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 Smitarani Sahoo, CVRGU, BBSR A DC machine has mainly four components. 1. Field magnet system 2. Armature 3. Commutator 4. 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: Smitarani Sahoo, CVRGU, BBSR It supports the field or exciting coils. They spread out the magnetic flux over the armature periphery more uniformly. It increases the cross-sectional area of the magnetic circuit, as a result, the reluctance of the magnetic path is reduced. Field pole of a DC machine Field or Exciting Coils: Each pole core has one or more field coils (windings) placed over it to produce a magnetic field. The coils are wound on the former and then placed around the pole core. When direct current passes through the field winding, it magnetizes the poles, which in turns produces the flux. The field coils of all the poles are connected in series in such a way that when current flows through them, the adjacent poles attain opposite polarity. Armature: Armature of DC machine The rotating part of the DC machine or a DC Generator is called the Armature. The armature consists of a shaft upon which a laminated cylinder, called Armature Core is placed. Smitarani Sahoo, CVRGU, BBSR 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. Smitarani Sahoo, CVRGU, BBSR 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: Smitarani Sahoo, CVRGU, BBSR Lap Winding: finish end of one coil is connected to a commutator segment and start end of the adjacent coil is situated under the same pole. Wave Winding: coil progressive passes every North and South pole till it returns to the coil side where it started. Equilizer ring: these are low resistance copper wires that connect points in armature winding which under ideal conditions should remain at the same potential. They relieve the brushes from circulating currents Dummy coils: these are not electrically connected to the rest of the winding. They only preserve mechanical balance. Smitarani Sahoo, CVRGU, BBSR 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. Smitarani Sahoo, CVRGU, BBSR 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: Smitarani Sahoo, CVRGU, BBSR The working principle of a DC generator is based on Faraday’s laws of electromagnetic induction. When a conductor is placed in a varying magnetic field, an electromotive force gets induced within the conductor. For production of dynamically induced EMF, three things are necessary. Magnetic field Conductor Motion of the conductor with respect to the magnetic field In DC generators, the magnetic field is provided by the field magnet system. Conductors are placed on the armature and armature is being rotated by prime-mover. As the armature rotates, it cuts the air gap flux.