N6 Electrotechnics

Electrotechnics Downloaded from hlayiso.com N6

Module 1: DC machines TRACTION Traction: the force used to generate motion between a body and a tangential surface using dry friction. Traction also applies to generators. It is also one of the main factors behind the motion of a wheeled vehicle on a surface. Downloaded from hlayiso.com www.futuremanagers.com

Module 1: DC machines (continued) SERIES, PARALLEL AND SPEED CONTROL Speed control ensures the smooth running of machines. A series circuit refers to the current through each of the components is the same, and the voltage across the circuit is the sum of the voltages across each component. A parallel circuit refers to the voltage across each of the components is the same, and the total current is the sum of the currents through each component. Downloaded from hlayiso.com www.futuremanagers.com

Module 1: DC machines (continued) BRIDGE CONNECTION When you power a motor, the shaft of the motor rotates in one direction. For the shaft to rotate in the opposite direction, the polarity of the voltage supply needs to be changed. If you need a bi-directional motor, you need a bridge connection between the motor and the power supply. The figure shows a bridge connection: Downloaded from hlayiso.com www.futuremanagers.com

Module 1: DC machines (continued) SHUNT TRANSITION Shunt transition is a way of changing motor connections from series to shunt by which one or more motors are first shorted at their terminals to stop them. When they stop rotating, they may be connected as desired. Downloaded from hlayiso.com www.futuremanagers.com

Module 1: DC machines (continued) TORQUE AND POWER Torque is the ability of a force to cause rotation of an object about a fixed point. As a motor rotates, it generates a rotating force. Torque of a motor is the measure of the effectiveness of a force generated by the rotating action of a motor. Downloaded from hlayiso.com www.futuremanagers.com

Module 1: DC machines (continued) SPEED AND TORQUE The speed (angular velocity) of a motor is a measure of how many revolutions the rotor makes in a certain time. As shown below, torque of a motor is indirectly proportional to its speed. Downloaded from hlayiso.com www.futuremanagers.com

Module 1: DC machines (continued) LOAD SHARING Load sharing is a method which divides a load among a set of parallel connected generators. The way in which the load is divided depends on the drooping characteristics of the generators The figure shows a representation of load sharing: Downloaded from hlayiso.com www.futuremanagers.com

Module 1: DC machines (continued) EQUALISING BARS AND CROSS-CONNECTION FIELDS Equalising bars stand as a way to avoid generators from supporting more of the load compared to another generator. This prevents internal emf from increasing, which prevents a voltage drop in connected field windings. Downloaded from hlayiso.com www.futuremanagers.com

Module 1: DC machines (continued) TESTS When testing machines, you can conduct the tests in two ways, namely directly and indirectly. Hopkinson’s test: Also known as the Regenerative test, back-to-back test or heat-run test and is used to determine the efficiency of a DC machine. Swinburne’s test: Is done in to predetermine the efficiency of a DC machine. In Swinburne’s test, the losses are measured separately and then used together inDownloaded an expression for the efficiency. from hlayiso.com www.futuremanagers.com

Module 1: DC machines (continued) BRAKING There are two ways of braking a DC motor: mechanically or electrically. Mechanical braking involves the use of a “shoe” in contact with the rotating shaft or wheel attached to the motor. Electrical braking is also known as non- contact braking because no physical contact is made with the shaft to stop the motor. Downloaded from hlayiso.com www.futuremanagers.com

Module 2: AC circuits COMPLEX WAVEFORMS A complex waveform is formed when two or more sine waves are added together to form one resultant waveform: Downloaded from hlayiso.com www.futuremanagers.com

Module 2: AC circuits (continued) COMPONENTS AND HARMONICS OF A WAVE Two sine waves are called the harmonics of the complex wave. Harmonics are the sinusoidal constituents of a complex wave which have integer multiples of the fundamental frequency. Downloaded from hlayiso.com www.futuremanagers.com

