Download DC GENERATOR and more Slides Construction in PDF only on Docsity!
DC GENERATOR
INTRODUCTION TO GENERATORS:
Electrical generators are standalone machines that provide electricity when power from the local grid is unavailable. These generators supply backup power to businesses and homes during power outages. Generators do not create electrical energy, but they convert mechanical or chemical energy into electrical energy. Based on the output, generators are classified into two types as AC generators and DC generators. DC GENERATOR: A DC generator is an electrical machine that converts mechanical energy into electricity. When a conductor cuts magnetic flux, an electromotive force (EMF) is produced in them based on the principle of electromagnetic induction The EMF so produced is called dynamically induced EMF as it is produced to rotation of conductors. The electromotive force can cause a flow of current when the conductor circuit is closed. The direction of the EMF can be obtained by Flemming’s Right hand rule. CONSTRUCTION: Cut-section of a DC Machine Front View of DC Machine
A DC machine has mainly four components.
- Field magnet system
- Armature
- Commutator
- Brush & Brush Gear Field Magnet System: The Field Magnet System is the stationary or fixed part of the machine. It produces the main magnetic flux. The magnetic field system consists of Mainframe or Yoke, Pole core and Pole shoes and Field or Exciting coils. Field magnet System of DC machine Magnetic Frame and Yoke: The outer hollow cylindrical frame to which main poles and inter-poles are fixed and by means of which the machine is fixed to the foundation is known as Yoke. It is made of cast steel or rolled steel for the large machines and for the smaller size machine the yoke is generally made of cast iron. The two main purposes of the yoke are as follows:- It supports the pole cores and provides mechanical protection to the inner parts of the machines. It provides a low reluctance path for the magnetic flux. Pole Core and Pole Shoes: The Pole Core and Pole Shoes are fixed to the magnetic frame or yoke by bolts. Since the poles, project inwards they are called salient poles. Each pole core has a curved surface. Usually, the pole core and shoes are made of thin cast steel or wrought iron laminations. The poles are laminated to reduce the Eddy Current loss. The shape of Pole shoe is referred to as cruciform shape. The poles core serves the following purposes given below:
The armature core of DC Generator is cylindrical in shape and keyed to the rotating shaft. At the outer periphery of the armature has grooves or slots which accommodate the armature winding. The armature core of a DC generator or machine serves the following purposes. It houses the conductors in the slots. It provides an easy path for the magnetic flux. As the armature is a rotating part of the DC Generator or machine, the reversal of flux takes place in the core, hence hysteresis losses are produced. The silicon steel material is used for the construction of the core to reduce the hysteresis losses. The rotating armature cuts the magnetic field, due to which an e.m.f is induced in it. This e.m.f circulates the eddy current which results in Eddy Current loss. Thus to reduce the loss the armature core is laminated with a stamping of about 0.35 to 0.55 mm thickness. Each lamination is insulated from the other by a coating of varnish. Armature Winding: The insulated conductors are placed in the slots of the armature core. This arrangement of conductors is called Armature Winding. The armature winding is the heart of the DC Machine. Armature winding is a place where the conversion of power takes place. In the case of a DC Generator here, mechanical power is converted into electrical power. Commutator: Commutator of DC machine The commutator, which rotates with the armature, is cylindrical in shape and is made from a number of wedge-shaped hard drawn copper bars or segments insulated from each other and from the shaft. The segments form a ring around the shaft of the armature. Each commutator segment is connected to the ends of the armature coils. It connects the rotating armature conductors to the stationary external circuit through brushes. It converts the induced alternating current in the armature conductor into the unidirectional current in the external load circuit in DC Generator action, whereas it converts the alternating torque into unidirectional (continuous) torque produced in the armature in motor action.
