Electric machine

 Three phase induction  motor

NTRODUCTION

Three-Phase
Synchronous Machines
Synchronous machines are called "synchronous" as their speed is directly related to the line
frequency.

 

          ns = 120 f/p
                                 revlm s p

where ns is the synchronous speed in rpm, 

p the number of magnetic poles designed into each
machine and

 f is the frequency in Hz of the supply voltage. Thus when two or more synchronous
machines are connected to the same supply line, they will all run in synchronism because they
are all operating at the same supply frequency. Of course, if one machine has 2 poles (Turboalternators)
and another has 12 poles, the 12-pole machine will run at precisely �th the speed of
the 2-pole machine.

 CONSTRUCTION

Rotating machines have an outside (stationary) part called the stator. The inner (rotating) part is
called the rotor. The rotor is centred within the stator so that the rotor axis is concentric with
that of the stator. The space between the outside of the rotor and inside of the stator is called the
air gap.
The rotor is mounted on a stiff rod (usually steel) called a shaft. The shaft is supported in
bearings so that the rotor is free to rotate. The shaft extends through one or both of the bearings
to provide a means to connect the machine to the mechanical system. It is to be noted that the
rotor is solidly fastened to the shaft so that the rotor and the shaft rotate at the same speed.
Therefore, the terms 'rotor speed', 'shaft speed' or machine speed all mean the same thing and are
used interchangeably.
The rotor and stator each have three basic parts, the core, the windings and the insulation.
Thus, it is proper to speak of "the rotor core", 'the stator windings' 'the rotor insulation' and so on.
The purpose of the rotor and stator cores is to conduct the magnetic field through the coils of the
windings. The cores are almost always made of iron or steel.
The stator winding is known as armature winding as it carries the load current whereas
the rotor winding is known as field winding as this winding provides exciting current or magnetiscurrent. Since the armature winding carries load current, it has to handle all of the power being

converted or transformed by the device, however, the magnetising power requirement is relatively
small. The steady state power input to a field winding is only about 1/2 to 2 per cent of the rated 
power of the machine. The input power to de field winding is all consumed as I2R loss except
during transient period which lasts for almost a second or two during which energy is being
stored in magnetic field.
The winding conductors are either of copper or aluminium and that could be in the form of
coils of wire or heavy bars depending upon the current carrying capacity required. Also each
winding consists of several coils or bars in series or in series-parallel combination depending upon
the voltage and current requirements of the machine. The ends of certain of the windings are
brought out to terminals to allow easy connection to the electrical system.
In some machines, the windings of either the rotor or the stator may be placed around
projecting magnetic pole pieces, called salient poles (salient means prominent). When a core has
salient poles the coils of the winding are wound around the waists of the pole pieces. These
narrower parts of the salient poles are called the pole cores. The shaped ends of the poles are called
the pole shoes. Their purpose is to provide the correct flux density distribution in the air gap.
Salient poles are used on the stator cores of de machines and on the rotor cores of many synchronous
machines and in both cases they carry de field windings. The mechanical weakness and air
resistance of salient poles prohibit their use on rotors oflarge high speed synchronous generator
designed to he driven hy steam or gas turbines. Turhoalternators as these are usually called
always have cylindrical rotors with the field windings embedded in slots cut into the rotor surface.
A machine having both its rotor and stator windings in slots is known as round rotor machine or
non-salient pole machine.
The insulation system consists of (i) the conductor or wire insulation e.g. superenamelled
wires or conductors ; (ii) the coil insulation usually some kind of tape or several layers of tapes
depending upon operating voltage ; and (iii) the slot lines when the windings consist of coils that
are located in slots, the coils are held in place in the slots by slot wedges. The insulation system
thus prevents short circuits between turns of a given winding coil and insulate the winding from
the iron core which is always grounded for safety's sake. The insulation system must protect the
machine against damage due to overvoltages and overcurrents that may occur during the operation
of the machine. 

Microprocessor and Microcontrollers

INSTRUCTION SET AND EXECUTION IN 8085

Based on the design of the ALU and decoding unit, the microprocessor manufacturer 

provides instruction set for every microprocessor. The instruction set consists of both 

machine code and mnemonics.

An instruction is a binary pattern designed inside a microprocessor to perform a specific 

function. The entire group of instructions that a microprocessor supports is called 

instruction set. Microprocessor instructions can be classified based on the parameters such 

functionality, length and operand addressing.

Classification based on functionality:

I. Data transfer operations:   This group of   instructions copies data from source to destination. The content of the source  is  not altered.

II. Arithmetic operations: Instructions       of    this group perform operations         like    addition,                                             subtraction,increment &                           decrement.    One    of the data used       in arithmetic     operation is 

     stored in accumulator and the                result   is  also stored in                              accumulator.

III. Logical operations: Logical operation include AND, OR, EXOR, NOT. The  operations like AND, OR and EXOR use   two operands, one is stored in 

  accumulator and other can be any         register or memory location. The result       is stored in accumulator. NOT  operation    requires single operand,  which is stored in           accumulator.

  IV. Branching operations: Instructions in   this group can be used to transfer        program sequence from one memory         location to another either conditionally      or unconditionally.

V. Machine control operations: Instruction       in this group control execution of other  instructions and control operations   like   interrupt, halt etc.

