TYPES OF INDUCTORS

TYPES OF INDUCTORS

By Engr. Aneel Kumar   August 26, 2015   No comments:

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

Some circuits need inductors that can have their values changed, some by screwdriver adjustment and others by changing the core material. Figure 3 shows the symbols for variable inductors. Note the differences for iron-core variable inductors. Iron-core (made of iron or steel) chokes are indicated by two straight lines over the loops. Dashed lines indicate powdered iron cores.

Figure 3 Symbol for variable inductors.

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TYPES OF CAPACITORS

TYPES OF CAPACITORS

By Engr. Aneel Kumar   August 26, 2015   No comments:

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Most paper capacitors have been replaced by those made of better materials, usually plastic.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 high-frequency applications with high voltages. The ceramic type is found in circuits that use high voltages, such as television sets and radar equipment.

Figure 1 Fixed capacitors.

ELECTROLYTIC CAPACITORS

The electrolytic capacitor has a very high capacitance value when compared with the types mentioned previously. These capacitors may be tubular or square in shape. They have cardboard or metal covers. Values are printed on the cardboard cover and stamped into the metal cover (see Figure 2). They are available in a variety of shapes and sizes. One characteristic of the electrolytic capacitor is its polarity. Its terminals will have - (negative) or + (positive) marked on them. This means that the circuit power must be connected correctly to avoid damage to the electrolytic capacitor. It is not to be used on ac unless it is an AC electrolytic capacitor and so identified.

Figure 2 Electrolytic capacitor with value marked on it.

Caution: If an electrolytic capacitor marked with a -ve and a +ve is connected to ac, it will explode, and can throw its contents over an area as large as 50 square feet. Thus it can be dangerous. Some are manufactured with a small hole in them so that their contents will spew out instead of exploding. However, safety dictates that you treat all electrolytic capacitors as firecrackers and a larger one as a piece of dynamite. This is another reason for wearing eye protection when working with electric circuits.

Note that the symbols for capacitors in Figure 3 indicate the electrolytic capacitor with polarity markings.

Figure 3 Capacitor symbols with electrical polarity marked.

Electrolytic capacitors are 1 microfarad (uf) and larger in size. They can be made to operate on ac by connecting two of them back-to-back as shown in Figure 4. AC electrolytic capacitors are used in electrical motors, crossover networks in speaker systems, and other places needing large capacitances in circuits that contain ac.

FIGURE 4 Back-to-back electrolytic for ac operation.

VARIABLE CAPACITORS

Variable capacitors are used for tuning purposes in radios and televisions. In most instances, you will not need them for air-conditioning and refrigeration circuits. However, in case you do see one utilized in the electronics control unit, you can identify it by using Figure 5.

Figure 5 Variable capacitors with symbols. 

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TYPES OF RESISTORS

TYPES OF RESISTORS

By Engr. Aneel Kumar   August 26, 2015   No comments:

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 easier it is for it to dissipate heat. Since resistors put up resistance to current flow, they also drop voltage. The energy has to be dissipated as heat. Thus, the surface of the resistor must be large enough to allow the heat to be dissipated.

Figure 2 shows some fixed wire-wound resistors. These are made of high resistance wire wound on an insulating core with a ceramic coating. They usually are large enough so that the resistance of the unit can be stamped on it.

Figure2 Fixed wire-wound resistors.

The symbol for a fixed resistor is shown in Figure 3. Note how the symbols vary for different users. A is the standard EIA (Electronics Industries Association) symbol. B is usually used by foreign manufacturers and occasionally by American makers of industrial equipment. C is seldom encountered, but it is sometimes used in schematics for industrial equipment and can be seen in some refrigeration and air-conditioning electrical schematics.

Figure 3 Symbols for a fixed resistor.

VARIABLE RESISTORS

Some resistors are variable. This means that the amount of resistance can be changed. Variable resistors may be either carbon composition or wire wound. These resistors are used for special circuits. On these circuits, the amount of voltage or current that is delivered must be varied. A common example is the volume control on your radio or television set (Figure 4).

Figure 4 Variable Resistor/ Potentiometer.

Variable resistors are easily identified because they have three connections for leads. The center lead is usually the variable contact. A variable resistor that is connected into a circuit at all three points is called a potentiometer (see Figure 5).

