Basic Electrical Engineering Interview Questions Part 1

Q:How tube light circuit is connected and how it works?
A:A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is provided ,the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will generate around 1000volts . This volt will capable of to break the electrons inside the tube to make electron flow. once the current passes through the tube the starter circuit will be out of part. now there is no change of supply causes choke voltage normalized and act as minimize the current.

Q:whats is MARX CIRCUIT?
A:It is used with generators for charging a number of capacitor in parallel and discharging them in series.It is used when voltage required for testing is higher than the available.

Q:What is encoder, how it function?
A:An encoder is a device used to change a signal (such as a bitstream) or data into a code. The code may serve any of a number of purposes such as compressing information for transmission or storage, encrypting or adding redundancies to the input code, or translating from one code to another. This is usually done by means of a programmed algorithm,especially if any part is digital, while most analog encoding is done with analog circuitry.

Q:What are the advantages of speed control using thyristor?
A:Advantages :
1. Fast Switching Characterstics than Mosfet, BJT, IGBT 2. Low cost 3. Higher Accurate.

Q:Why Human body feel Electric shock ?? n in an Electric train during running , We did nt feel any Shock ? why?
A:Unfortunately our body is a pretty good conductor of electricity, The golden rule is Current takes the lowest resistant path if you have insulation to our feet as the circuit is not complete (wearing rubber footwear which doing some repairs is advisable as our footwear is a high resistance path not much current flows through our body).The electric train is well insulated from its electrical system.

Q:what is the principle of motor?
A:Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movemnt is called as torque.It is based on farady's laws of electromagnet induction.

Q:What is electric traction?
A:Electric traction means using the electric power for traction system (i.e. for railways,trams, trolleys etc). Electric traction means use of the electricity for all the above machines. Now a days, magnetic traction is also used for bullet trains.and basically dc motors are used for electric traction systems.

Q:How can you start-up the 40w tube lite with 230v AC/DC without using any choke/Coil?
A:It's possible by means of Electronic chokes,otherwise it's not possible to ionize the particles in tube light with normal voltage.

Q:What is "pu" in electrical engineering?
A:Pu stands for per unit and this will be used in single line diagram of power distribution and it is like a huge electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the component is considered as base values, then all other component ratings will get back into this basis.Those values are called as pu values. (p.u=actual value/base value).

Q:Operation carried out in Thermal power stations?
A:The water is obtained in the boiler and the coal is burnt so that steam is obtained this steam is allowed to hit the turbine , the turbine which is coupled with the generator generates the electricity.

Q:Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?
A:Link is provided at a Neutral common point in the circuit from which various connection are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e load) only.So if any malfunction happen the fuse connected in the particular control circuit alone will blow off.

Q: What is the difference between electronic regulator and ordinary electrical rheostat  regulator for fans?
A:The difference between the electronic and ordinary electrical  regulator is that in electronic regulator power losses are less because as we decrease the speed the electronic regulator gives the power needed for that particular speed but in case of ordinary rheostat type regulator, the power wastage is same for every speed and no power is saved.In electronic regulator, triac is employed for speed control by varying the firing angle speed and it is controlled but in rheostatic ,control resistance is decreased by steps to achieve speed control.

Q:Why, when birds sit on transmission lines or current wires doesn't get shock? 
A:Its true that if birds touch the single one line (phase or neutral) they don't get electrical shock... if birds touch 2 lines than the circuit is closed and they get electrical shock.. so if a human touch single one line(phase) then he doesn't get shock if he is in the air (not touching - standing on the ground if he is standing on the ground then touching the line (phase) he will get a shock because the ground on what we standing is like line (ground bed - like neutral)। and in the most of electric lines the neutral is grounded..so that means that human who touch the line closes the circuit between phase and neutral.

Q:what is meant by armature reaction?
A:The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.

Q:what happen if we give 220 volts dc supply to d bulb r tube light?
A:Bulbs [devices] for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element.

Q:Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?
A:DC Series motor has high starting torque. We can not start the Induction motor and Synchronous motors on load, but can not start the DC series motor without load.

Q:what is ACSR cable and where we use it?
A:ACSR means Aluminium conductor steel reinforced, this conductor is used in transmission & distribution. 