       Classification based on length:

I. One-byte instructions: Instruction          having   one byte in machine code           Examples are depicted in table

Opcode ~Operand ~Machinecode/He                                              x code                      MOV      ~    A, B            ~   78

     ADD             ~      M           ~        8 6

2.Two-byte instructions: Instruction having two byte in machine code. Examples are 

depicted in Table 

3. Three-byte instructions: Instruction having three byte in machine code. Examples are depicted in Table 

Addressing Modes in Instructions:

   The process of specifying the data to     be operated on by the instruction is       called addressing.The various                 formats for specifying operands are       called addressing modes. The 8085         has the following five types of                 addressing:

I. Immediate addressing

II. Memory direct addressing

III. Register direct addressing

IV. Indirect addressing

V. Implicit addressing

1.Immediate Addressing:

In this mode, the operand given in the instruction - a byte or word – transfers to the destination register or memory location.

Ex: MVI A, 9AH

 The operand is a part of the                      instruction.

 The operand is stored in the register      mentioned in the instruction.

 2.   Memory Direct Addressing:

    Memory direct addressing moves a        byte or word between a memory            location and register. 

    The memory location address is              given in the instruction.

Ex: LDA 850FH

       This instruction is used to load the         content of memory address 850FH         in the accumulator.

3.Register Direct Addressing:

Register direct addressing transfer a copy of a byte or word from source register to destination register.

Ex: MOV B, C

        It copies the content of register C            to register B.

4.Indirect Addressing:

       Indirect addressing transfers a             byte or word between a register             and     a memory location.

Ex: MOV A, M

     Here the data is in the memory              location pointed to by the contents of HL pair. The data is 

       moved to the accumulator.

5.Implicit Addressing.

In this addressing mode the data itself specifies the data to be operated upon. 

Ex: CMA

      The instruction complements the            content of the accumulator. No                specific data or operand is 

      mentioned in the instruction

Microprocessor and Microcontroller

 INTRODUCTION TO MICROPROCESSOR AND MICROCOMPUTER ARCHITECTURE

    A microprocessor is a programmable            electronics chip that has computing and       decision   making capabilities similar to      centra   processing unit of a    computer     Any microprocessor   based systems             having limited number of    resources           are   called microcomputers.   Nowadays,     microprocessor can be seen in almost           all types of electronics devices like                   mobile phones,    printers, washing              machines etc.   Microprocessors are also     used in    advanced applications like             radars, satellites and flights. Due to the        rapid advancements in electronic                   industry and large scale integration of           devices results in a significant cost                 reduction and increase application of            microprocessors and their derivatives.

 Bit: A bit is a single binary digit.

 Word: A word refers to the basic data         size or bit size that can be processed by        the arithmetic and logic unit of the                processor. A 16-bit binary number is              called a word in a 16-bit processor.

 Bus: A bus is a group of wires/lines that          carry similar information.

 System Bus: The system bus is a group of wires/lines used for communication 

between the microprocessor and peripherals.

 Memory Word: The number of bits that can be stored in a register or memory element is called a memory word.

 Address Bus: It carries the address,              which is a unique binary pattern used           to   identify a memory location or an             I/O     port. For example, an eight bit              address bus      has eight lines and thus         it can address 28= 256 different                    locations. The locations in hexadecimal         format can be written as 00H – FFH.

 Data Bus: The data bus is used to                    transfer data between memory and                 processor or between I/O device and              processor. For example, an 8-bit                   processor will generally have an 8-bit           data bus and a 16-bit processor will                have 16-bit data bus.

 Control Bus: The control bus carry                control signals, which consists of signals       for selection of memory or I/O device           from the given address, direction of            data transfer and synchronization of             data transfer in case of slow devices.

Power Electronic

 Thyristors – Silicon Controlled Rectifiers (SCR’s)

A silicon controlled rectifier or semiconductor-controlled rectifier is a four-layer solidstate current-

controlling device. The name "silicon controlled rectifier" is General Electric's trade name for a type of thyristor

SCRs are mainly used in electronic devices that require control of high voltage and power. This makes 

them applicable in medium and high AC power operations such as motor control function.

An SCR conducts when a gate pulse is applied to it, just like a diode. It has four layers of 

semiconductors that form two structures namely; NPNP or PNPN. In addition, it has three junctions 

labeled as J1, J2 and J3 and three terminals(anode, cathode and a gate). An SCR is diagramatically 

represented as shown below.

The anode connects to the P-type, cathode to the N-type and the gate to the P-type as shown below.

In an SCR, the intrinsic semiconductor is silicon to which the required dopants are infused. However, 

doping a PNPN junction is dependent on the SCR application

Modes of Operation in SCR

 OFF state (forward blocking mode) −                 Here the anode is assigned a positive          voltage, the gate is 

       Nnassigned a zero voltage                              (disconnected) and the cathode is              assigned   a negative voltage. As a 

  result, Junctions J1 and J3 are in forward      bias while J2 is in reverse bias. J2                  reaches its 

     breakdown avalanche value and starts       to conduct. Below this value, the         resistance of J1 is 

    significantly high and is thus said to be        in the off state.

 ON state (conducting mode) − An SCR is        brought to this state either by increasing       the potential 

       difference between the anode and                   cathode above the avalanche voltage          or by applying a positive 

     signal at the gate. Immediately the SCR          starts to conduct, gate voltage is no                longer needed to 

maintain the ON state and is, therefore, switched off by −

      Decreasing the current flow through            it     to the lowest value called holding         current

       Using a transistor placed across the            junction.

 Reverse blocking − This compensates the        drop in forward voltage. This is due to           the fact that a 

      low doped region in P1 is needed. It is          important to note that the voltage                 ratings of forward and 

     reverse blocking are equal.

