static characteristics of a thyristor

Important Points About The V-I Characteristics of SCR

Forward Characteristics
When anode is positive w.r.t. cathode, the curve between V and I is called the forward characteristics.

In fig.1, OABC is the forward characteristics of SCR at IG=0.

If the supply voltage is increased from zero, a point reached (point A) when the SCR starts conducting.

Under this condition,the voltage across SCR suddenly drops as shown by dotted curve AB and most of supply voltage appears across the load resistance RL .

If proper gate current is made to flow, SCR can close at much smaller supply voltage.

Reverse Characteristics
When anode is negative w.r.t. cathode, the curve between V and I is known as reverse characteristics.

The reverse voltage does come across SCR when it is operated with a.c. supply.

If the reverse voltage is gradually increased, at first the anode current remains small (i.e. leakage current) and at some reverse voltage, avalanche breakdown occurs and the SCR starts conducting heavily in the reverse direction as shown by the curve DE.

This maximum reverse voltage at which SCR starts conducting heavily is known as reverse breakdown voltage.

SCR in Normal Operation
In order to operate the SCR in normal operation, the following points are kept in view:

The supply voltage is generally much less than breakover voltage.
The SCR is turned on by passing appropriate amount of gate current ( a few mA) and not by breakover voltage.
When SCR is operated from a.c.  supply, the peak reverse voltage which comes during negative half-cycle should not exceed the reverse breakdown voltage.
When SCR is to be turned OFF from the ON state, anode current should be reduced to holding current.
If gate current is increased above the required value, the SCR will close at much reduced supply voltage.
Important Terms In The V-I Characteristics of SCR
The following terms are much used in the study of SCR :

Breakover voltage
Peak reverse voltage
Holding current
Forward current rating
Circuit fusing rating
1.  Breakover Voltage
It is the minimum forward voltage, gate being open, at which SCR starts conducting heavily i.e. turned on.

Thus, if the breakover voltage of an SCR is 200 V, it means that it can block a forward voltage  (i.e. SCR remains open) as long as the supply voltage is less than 200 V. If the supply voltage is more than this value, then SCR will be turned on.

In practice, the SCR is operated with supply voltage less than breakover voltage and it is then turned on by means of a small voltage applied to the gate.

Commercially available SCRs have breakover voltages from about 50 V to 500 V.

2.  Peak Reverse Voltage (PRV)
It is the maximum reverse voltage  (cathode positive w.r.t. anode) that can be applied to an SCR without conducting in the reverse direction.

PRV is an important consideration while connecting an SCR in an a.c. circuit. During the negative half of a.c. supply, reverse voltage is applied across SCR. If PRV is exceeded, there may be avalanche breakdown and the SCR will be damaged if the external ciruit does not limit the current.

Commercially available SCRS have PRV ratings upto 2.5 kV.

3.  Holding Current
It is the maximum anode current, gate being open, at which SCR is turned OFF from ON condition.

When SCR is in the conducting state, it can not be turned OFF even if gate voltage is removed.

The only way to turn off or open the SCR is to reduce the supply voltage to almost zero at which point the internal transistor comes out of saturation and opens the SCR.

The anode current under this condition is very small (a few mA) and is called holding current.

Thus, if an SCR has a holding current of 5mA, it means that if anode current is made less than 5 mA, then SCR will be turned off.

4.  Forward Current Rating
It is the maximum anode current that an SCR is capable of passing without destruction.

Every SCR has a safe value of forward current which it can conduct. If the value of current exceeds this value, the SCR

SCR Thyristor summary

Silicon Controlled Rectifiers known commonly as Thyristors are three-junction PNPN semiconductor devices which can be regarded as two inter-connected transistors that can be used in the switching of heavy electrical loads. They can be latched-“ON” by a single pulse of positive current applied to their Gate terminal and will remain “ON” indefinitely until the Anode to Cathode current falls below their minimum latching level.

Static Characteristics of a Thyristor SCR

✔Thyristors are semiconductor devices that can operate only in the switching mode.
✔Thyristor are current operated devices, a small Gate current controls a larger Anode current.
✔Conducts current only when forward biased and triggering current applied to the Gate.
✔The thyristor acts like a rectifying diode once it is triggered “ON”.
✔Anode current must be greater than holding current to maintain conduction.
✔Blocks current flow when reverse biased, no matter if Gate current is applied.
✔Once triggered “ON”, will be latched “ON” conducting even when a gate current is no longer applied providing Anode current is above latching current.

What is silica gel and its purpose

Function of Silica Gel Breather

Most of the power generation companies use silica gel breathers fitted to the conservator of oil filled transformers. The purpose of these silica gel breathers is to absorb the moisture in the air sucked in by the transformer during the breathing process.

What is Transformer Breathing?

When load on transformer increases or when the transformer under full load, the insulating oil of the transformer gets heated up, expands and gets expel out in to the conservator tank present at the top of the power transformer and subsequently pushes the dry air out of the conservator tank through the silica gel breather. This process is called breathing out of the transformer.

When the oil cools down, air from the atmosphere is drawn in to the transformer. This is called breathing in of the transformer.