Q:What is vaccum currcuit breaker.define with cause and where be use it Device?
A:A breaker is normally used to break a ciruit. while breaking the circuit, the contact terminals will be separated. At the time of seperation an air gap is formed in between the terminals. Due to existing current flow the air in the gap is ionised and results in the arc. various mediums are used to quench this arc in respective CB's. but in VCB the medium is vaccum gas. since the air in the CB is having vaccum pressure the arc formation is interrupted. VCB's can be used upto 11kv.

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Electric Field and Magnetic Field

When charges are separated, a space is created where forces are exerted on the charges. An electric field is such a space. Depending upon the polarity of the charges, the force is either attractive or repulsive. Therefore, we can say that static charges generate an electric field. An electric field influences the space surrounding it. Electric field strength is determined in terms of the force exerted on charges. A capacitor is a reservoir of charge. The two parallel plates of a capacitor, when connected to a voltage source, establishes an electric field between the plates. The positive terminal, or pole of the voltage source will draw electrons from plate 1 whereas the negative pole will push extra electrons on to plate 2. Voltage across the capacitor will rise. The capacitor gets charged equal to the voltage of the source. The capacitance of a capacitor is a measure of its ability to store charge. The capacitance of a capacitor is increased by the presence of a dielectric material between the two plates of the capacitor.

A current-carrying conductor or a coil produces magnetic field around it. The strength of the magnetic field produced depends on the magnitude of the current flowing through the conductor or the coil. There is presence of magnetic field around permanent magnets as well.

A magnet is a body which attracts iron, nickel, and cobalt. Permanent magnets retain their magnetic properties. Electromagnets are made from coils through which current is allowed to flow. Their magnetic properties will be present as long as current flows through the coil.

The space within which forces are exerted by a magnet is called a magnetic field. It is the area of influence of the magnet.

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Atomic Structure and Electric Charge

Several theories have been developed to explain the nature of electricity. The modern electron theory of matter, propounded by scientists Sir Earnest Rutherford and Niel Bohr considers every matter as electrical in nature. According to this atomic theory, every element is made up of atoms which are neutral in nature. The atom contains particles of electricity called electrons and protons. The number of electrons in an atom is equal to the number of protons.

The nucleus of an atom contains protons and neutrons. The neutrons carry no charge. The protons carry positive charge. The electrons revolve round the nucleus in elliptical orbits like the planets around the sun. The electrons carry negative charge. Since there are equal number of protons and electrons in an atom, an atom is basically neutral in nature.

If from a body consisting of neutral atoms, some electrons are removed, there will be a deficit of electrons in the body, and the body will attain positive charge. If neutral atoms of a body are supplied some extra electrons, the body will attain negative charge. Thus, we can say that the deficit or excess of electrons in a body is called charge.

Charge of an electron is very small. Coulomb is the unit of charge. The charge of an electron is only 1.602 × 10^–19 Coulomb (C). Thus, we can say that the number of electrons per Coulomb is the reciprocal of 1.602 × 10^–19 which equals approx. 6.28 × 10¹⁸ electrons. Therefore, charge of 6.28 × 10¹⁸ electrons is equal to 1C. When we say that a body has a positive charge of 1C, it is understood that the body has a deficit of 6.28 × 10¹⁸ electrons.

Any charge is an example of static electricity because the electrons or protons are not in motion. You must have seen the effect of charged particles when you comb your hair with a plastic comb, the comb attracts some of your hair. The work of combing causes friction, producing charge of extra electrons and excess protons causing attraction.

Charge in motion is called electric current. Any charge has the potential of doing work, i.e., of moving another charge either by attraction or by repulsion. A charge is the result of separating electrons and protons. The charge of electrons or protons has potential because it likes to return back the work that was done to produce it.

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Introduction to Alternating Current

 Last time, we studied the first part of Learn Electrical Engineering for Beginners and this is all about DC Circuits. Today, we will be dealing with our Part 2 of our module and this is all about Alternating Current Circuits.

 

So, you may now start to learn what this ac is and how it behaves. Alternating Current does not flow through a conductor in the same direction as what dc does. Instead, it flows back and forth in the conductor at the regular interval, continually reversing its direction of flow and can do so very quickly. It is measured in amperes, just as dc is measured too. Remember, one couloumb of electrons is passing a given point in a conductor in one second. This definition also applies when ac is flowing- only now some of the electrons during that 1 second flow past the given point going in one direction, and the rest flow past it going in the opposite directions.