Module 3: Transformers INTRODUCTION Transformers are some of the most important (and expensive) equipment in power systems. A transformer is a static device which transforms electrical energy from one circuit to another without direct electrical connection. A transformer can be a step-up or a step-down transformer. The former is such that the voltage at the secondary windings is higher than the voltage at the primary windings. The latter describes a transformer where the voltage at the secondary is lower than that at the primary Downloaded from side. hlayiso.com www.futuremanagers.com

Module 3: Transformers (continued) CONDUCT TESTS Back-to-back test: Is done to determine the losses in a transformer. Delta-delta connection: Both the three-phase primary and secondary windings are connected in delta. Scott connection: May be made when a three-phase power supply is required to supply a two-phase or single-phase machine. Harmonics test: Used determine the harmonic frequencies at which the transformer is most or least efficient.from hlayiso.com Downloaded www.futuremanagers.com

Module 3: Transformers (continued) CONDUCT TESTS (CONT) Zig-zag connection: This type of transformer connection is also known as the “interconnected star” connection. It has features of the wye and delta hence the advantages of both are combined. Tertiary winding arrangement: A three-phase transformer may have a third winding in addition to the primary and secondary (this winding is called a tertiary winding). This type of transformer is called a three winding transformer. Downloaded from hlayiso.com www.futuremanagers.com

Module 3: Transformers (continued) REGULATION Transformer regulation is defined as the ratio of the difference between the no load voltage to the full voltage expressed as a percentage. It is given by the formula below: no load voltage − full load voltage Regulation % = × 100 full load voltage Downloaded from hlayiso.com www.futuremanagers.com

Module 4: AC machines THE SYNCHRONOUS MACHINE The term “synchronous machine” refers to either a synchronous generator or a synchronous motor. The generator converts mechanical energy input through a prime mover, to electrical energy, whereas the motor converts electrical energy to mechanical energy to drive a mechanical load such as a pulley or fan belt. Downloaded from hlayiso.com www.futuremanagers.com

Module 4: AC machines (continued) THE INDUCTION MACHINE The induction motor is the most widely used machine in industry. It is also known as the “workhorse of industry”. It gets its name from the fact that the stator induces the magnetic field in the rotor. There are two types of induction motors, namely the squirrel cage induction motor and the wound rotor induction motor. Downloaded from hlayiso.com www.futuremanagers.com

Module 5: Generation and supply of AC POWER FACTOR CORRECTION It is important to achieve a power factor of unity, because a low power factor increases your electricity bill. A low power factor means that less usable power is being delivered to your system and this may be unfavourable for your system. An improved power factor, which results in increased system capacity and reduced system losses, can be done by adding capacitors. Downloaded from hlayiso.com www.futuremanagers.com

Module 5: Generation and supply of AC (continued) METHODS OF POWER CORRECTION Capacitor banks can be used for power factor correction, because capacitors can generate the reactive power needed by inductive loads. Another method of power factor correction is using synchronous motors. Synchronous motors can be set to operate in lagging or leading mode. The ability to operate in leading mode makes them useful for power factor correction. Downloaded from hlayiso.com www.futuremanagers.com

Module 5: Generation and supply of AC (continued) TRANSMISSION LINES A transmission line is a long conductor specifically designed to carry a certain amount of generated power at high voltage from one station to another. There are three types of transmission lines: short, medium and long transmission lines. Downloaded from hlayiso.com www.futuremanagers.com

Module 6: Measuring instruments USE, CONSTRUCTION AND OPERATION OF MEASURING INSTRUMENTS Electrical measuring instruments are mainly found in two categories, namely absolute (gives the value of the quantity to be measured only in terms of the instrument’s constants and deflection) and secondary instruments (allows the value of the electrical quantity to be measured). Downloaded from hlayiso.com www.futuremanagers.com

Module 6: Measuring instruments (continued) MEASUREMENT CONNECTIONS AND DIAGRAMS Measuring the properties of electrical devices and circuits is a crucial aspect of their functioning. Measuring requires a great understanding of these properties as well as the relevant measurement methods. Measurement connections are better understood with the use of diagrams. Downloaded from hlayiso.com www.futuremanagers.com

