Electrical Trade Theory N 2 Module 1 Conductors

Electrical Trade Theory N 2

Module 1: Conductors and cables METHODS OF INSTALLING CABLES There are three general methods of installing armoured cables: • Open (in free air); • In ducts; and • Buried. www. futuremanagers. com

Module 1: Conductors and cables (continued) INDUCTANCE – SELF INDUCTION When there is a change of current flowing through a coil, there will also be a change of flux around the coil. Whenever there is a change in flux in a coil, an emf is induced in that coil. Since the emf is induced in the same coil through which the current responsible for setting up the flux flows, it is known as an emf of self induction. www. futuremanagers. com

Module 1: Conductors and cables (continued) TRUE POWER, APPARENT POWER AND REACTIVE POWER • This product of the voltage and the current is known as apparent power. • The true power or active power is a product of the voltage and the in-phase component. • The component of the current does no useful work and causes energy loss in the form of heat. The reactive component is said to produce reactive power. www. futuremanagers. com

Module 1: Conductors and cables (continued) CABLE SELECTION Following are a number of variables to consider when selecting a cable for a specific application: • Load to be supplied. • Permissible volt drop. • Prospective fault (short-circuit) current. • The circuit protection that is available. • Environmental conditions. www. futuremanagers. com

Module 2: Cable joints and switchgear JOINING CONDUCTORS Joints are made in conduit boxes, cable jointing boxes, appliances and light fittings in many smaller applications. When large diameter copper conductors are joined, crimping ferrules are almost always used. www. futuremanagers. com

Module 2: Cable joints and switchgear (continued) CABLE JOINTS (SPLICES) There are five basic steps in making a joint in a cable: 1. Prepare the ends (make off the ends and prepare the surfaces). 2. Join the conductors. 3. Insulate. 4. Reshield (earth sock, armouring, screening, metal sheaths). 5. Rejacket. www. futuremanagers. com

Module 2: Cable joints and switchgear (continued) TYPES OF JOINTS Low voltage joints exist, such as: High voltage joints exist, such as: • Acceptable couplers; • Resin joint with taped conductors; • Resin joint; • Hot shrink joint; • Taped joint; • Cold shrink joint. • Screw connectors in joint box; • Hot or cold shrink joint with resin; and • Strip connectors in joint box; and • Resin pressure joints. www. futuremanagers. com

Module 2: Cable joints and switchgear (continued) INSULATORS AND INSULATION The main function of insulating materials is to support and/or electrically separate conductors in order to prevent the flow of an electric current where a potential difference exists between two or more points. www. futuremanagers. com

Module 2: Cable joints and switchgear (continued) RETICULATION NETWORK Power stations are often situated at considerable distances from the consumers. In order to transmit electrical energy economically over long distances, it is necessary to do so at high voltages and by means of overhead conductors. www. futuremanagers. com

Module 2: Cable joints and switchgear (continued) DISCONNECTORS, RELAYS AND CONTACTORS A disconnector is a mechanical switching device that provides an isolating distance in accordance with specified requirements, and is capable of opening and closing a circuit either when negligible current is present. Relays are small electromagnetic switches. They are used to relay electrical signals from one circuit to another. www. futuremanagers. com

Module 2: Cable joints and switchgear (continued) MINIATURE CIRCUIT BREAKERS (MCB) Low tension circuit-breakers of the domestic and small industrial type may be of the single-phase or three-phase type. Miniature circuit-breakers can be of the magnetic-hydraulic type or of thermalmagnetic type. www. futuremanagers. com

Module 3: Direct current motors CONSTRUCTION The general construction of a four-pole dc motor is shown below. The main frame is referred to as the yoke and is made of a magnetic material. Field coils which produce the main magnetic field are wound around soft iron pole cores. www. futuremanagers. com

Module 3: Direct current motors (continued) INTERNAL CONNECTIONS (TYPES OF MOTORS) DC machines are named according to the method used to connect the field coils relative to the armature. There are two main types of internal connections: seperately excited and self excited. Self excitation can be subdivided into three categories; shunt, series and compound connected. www. futuremanagers. com