Power Electronics

  Introduction to power electronics:

            Power Electronics is a field which combines Power (electric power), Electronics and Control systems. 
Power engineering deals with the static and rotating power equipment for the generation, transmission 
and distribution of electric power. Electronics deals with the study of solid state semiconductor power 
devices and circuits for Power conversion to meet the desired control objectives (to control the output 
voltage and output power). Power electronics may be defined as the subject of applications of solid state 
power semiconductor devices (Thyristors) for the control and conversion of electric power. Power 
electronics deals with the study and design of Thyristorised power controllers for variety of application 
like Heat control, Light/Illumination control and Motor control - AC/DC motor drives used in industries, 
High voltage power supplies, Vehicle propulsion systems, High voltage direct current (HVDC) 
transmission. 
Power Electronics refers to the process of controlling the flow of current and voltage and converting it to 
a form that is suitable for user loads. The most desirable power electronic system is one whose efficiency    and reliability is 100%.
                   

     Power electronic applications 

  1.  Commercial applications  Heating                 Systems Ventilating, Air Conditioners,           Central Refrigeration, 

      Lighting, Computers and Office                     equipments, Uninterruptible Power              Supplies (UPS), Elevators, and 

        Emergency Lamps 

2.Domestic applications .Cooking                        Equipments, Lighting, Heating, Air                Conditioners, Refrigerators & 

    Freezers, Personal Computers,                       Entertainment Equipments, UPS

  3. Industrial applications .Pumps,                        compressors, blowers and fans                      Machine  tools, arc furnaces,                           induction 

        furnaces, lighting control circuits,                   industrial lasers, induction heating,               welding equipments 

  4. Aerospace applications .Space shuttle           power supply systems, satellite power          systems, aircraft power 

          systems. 

     5. Telecommunications Battery                          chargers,         power supplies (DC and          UPS), mobile         cell phone battery 

       chargers 

6. Transportation  .Traction control of               electric vehicles, battery chargers for           electric vehicles, electric 

      locomotives, street cars, trolley buses,          automobile electronics including                    engine controls 

7.Utility systems . High voltage DC                      transmission (HVDC), static VAR                    compensation (SVC), Alternative 

      energy sources (wind, photovoltaic),           fuel cells, energy storage systems,                induced draft fans and boiler feed 

     water pumps 

    Types of power electronic                                converters 

1. Rectifiers (AC to DC converters): These converters convert constant ac voltage to variable dc 

output voltage.

2. Choppers (DC to DC converters): Dc chopper converts fixed dc voltage to a controllable dc 

output voltage.

3. Inverters (DC to AC converters): An inverter converts fixed dc voltage to a variable ac output 

voltage.

4. AC voltage controllers: These converters converts fixed ac voltage to a variable ac output voltage 

at same frequency.

5. Cycloconverters: These circuits convert input power at one frequency to output power at a 

different frequency through one stage conversion.              

Electrical engineering

Mcq Electrical  engineering 

1. The insulating material suitable for   low temperature applications is

     (a) cork. 

     (b) diatomaceous earth.

      (c) asbestos paper

      (d) 75 percent magnesia,

2. The quantity of heat absorbed from          the heater by convection dopends              upon

   (A) the temperature of heating element        above the surroundings

   (b) the surface area of the heater.

   (c) the position of the heater

   (d) all of the above

3.The material of the heating element          should be

    (a) such that it may withstand the                     required temperature without getting            oxidised.

    (b) of low resistivity. 

    (c) of low melting point

    (d) of high temperature coefficient, 

4. The material to be used for heating           element should be of high resistivity         so as to

(a) increase the life of the heating element.   (b) reduce the length of the heating                     element.

 (c) reduce the effect of oxidation

 (d) produce large amount of heat.

5. The material to be used for heating              element should be of low                               temperature  coefficient so as to

  (a) void initial rush of current.

 (b) avoid change in kW rating with                    temperature

 (c) reduce the effect of oxidation

 (d) both (a) and (b) above.

6.Which of the following heating                   element will have the least                       temperature  range ?

 (a) Eureka

 (b) Silicon carbon

(c) Nichrome. (d: Kanthal. 

Utilization of Electrical energy

ELECTRICAL ENGINEERING 

Some important mcq 

( UEE, illumination) 

1.Luminous flux is

   (a) the rate of energy radiation in the                form  of light waves. 

   (b) the part of light energy. radiated by              Sun  that is received on

   (c)measured in lux

2. Glare is caused due to

(a) excessive luminance

(b) excessive lighting contrast in the field of vision 

(c) either (a) or (b)

3. Unit of illumination is

(a) lumen

 (c) lux.

(b) lambert 

(d) steradians.

4. The illumination at a surface due to a      source of light placed at a distance d'       from the surface varies as 

   (a) 1/d²

   (b) l/d. 

   (c) d

   (d) d³

5.The illumination at various points             on    a horizontal surface illu-                        minated by     the same source                     varies as

 (a)   cos(θ)

 (b) cos(θ²)

 (c) cos(θ³)

 d) 1/cos(θ)

 6. Carbon arc lamps are commonly                  used  in

(a) cinema projectors.

 (b) domestic lighting

 (c) factory lighting

 (d) street lighting

7. For two filaments of same material         operating at the same tem perature,         the diameter and current through             filament are related by

       (a) diameter directly proportional to            current. 

   (b) diameter proportional to (current²)

   (c) diameter proportional to (current)⅔

   (d) none of these.

8.The melting point of tungsten is

(a) 3,400°C.

 (b) 2,800°C.

 (C) 2,600°C.

(d) 2,400C. 

9. The vacuum inside an incandescent              lamp is of the order of 

  (a) 10‐2 mm Hg.

 (b) 10‐⁴mmHg. 

  (c) 10‐⁷mm Hg.

(d) 10 ‐⁸mmHg.

1o. The rate of evaporation of Upon                  tungsten filament in a lamp                          depends  Upon  

  (a) exhaust tube diameter. 

  (b) glass shell diameter. 

   (e) vapour pressure inside. 