Module 6: Measuring instruments (continued) IDENTIFY CABLE FAULTS Cable faults can occur at any location and under any condition that the cable is subjected to. Finding, identifying and correcting these faults are necessary procedures for the stability and the correct operation of the systems that the cables belong to. Cable faults can result for a number of reasons, such as: Moisture or dampness, mechanical damage and puncturing, overloading or power source failures. Downloaded from hlayiso.com www.futuremanagers.com

Module 6: Measuring instruments (continued) MURRAY LOOPS The Murray loop is a circuit that is used to locate faults, mostly in underground cables. The faulted cable is connected at one end to a resistor network and a voltage source. The other end of the cable is connected to a sound cable and ground or short circuit connection is made to evaluate the location of the fault. The Murray loop test identifies short circuit or earth (ground) faults. Downloaded from hlayiso.com www.futuremanagers.com

Module 7: Switchgear and protective devices FUNCTIONING AND OPERATION OF INDUCTION DISC RELAYS The induction disk relay is a relay that is activated by high currents rotating a disc that activated by a circuit breaker that pulls contacts open to break the circuit. It works on the principle of electromagnetic induction and, therefore, it only applies to alternating current systems. Downloaded from hlayiso.com www.futuremanagers.com

Module 7: Switchgear and protective devices (continued) APPLICATION OF INDUCTION DISC RELAYS The induction disc relay is usually installed by carrying out the following tasks: • Connect the current tap to the current transformer. • Earth the device. • Connect the circuit breaker. • Set the delay period. Downloaded from hlayiso.com www.futuremanagers.com

Module 7: Switchgear and protective devices (continued) CURRENT AND VOLTAGE BREAK CAPACITIES OF CONTRACTORS A contactor is an electrically powered switch that is used to establish and interrupt an electric power circuit repeatedly and under normal conditions. Circuit breakers, contactors and other interrupting devices have certain capabilities, such as a maximum current and voltage that they are able to function in. These capacities depend on the type of circuit that they are being used on. This is called the breaking capacity. Downloaded from hlayiso.com www.futuremanagers.com

Module 7: Switchgear and protective devices (continued) REVERSE PHASE RELAYS Reverse phase relays constantly monitor the phase rotation of a three-phase line. They operate at many different power levels, but they perform the same function. A reverse phase relay usually works by a solid-state sensing circuit that drives a relay. The relay is usually energised when the phase rotation is correct. The relay is de-energised if there is a phase reversal. Downloaded from hlayiso.com www.futuremanagers.com

Module 7: Switchgear and protective devices (continued) THE OPERATION OF HIGH VOLTAGE AND CURRENT CIRCUIT BREAKERS Circuit breakers come in many forms. High voltage and high current circuit breakers have the following four characteristics: • A quenching medium. • A suitable placement indoor or outdoor. • Operating mechanism. • Voltage and current level. Downloaded from hlayiso.com www.futuremanagers.com

Module 7: Switchgear and protective devices (continued) KVA RATINGS OF REACTORS Reactors are rated according to the kVA rating and voltage, which is calculated by multiplying the short circuit current by the rated voltage at full load. The rated voltage is the maximum voltage across the reactor. A safety factor is usually used to account for the performance degradation of the circuit components. Downloaded from hlayiso.com www.futuremanagers.com

Module 8: Static control ANALOGUE-TO-DIGITAL CONVERSION (ADC) Analogue-to-digital conversion is a system used to process raw data and send it to digital system. An example of analogue-to-digital conversion is the circuitry used to measure the vibration of the rotor. Most of the sensors are designed and constructed with an ADC circuit coupled to it or designed as a separate circuit. Downloaded from hlayiso.com www.futuremanagers.com

Module 8: Static control (continued) DIGITAL-TO-ANALOGUE CONVERSION DAC essentially is used to convert the fixed binary numbers to a continuous varying signal. During an ideal conversion, a reconstruction filter is used processes the conceptual sequence of impulses to create a continually sampled signal. A conventional practical DAC on the other hand produces a piecewise constant function during conversion of numbers. Downloaded from hlayiso.com www.futuremanagers.com

