What are the differences between an alternating current generator and a direct current generator?

Main Differences Between AC and DC Generators

An electrical machine is a device that converts mechanical energy into electrical energy and vice versa. A generator is a type of such machine that converts mechanical energy into electrical energy. However, the electrical energy generated can be in either alternating current (AC) or direct current (DC) form. Thus, the primary difference between AC and DC generators is that they generate alternating current and direct current respectively. While there are some similarities between the two, there exist quite a number of differences.

Before going into the list of differences between them, we are going to discuss how the generator generates electricity & how AC & DC is generated.

Electricity Generation

Electricity is generated based on Faraday’s Law of Electromagnetic Induction, which states that an electric current or electromotive force (EMF) will be induced in a conductor when it is placed in a changing magnetic field. Both AC and DC generators operate on this same principle to generate electric current.

There are two methods to vary the magnetic field acting on the conductors: either rotate the magnetic field around a stationary conductor, or spin the conductor within a stationary magnetic field. In both scenarios, the magnetic field lines interacting with the conductor change, thereby inducing an electric current in the conductor.

An alternator utilizes the concept of a rotating magnetic field around a stationary conductor, though this will not be discussed in the present article.

AC Generator: Slip Rings and Alternators

Since slip rings are continuous conductive rings, they transmit the alternating current generated in the armature as-is. As the brushes slide continuously over these rings, there is little risk of short circuits or sparking between the components. This results in a longer service life for the brushes in AC generators compared to DC generators.

An alternator is another type of AC-only generator, featuring a stationary armature and a rotating magnetic field. Because electrical current is generated in the stationary part, transmitting it to the stationary external circuit is simpler and more straightforward. In such designs, the brushes experience minimal wear, further enhancing durability.

DC Generator

A DC generator is a device that converts mechanical energy into direct current (DC) electrical energy, also known as a dynamo. It produces pulsating direct current, where the current magnitude may vary but the direction remains constant.

The current induced in the rotating armature conductors is inherently alternating. To convert this to DC, a split-ring commutator is used. The commutator not only transfers current from the rotating armature to the stationary circuit but also ensures the direction of the supplied current remains consistent.

Split-Ring Commutator in DC Generators

The split-ring commutator consists of a single ring-shaped conductor divided into two halves, with an insulating gap between them. Each half of the split ring is connected to a separate terminal of the armature winding, while two stationary carbon brushes make sliding contact with the rotating commutator to supply current to the external circuit.

As the armature rotates and the induced AC current reverses direction every half-cycle, the split-ring commutator ensures that the current supplied to the circuit maintains a consistent direction:

During one half-rotation, current flows through one brush to the circuit.
During the next half-rotation, the commutator segments switch contact with the brushes, reversing the internal current direction but maintaining the same external current flow.

However, the gap between the commutator segments introduces two key challenges:

Sparking: As the brushes transition between segments, they briefly bridge the gap, causing momentary short circuits and sparking.
Brush Wear: The repeated arcing and mechanical stress accelerate brush degradation, reducing generator efficiency and lifespan.

These factors necessitate regular maintenance and replacement of brushes in DC generators compared to AC generators with slip rings.