Difference between DC and AC

The industrial applications of alternating current are widespread. These include the many types of induction motor, ranging in size employed in wind tunnels and reclamation projects, transformer equipment used in connection with welders and many kinds of control devices, communication systems, and many others.

The advantages of ac generation are, however, apparent when it is recognized that it can be accomplished economically in large power plants where fuel and water are abundant. But nowadays, solar power is becoming popular as power plants through solar panels. Moreover, generators and associated equipment may be large, an important matter in so far as cost per kilowatt is concerned; also transmission over networks of high-voltage lines to distant load centers is entirely practicable.

Transmission Lines to distant load centers

In Part 2 of Learn Electrical Engineering for Beginners, you will study the nature, behavior and uses of time-varying or alternating current. You will study the for the first time two components - the inductor and the capacitor which are frequently used to control direct as well as alternating current and voltage. The resistors, in which we all know acted in such a way as to restrict the flow of current directly. In other words, the bigger the resistor you put in, the more you restrict the current flow. The inductor and the capacitor, on the other hand, act to control the current and voltage in different ways, and you will see that they do depends on how often the current is reversed. These three components - the resistor, inductor and the capacitor are basic elements of electric and electronic circuits.

Resistor, Capacitor and Inductor Behavior in AC Circuits

As of now, you will not understand the meaning of the behavior of the given diagram shown above. But as we started the first topic of AC Circuits on my next post, you will appreciate and understand gradually what really mean by AC Circuits.

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Generation of Alternating EMF's

A voltage can be developed in a coil of wire in one of the three ways:
1. By changing the flux through the coil.
2. By moving the coil through the magnetic field.
3. By altering the direction of the flux with respect to the coil.

The first one is that voltage is said to be induced emf and in accordance with Faraday's law, its magnitude at any instant of time is given by the formula as shown below:

e = N(dΦ/dt) x 10 ^-8 volts

where N is the number turns in a coil
dΦ/dt = rate at which the flux in maxwells changes through the coil

Please take note that in this method of developing an emf, there is no physical motion of coil or magnet; the current through the exciting coil that is responsible for the magnetism is altered to change the flux through the coil in which the voltage is induced. For the second and third method mentioned above, there is actual physical motion of coil or magnet, and in altered positions of coil or magnet flux through the coil changes. A voltage developed on these ways is called a generated emf and is given by the equation:

e = Blv x 10^-8 volts

where B is the flux density in lines per square inch
l is the length of the wire, in., that is moved relative to the flux
v is the velocity of the wire, in.per sec., with respect to the flux

Two-pole single AC Generator

The figure above illustrates an elementary a-c generator. The single turn coil may be moved through the magnetic field created by two magnet poles N and S. As you can see, the ends of the coil are connected to two collectors upon which two stationary brushes rest on it. For the clockwise rotation as shown, the side of the coil on north pole N is moving vertically upward to cut the maximum flux under north pole N, while the other side of the coil on south pole S is moving vertically downward to cut the maximum flux under south pole S. After the coil is rotated one quarter of a revolution to the position as shown below:

Rotated 90 degree

the coil sides have no flux to be cut and no voltage is generated. As the coil proceeds to rotate, the side of the coil on south pole S will cut the maximum flux on north pole N. Then, the side of the coil previously on north pole N will cut the maximum flux on south pole S. With this change in the polarity that are cut by the conductors, reversal in brush potential will occur. There are two important points that would like to emphasize in connection with the rotation of the coil of wire through a fixed magnetic field:

1. The voltage changes from instant to instant.
2. The electrical polarity (+) and minus (-) changes with alternating positions under north and south poles.

In actual, ac generator rotate a set of poles that is placed concentrically within a cylindrical core containing many coils of wires. However, a moving coil inside a pair of stationary poles applies equally well to the rotating poles construction; in both arrangements there is a relative motion of one element with respect to the other.