Module 8: Static control (continued) RC NETWORK PHASE CONTROL RC network phase control is a technique or set of techniques used to regulate the switching patterns of the switching devices. The phase control techniques linked to RC network are silicon-controlled rectifiers (SCR) and Triac. Downloaded from hlayiso.com www.futuremanagers.com

Module 9: Special characteristics DEFINING SPECIAL CHARACTERISTICS The special characteristics of an electrical system can be defined as features that can cause or make it operate much better than other devices in the same category. The conditions for special characteristics are such that they must complement what is already considered to be a standard characteristic. Downloaded from hlayiso.com www.futuremanagers.com

Module 9: Special characteristics (continued) SPECIAL CHARACTERISTICS OF ARC FURNACE TRANSFORMERS Arc furnace transformers are characterised as specific industrial transformers due to power requirements of the arc furnaces. These transformers are designed to deliver high currents at low voltages. The special characteristics of arc furnace transformers are based on their technical features and design options. Downloaded from hlayiso.com www.futuremanagers.com

Module 9: Special characteristics (continued) SPECIAL CHARACTERISTICS OF SWITCHGEARS AND CONTROL SYSTEMS The switchgears, like all other components on an electrical network, can be designed to operate in different environmental conditions. The introduction of control systems on the switchgear or overall electrical network has allowed for processes to be monitored and regulated from a remote area. The control system is mainly made up of a computer (to accommodate relevant software), a network and control system Downloaded from hardware. hlayiso.com www.futuremanagers.com

Module 9: Special characteristics (continued) SPECIAL CHARACTERISTICS OF ELECTRICAL AND HYDRAULIC SYSTEMS The electrical system is the fundamental component of the hydraulic system. For an effective operation of the hydraulic system, the systems have to run at optimal levels. The introduction of control and communication systems ensures that the hydraulic system can be regulated and monitored at any given time. Downloaded from hlayiso.com www.futuremanagers.com

Module 9: Special characteristics (continued) TYPES OF ELECTRODES Electrodes are generally used to create a contact with a non-metallic part of an object to make sure that the current flows. The types of electrodes are as follows: • Electrical discharge machining (EDM) electrodes. • Anode and cathode electrodes. • Alternating current (AC) electrodes. Downloaded from hlayiso.com www.futuremanagers.com

Module 10: Installation, care, operation and inspection of equipment INSTALLATION OF EQUIPMENT Before installing electric machines, you must make sure that you are working in line with the worksite procedures and the legal requirements. Electrical equipment needs to be isolated and an electrical lockout procedure must be put in place. In some cases, a circuit breaker will be pulled or isolated to make it impossible to operate the equipment being worked on. Downloaded from hlayiso.com www.futuremanagers.com

Module 10: Installation, care, operation and inspection of equipment (continued) LOCATING THE RIGHT TOOLS FOR THE JOBS Tips for locating the right tools and equipment: • Make sure that you know which equipment is needed for the task. • Keep your toolbox organised and clean. • Store your tools in a safe place at all times. • Store all tools where you can find them easily and without wasting time. • Keep tools clean, in good working order and ready for immediate use. • Handle sensitive equipment with from Downloaded care. hlayiso.com www.futuremanagers.com

Module 10: Installation, care, operation and inspection of equipment (continued) CONNECTING ELECTRICAL MACHINES Practical tips for connecting electrical machines: • Make sure that you use the correct size tools and equipment. • Make sure that you have the right size joints, because they are very expensive. • Check the size of logs. • Make sure that you have a crimping tool that is fit for the purpose of joining wires. Downloaded from hlayiso.com www.futuremanagers.com

Module 10: Installation, care, operation and inspection of equipment (continued) CARE AND MAINTENANCE OF EQUIPMENT Electrical machines are key to the operation of mines, factories, businesses and homes. Some of these machines are very strong and can operate heavy loads in difficult weather conditions like extreme heat, cold, and rain. However, it is important to remember that they also have a lifetime in which they will work in extreme conditions. Maintaining machines keeps them operating effectively for as long as possible. Downloaded from hlayiso.com www.futuremanagers.com