Module 3: Direct current motors (continued) THE NEED FOR MOTOR STARTERS The armature current of a motor is limited by the back emf and armature circuit resistance. In order to limit this high starting current to a safe value, the resistance of the armature circuit must be increased. This is done by adding a starting resistor to the armature circuit while the motor is being started. www. futuremanagers. com

Module 3: Direct current motors (continued) FACE PLATE STARTER FOR A SHUNT MOTOR A face plate starter is manually operated. It has a spring loaded contact arm which returns automatically to the off position when the no-volt release coil (NVR) de-energises. www. futuremanagers. com

Module 3: Direct current motors (continued) REVERSING THE DIRECTION OF ROTATION The direction of rotation of a dc motor may be reversed by reversing the direction of the current through the field windings or through the armature windings, but not both. www. futuremanagers. com

Module 4: Alternating current motors THE INDUCTION MOTOR In the induction motor there is no electrical connection between the stator and the rotor. As in a transformer, the energy is transferred entirely magnetically by means of the emf induced in the rotor conductors by the rotating field set up by the stator windings. www. futuremanagers. com

Module 4: Alternating current motors (continued) CONSTRUCTION The induction motor consists essentially of two main parts namely: • A stator, and • A rotor. www. futuremanagers. com

Module 4: Alternating current motors (continued) ROTATING MAGNETIC FIELD A rotating magnetic field is produced: The currents through each of the three stator windings in a three-phase motor differ in phase by a third of a cycle (120°). A pulsating magnetic field is set up by each of these windings. Since each magnetic field follows after the next, they combine to set up a resultant field which moves around the stator surface at a constant speed. www. futuremanagers. com

Module 4: Alternating current motors (continued) PRINCIPLE OF OPERATION When the stator winding (primary) is energised from a three-phase supply, a rotating magnetic field, as described above, is established, which rotates at synchronous speed. www. futuremanagers. com

Module 4: Alternating current motors (continued) SPEED AND SLIP The rotor speed, even at no-load, will always be slightly less than synchronous speed in order that current may be induced in the rotor which will produce a torque large enough to overcome friction and wind resistance. This difference between rotor speed and synchronous speed is called slip. www. futuremanagers. com

Module 4: Alternating current motors (continued) STAR AND DELTA CONNECTIONS Three-phase induction motors run with their stator windings connected in delta. For the starting of large squirrel-cage induction motors, the stator winding is often connected in star (which reduces the voltage across each phase winding) for starting and delta for running. This is accomplished by means of a star-delta starter. www. futuremanagers. com

Module 4: Alternating current motors (continued) SINGLE-PHASE INDUCTION MOTORS A single-phase motor is not self-starting and requires special means to get it started. In order to start a single-phase induction motor, the phase is split by means of an auxiliary (starting) winding. Because of this they are also referred to as split-phase motors. www. futuremanagers. com

Module 4: Alternating current motors (continued) OVERCURRENT PROTECTION OF MOTORS Overcurrent protection for induction motors are often set in the region of 20% above the full load current rating. The use of a starter reduces the starting current but increases the accelerating time because of the reduced value of the starting torque. www. futuremanagers. com

Module 4: Alternating current motors (continued) STARTING OF INDUCTION MOTORS The purpose of a motor starter is to start a motor manually or automatically from a position close to the motor or remotely. The function of a motor starter, other than a direct-on-line starter, is to reduce the current drawn by the motor while the rotor accelerates. www. futuremanagers. com

Module 4: Alternating current motors (continued) ELECTRICAL TESTS ON THE STATOR There are three electrical tests that are carried out on the stator windings of a three-phase motor: • Insulation resistance test between windings; • Insulation resistance to earth test; and • Short-circuit and open-circuit tests. www. futuremanagers. com

Module 5: Earthing EARTHING The purpose of earthing is to guard every electrical installation, machine, appliance or apparatus against the effects of leakage currents, static charges and lightning discharges. www. futuremanagers. com

Module 5: Earthing (continued) EARTHING OF UNDERGROUND CABLES The wire armouring of an armoured cable may be used as an earth continuity conductor. www. futuremanagers. com

Module 5: Earthing (continued) OVERHEAD LINES Earth continuity conductors are erected above overhead lines in order to protect them from lightning. It is also a useful means of earthing the pylons and to supply the consumer with a reliable earth. www. futuremanagers. com