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Generation of a Sine Wave of Voltage

There are two facts that the voltage developed in a coil of a generator changes; the first one is it changes in magnitude from instant to instant as varying values of flux are cut per second and the other one is it changes in direction as coil side change positions under north and south poles, implies that alternating emf is generated. This means that the voltage is maximum as mentioned in our last topic here when the position of the coil is just like shown in the figure below:

Initial position of the coil

and will diminish to zero as the coil rotates clockwise toward the position as shown below:

As the coil rotates clockwise

Then, as the coil continues to rotate clockwise, the polarities will change. Assuming uniform flux distribution between north and south poles, the generated voltage in a coil located from the vertical will be:

e = Em sin α

Consider the figure below for us to analyze why this relationship mentioned above happened.

Illustrating the generated voltage is proportional to sin alpha 

It was come up to the relationship between instantaneous voltage e and maximum voltage Em is that a coil side such as a, moving tangentially to a circle as indicated, cut lines of force in proportion to its vertical component of the motion. If the vector length ay in the figure above represents a constant rotating velocity, it should be obvious that vector xy is, its vertical component; the vector length ax is the horizontal component and it emphasize that motion in this direction involves no flux- cutting action. Since the velocity ratio xy/ay=sinα is also a measure of the voltage in coil side a with respect to the maximum voltage (when the coil is located horizontally) it follows that sinα is a varying proportionality factor that equates e to Em.

The equation above may be used to determine a succession of generated voltage values in a coil as it rotates through a complete revolution. This is just by computing with its selected angular displacements.

A more convenient way of representing the instantaneous voltage equation mentioned above is to draw a graph to illustrate a smooth variation of voltage with respect to the angular position of the coil, this graph is called a sine wave. The wave repeats itself and it is called a periodic, then each complete succession of values is called a cycle, while each positive or negative half of the cycle is called alternation.

Sinusoidal Voltage Wave

Now, we can say that an alternating voltage as an emf that varies in magnitude and direction periodically. Then, when the emfs are proportional to the trigonometric sine function, it is referred to a sinusoidal alternating voltage. However, there are also some periodic waves which do not follow this shape and they are called non sinusoidal waves. This topic will be covered when we reached more complicated analysis is AC Circuits.

Lets have a practical example of a problem using the equation above just for you to appreciate the presented  formula above:

Problem : The voltage in an ac circuit varies harmonically with time with a maximum of 170V. What is the instantaneous voltage when it has reached 45 degree in its cycle?

Using, e = Em sin α = 170V x sin (45 degree) = 170V x 0.71 = 120 V.

In the common 60 cycle ac circuit, there are 60 complete cycle each second; i.e. the time interval of 1 cycle is 1/60 sec. It should be noted that this corresponds to a reversal in a direction of the current every 1/120 sec. (since the direction reverses twice during each cycle).

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Introduction to Electrical Engineering

INTRODUCTION

Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical power supply. It now covers a range of subtopics including power, electronics, control systems, signal processing and telecommunications.

Electrical engineering may or may not include electronic engineering. Where a distinction is made, usually outside of the United States, electrical engineering is considered to deal with the problems associated with large-scale electrical systems such as power transmission and motor control, whereas electronic engineering deals with the study of small-scale electronic systems including computers and integrated circuits. Alternatively, electrical engineers are usually concerned with using electricity to transmit energy, while electronic engineers are concerned with using electricity to transmit information. More recently, the distinction has become blurred by the growth of power electronics.

POWER SYSTEM

Power engineering, also called power systems engineering, is a subfield of engineering that deals with the generation, transmission and distribution of electric power as well as the electrical devices connected to such systems including generators, motors and transformers. Although much of the field is concerned with the problems of three-phase AC power - the standard for large-scale power transmission and distribution across the modern world - a significant fraction of the field is concerned with the conversion between AC and DC power as well as the development of specialized power systems such as those used in aircraft or for electric railway networks.

BASICS OF ELECTRIC POWER

Electric power is the mathematical product of two quantities: current and voltage. These two quantities can vary with respect to time (AC power) or can be kept at constant levels (DC power).

Most refrigerators, air conditioners, pumps and industrial machinery use AC power whereas most computers and digital equipment use DC power (the digital devices you plug into the mains typically have an internal or external power adapter to convert from AC to DC power). AC power has the advantage of being easy to transform between voltages and is able to be generated and utilized by brushless machinery. DC power remains the only practical choice in digital systems and can be more economical to transmit over long distances at very high voltages.