A potentiometer is often referred to as a pot. Usually, a potentiometer is used to vary voltage. The device is connected across a voltage source by placing it directly across the battery or power source. The variable arm is then used to change the voltage that is available from the potentiometer. The rheostat is a variable resistor. It is used by connecting it in series not across the voltage source, as was the case with the potentiometer.

Rheostats are designed to handle higher currents than potentiometers. Very few rheostats are used today because their jobs are being done by semiconductors. Usually, a rheostat is connected to a circuit at only two points. A symbol for the rheostat is shown in Figure 5.

Figure 5 A rheostat symbol.

Variable resistors have a wide range of adjustments. For example, volume controls typically use carbon resistors. Resistance ratings can be adjusted from 0 to 10 million ohms. Another way to state these values is from 0 to 10 mega-ohms (mega means million).

Many potentiometers have what is called a nonlinear resistance element. This simply means that resistance does not change at a fixed, or uniform, rate as adjustments are made. Usually, they are small, or fine, changes at the low end. At the high end, settings lead to large resistance changes. This non-uniform resistance leads to what is called a tapered control. Such devices are usually used to adjust sound volumes and are called audio taper resistors.

There are also linear taper potentiometers. They have a uniform change of resistance as the settings are adjusted. They look exactly the same as the audio taper. When replacing a potentiometer, you must be very careful not to use a linear taper one in a volume control circuit or, worse yet, an audio taper in a control circuit. This is one of the things that you, as a technician, must be aware of in making repairs. Do not try to substitute a volume control of the same resistance for a control circuit potentiometer. You will find it very difficult to make the required adjustments in the control circuit.

Wattage ratings are usually marked on the rheostat or potentiometer. It is difficult to tell the wattage rating by just observing the device. It takes practice to be able to tell the difference between various wattage ratings.

TAPPED RESISTORS

Tapped resistors are used in some circuits. They have taps for easy connections. They are usually wire wound, although some are carbon. Figure 6(a) shows samples of the tapped resistor. Figure 6(b) shows the schematic representation of tapped resistors.

The ceramic coating is left off the wire where the tap is to be made. This allows a sliding connection so that the tapped resistor can be made into a variable resistor or adjusted as needed.

Figure 6 (a) Tapped resistors. (b) Schematic representation of tapped resistors.

VARIABLE RESISTORS

A variable resistor may be made of carbon or it may be wire wound. The idea behind the variable is to make it adjustable to meet the needs of the circuit. You are most familiar with the variable resistor as a volume control on a radio or television set. This is a variable carbon-composition type of resistor and controls a circuit to allow for increases or decreases in volume, as you desire.

A variable resistor has a movable contact that is used to adjust or select the resistance value between two terminals. In most uses, the variable resistor is a control device. It is made in many sizes and shapes. Figure 7(a) shows some of these types. The shafts of most variable resistors have knobs placed on them to make them easier to use. However, some are made to be adjusted by the insertion of a screwdriver blade in a slot on the resistor. Many adjustable resistors are used in controls for air-conditioning and refrigeration systems. Figure 7(b) shows the schematic representation for variable resistors.

Figure 7 (a) Types of variable resistors. (b) Schematic representation of variable resistors.

FUSIBLE RESISTORS

In some cases, the resistor has a purpose other than providing resistance. One type is used to protect the equipment or circuit against excess current surges.

This type of resistor, called a fusible resistor, is built to fail before damage is done to more expensive parts. Such units are often made to plug into a socket {see Fig. 8(a)}. Figure 8(b) shows the schematic symbol for a fusible resistor.

Figure 8 (a) Fusible resistors. (b) Fusible-resistor symbol.

TEMPERATURE COMPENSATING RESISTORS

Another type of special resistor is the temperature-compensating resistor. These are designed so that the resistance value changes in a direct or inverse relation with temperature changes. Such resistors are used to provide special control of circuits that must be extremely stable in operation. The symbol is shown in Figure 9.

Figure 9 Symbol for temperature-compensating resistor.

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SOURCES OF ELECTRICITY

SOURCES OF ELECTRICITY

By Engr. Aneel Kumar   August 24, 2015   No comments:

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 electricity. The crystal cartridge of an inexpensive record player is a good example. The needle causes a changing pressure. The crystal produces a changing voltage in step with the grooves in the record.

5) MAGNETISM

The most common method of generating electrical power is by turning a coil of wire in a magnetic field. This is the method the power station uses to generate the electric power that is used in homes, business, and industry.

