Unit 4: Synchronous Machine
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Topics Covered:
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Synchronous Machine
Synchronous Machine constitutes of both synchronous motors as well as synchronous generators. An AC system has some advantages over a DC system. Therefore, the AC system is exclusively used for the generation, transmission, and distribution of electric power. The machine which converts mechanical power into AC electrical power is called a Synchronous Generator or Alternator. However, if the same machine can be operated as a motor is known as Synchronous Motor.
Synchronous machine is an AC machine whose satisfactory operation depends upon the maintenance of the following relationship.
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Where,
When connected to an electric power system, a synchronous machine always maintains the above relationship shown in equation (1).
If the synchronous machine working as a motor fails to maintain the average speed (Ns) the machine will not develop sufficient torque to maintain its rotation and will stop. Then the motor is said to be Pulled Out of Step.
In case, when the synchronous machine is operating as a generator, it has to run at a fixed speed called Synchronous speed to generate the power at a particular frequency. As all the appliances or machines are designed to operate at this frequency. In some countries, the value of the frequency is 50 hertz.
Basic Principles of Synchronous Machine
A synchronous machine is just an electromechanical transducer that converts mechanical energy into electrical energy or vice versa. The fundamental phenomenon or law which makes these conversions possible is known as the Law of Electromagnetic Induction and Law of interaction.
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The detailed description is explained below.
Law of Electro-Magnetic Induction
This law is also called Faraday’s First Law of Electromagnetic Induction. This law relates to the production of emf, i.e.; emf is induced in a conductor whenever it cuts across the magnetic field as shown below:
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Law of Interaction
This law relates to the production of force or torque, i.e., whenever a current-carrying conductor is placed in the magnetic field, by the interaction of the magnetic field produced by the current-carrying conductor and the main field, force is exerted on the conductor producing torque. The figure is shown below:
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Three-Phase Synchronous Machine
In a small synchronous machine, the fielding winding is placed on the stator, and the armature winding is placed on the rotor whereas for the large synchronous machine the field winding is placed on the rotor, and the armature winding is placed on the stator.
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Construction
Construction of a Synchronous Machine, i.e. alternator or motor consists of two main parts, namely the stator and the rotor. The stator is the stationary part of the machine. It carries the armature winding in which the voltage is generated. The output of the machine is taken from the stator. The rotor is the rotating part of the machine. The rotor produces the main field flux.
he important parts of the Synchronous Machine are given below:
Stator Construction
The stationary part of the machine is called Stator. It includes various parts like stator frame, stator core, stator windings, and cooling arrangement. They are explained below in detail.
Stator Frame
It is the outer body of the machine made of cast iron, and it protects the inner parts of the machine.
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Stator Core
The stator core is made of silicon steel material. It is made from a number of stamps that are insulated from each other. Its function is to provide an easy path for the magnetic lines of force and accommodate the stator winding.
Stator Winding
Slots are cut on the inner periphery of the stator core in which 3 phase or 1 phase winding is placed. Enameled copper is used as a winding material. The winding is star-connected. The winding of each phase is distributed over several slots. When the current flows in a distributed winding it produces an essentially sinusoidal space distribution of EMF.
Rotor Construction
The rotating part of the machine is called Rotor. There are two types of rotor construction, namely the salient pole type and the cylindrical rotor type.
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Salient Pole Rotor
The term salient means projecting. Thus, a salient pole rotor consists of poles projecting out from the surface of the rotor core. The end view of a typical 6 pole salient pole rotor is shown below in the figure:
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Since the rotor is subjected to changing magnetic fields, it is made of steel laminations to reduce eddy current losses. Poles of identical dimensions are assembled by stacking laminations to the required length. A salient pole synchronous machine has a non-uniform air gap. The air gap is minimized under the pole centers and it is maximum in between the poles.
They are constructed for medium and low speeds as they have a large number of poles. A salient pole generator has a large diameter. The salient pole rotor has the following important parts.
Spider: It is made of cast iron to provide an easy path for magnetic flux. It is keyed to the shaft and at the outer surface, pole core and pole shoe are keyed to it.
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Pole Core and Pole Shoe: It is made of laminated steel sheet material. The Pole core provides the least reluctance path for the magnetic field and the pole shoe distributes the field over the whole periphery uniformly to produce a sinusoidal wave.
Field Winding or Exciting Winding: It is wound on the former and then placed around the pole core. DC supply is given to it through slip rings. When direct current flows through the field winding, it produces the required magnetic field.
Damper Winding: At the outermost periphery, holes are provided in which copper bars are inserted and short-circuited at both sides by rings forming Damper winding.
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Non- Salient Pole Rotor or Cylindrical Rotor
In this type of rotor, there are no projected poles, but the poles are formed by the current flowing through the rotor exciting winding. Cylindrical rotors are made from solid forgings of high-grade nickel chrome-molybdenum steel. It has a comparatively small diameter and long axial length.