The ability to easily transform the voltage of AC power is important for two reasons: Firstly, power can be transmitted over long distances with less loss at higher voltages. So in power networks where generation is distant from the load, it is desirable to step-up the voltage of power at the generation point and then step-down the voltage near the load. Secondly, it is often more economical to install turbines that produce higher voltages than would be used by most appliances, so the ability to easily transform voltages means this mismatch between voltages can be easily managed.

Solid state devices, which are products of the semiconductor revolution, make it possible to transform DC power to different voltages, build brushless DC machines and convert between AC and DC power. Nevertheless, devices utilizing solid state technology are often more expensive than their traditional counterparts, so AC power remains in widespread use.

POWER

Power Engineering deals with the generation, transmission and distribution of electricity as well as the design of a range of related devices. These include transformers, electric generators, electric motors and power electronics.

The power grid is an electrical network that connects a variety of electric generators to the users of electric power. Users purchase electricity from the grid avoiding the costly exercise of having to generate their own. Power engineers may work on the design and maintenance of the power grid as well as the power systems that connect to it. Such systems are called on-grid power systems and may supply the grid with additional power, draw power from the grid or do both.

Power engineers may also work on systems that do not connect to the grid. These systems are called off-grid power systems and may be used in preference to on-grid systems for a variety of reasons. For example, in remote locations it may be cheaper for a mine to generate its own power rather than pay for connection to the grid and in most mobile applications connection to the grid is simply not practical.

Today, most grids adopt three-phase electric power with alternating current. This choice can be partly attributed to the ease with which this type of power can be generated, transformed and used. Often, the power is split before it reaches residential customers whose low-power appliances rely upon single-phase electric power. However, many larger industries and organizations still prefer to receive the three-phase power directly because it can be used to drive highly efficient electric motors such as three-phase induction motors.

Transformers play an important role in power transmission because they allow power to be converted to and from higher voltages. This is important because higher voltages suffer less power loss during transmission. This is because higher voltages allow for lower current to deliver the same amount of power, as power is the product of the two. Thus, as the voltage steps up, the current steps down. It is the current flowing through the components that result in both the losses and the subsequent heating. These losses, appearing in the form of heat, are equal to the current squared times the electrical resistance through which the current flows, so as the voltage goes up the losses are dramatically reduced.

For these reasons, electrical substations exist throughout power grids to convert power to higher voltages before transmission and to lower voltages suitable for appliances after transmission.

COMPONENTS

Power engineering is a network of interconnected components which convert different forms of energy to electrical energy. Modern power engineering consists of three main subsystems: the generation subsystem, the transmission subsystem, and the distribution subsystem. In the generation subsystem, the power plant produces the electricity. The transmission subsystem transmits the electricity to the load centers. The distribution subsystem continues to transmit the power to the customers.

GENERATION

Generation of electrical power is a process whereby energy is transformed into an electrical form. There are several different transformation processes, among which are chemical, photo-voltaic, and electromechanical. Electromechanical energy conversion is used in converting energy from coal, petroleum, natural gas, uranium, water flow, and wind into electrical energy. Of these, all except the wind energy conversion process take advantage of the synchronous AC generator coupled to a steam, gas or hydro turbine such that the turbine converts steam, gas, or water flow into rotational energy, and the synchronous generator then converts the rotational energy of the turbine into electrical energy. It is the turbine-generator conversion process that is by far most economical and consequently most common in the industry today.

The AC synchronous machine is the most common technology for generating electrical energy. It is called synchronous because the composite magnetic field produced by the three stator windings rotate at the same speed as the magnetic field produced by the field winding on the rotor. A simplified circuit model is used to analyze steady-state operating conditions for a synchronous machine. The phasor diagram is an effective tool for visualizing the relationships between internal voltage, armature current, and terminal voltage. The excitation control system is used on synchronous machines to regulate terminal voltage, and the turbine-governor system is used to regulate the speed of the machine.

The operating costs of generating electrical energy is determined by the fuel cost and the efficiency of the power station. The efficiency depends on generation level and can be obtained from the heat rate curve. We may also obtain the incremental cost curve from the heat rate curve. Economic dispatch is the process of allocating the required load demand between the available generation units such that the cost of operation is minimized.

