- Definition of Alternator: An alternator is defined as a type of AC generator that converts mechanical energy into electrical energy using a rotating field and stationary armature.
- Parts of an Alternator: An alternator consists of two main parts: a rotor (which rotates) and a stator (which is stationary).
- Construction of Alternator: The construction includes field poles on the rotor and armature conductors on the stator, which induce a three-phase voltage.
- Types of Rotors: There are two types of rotors used in alternators: salient pole type (for low-speed) and cylindrical rotor type (for high-speed).
- Salient and Cylindrical Rotors: Salient pole rotors have a large diameter and short length, suitable for low-speed operations, while cylindrical rotors are smooth, balanced, and used for high-speed operations.
A conventional large alternator has a stationary armature on the stator and a magnetic field system on the rotor. The rotor also includes its shaft, body, field winding and retaining parts. The stator supports an insulated three-phase winding in a laminated core. As the rotor field turns, it induces three phase voltages in the stator winding. A connected network can then receive three-phase electrical power. Generators in electrical power generating stations also need bearings, cooling, excitation, protection and terminal systems beyond the two electromagnetic members shown here.
An alternator is a synchronous AC generator. Its mechanical synchronous speed is Ns = 120f/P rpm, where f is electrical frequency in hertz and P is the rotor pole count. The prime mover and grid operating conditions hold this speed during normal power generation.
Unlike the elementary rotating-armature model and the usual commutator arrangement in a DC generator, a large alternator keeps its load winding stationary. Stationary high-voltage armature windings are easier to insulate, brace, cool and connect without carrying load current through moving contacts. The rotor carries the lower-power excitation field, either through slip rings or a brushless exciter. Rotor design must still withstand centrifugal, thermal and electromagnetic stresses.
Alternator rotors are commonly grouped by magnetic geometry:
- Salient pole type.
- Cylindrical rotor type.
Salient Pole Type
Salient means projecting. A salient-pole rotor has distinct poles attached to a rotor rim or spider. It is well suited to lower-speed machines that need many poles, so hydro-generators often have a large diameter and short axial length. Pole bodies and shoes may use laminated steel to control eddy-current loss, with construction chosen for mechanical and electromagnetic duty.
A grid-connected alternator can undergo rotor-angle oscillations after a load or system disturbance. Salient-pole machines may include damper bars in their pole faces. These bars provide damping torque during relative motion between the rotor and the stator’s rotating magnetic field. Damper design is not determined by generator power alone.
Damper windings use conductive bars in the pole faces, joined by short-circuiting end connections. In ideal balanced steady state, the fundamental stator field is stationary relative to the synchronous rotor and induces no fundamental-frequency damper current. Rotor-angle or speed oscillation creates relative motion, inducing bar current and a torque that opposes the oscillation. Harmonics and unbalance can also produce damper currents and heating.
Typical salient-pole construction has these features:
- Large diameter and comparatively short axial length for many-pole, lower-speed designs.
- Diagrams often show a pole arc near 2/3 of pole pitch, but the actual ratio is design-specific.
- Pole-face laminations can reduce eddy current loss from harmonics and transient fields.
- Use with lower-speed prime movers such as hydraulic turbines and many diesel engines; exact speed follows frequency and pole count rather than a universal 100-to-400 rpm range.
A salient-pole alternator driven by a water turbine is commonly called a hydro-generator. Its pole count is selected to match turbine speed and grid frequency.
Cylindrical Rotor Type
Cylindrical rotors suit high-speed turbine-driven alternators, often called turbogenerators. A forged steel rotor has a nearly smooth outer surface with axial slots for the field winding. The long, small-diameter shape and retained winding support high rotational speed. Available ratings vary by manufacturer, cooling system and application, so a generic MVA range is not a design limit.
At 50 Hz, a 2-pole cylindrical-rotor alternator has a synchronous speed of:
At the same frequency, a 4-pole design has a synchronous speed of:
The protected examples use f = 50 Hz. Other grid frequencies and pole counts produce different synchronous speeds through N = 120f/P.
A cylindrical rotor has no projecting pole bodies. Field-winding conductors lie in machined axial slots, while retaining systems secure the end windings against centrifugal force. Unslotted pole-centre regions help form the required rotor magnetic poles.
The slot and winding distribution shapes rotor magnetomotive force so air-gap flux density approaches the intended waveform. The smooth rotor contour supports good mechanical balance and quieter high-speed operation with lower windage than a projecting-pole geometry of similar duty. Final performance still depends on electromagnetic design, cooling and rotor dynamics.