Module 5: Earthing (continued) THE EARTHING CHAIN Earthing is a serious matter and has many sections from the point which requires to be earthed up to the final connection with general mass of the earth. These can be compared to the links of a chain and just as with a chain; an earthing chain is as strong as its weakest link. www. futuremanagers. com

Module 5: Earthing (continued) SYSTEM EARTHING System earthing relates to the earthing of the power system such as earthing the windings of transformers. The same applies to the star point of an alternator. Proper system earthing is required to ensure proper surge protection and the proper functioning of other protective relays and devices. www. futuremanagers. com

Module 5: Earthing (continued) EQUIPMENT EARTHING Equipment earthing relates to an earthing connection to the metal frames of electrical machines, the metal housings that contain switching equipment or other electrical apparatus, metal raceways containing electrical conductors, closely adjacent conducting structures and any other extraneous conductive parts judged to be vulnerable to contact by an energised conductor. www. futuremanagers. com

Module 5: Earthing (continued) COMBINED SYSTEM AND EQUIPMENT EARTHING Proper electrical continuity between system and equipment earthing is accomplished by means of earth continuity conductors. An earth continuity conductor is a conductor, which includes any clamp or terminal, that connects the consumer’s earth terminal to the exposed conductive parts of an installation for the purpose of earthing such parts and for carrying fault currents. www. futuremanagers. com

Module 5: Earthing (continued) EARTHING AT POWER STATIONS AND SUBSTATIONS Good substation earthing is very important for effective protection and operation of protective devices. When designing substation earthing, the safety of people is the most important consideration taken into account. www. futuremanagers. com

Module 6: Protection EARTH LEAKAGE PROTECTION The purpose of an earth leakage unit is to detect an earth fault current and to automatically disconnect an installation or circuit from the supply, within a specified time, when it exceeds a specified or predetermined value. www. futuremanagers. com

Module 6: Protection (continued) CORE BALANCE EARTH LEAKAGE RELAY Core balance earth leakage relays are basically current transformers which operate tripping devices when they detect an earth fault current which exceeds a predetermined value. www. futuremanagers. com

Module 6: Protection (continued) PHASE IMBALANCE In a multiphase installation, the circuits shall be so arranged that the total load is, as nearly as is practically possible, balanced over the three phases. With a rigidly balanced three-phase load such as a three-phase motor, any imbalance in the supply current will indicate a fault. Single-phasing is a severe case of phase imbalance. www. futuremanagers. com

Module 6: Protection (continued) SURGE PROTECTION Lightning is a major cause of short-circuits on overhead transmission and distribution lines. Transient voltages on the lines caused by lightning strikes may exceed the insulation level of the line. This would cause a flashover between a phase and earth or between phases. The function of a lightning arrester is to provide a path by which these surges are discharged to earth before a flashover on the line can occur. www. futuremanagers. com

Module 6: Protection (continued) EARTH CURRENT COMPENSATION If the system is earthed at more than one point, a proportion of the earth fault current will return by a path other than that at which the relays are located. Earth compensation relays are required to correct this. www. futuremanagers. com

Module 6: Protection (continued) OVERLOAD RELAYS An overload (overcurrent) relay is a protective device designed to protect a motor from overload during starting and running. www. futuremanagers. com

Module 6: Protection (continued) SEVERE STARTING Severe starting conditions can cause the contacts of contactors to overheat and splatter or even weld together. Fuses in the supply line could deteriorate and even melt. Severe starting can affect time delay features which could cause nuisance tripping. Supply cables could be damaged unless severe starting was taken into account when the cables were rated. www. futuremanagers. com

Module 6: Protection (continued) FUSES The function of a fuse is to detect overcurrents or short-circuit currents and to then automatically disconnect the faulty equipment or circuit from the supply. www. futuremanagers. com

Module 7: Measuring instruments WATTMETER (ELECTRODYNAMOMETER TYPE) It is often necessary to measure the rate at which electrical energy is being consumed. Power is defined as the rate of expending (converting energy from one form to another) energy and is measured by means of a wattmeter. www. futuremanagers. com

Module 7: Measuring instruments (continued) CONNECTION OF A WATTMETER OR WATT-HOUR METER IN A SINGLE-PHASE SYSTEM There is: • Direct connection; and • Indirect connection. www. futuremanagers. com