TRANSMISSION

The electricity is transported to load locations from a power station to a transmission subsystem. Therefore we may think of the transmission system as providing the medium of transportation for electric energy. The transmission system may be subdivided into the bulk transmission system and the sub-transmission system. The functions of the bulk transmission are to interconnect generators, to interconnect various areas of the network, and to transfer electrical energy from the generators to the major load centers. This portion of the system is called "bulk" because it delivers energy only to so-called bulk loads such as the distribution system of a town, city, or large industrial plant. The function of the sub-transmission system is to interconnect the bulk power system with the distribution system.

Transmission circuits may be built either underground or overhead. Underground cables are used predominantly in urban areas where acquisition of overhead rights of way are costly or not possible. They are also used for transmission under rivers, lakes and bays. Overhead transmission is used otherwise because, for a given voltage level, overhead conductors are much less expensive than underground cables.

The transmission system is a highly integrated system. It is referred to the substation equipment and transmission lines. The substation equipment contain the transformers, relays, and circuit breakers. Transformers are important static devices which transfer electrical energy from one circuit with another in the transmission subsystem. Transformers are used to step up the voltage on the transmission line to reduce the power loss which is dissipated on the way. A relay is functionally a level-detector; they perform a switching action when the input voltage (or current) meets or exceeds a specific and adjustable value. A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. A change in the status of any one component can significantly affect the operation of the entire system. There are three possible causes for power flow limitations to a transmission line. These causes are thermal overload, voltage instability, and rotor angle instability. Thermal overload is caused by excessive current flow in a circuit causing overheating. Voltage instability is said to occur when the power required to maintain voltages at or above acceptable levels exceeds the available power. Rotor angle instability is a dynamic problem that may occur following faults, such as short circuit, in the transmission system. It may also occur tens of seconds after a fault due to poorly damped or undamped oscillatory response of the rotor motion.

DISTRIBUTION

The distribution system transports the power from the transmission system to the customer. The distribution systems are typically radial because networked systems are more expensive. The equipment associated with the distribution system includes the substation transformers connected to the transmission systems, the distribution lines from the transformers to the customers and the protection and control equipment between the transformer and the customer. The protection equipment includes lightning protectors, circuit breakers, disconnectors and fuses. The control equipment includes voltage regulators, capacitors, relays and demand side management equipment.

ELECTRICAL MACHINES

An Electrical machine is a device that converts mechanical energy to electrical energy or vice versa, and changes AC voltage from one level to another level.

Electrical machines are divided into three parts:

GENERATOR

A generator is the device that converts mechanical energy at its prime mover to produce constant electrical energy at its output. In more technical words, it is a dynamic electrical energy machine. Generator is classified into two types: AC generator and DC generator.

The basic requirements for a dynamically induced emf to exist are the following: (1) A steady magnetic field (2) A conductor capable of carrying current (3) The conductor to move in the magnetic field

AC Generator

AC generator is the generator that converts mechanical energy at its prime mover into AC electricity.

AC generator is classified into several types:

• Asynchronous AC generator or induction AC generator, an AC generator whose field current is supplied by magnetic induction into the field windings.
• Synchronous AC generator, an AC generator whose magnetic field current is provided by a separate DC current source, either external DC source or mounted DC source.

 DC Generator

DC generator is the generator that produces DC power i.e., constant power P=V*I by taking mechanical energy as input. Example of a DC generator is dynamo.

MOTOR

Motor is the device that converts electrical energy at its input to produce mechanical energy. Motor is classified into two types: AC motor and DC motor.

AC motor is the motor that converts AC electrical energy at its input into mechanical energy. AC motor is classified into several types:

• Asynchronous motor or induction AC motor
• Synchronous motor

DC motor is the motor that converts DC electricity into mechanical energy. Its main components are stator, rotor, windings (field windings and armature windings) and commutator.

DC motor is classified into five types:

• Compounded DC motor
• Permanent magnet DC motor
• Separately excited DC motor, a DC motor whose field circuit receives power from a separate constant voltage supply.
• Series DC motor, a DC motor whose field windings consist of relatively few turns and connected in series with the armature circuit.
• Shunt DC motor, a DC motor whose field circuit receives power directly across the armature terminals.