Use of Silica gel breather

During the breathing process, the incoming air may consist of moisture and dirt which should be removed in order to prevent any damage. Hence the air is made to pass through the silica gel breather, which will absorb the moisture in the air and ensures that only dry air enters in to the transformer. Silica gel in the breather will be blue when installed and they turn to pink colour when they absorb moisture which indicates the crystals should be replaced. These breathers also have an oil cup fitted with, so that the dust particles get settled in the cup.

Thus Silica gel breathers provide an economic and efficient means of controlling the level of moisture entering the conservator tank during the breathing process.

AC bridge circuit

The bridge circuit :
                                  These are used for measuring component values, such as resistance inductance , capacitance etc. the bridge are compare the value of an unknown
components its measured accuracy is very high

Wheatstone Bridge :
                                      It is used for the measurement of d.c. resistance and a simple version the bridge has four resistor arms together with source of emf and null dector . The bridge is said to be balanced when the potential difference across the galvanometer is zero volts so there is no current through the galvanometer hence the bridge is balanced when R1 R4=R2R3

the Wheatstone widely used for precision measurement of resistance from approximately 1ohm to the low mega-ohm range

AC Bridge :
                     AC bridge are consist four arms a source of excitation and a null detector . The power source supplies an a.c. voltage to the bridge at the desired frequency . At higher frequencies. an oscillator generally supplies the excitation voltage . The null detector respond to an ac unbalance current and in its cheapest form consist of a repair headphone . other null dector maybe ac amplifier with an output meter or an electron ray tube indicator  the condition for bridge balances is 

ż1ż4=ż2ż3

Maxwell Bridge:
                          The Maxwell bridge is used to measures an unknown inductance in term of known capacitance . Once of the ratio arms has a resistance and capacitance in parallel. 

Maxwell bridge is limited to the measurement of medium Q-Coils (1<Q<10).  It is unsuit for the meaurement of coils with very low Q value (Q<1)

Hey Bridge circuit:
                                   It is different from Maxwell bridge is having R1 in series with standard capacitor C1 instead of being in parallel.
the value or Rs and Ls are given

Hey  Bridge is suited for the measurement  of high Q inductors having Q more then 10

Schering Bridge :
                                 It is used for measurement of capacitors particularly for measuring insulating properties .In other word for phase angle very nearly 90° . arm I has a parallel combination of a capacitor and combination of a resistor and a ca0acitor and standard arm contain only a capacitor

Wein Bridge :
                         The wein bridge has a series RC combination in one arm and a parallel RC combination in the adjoining arm. The frequecy of the applied voltage is

Wein bridge is used as a notch filter in harmonic distortion analyzer . 

It is used in audio and HF oscillator as frequency determining element
   

what is Tertiary winding

What is Tertiary Winding? What is Three Winding Transformer?
In some high rating transformer, one winding in addition to its primary and secondary winding is used. This additional winding, apart from primary and secondary windings, is known as Tertiary winding of transformer. Because of this third winding, the transformer is called three winding transformer or 3 winding transformer.

Advantages of Using Tertiary Winding in Transformer
Tertiary winding is provided in electrical power transformer to meet one or more of the following requirements-

a.) It reduces the unbalancing in the primary due ato unbalancing in three phase load.

b.) It redistributes the flow of fault current.

c.) Sometime it is required to supply an auxiliary load in different voltage level in addition to its main secondary load. This secondary load can be taken from tertiary winding of three winding transformer.

d.) As the tertiary winding is connected in delta formation in 3 winding transformer, it assists in limitation of fault current in the event of a short circuit from line to neutral.

Rating of Tertiary Winding of Transformer:
Rating of tertiary winding of transformer depends upon its use. If it has to supply additional load, its winding cross - section and design philosophy is decided as per load, and three phase dead short circuit on its terminal with power flow from both sides of HV and MV.
In case it is to be provided for stabilizing purpose only, its cross-section and design has to be decided from thermal and mechanical consideration for the short duration fault currents during various fault conditions single line to ground fault being the most onerous.

Purpose of Transformer Core

In an electrical power transformer, there are primary, secondary and may be tertiary windings. The performance of a transformer mainly depends upon the flux linkages between these windings. For efficient flux linking between these windings, one low reluctance magnetic path common to all windings should be provided in the transformer. This low reluctance magnetic path in transformer is known as core of transformer.

Measuring instrument

MEASURING INSTRUMENTS
1.1 Definition of instruments
An instrument is a device in which we can determine the magnitude or value of the quantity to be measured. The measuring quantity can be voltage, current, power and energy etc.

Generally instruments are classified in to two categories.
a.) Absolute Instrument
b.) Secondary Instrument

1.2 Absolute instrument
An absolute instrument determines the magnitude of the quantity to be measured in terms of the instrument parameter. This instrument is really used, because each time the value of the measuring quantities varies. So we have to calculate the magnitude of the measuring quantity,
analytically which is time consuming. These types of instruments are suitable for laboratory use.
Example: Tangent galvanometer.

1.3 Secondary instrument
This instrument determines the value of the quantity to be measured directly. Generally these instruments are calibrated by comparing with another standard secondary instrument.
Examples of such instruments are voltmeter, ammeter and wattmeter etc. Practically secondary instruments are suitable for measurement.

