Comprehensive Guide to Alternators: The Workhorses of Power Generation
Table of Contents
- Introduction to Alternators and How They Generate Electricity
- Key Components of Alternators
- Relationship Between Rotor Speed, Number of Poles and Frequency
- Three-Phase Power Generation in Alternators
- Speed and Pole Considerations for Mechanical Integrity
- Voltage Regulation Using Automatic Voltage Regulators
Introduction to Alternators and How They Generate Electricity
Alternators are the workhorse of the power generation industry. They are capable of generating AC power at a specified frequency. Alternators are also called synchronous generators. Electricity is produced in alternators by electromagnetic induction. To generate electricity in a coil, either the coil should rotate with respect to a magnetic field or a magnetic field should rotate with respect to the coil. In alternators, the latter approach is used where the rotor produces a rotating magnetic flux and the armature coils are stationary.
The rotating magnetic flux associated with the rotor induces electricity in the armature coils. This method of generating electricity by the interaction between a rotating magnetic field and stationary conductors is known as the dynamo effect.
Definition and Capabilities of Alternators
An alternator is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current (AC). Alternators generate electricity based on the principles of electromagnetism and electromagnetic induction. The key capabilities of alternators include:
- Generate AC electricity at a specified voltage and frequency based on design
- Output single-phase or multi-phase AC power
- Scale up to large capacity ratings for industrial and utility scale power generation
- Integrate with various mechanical drive systems like gas/diesel engines, steam/gas turbines, hydro turbines etc.
Working Principle of Alternators
The working principle of alternators involves producing a rotating magnetic field by means of the rotor, which induces alternating voltages in the stationary armature windings due to electromagnetic induction. This induced EMF (electromotive force) constitutes the electrical output. Specifically, the rotor contains field windings supplied with direct current (DC). This DC excitation current produces a magnetic field oriented along the rotor axis with alternating North and South poles. As the rotor spins at a particular speed by the driving mechanical force, the magnetic field lines also rotate at synchronous speed. The rotating magnetic field from the rotor intercepts the armature conductors placed around the rotor. This changing magnetic flux induces an alternating EMF in the armature windings as per Faraday's Law of Electromagnetic Induction. The frequency of the output electricity depends on the rotor speed and number of poles as per the fundamental relationship.
Key Components of Alternators
Rotor and Armature Coils
The two main components of an alternator are the rotor and armature coils (stator windings).
- Rotor - The rotor houses the field windings and produces the rotating magnetic flux when DC excitation current is applied. Based on design, rotors can be salient pole type or cylindrical type.
- Stator/Armature - The stator contains the armature windings (conductor coils) in which AC voltage is induced. The coils are wound on a laminated steel core. In summary, the rotor rotates to create a rotating magnetic field which intercepts the stator conductors, inducing an EMF as per the dynamo effect.
Salient Pole Rotors
The rotor type shown in the diagram is known as a salient pole rotor. It consists of projecting magnetic poles wound with field coils to carry DC excitation current. When energized, the coils establish alternate North and South poles magnetically around the rotor periphery. As the rotor spins by prime mover drive, the rotor magnetic poles rotate at synchronous speed causing a revolving magnetic field pattern. This rotating field induces AC across the stationary armature coils due to electromagnetic induction. The salient pole rotor construction directs the magnetic flux effectively from rotor to stator to generate voltage.
Relationship Between Rotor Speed, Number of Poles and Frequency
There is a fundamental relationship that connects the rotational speed of the alternator rotor, number of magnetic poles on the rotor and the frequency of the electricity generated:
Frequency (in Hz) = (Rotor Speed in RPM) X (Number of Poles) / 120
This shows that the output frequency of the alternator depends directly on the rotor shaft speed if the pole number is constant. For example, a 2-pole alternator would need to spin at 3000 RPM to produce 50 Hz electricity while a 10-pole one would require only 600 RPM for the same frequency.
Therefore, higher pole numbers allow lower operating speeds to obtain the same electrical frequency. But more poles mean greater magnetic leakage between poles. Hence an optimal pole number is chosen based on speed, frequency and output power considerations.
Three-Phase Power Generation in Alternators
In order to generate three-phase AC power, three sets of armature windings spaced 120 electrical degrees apart are incorporated in the stator core. The phase windings are usually connected in star (wye) or delta configuration.
As the multi-pole rotor spins inside the stator creating a rotating magnetic field, three symmetric sine waves of voltage are induced in the three phase windings with 120 degree phase difference constituting balanced three-phase power.
Speed and Pole Considerations for Mechanical Integrity
For a 4-pole alternator operating at 1800 RPM, the output frequency works out to be 60 Hz as per the fundamental formula. However, spinning the rotor at such a high speed imposes large centrifugal forces that can compromise mechanical reliability over longer operation.
Hence alternators aimed at direct-drive power generation often have higher number of poles, typically 8 to 24. This allows achieving 60 Hz output at lower rotor speeds of 900 to 1200 RPM, ensuring better mechanical integrity.
Voltage Regulation Using Automatic Voltage Regulators
The output voltage of an alternator varies with changing electrical load conditions. Thus voltage control is important to maintain consistent terminal voltage.
Automatic voltage regulators (AVRs) sense alternator terminal voltage and automatically adjust DC excitation to the rotor field winding to regulate voltage. Specifically, if voltage drops below set point, the AVR increases rotor field current to boost output voltage. If terminal voltage is high, AVR lowers excitation thereby lowering terminal voltage.
This allows alternators to maintain steady AC voltage regulation in the face of fluctuating loads for high quality power.
FAQ
Q: What are the main components of an alternator?
A: The two main components are the rotor, which produces a rotating magnetic flux, and the armature coils, which are stationary and generate electricity from the rotating magnetic flux.
Q: How is the generated voltage regulated in alternators?
A: Automatic voltage regulators control the field current to the rotor to increase or decrease the magnetic field strength and terminal voltage as needed.
Q: Why are more poles used in alternator rotors?
A: Using more poles, such as 10-20, allows lower rotor speeds while still generating standard frequencies like 60 Hz, reducing mechanical stress.
Q: What is self-excitation in alternators?
A: Self-excited alternators have a small DC generator fitted on the same shaft to power the rotor field coils instead of an external DC source.