Losses in DC motor are brush drop losses, core losses, mechanical losses and stray losses.

TRANSFORMER

Transformer is the device that converts AC voltage from one level to another level higher or lower, or even to the same level without changing the frequency. It works based on the principle of mutual induction, so its power remains approximately constant, where as frequency also remains the same.

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Multiple Choice Interview Questions On Special Motor Part 2

Ques.[1] For torque transmission over a long distance with the help of electrical wires only, which of the following two synchros are used ?

(a)CX and CT
(b)CX and CR
(c)CX and CD

(d)CT and CD

Answer :B 

Ques.[2] Squirrel-cage induction motor is finding increasing application in high-power servo systems because new methods have been found to

(a) increase its rotor resistance
(b) control its torque
(c) decrease its intertia
(d) decouple its torque and flux. 
Answer:D 

Ques.[3] The control ........................ synchro has three- phase winding both on its stator and rotor.

(a) differential
(b) transformer
(c) receiver
(d) transmitter

Answer:A 

Ques.[4] A dc servomotor is similar to a regular d.c. motor except that its design is modified to cope with

(a) electronic switching
(b) slow speeds
(c) static conditions
(d) both (b) and (c). 

Answer:D 

Ques.[4] The low-torque synchros cannot be used for

(a) torque transmission
(b) error detection
(c) instrument servos
(d) robot arm positioning.

Answer:D 

Ques.[5].Which of the following synchros are used for error detection in a servo control system ?

Must Read

•  Multiple Choice Interview Questions On Special Motor Part 1
•  Basic Interview Question Of Alternator Part 1

(a) control transmitter
(b) control transformer
(c) control receiver
(d) both (a) and (b).

Answer:D  

Ques.[6] The arrangement required for producing a rotation equal to the sum or difference of the rotation of two shafts consists of the following coupled synchros.

(a) control transmitter
(b) control receiver
(c) control differential transmitter
(d) all of the above.

Answer:D 

Ques.[7] Which of the following motor would suit applications where constant speed is absolutely essential to ensure a consistent product ?

(a) brushless dc motor
(b) disk motor
(c) permanent-magnet synchronous motor
(d) stepper motor.

Answer:C 

Ques.[8] A switched reluctance motor differs from a VR stepper motor in the sense that it

(a) has rotor poles of ferromagnetic material
(b) rotates continuously
(c) is designed for open-loop operation only
(d) has lower efficiency.

Answer:B 

Ques.[9] The electrical displacement between the two stator windings of a resolver is

(a) 120º
(b) 90º
(c) 60º
(d) 45º
Answer:B 

Ques.[10] Which of the following motor runs from a low dc supply and has permanently magnetized salient poles on its rotor ?

(a) permanent-magnet d.c. motor
(b) disk d.c. motor
(c) permanent-magnet synchronous motor
(d) brushless d.c. motor.

Answer:A 

Ques.[11] Regarding voltages induced in the three stator windings of a synchro, which statement is false ?

(a) they depend on rotor position.
(b) they are in phase.
(c) they differ in magnitude.
(d) they are polyphase voltages. 
Answer:D

Ques.[12] A dc servomotor is similar to a regular d.c. motor except that its design is modified to cope with

(a) electronic switching
(b) slow speeds
(c) static conditions
(d) both (b) and (c).

Answer:D 

Ques.[13] One of the basic requirements of a servomotor is that it must produce high torque at all

(a) loads
(b) frequencies
(c) speeds
(d) voltages.

Answer:C 

Ques.[14] The most common two-phase ac servomotor differs from the standard ac induction motor because it has

(a) higher rotor resistance
(b) higher power rating
(c) motor stator windings
(d) greater inertia.

Answer:A 

Ques.[15] Though structurally similar to a control transmitter, a control receiver differs from it in the following way :

(a) it has three-phase stator winding
(b) it has a rotor of dumbell construction
(c) it has a mechanical damper on its shaft
(d) it has single-phase rotor excitation.

Answer:C

Ques.[16] A switched reluctance motor differs from a VR stepper motor in the sense that it

(a) has rotor poles of ferromagnetic material
(b) rotates continuously
(c) is designed for open-loop operation only
(d) has lower efficiency. 

Answer:B 

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