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Difference Between Alternator and Generator with Comparison

Key Differences between an Alternator and a Generator

Alternator and generator are both mechanical devices that convert mechanical energy into electrical energy. Apart from that, they are different from each other in every other aspect. The major point that differentiates between an alternator and generator is that in an alternator, the magnetic field rotates around a stationary armature and in generator; the armature rotates inside a stationary magnetic field.

Difference between Alternator and Generator

Before going into the differences between alternator and generator, let’s look into what alternator and generator are and how electricity is generated.

Electricity Generation

The electricity is generated using Faraday’s law of electromagnetic induction. It states that a current (EMF) will be induced in a conductor if it is placed in a continuously changing magnetic field.

The alternator and generator apply the same principle to generate electric current. The current is generated by changing the magnetic field acting upon a conductor. However, there are two ways to do it. Either rotate the magnetic field (in housing) around a stationary conductor or spin the conductor (in form of a rectangular coil) inside the stationary conductor.  In both cases, the magnetic field lines intersecting the conductor varies and it induces an electric current in the conductor.

The part of the machine that rotates is called the rotor and the part that is stationary is called the stator.

What is an Alternator?

An alternator is a mechanical device that converts mechanical energy into AC electrical energy.

It is a type of generator where the magnetic field (rotor) rotates around the armature (stator).


The magnetic field is generated by either permanent magnet or electromagnet using DC supply. The electromagnet is in housing surrounding the armature (conductor windings). The magnetic field is rotated by any means (steam turbines, gas turbines, and combustion engines) known as the prime mover. Its rotation causes the change in the magnetic field lines intersecting the armature conductors. As a result, an electric current is induced in the armature.

The armature supplies the electric current to the output load through brushes. Since the armature is stationary, the brushes do not wear out. Thus increasing its mechanical life and reducing the maintenance requirement.

The revolving magnetic field continuously reverses its polarity (north and south) acting on the stationary armature, causing a continuous change in the direction of the induced current. This is the reason the output of the alternator is always Alternating Current.  

The alternator conserves energy. They are the enhanced and efficient version of a generator that generates energy when it is needed (depends on the load). Therefore, the batteries in the vehicle do not die due to overcharging (because the alternator stops supplying power). Hence an alternator does not waste energy.

The alternator cannot be used for charging a completely drained battery.

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What is a Generator?

A generator is a mechanical device that converts mechanical energy into AC or DC electrical energy.


In the generator, the outer housing that surrounds the armature is made of either permanent magnet or an electromagnet that is stationary known as the stator. The armature is made up of coils of conductors that rotate around its axis inside this stationary magnetic field. Because of the rotating armature, the magnetic field line intersecting the conductors varies. Hence a current is induced in the rotating armature.

The armature is connected with either a slip ring or a commutator. They both transfer the electric current (which is continuously changing direction) from the rotating armature to the static output circuit. But the slip ring has a full circular connection that allows the continuous transfer of the electric current from the rotating shaft which results in an Alternating Current AC. On the other hand, the commutator has at least 2 breaks between them, which reverses the direction of current after each half rotation thus, the output current stays in one direction also known as the Direct Current DC.

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It provides energy constantly even without the need for it. This is the reason it has less efficiency than the Alternator. But it can generate AC as well as DC and the output voltage remains constant throughout its operation. Therefore the generator is perfect for being used as a backup power supply in homes, offices and construction sites, etc.

It can be used for charging a completely drained battery.

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Main Differences between Alternator and Generator

A machine that converts mechanical energy into AC electrical energy.A machine that converts mechanical energy into AC or DC (electrical).
It can only generated Alternating Current “AC”.It can generate either of both Alternating current AC or Direct Current “DC”.
The rotating part or rotor is the magnetic field.The rotating part or rotor is the armature.
The stationary part or stator is the armature.The stationary part or stator is the surrounding magnetic field.
The generated output current is taken from the stator.The generated output current is taken from the rotor.
The output voltage of Alternator is always higher then Generator.The output voltage of the Generator is always higher then Alternator.
The brushes last longer because the armature is stationary and there is no friction.The brushes wear out due to friction against the rotor thus they do not last very long.
It does not need polarization after install.The generator needs polarization after install.
It has a wide range of RPM (Rotations Per Minute).It has a low range of RPM.
The generated voltage or energy varies depending on the load.The output voltage is constant irrespective of the load.
It cannot charge a completely drained battery (worst-case scenario, it will burn up).It can be used for charging a completely drained battery.
It conserves energy and is more efficient then Generator.It wastes some energy and is less efficient than Alternator.
They are smaller in size then Generator.They are larger in size then Alternator.
Alternators are mostly used in vehicles to charge its battery.The generator is widely used as a backup power supply in every sector such as domestic, industrial, construction sites, etc.

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  1. Hi
    Le me discus some additional notes about alternator and generator.
    In a typical alternator on a car the rotor receives a regulated current via several brushes to create a regulated magnetic field, so the magnetic field rotates INSIDE the induced coils at the stator.
    This configuration, unlike dynamos, allow smaller collectors at the rotor so the alternator can spin at higher rpm and the brushes only hold a very low current and not the load current.
    Small engines uses alternator described in the article since a permanent magnet rotor spins around the induced coils.
    Voltage is then regulated and rectified on an external device.

    Regarding the output voltage, the article says that on the generator is higher then the alternator and the alternator higher then the generator.????

    The brushes last longer because they carry low current. Nothing to do with the stationary armature. There is never friction with the stator. Only with the rotor, despite it is the inductor or the induced coils.

  2. I have a standby genset at my home, and I understand the “generator” is a Newage brand. But I noticed that since Cummins owns them, they are called Alternators. It is 35 kvA. So what is it? An alternator or a generator?

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