  • Gas engine.
  • Diesel engine.
  • Wind energy.
  • Ocean waves energy.

Why is AC generator used over DC generator?

AC generators are used over DC generators because, transformers cannot be utilized the DC directly, and the output of DC generators is difficult to distribute.

AC generators are particularly efficient and used widely because, they have low energy losses and also It reduces the size of the transmission link size.

The output of AC generators is easily distributed, stepped up, and stepped down using a transformer. Sparking and other losses such as copper, eddy current, mechanical, and hysteresis make DC generators less efficient.

Some other major reasons that’s attract users to use AC generators over DC generators are listed below:

  • Commutators are not used in AC generators but are used in DC generators to solve the problem of switching polarities. Furthermore, AC generators generate extremely high voltages, whereas DC generators generate relatively modest voltages.
  • It powers small engines and household electrical devices. On the other hand, DC generators are used to power massive electric engines, such as those used in subway lines.
  • AC generators provide a high voltage with changing intensity and frequency. The emission frequency varies. In contrast to DC generators, which generate a constant amplitude and duration, i.e., almost zero output frequency, DC generators produce a low value.
  • While the magnet is moving, the coil through which the current travels in an AC generator remains motionless. The structure is simple, and the prices are reasonable. A DC generator, on the other hand, has a set domain through which current flows. The overall concept is easy due to the commutators and slips rings; nevertheless, the assembly is difficult.

Is AC generator is called alternator?

Yes, An AC generator is called an alternator.

Electrical AC generators are an electrical device that converts mechanical energy into alternating electrical energy.

A linear alternator or moving in the armature with the magnet’s stationary field is utilized in a variety of areas.

One of the main distinctions between generators and the alternator is the fact that in an alternator, there is an armature that is stationary, and the field turns. In contrast, in the generator, the armature turns, and the field remains stationary.

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