AC Generator Basics

AC Generator Basics

• A simple generator without a commutator produces an alternating current (AC) that changes direction as the armature revolves.

AC Generator vs DC Generator

• AC generators differ from DC generators in two main aspects:
  • Armature winding ends are connected to solid unsegmented slip rings on the generator shaft instead of commutators.
  • Field coils are energized by an external DC source rather than by the generator itself.

AC Generator Design

• Low-speed AC generators can have up to 100 poles to improve efficiency and achieve desired frequency.
• High-speed AC generators (alternators) driven by turbines are often two-pole machines.

Frequency of AC Generator

• The frequency of the current delivered by an AC generator is equal to half the product of the number of poles and the number of revolutions per second of the armature.

Alternator Construction

• Alternators are constructed with a stationary armature and a rotating rotor composed of field magnets.
• The principle of operation is the same as that of an AC generator, but with the magnetic field in motion instead of the conductors.

Single-Phase Alternating Current

• The current generated by an alternator rises to a peak, sinks to zero, drops to a negative peak, and rises again to zero a number of times each second, depending on the frequency.

Multi-Phase Alternating Current

• If the armature has two windings at right angles, two current waves are produced, forming two-phase alternating current.
• If the armature has three windings at 120° to each other, a triple wave is produced, forming three-phase alternating current.
• Three-phase alternating current is the most commonly used in modern electrical engineering.

Voltage and Applications

• Voltages as high as 13,200 are common in alternators.
• Three-phase alternators are typically used for generating electric power.