  (d) none of the above

11. Heat from the lamp filament is                 transmitted to the surrounding                   mainly through

 (a) circulation 

  (b) conduction

  (c) convection. 

   (d) radiation 

12. Which of the following material is            most commonly used for the                      filaments  in incandscent lamps ?

      (a) Tungsten 

      (b) Tantalum

     (c) Osmium 

     (d) Silver

13. A zero watt lamp consumes power of 

(a) zero watt.

 (b) 5-10 W.

(c) about 15 W. 

(d) about 25 W.

 14. The safe operating temperature of a        tungsten filament

(a) 1,000°C 

(b) 3,000°C. 

(c) 2,000°C: 

(d) 3,500°C

15. The output of a tungsten filament                 depends on 

     (a) size of the shell. 

      (b) size of the lamp

     (c) temperature of filament 

     (d) all of the above 

16. What percentage of the input energy         is radiated by filament lamps 

       (5) 60-70 per cent 

        (b) 40-50 per cent

       (c) 25 - 30 per cent.

        (d) 10-15 per cent.

 18. An electric bulb, when broken,                   produces bang It is on account of 

   (a) vacuum inside the bulb.

   (b) pressure inside is equal to that                      outside

   (c) pressure of air in the bulb 

     (d) none of the above. 

19. In an incandescent lamp, bird cage            filament is usually used in Vacuum            bulb so as to

    (a) increase the life span of the filament 

    (b) give uniform radiations

    (c) reduce the oxidation phenomenon 

    (d) reduce the convection loSSes.

20. Filament lamp at starting will take                 current

   (a) equal to its full running current 

   (b) more than its full running current

   (c) less than its full running current.

     (d) none of these 

21. Most affected parameter of a                      filament   lamp due to variation in            operating     voltage is 

    (a) life.

    (b) light output. 

    (c)luminous  efficiency 

   (d) wattage.

22. Which gas is sometimes used in                   filament lamps

(a) Nitrogen.

 (b) Argon

 (c) Curbon dioxide.

 (d) Krypton

23. Which of the following vapour/gas             will give yellow colour in a filamen             lamp ? 

     (a) Helium. 

    (b) Mercury. 

    (c) Magnesium. 

    (d) Sodium.

24. Magnesium vapour in a filament                  lamp    gives 

(a) green colour light.

 (by pink colour light 

(c) blue colour light. 

(d) white colour light.

25. The gas used in a gas-filled filament             lamp is

(a) helium

 (b) oxygen 

(c) nitrogen. 

(d) ozone. 

26. In filament lamps, coiled coil                         filaments are used in

(a) coloured lamps 

(b) gas-filled lamps:

(c) low wastage lamps. 

(d) higher wattage lamps. 

27. In electric discharge lamps, light is               produced by

 (a) magnetic effect of current.

 (b) heating effect of current. 

 (c) cathode ray emission

 (d) ionisation in a gas or vapour

 28. In electric discharge lamps for                     stabilization of are

(a) a choke is connected in series with the          supply

 (b) a variable resistance is connected in            series with the circuit 

(c) a condenser is connected across the            supply

Ac machine

Ac machine  , some  important mcq  

( 2018 EVEN,  polytechnic  5th sem)

1. The short-circuit characteristics of      an alternator is

      (a) Always linear

      (b) Always non-linea

     (0) Either (a) or (b)

     (d) None of the above 

2. 6-pole, 50 HZ 3-phase induction motor       is running at 950 rpm and has rotor           Cu-loss of 5 kw. Its rotor input is

    (a) 100 kw

       

     (b) 10 kw

    (c) 95 kw 

     (d) 5.3 kw

3. Alternator can be excited by

(a) A.C.

(b) None of these

(c) D.C.

(d) Both (a) and (b)

4. No. of slips-ring in a 3-phase                       synchronous motor will be

    (a) 0

    (b) 1

    (c) 2

     (d) 4

5.The power factor of an alternator is          determined by its 

   (b) Load

   (b)  Prime mover

    (c) load 

    (d) excitation 

6. A4-pole, 1200 rpm alternator will          generate emf at  

(a) 50 Hz
 (b) 40 Hz
 (c)  60 Hz
(d) 25 Hz

7. In an alternator the armature flux       helps the main field flux when the      load power factor is

 (a) Unity

 (b) zero lagging

 (c) 0.8 lagging 

(d) Zero leading

8. In an alternator damper winding are       used to

 (a) Reduce eddy current loss

(b) Prevent hunting

(c) Reduce armature reaction 

(d) None of these

9.The maximum possible speed at                 which  alternator can be driven to             generate 60 Hz and 4000 volts is

(a) 3600 rpm

(b) 3000 rpm

(c) 4000 rpm 

(d) 1500 rpm

10. In a 3-phase synchronous motor, the       magnitude of field  flux 

(a) Remains constant at all loads 

(b) Varies with load 

(c) Varies with speed 

(d) Varies with power factor

 11.When a synchronous motor is                    running at synchronous speed, the            damper winding produces 

 (a) Damping torque

 (6) Eddy current torque

(c) Torque adding the developed torque

(d) No torque

12. A 3-phase, 4 pole , 50 Hz induction              motor is    running at 1425 rpm                    while supplying full load. Its slip  is