Brushes & Brush Gear: Brush of DC Machines Carbon brushes are placed or mounted on the commutator and with the help of two or more carbon brushes, current is collected from the armature winding. Each brush is supported in a metal box called a brush box or brush holder. The brushes are pressed upon the commutator and form the connecting link between the armature winding and the external circuit. They are usually made of high-grade carbon because carbon is conducting material and at the same time in powdered form provides a lubricating effect on the commutator surface. Bearings: The ball or roller bearings are fitted in the end housings. The function of the bearings is to reduce friction between the rotating and stationary parts of the machine. Mostly high carbon steel is used for the construction of bearings as it is a very hard material. Shaft: The shaft is made of mild steel with a maximum breaking strength. The shaft is used to transfer mechanical power from or to the machine. The rotating parts like armature core, commutator, cooling fans, etc. are keyed to the shaft. Armature Winding:
Terms Associated with Conductors: Conductor (Z): The length of a wire lying in a magnetic field in which EMF is induced is called conductor. Turn (T): When two conductors are connected in series, so that the EMF induced in them help each other is known as a turn. Coil: Two coils along with their end connections constitute one coil. A coil may be single turn or multi-turn. Single turn coils have two conductors but multi-turn conductors have many conductors per coil side. Winding: Number of coils arranged in coil group is called winding. Pole pitch: No. of conductors per pole Coil Pitch: it is the distance measured in terms of armature slots between two sides of a coil. Front pitch (Yf): Distance in terms of no. of armature conductors between the second conductor of one coil and the first conductor of the next coil which are connected to the same commutator segment.
Back pitch (Yb): Distance in terms of no. of armature conductors between the first and last conductor of the coil Resultant pitch (YR): Distance in terms of no. of armature conductors between the start of one coil and start of the next coil to which it is connected. Commutator pitch (YC): Distance in terms of no. of commutator segments between the segments to which two ends of a coil are connected. WORKING PRINCIPLE : Flemming’s Right Hand Rule: “Hold the right hand fore-finger, middle finger and the thumb at right angles to each other. If the forefinger represents the direction of the magnetic field, the thumb points in the direction of motion or applied force, then the middle finger points in the direction of the induced current.” Principle of Operation:
When the coil is rotated in clockwise direction, the coil sides begin to cut the field first slowly then at gradually increasing rate. So, the EMF gradually increases and becomes maximum when the loop rotates through 90^0. Direction of induced EMF is B-A & C-D. In the next quarter cycle, between (90^0 - 1800 ), the rate at which conductors cut the flux gradually decreases. So, the EMF reduces gradually and becomes zero when the loop again becomes parallel to the magnetic field. In the third quarter of revolution, i.e. between 1800 - 2700 , the rate at which conductors cut across the magnetic field increases and the EMF induced also becomes maximum gradually. But the direction of induced EMF is now A-B & D-C. In the fourth quarter of revolution, i.e. between 270^0 - 3600 , the induced EMF decreases as the coil moves and becomes zero when it reaches 360^0. This cycle is repeated in each revolution of armature. The EMF generated is of pulsating nature and hence called as Alternating EMF.
The current induced in the coil is collected and conveyed to the external load circuit through slip rings. To obtain unidirectional current, split rings or commutators are used. In the first half cycle, current flows along ‘B-A-M-L-D-C-B’. ‘a’ acts as negative pole and ‘b’ acts as positive pole. In the next half cycle, position of segments ‘a’ and ‘b’ are also reversed. So, M and L are again in contact with negative and positive segment respectively. So, the current collected or the voltage appearing across the brushes is unidirectional. This EMF has ripples. To have a constant waveform, large number of commutator segments are used. The voltage generated by one single coil is small. Hence, several turns in series are used.
Therefore, the average induced EMF across each parallel path or the armature terminals is given by:
Eg =
Pɸ𝑁 60
x
𝑍 𝐴
Pɸ𝑁𝑍 60 𝐴 Volts If we take, n= 𝑁 60 i.e., Number of rotations per minute, then the EMF equation becomes:
Eg =
Pɸ𝑛 𝐴 Volts In case of lap winding, A=P, So;
Eg =
ɸ𝑁𝑍 60 Volts In case of wave winding, A=2, So,
Eg =
Pɸ𝑁𝑍 120 Volts In the above equation, we can see that, P, Z, A are constants, Hence, Eg α Nɸ Classification of DC Generators: Depending on the manner in which the field winding gets supply, DC generators are of the following types.
