ENGINEERING ARTICLES

Coils, inductors, and chokes are the names used to indicate a coil of wire. “Inductor” is preferred because inductors have inductance, a property that is utilized in many electrical circuits. FIXED INDUCTORS The simplest coil or inductor has an air core and is made by winding a wire in a series of loops, which may or may not have a form to hold them in place. Coils are seldom color coded for value, so we look at the schematic or a parts list for the inductance value of a coil. Inductance is the electrical property of a coil, just as resistance is the electrical property of a resistor. Many coils are wound on plastic forms that support the loops of wire. The form has no effect on the operation of the coil. The symbols for air-core coils are shown in Figure 1. Figure 1 Symbols for air-core coils. Other types are powdered iron core and iron core. Symbols for these types are shown in Figure 2. Figure 2 Powdered-iron-core and iron-core inductors. VARIABLE INDUCTORS ...

The capacitor is used in many electric circuits in both electronics and in air conditioning and refrigeration circuits. Two types of capacitors are used in these circuits: fixed and variable. FIXED CAPACITORS The fixed capacitor is made for a certain value and is not adjustable. The fixed capacitor is divided into several groupings. It may be made with paper separating two plates of aluminum foil, or it may use plastic, mica, ceramic, or electrolytes. Most paper capacitors have been replaced by those made of better materials, usually plastic. A typical capacitor is shown in Figure 1. Capacitors are large enough to have their values printed on them. The smaller capacitors use a color code to indicate their value and working voltage. Capacitors come in hundreds of sizes and shapes. It takes a good half-hour to thumb through an electronics catalog that shows all the various types. Each type has a special or particular application. Mica types, for instance, are used for some ...

The resistor is the most widely used electrical and electronic device. Every radio, television set, and control circuit has a resistor or resistors. This component is used to provide resistance. It is designed to be used at a fixed value or as a variable-value device. FIXED RESISTORS The fixed resistor is the simpler of the two types. It is made so that you cannot change the resistance. Some carbon fixed resistors are shown in Figure 1. These are carbon composition and have a cover of black, brown, or green plastic. A color code is used to give the value of the resistor. Fixed wire-wound resistors are available for use when the wattage rating is higher than 2 watts. Carbon-composition resistors come in 1/8, 1/4, 1/2, 1, and 2 watt sizes. The physical size tells the rating. You get used to the wattage rating when working with resistors. The larger the resistor, the higher the wattage rating is. Figure 1 Fixed carbon composition resistors. The larger the resistor, the ...

The five most important sources of electricity for technicians are chemical action, heat, light, pressure, and magnetism. 1) CHEMICAL ACTION In the electrical and electronics fields, battery is the backup source of electricity. Batteries produce electrical energy by a chemical action. 2) HEAT Heat can be used to free electrons from some metals and from specially prepared surfaces. When some materials are heated to a high temperature, electrons are freed from their surfaces. Any nearby metallic surface, if positively charged, attracts these electrons and produces electron flow. The freeing of electrons by heat is called thermal emission. 3) LIGHT Light striking the surface of certain materials can be used to free electrons. This is called photoemission. With a suitable collecting surface, useful electron flow can result. Photoemission is used in photoelectric devices and television camera tubes. 4) PRESSURE Mechanical pressure on certain crystals can be used to produce electr...

Unlike in square wave inverters the switches of PWM inverters are turned on and off at significantly higher frequencies than the fundamental frequency of the output voltage waveform. The typical pole voltage waveform of a PWM inverter is shown in below figure over one cycle of output voltage. In a three-phase inverter the other two pole voltages have identical shapes but they are displaced in time by one third of an output cycle. Compared to the square pole voltage waveform, the pole voltage waveform of the PWM inverter changes polarity several times during each half cycle. The time instances at which the voltage polarities reverse have been referred here as notch angles. It may be noted that the instantaneous magnitude of pole voltage waveform remains fixed at half the input dc voltage (Edc). When upper switch (SU), connected to the positive dc bus is on, the pole voltage is + 0.5 Edc and when the lower switch (SL), connected to the negative dc bus, is on the instantaneous pole volta...

Pulse width modulated (PWM) inverters are among the most used power-electronic circuits in practical applications. These inverters are capable of producing ac voltages of variable magnitude as well as variable frequency. The quality of output voltage can also be greatly enhanced, when compared with those of square wave inverters. The PWM inverters are very commonly used in adjustable speed ac motor drive loads where one needs to feed the motor with variable voltage, variable frequency supply. For wide variation in drive speed, the frequency of the applied ac voltage needs to be varied over a wide range. The applied voltage also needs to vary almost linearly with the frequency. PWM inverters can be of single phase as well as three phase, principle of operation is same for both. There are several different PWM techniques, differing in their methods of implementation. However in all these techniques the aim is to generate an output voltage, which after some filtering, would result...

In a single-phase ac circuit, instantaneous power to a load is of a pulsating nature. Even at unity power factor (i.e., when the voltage and the current are in phase with respect to each other), the instantaneous power is less than unity (i.e., when the voltage and the current are not in phase). The instantaneous power is not only zero four times in each cycle but it is also negative twice in each cycle. Therefore, because of economy and performance, almost all electrical power is produced by polyphase sources (i.e., by those generating voltages with more than one phase. A polyphase generator has two or more single phases connected so that they provide loads with voltages of equal magnitudes and equal phase differences. For example, in a balanced n-phase system, there are n voltage sources connected together. Each phase voltage (or source) alternates sinusoidally, has the same magnitudes, and has a phase difference of 360/n° (where n is the number of phases) from its adjacent...