      (a) 5%

       (b) 4% 

       (c) 3%

      (d) 10%

13. Rotating magnetic field is produced           in 

(a) 3 phase  induction  motor 

(b) dc series  motor 

(c) Ac series  motor 

(d) none of these

15 .The power factor of an induction               motor at full load  is around 

(a) Unity 

(b) 0.85 lagging

(c) 0.5 leading 

(d) 0.85 Leading 

15.A ceiling fan uses

(c) Cupacitor start motor

 (d ) Capacitor start capacitor run motor

(d) None of these

16. Stator of an induction motor is made         of 

(a) Carbon 

(b) Wood

(c) Silicon steel laminations

 (d) None of these 

17. The motor used in electric toys

(a) Capacitor start motor

 (c) Shaded pole motor

(c) shaded pole motor 

(d)  none of above 

18 .The short circuit characteristic of an        alternator is

(a) always  linear 

(b) Always  non linear 

(c) Either  A ans B 

(d) none  of  the  above 

19. Alternator are rated  in

(a) MW

(b) KVAR

(c) KWH 

(d)KWA

20.No of slips rings  in a 3 phase    synchronous  motor  will be 

(a) 0 

(b) 1 

(c) 2

(d) 4

Utilization of Electrical energy

         Some  important  questions 

(20 . Mcq for Electrical engineering              students ,  ILLUMINATION)

1. The filament of a GLS lamp is made      of

(a) aluminium. (c) carbon

(b) tungsten.     (d) copper

2.Which of the following is not the            GLS lamp of the standard rating ?

(a) 15 W  (b) 40 W  (c) 75 W  (d) 100 W. 

3. The luminous efficiency of GLS               lamps is normally in the range                of....lumens/watt

       (a) 100 to 150 

        (b) 75 to 100

         (c) 50 to 75

         (d) 10 to 18

4. Nitrogen or argon is filled in GLS           lamps so as to

 (a) retard evaporation of tungsten                       filament.

(b) improve efficiency

 c) change the colour of the light

(d) reduce the glare.

5. In case of frosted GLS lamps,                  frosting of shell is done by 

   (a) acid etching .

   (b) ammonia

    (c) ozone

   (d) salt water

6. In case of GLS lamps the increase         in   supply voltage reduces

  (a) power consumption 

  (b) lumens output

  (c) life: 

(d) efficiency

7.Sodium vapour lamp needs an               ionisation voltage of about 

(a) 5 V. 

(b) 20 V. 

(C) 50 V. 

(d) 100 V

8. The ignition voltage for a sodium           lamp is about

(a) 100-125 V 

(c) 300-400 V

(b) 200-240 V 

(d) 400-600 V

9. When a sodium vapour lamp is               switched on initially the colour is

  (a) red.

  (b) pink. 

  (c) yellow. 

  (d) blue.

10. The colour of light given out by a           sodium vapour discharge lamp is

(a) pink

 (b) bluish green  

(c) yellow. 

(d) blue. 

11.An auto transformer used with             sodium vapour lamp should have

(a) high step-up ratio

 (b) high leakage reactance.

 (c) high step-down resistance.

(d) high efficiency

12.Leak transformer in a sodium                vapour lamp initially provides

     (b) low voltage

     (a) high current

      (c) high voltage,

13. In a sodium vapour discharge                 lamp the neon gas 

(a) changes the colour of light,

(b) prevents Vapourization of filament,

 (c) acts as a shield around the filament,

(d) assists in developing enough heat to               vapourize the sodium 

Q

14.The capacitor is used in auto-                 transformer circuit of a sodium             vapour lamp in order to

(a) regulate discharge voltage

(b) improve the circuit power factor.

(c) control lamp illuminution level. 

(d) protect the lamp against overvoltage,

15.The sodium vapour lamp (a) is               only suitable for ac and so needs          choke control

(a) is only suitable for ac and so needs                choke  control 

(b) needs capacitor in its auto-transformer        circuit to improve the power factor              which is very low (about 0.3 lagging).

(C) comes up to its rated   output in                     approximately

(d) all of the above.

16. The luminous efficiency of a                  sodium vapour lamp is....lumens           per watt

 (a) 40-50

 (b) 50-100

 (c) 120-200 

 (d) 10-12

17. Phosphor zinc silicate produces            light of 

(a) pink colour.

(b) red colour.

(c) green colour

(d) yellow colour,

18. A mercury vapour lamp gives                light o

(a) pink colour 

(b) yellow colour 

(c) greenish-blue colour 

(d) red colour

19. High pressure mercury vapour              lamps 

(a) similar in construction to sodium                  vapour lamps:

(b) need 4 or 5 minutes to attain full                   brilliancy

 (c) are generally used for general                         industrial lighting

        yards, work areas, shoping centres etc.

 (d) all of the above

20.In high pressure mercury vapour          lamps

 (a) the main electrodes are made of                      tungsten wire in from of helix

(b) the choke is provided to limit the                 current to a safe value.

(e) in addition to two main electrodes a             starting (auxiliary) elec trode is                     connected through a high resistance

(d) all of the above.

Synchronous machine

  SOME IMPORTANT MCQ FOR  ELECTRICAL STUDENTS 

(20 mcq, synchronous machine) 

1. Alternator operates  on the 

  principle of 

(a) electromagnetic induction 

(b) self induction. 

(c) mutual  induction 

(d) self or mutual  induction 

2.The armature bof an alternator 

(a) is a stationary member 

(b) is revolving  member 

(c)  is the frame  

(d) consisting of the winding  into which            current is induced 

3. In morden  alternator, the rotating      part  is 

(a) field  system 

(b) armature 

(c) armature  as well as field system

(d) none of the above 

4.The current from the stator of an         alternator is taken  out the external     load circuit  through 

(a) slip rings   

(b) commentator segments  

(c) solid  connections 

(d) carbon  brushes 

5.An alternator  is . 

(a) a polyphase synchronous machine  

     operated with dc exciter

(b) a polyphase synchronous machine                operated with ac exciter

(c) a three phase  induction machine with           prime mover.

