RFAC Generator vs. DC Generator: Key Differences Explained
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This article explores the fundamental differences between AC (Alternating Current) and DC (Direct Current) generators. Both types of generators, regardless of size, rely on the principle of magnetic induction.
In DC generators, the armature is always the rotor. However, in AC generators (alternators), the armature can be either the rotor or the stator.
AC Generator
An AC generator is a machine that converts mechanical energy into AC electrical energy.
It operates based on the principle of electromagnetic induction: “Whenever a conductor cuts magnetic lines of force, an electromotive force (EMF) is induced in it.”
Figure 1: AC Generator Types
As shown in Figure 1, there are two main types of AC generators:
- Rotating Armature Alternator: The armature rotates within a stationary magnetic field.
- Rotating Field Alternator: The magnetic field rotates around a stationary armature.
The generated AC voltage from the alternator is supplied to the external circuit via slip rings and brushes.
DC Generator (Dynamo)
A DC generator, also known as a dynamo, converts mechanical energy into DC electrical energy.
The EMF generated by the dynamo is supplied to an external circuit in the form of DC voltage using a commutator and brushes.
Dynamo efficiency is calculated as follows:
Dynamo efficiency = (Output electrical power / Input mechanical power) x 100%
Figure 2: DC Generator
An elementary generator consists of a single coil rotating in a magnetic field, producing AC voltage. A basic DC generator is created by replacing the slip rings of the elementary generator with a two-piece commutator. This converts the output voltage to pulsating DC voltage, as depicted in Figure 2.
Using a multiple coil armature decreases ripple and increases the output voltage of the DC generator.
Comparison Table: AC Generator vs. DC Generator
The following table highlights the key differences between AC and DC generators:
| Features | AC Generator | DC Generator |
|---|---|---|
| Function | Generates AC output. | Generates DC output. |
| Output Connections | The generated AC is brought to the load unchanged by means of slip rings. | The EMF generated in the armature windings is converted from AC to DC by means of the commutator. |
| Types/Classifications | - Rotating armature alternator and rotating field alternator. - Single phase (<=25 KW): generates at specific voltage. - Polyphase (2, 3, or 6 phases). | Classified based on field excitation: permanent magnet, separately excited, self-excited, shunt/series, compound. |
| Advantages | - Simple in construction. - More powerful three-phase generators can be constructed. - Easy to distribute AC voltage with transformers. | - Many devices rely on DC current for operation. - At a given voltage, DC current is powerful compared to equivalent AC current. |
| Disadvantages | - Electricity grid requires precise coordination to keep generators in phase. - AC current is more dangerous than DC due to electric shock. | - More complex construction with split ring commutator. - Sparking occurs in the commutator gap, wasting energy. - More difficult to distribute DC efficiently. |