They are useful in high-speed machines. The cylindrical rotor type alternator has two or four poles on the rotor. Such a construction provides greater mechanical strength and permits more accurate dynamic balancing. The smooth rotor of the machine makes fewer windage losses and the operation is less noisy because of the uniform air gap.
The figure in the next slide shows the end view of the 2 poles and 4 pole cylindrical rotors.
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They are driven by steam or gas turbines. Cylindrical synchronous rotor synchronous generators are called turbo-alternators and turbo generators. The machines are built in a number of ratings from 10 MVA to over 1500 MVA. The biggest size used in India has a rating of 500 MVA installed in the super thermal power plant.
Non-salient pole-type rotors have the following parts. They are as follows:
Rotor Core: The rotor core is made of silicon steel stampings. It is placed on the shaft. At the outer periphery, slots are cut in which exciting coils are placed.
Rotor Winding or Exciting Winding: It is placed on the rotor slots, and the current is passed through the winding in such a way that the poles are formed according to the requirement.
Slip Rings: Slip rings provide DC supply to the rotor windings.
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Miscellaneous Parts
The miscellaneous parts are given below:
Brushes: Brushes are made of carbon, and they slip over the slip rings. A DC supply is given to the brushes. Current flows from the brushes to the slip rings and then to the exciting windings.
Bearings: Bearings are provided between the shaft and the outer stationary body to reduce the friction. They are made of high carbon steel.
Shaft: The shaft is made of mild steel. Mechanical power is taken or given to the machine through the shaft.
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Pitch Factor & Distribution Factor
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Before knowing about, winding factor, we should know about pitch factor and distribution factor, since winding factor is the product of pitch factor and distribution factor.
If we denote winding factor with Kw, pitch factor with Kp and distribution factor with Kd, we can write
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The pitch factor and distribution factor are explained below one by one.
Pitch Factor
In short pitched coil, the induced emf of two coil sides get vectorially added and give resultant emf of the loop. In short pitched coil, the phase angle between the induced emf of two opposite coil sides is less than 180o (electrical). But we know that, in full pitched coil, the phase angle between the induced emf of two coil sides is exactly 180o (electrical).
Hence, the resultant emf of a full pitched coil is just the arithmetic sum of the emfs induced on both sides of the loop. We well know that vector sum or phasor sum of two quantities is always less than their arithmetic sum. The pitch factor is the measure of resultant emf of a short-pitched coil in comparison with resultant emf of a full pitched coil.
Hence, it must be the ratio of phasor sum of induced emfs per coil to the arithmetic sum of induced emfs per coil. Therefore, it must be less than unity.
Let us assume that, a coil is short pitched by an angle α (electrical degree). Emf induced per coil side is E. The arithmetic sum of induced emfs is 2E. That means, 2E, is the induced voltage across the coil terminals, if the coil would have been full pitched.
Now, come to the short pitched coil. From the figure below it is clear that, resultant emf of the short pitched coil
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Distribution Factor
If all the coil sides of any one phase under one pole are bunched in one slot, the winding obtained is known as concentrated winding and the total emf induced is equal to the arithmetic sum of the emfs induced in all the coils of one phase under one pole.
But in practical cases, for obtaining smooth sinusoidal voltage waveform, armature winding of alternator is not concentrated but distributed among the different slots to form polar groups under each pole. In distributed winding, coil sides per phase are displaced from each other by an angle equal to the angular displacement of the adjacent slots. Hence, the induced emf per coil side is not an angle equal to the angular displacement of the slots.
So, the resultant emf of the winding is the phasor sum of the induced emf per coil side. As it is phasor sum, must be less than the arithmetic sum of these induced emfs.
Resultant emf would be an arithmetic sum if the winding would have been a concentrated one.
As per definition, distribution factor is a measure of resultant emf of a distributed winding in compared to a concentrated winding.
We express it as the ratio of the phasor sum of the emfs induced in all the coils distributed in some slots under one pole to the arithmetic sum of the emfs induced. Distribution factor is,
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As pitch factor, distribution factor is also always less than unity.
Let the number of slots per pole is n.
The number of slots per pole per phase is m.
Induced emf per coil side is Ec.
Angular displacement between the slots,
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Induced Emf Equations
EMF Equation of a Synchronous Generator
The generator which runs at a synchronous speed is known as the synchronous generator. The synchronous generator converts the mechanical power into electrical energy for the grid. The Derivation of EMF Equation of a synchronous generator is given below.
Let,
Flux cut by each conductor during one revolution is given as Pϕ Weber. Time taken to complete one revolution is given by 60/N sec
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The average EMF equation is derived with the following assumptions given below.
Root mean square (R.M.S) value of the EMF induced per phase is given by the equation shown below:
Eph = Average value x form factor
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Equivalent Circuit
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Phasor Diagrams
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Power Relations
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Power Angle Curve
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OCC & SCC Characteristics
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Salient Pole & Cylindrical Rotor Machines & Phasors
Phasor Diagram of Salient Pole machine
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Effect of Excitation on Synchronous Generator
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V Curves
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Power Factor Correction
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Parallel Operation Of Synchronous Generator
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