Synchronous Generator Basics

Basic Parts of a Synchronous Generator

• A synchronous generator consists of a rotor with a DC excited winding and a stator with a 3-phase winding in which the AC emf is generated.
• There are two types of rotors: salient or projecting poles and cylindrical or round rotors.

Types of Rotors

• Salient pole rotors have a 2-pole or 4-pole design and are typically used in low-power applications.
• Cylindrical rotors have a 2-pole design and are typically used in high-power applications.

Principle of Operation

• In a rotating armature alternator, the armature rotates through a stationary magnetic field, and the generated AC voltage is brought to the load through slip rings and brushes.
• In a revolving-field alternator, the stationary armature has a rotating magnetic field, and the generated voltage can be connected directly to the load without using slip rings and brushes.
• The majority of alternators use the revolving-field design.

Advantages of Stationary Armature and Rotating Field

• The stationary armature is easier to insulate, with voltages up to 30 kV or more.
• It is easier to collect current from the stationary parts of the generator.
• Slip ring voltages are lower.
• The simple construction allows for high-speed rotating DC fields, resulting in high efficiency.

Single-Phase Alternator

• A single-phase alternator has all armature conductors connected in series.
• The stator has two poles, and the winding is wound in two distinct pole groups, both poles being wound in the same direction around the stator frame.
• The rotor also consists of two pole groups, adjacent poles being of opposite polarity.

Two-Phase Alternator

• A two-phase alternator has two single-phase windings spaced to produce a 90-degree phase shift between the two phases.

Three-Phase Alternator

• A three-phase alternator has three single-phase windings spaced to produce a 120-degree phase shift between the three phases.
• The voltage waveforms generated across each phase are phase-displaced by 120 degrees from each other.

Three-Phase Stator Connection

• The three-phase stator can be connected in a star (wye) or delta configuration.
• In a star connection, the line voltage is 1.73 times the phase voltage, and the line and phase currents are equal.
• In a delta connection, the line voltage is equal to the phase voltage, but the line current is 1.73 times the phase current.

Frequency of AC Voltage

• The frequency of the AC voltage generated by an alternator depends on the number of poles and the speed of the rotor.
• The frequency can be calculated using the formula: f = (nrp)/120, where nr is the speed of the rotor in rpm, p is the number of poles, and f is the electrical line frequency.

Voltage Regulation

• The output voltage of an alternator varies with the load due to resistive and inductive voltage drops.
• The voltage regulation is the change in voltage from no-load to full-load.
• A constant output voltage can be maintained by varying the field strength as required by the load.

Comparison with DC Generator

• DC generators do not have a synchronous speed and operate at whatever speed is needed to obtain the desired output voltage.
• Synchronous generators rotate and generate EMF at the same rotor frequency, whereas DC generators generate DC voltage with no frequency.
• Structurally and mechanically, synchronous generators are built with the field windings on the rotor shaft and the armature winding built on the stator, whereas DC generators are built with the armature on the rotor shaft connected to the load through a commutator and the field windings in the stator.