Separately Excited DC Generator A DC generator whose field winding or coil is energized by a separate or external DC source is called a separately excited DC Generator. Here field current is independent of armature current. Here, If=Field current Ia=Armature current IL=Load current V= Terminal voltage Eg= EMF generated Ra= Armature resistance Rf=Field Resistance Ia= IL V= Eg- Ia Ra Or V= Eg- Ia Ra-Brush Drop Electrical power developed in the armature= EgIa Watt Electrical power delivered to the load= VIL Watt Self Excited DC Generator Self-excited DC Generator is a device, in which the current to the field winding is supplied by the generator itself. In self-excited DC generator, the field coils may be connected in parallel with the armature in the series, or it may be connected partly in series and partly in parallel with the armature windings.
Here, Rsh= shunt field winding resistance Ish= Field winding current Ish= 𝑉 Rsh Ia= IL+ Ish V= Eg- Ia Ra Or V= Eg- Ia Ra-Brush Drop Electrical power developed in the armature= EgIa Watt Electrical power delivered to the load= VIL Watt Compound Wound Generator In a compound-wound generator, there are two field windings. One is connected in series, and another is connected in parallel with the armature windings. There are two types of compound- wound generator. If the magnetic flux produced by the series winding assists the flux produced by the shunt winding, then the machine is said to be cumulative compounded. If the series field flux opposes the shunt field flux, then the machine is called the differentially compounded. It is connected in two ways. One is a long shunt compound generator, and another is a short shunt compound generator. If the shunt field is connected in parallel with the armature alone then the machine is called the short compound generator. In long shunt compound generator, the shunt field is connected in series with the armature. Short Shunt Compound Wound Generator In a Short Shunt Compound Wound Generator, the shunt field winding is connected in parallel with the armature winding only.
Here, Ise= IL Ish= 𝑉+𝐼𝐿Rse Rsh = Eg− Ia Ra Rsh Ia= IL+ Ish V= Eg- Ia Ra-ILRse Or, V= Eg- Ia Ra-ILRse-Brush Drop Electrical power developed in the armature= EgIa Watt Electrical power delivered to the load= VIL Watt Long Shunt Compound Wound Generator In a long shunt-wound generator, the shunt field winding is parallel with both armature and series field winding.
turns are required and the increase in flux is negligible. This point is called saturation. Hence relationship becomes non-linear. This curve giving relationship between flux per pole and field ampere turns per pole is called magnetization curve. Since the generated e.m.f in DC machine depends on flux and speed, the generated e.m.f is directly proportional to flux per pole at constant speed. If a curve is drawn between the generated e.m.f on no load and field current when the machine is running at a constant speed, the curve obtained is similar to saturation curve. These curves are called magnetic characteristics or open circuit characteristics. This curve does not start from zero due to residual magnetism. Characteristics of separately excited DC generators: As there is no connection between the field and armature windings, field current or exciting current is independent of load current. If a curve is drawn between the flux per pole and load current keeping field current constant, it is a straight line. However, due to armature reaction, the actual flux is less than ideal flux, so is the actual EMF generated in the armature. From the generated EMF some voltage is dropped in the armature winding resistance which is directly proportional to the load current.
Characteristics of DC series generators: Internal characteristics: the load current, field current and armature current are same in DC series generator. The internal characteristic lies below the open circuit characteristics due to demagnetizing effect of armature reaction. External characteristics: the terminal voltage of the generator is obtained by subtracting the voltage drop in armature and series field winding. i.e. V=Eg-Ia(Ra+Rse) In the initial portion of the curve, the relationship between the voltage and current is linear or directly proportional due to simultaneous increase in flux. However, when saturation approaches, the increase in flux is less as compared to Ohmic drop. So, in the later stage of the curve, Ohmic drop dominates and hence terminal voltage reduces or shows a drooping characteristics. The maximum value of load resistance for which the generator will be able to excite is called critical load resistance. So, it is clear from the external characteristics that, the terminal voltage first increases and then reaches maximum and reduces finally. If the generator is operated in the drooping portion of the characteristics, it gives approximately constant current irrespective of the load resistance.
