AC Generators

THREE PHASE SYNCHRONOUS GENERATORS

Introduction:

An electrical machine, which converts mechanical energy into electrical energy of alternating current in nature, is called an ALTERNATOR or AC GENERATOR.

It is also called as SYNCHRONOUS GENERATOR as its operation is in synchronization with other generators or other AC sources when it is operated along with them.

Principle of operation:

• Synchronous Generator (Alternator) operates on the fundamental principle of Faraday Laws of electromagnetic induction, i.e. whenever the magnetic flux linking the armature conductor changes, an Electro Motive Force (EMF) is induced in the conductor.
• The direction of induced E.M.F. is given by Fleming Right-hand rule.
• When there is a relative motion between the conductor and magnetic field, the induced EMF is called as dynamically induced EMF.
• In alternators, there are two types of construction:
  i) Stationary field winding and rotating armature winding-like DC generators (only for small capacity alternators of few KW rating)
  ii) Stationary armature winding and rotating field winding (Suitable for MW size Synchronous Generators)
• The field windings of alternators require direct current for excitation.
• Excitation is supplied by a DC generator called an exciter which is mechanically coupled with the rotor shaft.
• The DC exciter supplies required power for the rotating field winding to produce magnetic field.
• As the prime mover rotates, the rotor of an alternator also rotates and the stator conductors being stationary are cut by magnetic flux of rotor poles, hence an E.M.F. is induced in the stator conductors.
• As the rotor magnetic poles are alternatively North and South, they induce an alternating E.M.F in the stator conductors.
• The frequency of alternating E.M.F. depends on the number of North and South poles moving past the conductors in one second.

Constructional features and types of 3 phase Synchronous generators:

• Compared to the DC generators, the major differences in construction of Synchronous Generators of MW size are:
• a) In a DC generator the armature winding rotates and the field system is stationary whereas in an Alternator of MW size, the armature winding is mounted on a stationary frame and the field winding on a rotating frame.
• b) The Stationary armature of an alternator is connected directly to load which receives AC supply and the rotating field winding is connected to DC supply using two low capacity slip rings. This makes the construction of alternator simpler.

Advantages of having the stationary armature and rotating field:

• Insulation of stationary armature conductors working at high voltage is easier.
• Tapping of electrical energy from a stationary armature is simpler.
• The use of slip rings and brushes are eliminated as the load is directly connected to the alternator terminals
• The machine can operate at higher speeds enabling a larger output from the alternator.

Construction of Alternator (Synchronous generator):

• Basically an alternator consist of two parts: a) Stator b) Rotor

• a) Stator:
• It consists of stator frame, stator core and windings.
  • i) Stator Frame: It is a cast iron or welded steel protective frame and gives support to the entire machine assembly.
• In small machines it is made of a single piece of cast iron.
• In large sized machines, the frame is fabricated by sections of cast iron sheet steel welded together to form a cylindrical drum.
  • ii) Stator Core: It is made of special magnetic iron or steel alloy laminations.
• They are laminated to minimize the core losses.
• These laminations are insulated from one another and pressed together to form the core.
• Slots are provided on its inner periphery to house the stator conductors.
• Slots provided on the stator are of three types: Wide Open, Semi-Closed and closed are shown in Figure.
• The wide open type slots are more commonly used because the defective coils can be easily removed and replaced.
• The laminations also have openings which make axial and radial ventilations for the purpose of cooling.

• (iii) Stator Windings (armature windings): These are insulated copper conductors housed in stator slots in some specific manner of inter connections.
• b) Rotor:
• It is the rotating part, with North and South poles attached to it.
• Poles carry field windings which are supplied with direct current through two slip rings and brushes.
• Rotors are of two types:
  i) Salient Pole Type
  ii) Smooth Cylindrical or Non-salient Pole Type

i) Salient Pole Type:

• This type of rotor is like a magnetic fly wheel made of cast iron or steel and a number of alternate North and South poles are bolted to it as shown in Figure.
• The Salient or projecting poles are made of thick steel laminations, riveted together and fixed to the rotor poles.
• The ends of the field windings are connected to the DC supply through slip rings carrying brushes.
• Such rotors have large diameter and small axial length.

• Advantages:
  – a) Less expensive
  – b) It provides sufficient space for field coils
• Disadvantages:
  – a) If the salient poles are driven at high speed, it will cause excessive windage losses and would tend to produce noise.
  – b) As the rotor is not robust in construction, it cannot withstand mechanical stress if driven at high speed.
• Application: Used for low and medium speed alternators
• Examples: Hydro Electric Power Plants, Diesel Power Plants and Gas Turbine Power Plants.

ii) Smooth Cylindrical or Non-salient Pole Type:

• These rotors are cylindrical in construction and are made from solid forged steel alloy having a number of slots on its outer periphery at regular intervals for accommodating field coils.
• Field coils are connected to a DC supply by means of slip rings and brushes for excitation purpose.
• The regions forming the central polar areas are left unslotted.
• The field coils are so arranged around these polar areas that flux density is maximum on the central polar areas.
• These types of rotors are characterized by small diameter and very long axial length.

• Advantages:
  – a) Gives better balance
  – b) Noiseless operation
  – c) Less windage loss
  – d) Better E.M.F. waveform
• Application: Used in very high speed turbo alternators.
• Example: Steam turbines

Frequency of the induced E.M.F. or Relationship between Speed and Frequency:

• Consider an alternator whose rotor is being driven at a constant speed of N r.p.m.
• Let P be Number of poles, f be frequency of induced E.M.F.
• In one complete revolution of the rotor, each of the North and South poles move past all the stator conductors.
• When one pair of North and South poles moves past the armature conductor, the induced E.M.F. undergoes one full cycle.
• Therefore in a P pole machine, in one complete revolution of the field system, the induced E.M.F. in the armature conductors will complete P divided by 2 cycles of waveform.

• In order to keep the frequency constant, the speed N must remain unchanged.
• Synchronous generator connected to grid runs at a constant speed known as synchronous speed in steady state.

E.M.F. Equation:

• Consider a 3-Phase alternator with P-Poles driven at a constant speed of N r.p.m.

• Winding factors:(Kw=KpKd)
• For the following advantages windings are always short pitched by 1 or 2 slots
  – The primary advantage of short-pitch coils is we achieve saving of copper.
  – Short pitched winding reduces the MMF harmonics produced by the armature winding .
  – Short pitched winding reduces the EMF harmonics induced in the windings, without reducing the magnitude of the fundamental EMF wave to a great extent.
  Pitch factor Kp is defined as the ratio of emf induced in a short pitched coil to emf induced in a full pitched coil.

• Concentrated windings:
  Concentrated windings in which all conductors of a given phase per pole are concentrated in a single slot, are not commercially used because they have the following disadvantages:

1. They fail to use the entire inner periphery of the stator iron efficiently
2. They make it necessary to use extremely deep slots where the windings are concentrated. This causes an increase in the mmf required to setup the desired airgap flux.
3. Deep slots also increase the armature leakage flux and the armature reactance.
4. They result in low copper-to-iron ratios by not using the armature iron completely.
5. They fail to reduce harmonics as effectively as distributed windings.

• Distribution factor Kd is the ratio of emf induced in a distributed winding to the emf induced in a concentrated winding