Module 7: Measuring instruments (continued) WATT-HOUR METER (ENERGY METER) The amount of electricity consumed is referred to as electrical energy and is measured by means of a watt-hour meter. Since energy is a product of power and time, both quantities must be taken into account in energy measurement. www. futuremanagers. com

Module 7: Measuring instruments (continued) FREQUENCY METER A frequency meter is used to measure the frequency of ac cycles per second in an alternating current circuit. The unit in which frequency is measured, is hertz (Hz). www. futuremanagers. com

Module 7: Measuring instruments (continued) POWER FACTOR METER Power factor is the ratio of the true power to apparent power of an alternating current circuit. Power factor meters are used to measure the power factor of an alternating current circuit. www. futuremanagers. com

Module 7: Measuring instruments (continued) MAXIMUM DEMAND Electrical demand comes at different times for entities such as consumers and industries. In order to measure the above-mentioned energy consumption, a maximum demand meter is used. Maximum demand meters register not only the effective power consumption in k. Wh, but also the highest average value of power in k. W during the time period of metering. www. futuremanagers. com

Module 8: Transformers www. futuremanagers. com

Module 8: Transformers (continued) THREE-PHASE TRANSFORMERS Three-phase transformers may be regarded as three single-phase transformers with their primary and secondary windings connected in either star or delta. www. futuremanagers. com

Module 8: Transformers (continued) TRANSFORMER TAPPINGS In order to compensate for volt drops which might happen owing to extra loads, tappings are made on the secondary side of supply transformers. Tap changing switches are fitted to increase the voltage during peak periods and to reduce it when the load reduces. Tap changing switches can be of the manual or automatic types. www. futuremanagers. com

Module 9: Electronics DIODES Semiconductor components are generally made in such a way that a p-type material is in close contact with an n-type material. The interface between the two is termed a p-n junction. Diodes are p-n junction devices. www. futuremanagers. com

Module 9: Electronics (continued) RATING OF DIODES Following are four different ratings which are applied to diodes: • Continuous current rating; • Intermittent current rating; • Power rating; and • Reverse voltage rating. www. futuremanagers. com

Module 9: Electronics (continued) BIASING Biasing refers to the application of a specific voltage polarity and value to achieve desired working conditions. www. futuremanagers. com

Module 9: Electronics (continued) TESTING A diode is tested to ensure that it will, within specified limits, conduct with the polarity in one direction and be non-conductive when the polarity is reversed. www. futuremanagers. com

Module 9: Electronics (continued) DC OPERATION A diode can be used to prevent a feedback signal or reverse voltage from being applied to a sensitive circuit thus protecting the circuit from damage. This is made possible by the very high impedance in reverse polarity. www. futuremanagers. com

Module 9: Electronics (continued) AC OPERATION Owing to the reverse-blocking characteristics of a diode, it may be used as a rectifier to convert ac to dc by allowing conduction during one half cycle and blocking conduction during the following half cycle. www. futuremanagers. com

Module 9: Electronics (continued) DIODES AS RECTIFIERS One of the first and still most popular uses of a diode, is to rectify alternating current by converting it to direct current for a wide range of purposes. These come in the forms of: • Half-wave rectifier; • Full-wave rectification with a centre-tap transformer; and • Full-wave bridge rectifier. www. futuremanagers. com

Module 9: Electronics (continued) www. futuremanagers. com

Module 9: Electronics (continued) ZENER DIODES Where constant voltage power supplies are required, use is often made of Zener diodes in order to stabilise the voltage. www. futuremanagers. com

Module 9: Electronics (continued) NPN AND PNP TRANSISTORS The circuit symbols of an NPN and a PNP transistor with their relative current flows are seen: www. futuremanagers. com

Module 9: Electronics (continued) TRANSISTOR IN A COMMON EMITTER CIRCUIT These can be formed as: • Common emitter amplifiers; • Common emitter switches (inverting mode); and • Common emitter switches (non-inverting mode). www. futuremanagers. com

Module 9: Electronics (continued) OPERATION OF A THYRISTOR Thyristors are used mainly as converters and for controlling electrical power. Some examples of the applications of thyristors are speed control for motors, self regulating battery chargers and static inverters. www. futuremanagers. com