Unit 1, DC Generator, Unit 2 DC Motors, Unit 3 Losses and Efficiency and Unit 4 Testing of DC Machines
Unit 5 to 8 Synchronous Machines, Voltage Regulation and Synchronous Motor
Notes Credit – ELearning
Unit 1 Sample Notes
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CLASSIFICATION OF DC MACHINES
METHODS OF EXCITATION: depending on the type of excitation of field winding, there are two basic types of DC machine.
1. Separately excited machine: In this type of machines the field flux is produced by connecting the field winding to an external source.
2. Self excited machine: The field flux is produced by connecting the field winding with the armature in this type. A self excited machine requires residual magnetism for operation.
Depending on the type of field winding connection DC machines can be further classified as:
1. Shunt machine: The field winding consisting of large number of turns of thin wire is usually excited in parallel with armature circuit and hence the nameshunt field winding. This winding will be having more resistance and hence carries less current.
2. Series machine: The field winding has a few turns of thick wire and is connected in series with armature.
3. Compound machine: Compound wound machine comprises of both series and shunt windings and can be either short shunt or long shunt, cumulative, differential or flat compounded.
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CHARACTERISTICS OF D.C GENERATORS
The three important characteristics of DC generator are
1. Open circuit characteristic or Magnetization curve or No – load saturation Curve
Open circuit characteristic is the relation between the No-load generated emf in the armature, and the field exciting current at a fixed speed. It is the magnetization curve for the material of electromagnets.
It is same for separately excited or self excited machine.
2. Internal or total characteristic
This characteristic curve gives the relation between the emf generated in the armature and the armature current.
3. External characteristic
This gives the relation between the terminal voltage and the load current. This characteristic takes into account the voltage drop due to armature circuit resistance and the effect of armature reaction.
This characteristic is of importance in judging the suitability of generator for a particular purpose.
This characteristic is also referred to as performance characteristics or voltage regulating curve.
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Synchronous Machines Sample Notes
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Synchronous machines are principally used as alternating current generators. They supply the electric power used by all sectors of modern society. Synchronous machine is an important electromechanical energy converter. Synchronous generators usually operate in parallel forming a large power system supplying electrical power to consumers or loads. For these applications the synchronous generators are built in large units, their rating ranging form tens to hundreds of Megawatts. These synchronous machines can also be run as synchronous motors.
Synchronous machines are AC machines that have a field circuit supplied by an external DC source.
Synchronous machines are having two major parts namely stationary part stator and a rotating field system called rotor.
In a synchronous generator, a DC current is applied to the rotor winding producing a rotor magnetic field. The rotor is then driven by external means producing a rotating magnetic field, which induces a 3-phase voltage within the stator winding.
Field windings are the windings producing the main magnetic field (rotor windings for synchronous machines); armature windings are the windings where the main voltage is induced (stator windings for synchronous machines).
Types of synchronous machines
According to the arrangement of armature and field winding, the synchronous machines are classified as rotating armature type or rotating field type.
In rotating armature type the armature winding is on the rotor and the field winding is on the stator.
The generated emf or current is brought to the load via the slip rings. These type of generators are built only in small units.
In case of rotating field type generators field windings are on the rotor and the armature windings are on the stator. Here the field current is supplied through a pair of slip rings and the induced emf or current is supplied to the load via the stationary terminals.
Based on the type of the prime movers employed the synchronous generators are classified as
1. Hydrogenerators : The generators which are driven by hydraulic turbines are called hydrogenerators. These are run at lower speeds less than 1000 rpm.
2. Turbogenerators: These are the generators driven by steam turbines. These generators are run at very high speed of 1500rpm or above.
3. Engine driven Generators: These are driven by IC engines. These are run at aspeed less than 1500 rpm.
Hence the prime movers for the synchronous generators are Hydraulic turbines, Steam turbines or IC engines.
Hydraulic Turbines: Pelton wheel Turbines: Water head 400 m and above
Francis turbines: Water heads up to 380 m
Keplan Turbines: Water heads up to 50 m
Steam turbines: The synchronous generators run by steam turbines are called turbogenerators or turbo alternators. Steam turbines are to be run at very high speed to get higher efficiency and hence these types of generators are run at higher speeds.
Diesel Engines: IC engines are used as prime movers for very small rated generators. Construction of synchronous machines
1. Salient pole Machines: These type of machines have salient pole or projecting poles with concentrated field windings. This type of construction is for the machines which are driven by hydraulic turbines or Diesel engines.
2. Nonsalient pole or Cylindrical rotor or Round rotor Machines: These machines are having cylindrical smooth rotor construction with distributed field winding in slots. This type of rotor construction is employed for the machine driven by steam turbines.
1. Construction of Hydro-generators: These types of machines are constructed based on the water head available and hence these machines are low speed machines. These machines are constructed based on the mechanical consideration. For the given frequency the low speed demands large number of poles and consequently large diameter. The machine should be so connected such that it permits the machine to be transported to the site. It is a normal to practice to design the rotor to withstand the centrifugal force and stress produced at twice the normal operating speed.
The stator is the outer stationary part of the machine, which consists of
• The outer cylindrical frame called yoke, which is made either of welded sheet steel, cast iron.
• The magnetic path, which comprises a set of slotted steel laminations called stator core pressed into the cylindrical space inside the outer frame. The magnetic path is laminated to reduce eddy currents, reducing losses and heating. CRGO laminations of 0.5 mm thickness are used to reduce the iron losses.
A set of insulated electrical windings are placed inside the slots of the laminated stator. The crosssectional area of these windings must be large enough for the power rating of the machine. For a 3- phase generator, 3 sets of windings are required, one for each phase connected in star. Fig. 1 shows one stator lamination of a synchronous generator. In case of generators where the diameter is too large stator lamination can not be punched in on circular piece. In such cases the laminations are punched in segments. A number of segments are assembled together to form one circular laminations. All the laminations are insulated from each other by a thin layer of varnish.
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