(d) any ac generator 

6. Which  type of alternator is used in       hydroelectric power stations ?

(a) Non salient pole alternator 

(b)Turbo generator 

(c) salient pole alternator 

(d) stream  turbine  alternator 

7. In a synchronous machine the,                stator frame is made of

(a) stainless steel 

(b) CRGOS

(c) cast iron 

(d) cast steel 

8. The stator core of a synchronous          machine is built up of   .................          laminations 

(a) stainless steel 

(b) silicone steel 

(c) cast iron  

(d) cast steel 

9. The slip rings employed in a 3                phase synchronous machine are            insulated for

(a) output  rated  voltage 

(b) low voltage 

(c) very  low  voltage 

(d) very high  voltage 

10. Hydro-generators are generally            employed to run at 

(a) 500

(b) 1,000

(c) 1,500

(d) 3,000

11. The salient pole rotors have 

(a) two or four projecting poles carrying            the field  coils 

(b) a large number of projecting poles                carrying the field coils 

(c) thick steel laminations riveted together       and fixed to the rotor by a dova-tail                joint 

(d) pole faces usually  provided with slots          for damper winding 

(e) all of the above  except  (a) 

12. Large  diameter  of salient pole            field  structure,  the pole  shoes              cover about... of pole pitch 

(a) it occupies  much  less space  

(b) it saves lot of copper in stator winding 

(c) it reduces weight of the motor 

(d) number  of conductors held  in large           circumference of rotor  is very large             and so they need  not be  long 

13. The stator of modern  alternator          are wound for...... phase group 

(a) 180⁰

(b) 120⁰

(c) 60⁰

(d) 240⁰

14. For a two layer  winding  the                  number  of stator slots  is equal            to the number  of 

(a) poles  

(b) conductor 

(c) coils  side

(d) coils 

15. The armature reaction effect in a        synchronous machine depends on

(a) load current  

(b) power  factor  of  the  load 

(c) speed of  machine  

(d) both  a and b 

16. A leading power factor load on an alternator implies that it's voltage regulation shall be

(a ) positive

(b) negative 

(c) zero 

(d) any of these 

17. A pony  motor is basically a 

(a) small  induction motor 

(d) dc series motor 

(c) dc shunt  motor 

(d) double  winding ac/dc motor 

18. When a synchronous motor is                 running at synchronous     speed          ,damper winding produces

(a) damping  torque 

(b) eddy current torque 

(c) torque  aiding the developed torque 

(d) no torque 

19. While  starting  synchronous                   motor its field winding should be 

(a) kept open 

(b) short circuited 

(c) connected to a  dc source 

(d) none  of the above 

20. which of the  following  motor is           not a synchronous motor ?

(b) Hysteresis 

(b) Repulsion 

(c) inductors 

(d) Reluctance 

Utilization of Electrical Energy

Some Important MCQ for Electrical Students

1. The basic elements of an electric         drive are 

(a) electric motor and the transmission              system .

(b) electric motor, the transmission and                  control system .

(c)  the transmission and control  system.

(d)  electric motor  and conversion equipment. 

2. Electric drive is becoming more and             more popular because 

(a)  it simple, clean, compact and reliable. 

(b)  it provides  easy  and smooth control,                 flexibility in layout ,easy starting and                   facility for remote control 

(c)  it is cheaper in initial   as well  as in                     maintenance  cost

(d)  all of the above 

  3. The main  drawback of electric  drive           is    that

(a)   it is  cumbersome  drive  

(b)   it is costlier in initial  as well as in    

         maintenance cost 

(c)  Electrical  power  supply  failure makes             the drive  standstill 

(d)  all of the above  

4. An existing  workshop  is  to be                    changed  over  from  an engine  drive to      an electric  drive  . The  type  of drive          likely  to be adopted  

(a) individual  drive  

(b) group  drive   

(c) multi-motor drive  

(d) any of the  above  

5. The  type  of drive used for a paper      mill requiring  constant  speed              operation  and flexibility  of                     control  is  

(a) group  drive 

(b) multi-motor 

(c) individual  drive  or multimulti-motor 

(d) individual  drive 

6. Introduction  of automation in production  of processes  has become  possible  only  because  of  use of 

(a) an electric  drive  

(b) group  drive  

(c) individual  drive or multi-motor drive 

(d) individual drive 

7. The selection of an electric  motor         is governed  by 

(a) nature of  load  to be handled 

(b) environment  conditions 

(c) nature  of electric  supply  available 

(d) all of the  above 

8. Dc motor are still preferred  for             use in

(a) Electrical  excavator, steel  mills  and            cranes 

(b) lathe and machine  tools  

(c) flour  mills  and jaw crushers 

(d) paper  industry 

9. The least  significant  electrical               characteristic  in selection  of                 electric  motor  flour  mills  is 

(a) starting  characteristics 

(b) breaking 

(C) running  characteristics  

(d) efficiency 

10. The least significant feature                     while  selecting  a motor  for                    centrifugal  pumps  is 

(a) speed  control 

(b) power  rating  of motor 

(c) operation  speed 

(d) starting  characteristic 

11. A typical  active  load  

(a) hoist  

(b) blower 

( c) pump 

( d) lathe 

12. An elevator  drive  is required  to       operate  in 

(a) one quadrant  

(b) two quadrant  

(c) three  quadrant 

(d) four quadrant 

13. Load  torque  constant  at all                  speeds is represented  by a 

(a) fan 

(b) compressor 

(c) centrifugal  pump  load  

14. The load  for which  the motor               always  starts  on load  is  

(a) fan motor  

(b) conveyor  motor  

(c) flour  mill motor 

(d) all of  the  above 

15.Belt conveyor  affer ...... starting           torque 

(a) high  

(b) medium 

(c) low 

(d) zero 

16. The travelling   speed of a crane             varies  from  

(a) 20 to 25m/s. 