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NATURE OF POLE VOLTAGE WAVEFORMS OUTPUT BY PWM INVERTERS

NATURE OF POLE VOLTAGE WAVEFORMS OUTPUT BY PWM INVERTERS

By Engr. Aneel Kumar   August 24, 2015   No comments:

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 voltage is - 0.5 Edc. The switching transition time has been neglected in accordance with the assumption of ideal switches. It is to be remembered that in voltage source inverters, meant to feed an inductive type load, the upper and lower switches of the inverter pole conduct in a complementary manner. That is, when upper switch is on the lower is off and vice-versa. Both upper and lower switches should not remain on simultaneously as this will cause short circuit across the dc bus. On the other hand one of these two switches in each pole (leg) must always conduct to provide continuity of current through inductive loads. A sudden disruption in inductive load current will cause a large voltage spike that may damage the inverter circuit and the load.

Figure: A typical pole-voltage waveform of a PWM inverter 

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PULSE WIDTH MODULATED INVERTER

PULSE WIDTH MODULATED INVERTER

By Engr. Aneel Kumar   August 24, 2015   No comments:

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 good quality sinusoidal voltage waveform of desired fundamental frequency and magnitude. For the inverter topology considered here, it may not be possible to reduce the overall voltage distortion due to harmonics but by proper switching control the magnitudes of lower order harmonic voltages can be reduced, often at the cost of increasing the magnitudes of higher order harmonic voltages. Such a situation is acceptable in most cases as the harmonic voltages of higher frequencies can be satisfactorily filtered using lower sizes of filter chokes and capacitors. Many of the loads, like motor loads have an inherent quality to suppress high frequency harmonic currents and hence an external filter may not be necessary. To judge the quality of voltage produced by a PWM inverter, a detailed harmonic analysis of the voltage waveform needs to be done.

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THREE PHASE SYSTEMS

THREE PHASE SYSTEMS

By Engr. Aneel Kumar   August 21, 2015   No comments:

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 voltage phasors, except in the case of two-phase systems. Generators of 6, 12, or even 24 phases are sometimes used with polyphase rectifiers to supply power with low levels of ripples in voltage on the do side in the range of kilowatts. Today, virtually all the power produced in the world is three-phase power with a frequency of 50 or 60 Hz. In the United States, 60 Hz is the standard frequency. Recently, six-phase power transmission lines have been proposed because of their ability to increase power transfer over existing lines and reduce electrical environmental impact. Even though other polyphase systems are feasible, the power utility industry has adopted the use of three-phase systems as the standard. Consequently, most of the generation, transmission, distribution, and heavy-power utilization of electrical energy are done using three-phase systems. A three-phase system is supplied by a three-phase generator (i.e., alternator), which consists essentially of three single-phase systems displaced in time phase from each other by one-third of a period, or 120 electrical degrees. The advantages of three-phase systems over single-phase systems are as follows:

• Less conductor material is required in the three-phase transmission of power and therefore it is more economical.

• Constant rotor torque and therefore steady machine output can be achieved.

• Three-phase machines (generators or motors) have higher efficiencies.

• Three-phase generators may be connected in parallel to supply greater power more easily than single-phase generators.

Figure 1(a) shows the structure of an elementary three-phase and two-pole ac generator (also called an alternator). Its structure has basically two parts: the stationary outside part which is called the stator and the rotating inside part which is called the rotor. The field winding is located on the rotor and is excited by a direct current source through slip rings located on the common shaft. Thus, an alternator has a rotating electromagnetic field; however, its stator windings are stationary. The elementary generator shown in Figure (1)a has three identical stator coils (aa', bb', and cc'), of one or more turns, displaced by 120° in space with respect to each other. If the rotor is driven counterclockwise at a constant speed, voltages will be generated in the three phases according to Faraday’s law, as shown in Figure (1)b. Notice that the stator windings constitute the armature of the generator (unlike dc machines where the armature is the rotor). Thus, the field rotates inside the armature. Each of the three stator coils makes up one phase in this single generator. Figure (1)b shows the generated voltage waveforms in time domain, while Figure (1)c shows the corresponding phasors of the three voltages. The stator phase windings can be connected in either wye or delta. In wye configuration, if a neutral conductor is brought out, the system is defined as a four-wire three-phase system; otherwise, it is a three-wire, three-phase system. In a delta connection, no neutral exists and therefore it is a three-wire three-phase system.

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