Difference between AC and DC Generator

Electric Traction Power Systems Utilisation of Electrical Power

This article is about AC generator and DC generator, and the main differences between the AC and DC generators.

What is a Generator?

An electrical machine which converts the mechanical energy input into electrical energy output is called the generator or electric generator.

Though, the electrical energy generated by the machine can be of either AC (Alternating Current) or DC (Direct Current). Hence, based on the type of supply produced by the generator, they are of two types viz. −

AC Generator (or Alternating Current Generator)
DC Generator (or Direct Current Generator)

Both of these alternators apply same principle (Faraday’s law of electromagnetic induction) to generate electricity. However, there are a lot of differences between the AC generator and DC generator in their construction, operating and performance.

What is an AC Generator?

The type of electric generator which converts mechanical energy input into AC electrical energy output is known as AC generator. It is also known as alternator, as it produces alternating current electricity. In practice, the AC generator is a type of generator designed to generate alternating current with a frequency of 50 Hz or 60 Hz.

The AC generator consists of fixed armature and a rotating magnetic field. When rotating magnetic field links with the stationary armature winding, produces alternating current in the winding by the principle of electromagnetic induction.

What is a DC Generator?

A type electric generator which converts input mechanical energy into DC electrical energy is called the DC generator or direct current generator. DC generator is also known as dynamo.

A DC generator consists of a rotating armature and static magnetic field. When the rotating armature moves in the stationary magnetic field, an alternating current is induced in the armature winding and it is converted into direct current by using a commutator (a mechanical rectifier) and supplied to the external circuit.

Differences between AC generator and DC Generator

The following table shows the key differences between the AC generator and DC generator.

Parameter	AC Generator	DC Generator
Definition	An electromechanical energy conversion device that converts mechanical energy into AC electrical energy is called AC generator.	An electromechanical energy conversion device that converts mechanical energy into DC electrical energy is called DC generator.
Armature position	In AC generator, the alternating current is induced in the stator winding. Thus, stator winding acts as armature winding.	In DC generator, current is induced in the rotor winding. Hence, rotor winding acts as armature winding.
Nature of current generated	AC generator produces alternating current whose magnitude changes continuously and direction changes periodically.	DC generator produces direct current, whose magnitude may be pulsating or constant, but flows in the one direction.
Frequency of generated output	In AC generator, the frequency of the generated output depends upon the speed of rotating magnetic field (synchronous speed).	In case of DC generator, there is no frequency, hence it is independent of rotational speed.
Commutator	No commutator is required in the AC generator.	As induced current in the armature of DC generator is alternating. Hence, to convert it into direct current, the commutator is used in the DC generator.
Energy losses	The energy losses are low in AC generator due to absence of commutator and carbon brushes.	In DC generator, commutator and brushes are used. Thus, the energy losses are high in the DC generator due to sparking between the commutator segments and brushes.
Limit of generated voltage	There is no limit of voltage produced by the AC generator.	With the increase in the generated voltage, sparking increases which can damage the commutator. Thus, a DC generator has a voltage limit.
Replacement of brushes	No need of brush replacement in AC generator.	The commutator wears out the brushes due to sparking between them. Hence, the periodic replacement of carbon brushes is required in the DC generator.
Maintenance & maintenance cost	AC generator does not need frequent maintenance. Thus, the maintenance cost is low.	DC generator requires frequent maintenance due to wear & tear of commutator and brushes. Therefore, their maintenance cost is comparatively higher.
Reliability	AC generators are more reliable.	DC generators are not reliable as they require more maintenance.
Efficiency	Efficiency of AC generators is high.	DC generators have relatively lower efficiency.
Design	AC generators are simple in design.	DC generators have complex design.
Initial cost	For AC generator, the initial cost is higher.	The initial cost of DC generators is relatively lower.
Overall cost	In long turn, the overall cost of AC generator is low.	Due to costly maintenance, the overall cost of DC generator is high.
Applications	AC generators are employed for suppling power for domestic, commercial and industrial purposes such as lighting, fans, etc.	DC generators are mainly used for supplying power to large DC motors such as in electric traction, charging batteries, etc.

Conclusion

From the above comparison, we can conclude that AC generators are more reliable, simple in design and construction, easy and cheap maintenance and less expensive in the long turn. In contrast, DC generators are suitable for supplying DC power to large DC motors such as in electric tractions. As the DC supply has zero frequency, the output of the DC generator remains unaffected from the variation in the speed of the prime mover.