(b) 10 to  20 m/s. 

(c) 1.0 to 2.5 m/s. 

(d) 0.2 to 1.0 m/s.

17. The diameter  of the motor  shaft            for an electric  motor  depends                upon  

(a) speed  only 

(b) power output 

(c) speed and power  output  

(d) speed , power  output  and  power                   factor 

18. When an electric  train  is moving         down  hill , the dc motor  will be            operate as a dc 

(a) series  motor  

(b) series  generator 

(c) shunt  motor 

(d) shunt  generator 

19. A dc series motor  is used for an             overhauling  load. It can work                stable  if 

(a) the armature is shunted  by a resistor 

(b) the field  winding  is reversed 

(c)  a resistor  is put in series  with the                  machine 

(d) a divertor is put across  the  field 

20.which motor should  not be used           for centrifugal  pump 

(a) series  motor 

 

Electric heating and welding

 Electric  Heating 

Introduction .      When the current  is pass through  a  conductor , it gets heated  up  due  to   i^2R  losses   and this  heating   characteristic   of the  electric  current  is being  utilized  in industrial  and  domestic  appliances  

                             Heating  ia required  for domestic  purposes  such  as  cooking  and heating  of building  where as for industrial  purposes  and heating  is  required  for melting  of metals , Harding  and tempering  and in welding  . 

  

Advantage  of Electric heating  and over other  system  of  heating  :

The main advantage of electric heating over other system of heating (i.e. coal, gas  or oil)  heating  are : 

1. Economical. Electric heating is economical as Electrical furnaces are cheaper in initial cost as well as maintenance cost .it does not require any attention so there is a considerable savings in labour cost over other  systems of heating more more over the electric energy is also cheap as it is produced on large scale

2.cleanliness.  since  dust and ash are completely  eliminated in electric heating  system  so it  is clean  system and  cleaning  costs  are rendered  to a minimum. 

3. Absence  of fuel  gases.  Since  no fuel  gases  are produced in this  system.  The atmosphere  around  is clean  and pollution  free.

4.Ease of cantrol : Simple accurate and reliable temperature of furnace can be had with the help of manual or automatic devices .desired temperature can be had in electric heating system which is not convenient  in other  heating  systems 

 

5. Efficiency. It has  been  practically  found that 75 to 100%  of heat produced  by electric  heating  can be successfully  utilized  as the source  can be bought  directly  to the point where  heat  is required  there  by reducing  the losses  .

6. Automatic  protection. Automatic protection against over current or 

Overheating can be provided through suitable Switchgear in electric heating system

 

7. Better  working  conditions: Electric heating system produces no irritation noise and also the radiation losses are low . Thus working with electric furnace is convenient and cool

8.safety .  Electric heating  is  quite  safe  and responds quickly.  

9. Upper limit  temperature. There is is no upper limit to the temperature  obtainable of the  except the ability of the material to with stand heat 

10.special heating requirement: certain requirements  of heating such as uniform heating of one particular  portion of the  job without effecting others, heating of non  conducting materials, heating with no  oxidation can be met only in electric  heating system 

Heating element:

The heating  effect of the electric current  can be produced  by passing electric current  through. Heating  element  and the material  used for heating element  must  have following  properties 

1.if should have  high specific  resistance  so that  a small length of wire (R=pl/a,o=Ra/l) is sufficient to produced the required amount  heat 

2. It should have high melting point so that high temperature can be  obtained 

3. It should have low temperature Coefficient since for accurate temperature control the resistance ,should have nearly constant at all temperature and this possible only if the resistance does not change with temperature

4. It should not oxidize at Higher temperatures otherwise it's life is shortened and needs frequent replacement

The most commonly used the heating elements are either lalloy of Nickel and chromium or Nickel chromium iron, Nickel chromium aluminium ,Nickel copper. the use of iron reduces the cost but lower 

 the life of element

Design  of Heating Element. 

The heating element is used for electrical heating may be a circular or rectangular like Ribbon but Ribbon type of  element requires more wattage per unit area. hence circular heating element is preferred. by knowing the electrical input  and its voltage size and length of heating element required to produce the temperature can be calculated 

Causes of failure of heating element 

There are so many causes are there for the failure of heating element some of them are explained below

1.formation of Hot spots:  hot spots are the points  in heating element which are at Higher temperature then the main body of the element .hot spots may be due to any of the following causes;

(a).  High rate of locker oxidation may reduce the cross section of the element wire thereby increasing the resistance at that spot the thus more heat will be produced locally giving rise to the the break dawn of the element

(b) shielding of element by support  etc.will reduce the  local heat losses by radiation and causes  a rise of temperature of shielded  portion  of the  element   therefore  minimum  number  of supports  without  producing  distortion  of the  element  should  be used 

(a).Due to  too high  element    temperature   insufficient  support  for the element  or selection  of  wrong   material . Sagging  and  wrapping  of element   may  result  which may cause  uneven  spacing  of section  there by producing  hot spots.

2.Oxidation of intermittency of operation  . At high  temperature , oxide scale  is the formed  on the  heating  element  which  is continuous  and tenacious  and is so starting  that it prevents  further  oxidation  of inner   metal of element  , However  ,if the element  used quit often  layer is subjected  to thermal  stresses  due to frequent  cooling  and  heating  thereby  the  oxide  layer  cracks  and   flakes  off exposing  further  fresh  metal  to oxidation   thereby  producing  hot spots 

3 Embrittlement due to grain ,growth:    All heating  alloys  containing  iron  tend to form large brittle  grains at high  temperatures . When cold  the elements are very brittle  and  liable to rupture  easily  on slightest  handling  and jerks . 

