Synchronous Generator – Construction and Working Principle

Through the process of Electromagnetic Induction, Synchronous Generators Transform Mechanical power into AC electric power.

Alternators or AC Generators are other names for Synchronous Generators. Since it Generates AC power, it is referred to as a “Alternator.” Because it needs to be driven at Synchronous speed in order to Generate AC power at the desired Frequency, this Generator is known as a Synchronous Generator.

A Synchronous Generator can be either Single-phase or Poly-phase (Generally 3phase).

Construction of Synchronous Generator or Alternator

As Alternator Consists of two main parts viz.

• Stator – The stator is the Stationary part of the Alternator. It carries the Armature winding in which the voltage is generated. The output of the alternator is taken form the stator.
• Rotor – The rotor is the Rotating part of the Alternator. The rotor produces the main field flux.

Stator Construction of Alternator

The stator of the Alternator includes several parts, viz. the frame, stator core, stator or Armature Windings, and cooling Arrangement.

• For small machines, the stator frame may be made of cast iron, and for larger machines, it may be made of welded steel.
• High-grade steel laminations with a silicon content are used to Assemble the stator core. The hysteresis and eddy-current losses in the stator core are decreased by these silicon steel laminations.
• The stator core’s inner perimeter is where the slots are cut. These slots are filled with a 3-phase armature winding.
• The Alternator’s Armature winding is star connected. Each phase’s winding is split among a number of slots. An essential sinusoidal space distribution of EMF is created when current passes through the distributed armature winding.

Rotor Construction of Alternator

The field winding of the alternator is carried by the rotor and is powered by a separate DC source through two slip rings (also called exciter). Typically, the exciter is a tiny DC shunt generator mounted on the alternator’s shaft.

For the alternator, there are two types of rotor constructions are used viz. the salient-pole type and the cylindrical rotor type.

Salient Pole Rotor

Projecting is what the word salient means. As a result, a salient pole rotor consists of poles that extend from the rotor core’s surface. According to the figure, the entire setup is attached to the alternator’s shaft. When the field winding is energized by the DC exciter, the adjacent poles have opposite polarities because the individual field pole windings are connected in series.

The salient pole type rotor is used in the low and medium speed (from 120 to 400 RPM) alternators such as those driven by the diesel engines or water turbines because of the following reasons −

• The construction of salient pole type rotor cannot be made strong enough to withstand the mechanical stresses to which they may be subjected at higher speed.
• If the salient field pole type rotor is driven at high speed, then it would cause windage loss and would tend to produce noise.

The alternators’ low speed rotors have a large diameter to accommodate the poles’ space requirements. Salient pole type rotors have a large diameter and a short axial length as a result.

Cylindrical Rotor

High-grade Nickel-chrome-molybdenum steel solid Forgings are used to create the Cylindrical rotors.

• The construction of the Cylindrical rotor is such that there are no-physical poles to be seen as in the salient pole rotor.
• Slots parallel to the rotor shaft are cut at regular intervals in about two-thirds of the cylindrical rotor’s outer perimeter.
• The field windings are placed in these slots and is excited by DC supply. The field winding is of distributed type.
• The Unslotted portion of the rotor forms the pole faces.
• The Cylindrical rotor’s figure makes it obvious that the formed poles are non-salient, or that they don’t protrude from the rotor surface.

The Cylindrical type rotor construction is used in the high-speed (1500 to 3000 RPM) Alternators such as those driven by steam turbines because of the following reasons −

• The construction of Cylindrical type rotors offers greater mechanical strength and enables more precise dynamic balancing.
• It gives noiseless operation at high speeds because of the uniform air gap.
• Since the flux distribution around the rotor’s perimeter is almost sine-wavelike, a better EMF waveform is produced.

A cylindrical rotor alternator has a large axial length and a relatively small diameter. Turbo-alternators and turbo-generators are two names for cylindrical rotor alternators. The installation of an alternator with a cylindrical rotor is always horizontal.

Working Principle and Operation of Alternator

An alternator or synchronous generator works on the principle of electromagnetic induction, i.e., An EMF is induced in a conductor when the flux connecting it changes. When the alternator’s armature winding is exposed to a rotating magnetic field, the winding will produce voltage.

The alternate N and S poles develop on the rotor when the alternator’s rotor field winding is powered by a DC exciter. The magnetic field of the rotor poles cuts the armature conductors placed on the stator when the rotor is rotated counterclockwise by a prime mover. As a result, electromagnetic induction induces the EMF in the armature conductors. Because the rotor’s N and S poles pass the armature conductors alternately, the induced EMF is alternating.

The direction of the generated EMF can be determined by the Fleming’s right rule and the frequency of it is given by,

f=NsP/120

Where,

• Ns is the synchronous speed in RP
• P is the number of rotor poles.

The rotor’s Rotational speed and the DC field Excitation current both affect how much voltage is Generated. When the winding is Balanced, the voltage Generated in each phase is the same but differs Electrically by 120° in phase.