 Modes of Transfer of heat :

  The heat from one body  to another  body  can be transferred  by any one of following  method 

1. Conduction       2.convection      3.Radiation 

1.conduction : in this method  ,heat is traveles  without  the actual  movement  of practice  (molecules) .The  flow of  heat from one part  to other  part is dependent  upon  the temperature  difference  between  these parts .  It is also  applicable   when  two bodies  at different  temperature  are joined  together  . The  hearted  molecules  of the substance  tramsfer their heat to the adjacent  molecules  and this heat  flow  will invariably   take place so long  as tnere is different  in temperature .

              For example when one end of solid is heated the molecules at that end absorb  the heat  energy and  begin to vibrate rapidly when these is molecule collide with during molecular energy is per set them with internal begin to vibrate faster to pass some on energy to their molecule the heat is transfer one molecule to another molecule without their actual movement

            If the heat is to be conducted from one object to another object, the following conditions must be met

       1.  The objects should be bodily in conctact with each other.

2.   The temperature ofthe two bodies should be different i.e. temperature gradient should exixt.

Definition of conduction:  the process in which heat is transferred from one practical to another in direction of fall of temperature without the actual movement of particles of medium is called conduction. 

  The rate of conduction of heat along a substance depends upon the temperature gradient and expressed in Mj/hr/m^2/m/c^0 or in watts /cm^2 in case of electric heating.

   In a place of thickness t meters having X-sectional area of its two parallel faces A sq.meters and temperature of two faces is T1and T2 absolute, the quantity of heat transferred through it during T hours given by 

            Q=kA

     Where K is coefficient of thermal conductivity for material in Mj/hr/m^2/m/c^0

2. Convection:

                 Def. The process of heat transference in which heat is transferred from one place another  (from hotter to colder one) by actual movement of particles of medium is calles convection. 

        For example: in cases of heater used for heating buildings,  the air in contact with a heat radiator element in a room receives heat from contact with the element. The heated air expands and rises, cold air flowing into takes place. Thus in this way the room gets heated up.

          A similar action takes place in an electric water heater, a continuous floe of water passing upwards across the immersed heating element, with the result that the whole of the water in the tank becomes hot.

        The quantity of heat absorbed from the heater bay convection depends mainaly upon the temperature of the heating element above the surrounding  and upon the size of surface of the heater. It also depends partion of the heater.

            Heat dissipation is given the following expression 

                           H=a(T2-T2)b  W/m^2 

Where a, b - constants whose values depends on the heating surface facilitates for heating etc.

       T1, T2- temperature of the heating surface and fluid in 0c.

3. Radiation:

          Def.  The process of transmission of heat in which heat energy is transferred from hotter body to colder body without heating the medium in between is called radiation. 

   For example: we receive energy from the sun by radiation through there in distance of about 150

 million Kms between sun 

Add cap


Arc heating  and Resistance  heating 

Control system ( classification of sensor)

 

Control system

  Introduction to Control Systems : 

In modern times, control systems play a very important role in our daily life. The concepts of automatic 

conllOl are fundamental and well mixed in every aspect our life touches. From a simple bread ,toaster to a 

complex modem power plant, there is a series of control principles that affect our life. Elementary acts such ru. 

walking or moving a chalk to a particular point on the blackboard illustrate control principles. Advanced 

examples of launching a satellite, regulating the generation in a power plant, tracking an enemy plane on radar 

etc., show exhaustive application of control principles. Practically an engineer has to deal with-these systems in 

any industry he lands in, and so it is necessary to be familiar with the analysis and design aspects of modern 

control systems. The principles of control theory is applicable to engineering as well as non engin~_ring fields. 

In this chapter we get familiar with elementary definitions, classification, applications and control 

tenninology. 

 $  Important Definitions : 

(1) System : "A system is an arrangement of or a combination of different physical components 

connected or related in such a manner so as to form an entire unit to attain a certain objective". Thuc; 

a system is a collection of objects etc. in such a manner so as to achieve an aim or output. Thus a 

system has an input, an output and a way to achieve this input-output combination. 

(2) Output : "The actual response obtained from a system is C;alled output": 

(3) Input : "The stimulus or excitation applied to a control system from an external source in order 

to produce the output is called input". 

(4) Control : "It means to regulate, direct or command a system so that the desired objective is 

attained". 

Combining the above definitions, 

(5) Control System : "It fs an arrangement of different physical elements  connected  in such  a manner  so as to regulate  direct or command  itself  to achieve a certain  objective 

Requirements of a Good Control System 

1..Accuracy : Accuracy . is very high feedback as any error  arising should  be connected  . Accuracy  can be improved  by  using feedback element, system becomes of feedback  element closed loop system 

improved by using feedback element. Because O · 

system In closed loop control system, steady state error tends to zero. 

2. Sensitivity : A co~trol system senses changes in output due to environmental or parametric changes, 

internal disturbances or any other parameters and corrects the same. Any control system should be 

insensitive to such parameters but very sensitive to the input signals. 

3  Noise : Noise is an undesired input signal. A good control system should be insensitive to such input 

signal. A good control system should be able to reduce the effects of wise or undesired inputs. 

4. Stability : Stability means bounded input and bounded output. In the absence of the inputs, the 

outpu!._ should tend to zero as time increases. A good control system response is stable for all variations. 

5.Bandwidth : Operating frequency range decides the bandwidth of any system. For frequency 

response of good control system, bandwidth should be large. The required output means maximum 

possible output without overshoots and it should be stable for required input frequency range. 

6.Speed : A good control system should have high speed. That is output of system should be as fast 

as possible.

  7..Oscillation : For a good control system, oscillations of output should be constant or sustained 

oscillation which follows the barkhausein's criteria.