Secondary instruments
a) Indicating instruments
b) Recording Integrating c)Electromechanically
d) Indicating instruments

1.3.1 Indicating instrument
This instrument uses a dial and pointer to determine the value of measuring quantity. The pointer indication gives the magnitude of measuring quantity.

1.3.2 Recording instrument
This type of instruments records the magnitude of the quantity to be measured continuously over a specified period of time.

1.3.3 Integrating instrument
This type of instrument gives the total amount of the quantity to be measured over a specified period of time.

1.3.4 Electromechanical indicating instrument
For satisfactory operation electromechanical indicating instrument, three forces are necessary.
They are
(a) Deflecting force
(b) Controlling force
(c)Damping force

1.4 Deflecting force
When there is no input signal to the instrument, the pointer will be at its zero position. To deflect the pointer from its zero position, a force is necessary which is known as deflecting force. A system which produces the deflecting force is known as a deflecting system. Generally a deflecting system converts an electrical signal to a mechanical force.

1.4.1 Magnitude effect
When a current passes through the coil it produces a imaginary bar magnet. When a soft-iron piece is brought near this coil it is magnetized. Depending upon the current direction the poles are produced in such a way that there will be a force of attraction between the coil and the soft iron piece. This principle is used in moving iron attraction type instrument.
If two soft iron pieces are place near a current carrying coil there will be a force of repulsion between the two soft iron pieces. This principle is utilized in the moving iron repulsion type instrument.
1.4.2 Force between a permanent magnet and a current carrying coil
When a current carrying coil is placed under the influence of magnetic field produced by a permanent magnet and a force is produced between them. This principle is utilized in the moving
coil type instrument.
1.4.3 Force between two current carrying coil
When two current carrying coils are placed closer to each other there will be a force of repulsion between them. If one coil is movable and other is fixed, the movable coil will move away from
the fixed one. This principle is utilized in electrodynamometer type instrument.

1.5 Controlling force
To make the measurement indicated by the pointer definite (constant) a force is necessary which will be acting in the opposite direction to the deflecting force. This force is known as controlling
force. A system which produces this force is known as a controlled system. When the external signal to be measured by the instrument is removed, the pointer should return back to the zero position. This is possibly due to the controlling force and the pointer will be indicating a steady
value when the deflecting torque is equal to controlling torque.
Td = Tc

1.5.1 Spring control
Two springs are attached on either end of spindle .The spindle is placed in jewelled
bearing, so that the frictional force between the pivot and spindle will be minimum. Two springs are provided in opposite direction to compensate the temperature error. The spring is made of
phosphorous bronze. When a current is supply, the pointer deflects due to rotation of the spindle. While spindle is
rotate, the spring attached with the spindle will oppose the movements of the pointer. The torque produced by the spring is directly proportional to the pointer deflectionθ .
TC ∝θ (1.2)

The deflecting torque produced Td proportional to ‘I’. WhenTC = Td
, the pointer will come to a
steady position. Therefore
θ ∝ I
Since, θ and I are directly proportional to the scale of such instrument which uses spring
controlled is uniform.
1.6 Damping force
The deflection torque and controlling torque produced by systems are electro mechanical. Due to inertia produced by this system, the pointer oscillates about it final steady position before coming to rest. The time required to take the measurement is more. To damp out the oscillation is quickly, a damping force is necessary. This force is produced by different systems.
(a) Air friction damping
(b) Fluid friction damping
(c) Eddy current damping
1.6.1 Air friction damping
The piston is mechanically connected to a spindle through the connecting rod  The
pointer is fixed to the spindle moves over a calibrated dial. When the pointer oscillates in clockwise direction, the piston goes inside and the cylinder gets compressed. The air pushes the
piston upwards and the pointer tends to move in anticlockwise direction.
If the pointer oscillates in anticlockwise direction the piston moves away and the pressure of the
air inside cylinder gets reduced. The external pressure is more than that of the internal pressure.
Therefore the piston moves down wards. The pointer tends to move in clock wise direction.
1.6.2 Eddy current damping
An aluminum circular disc is fixed to the spindle . This disc is made to move in the
magnetic field produced by a permanent magnet.When the disc oscillates it cuts the magnetic flux produced by damping magnet. An emf is induced in the circular disc by faradays law. Eddy currents are established in the disc since it has
several closed paths. By Lenz’s law, the current carrying disc produced a force in a direction opposite to oscillating force. The damping force can be varied by varying the projection of the
magnet over the circular disc.

Single phase motor working theory

Single Phase Induction motor:-

》The single phase induction motor operation can be described by two methods double revolving field theory and cross field theory 113 is perhaps the easier of the two explanation to understand so we will discuss the double revolving theory only

Double revolving field theory:-

》》A single phase AC current supplies the man winding that produce a pulsating magnetic field 》》Mathematically Rakul setting friend would be divided into two field which are rotating in opposite direction
》》 the interaction between the field and current induced in the road Charbagh generates opposing torque

Starting torque:-

》》Single phase motor starting torque is zero  of the pulsating  single phase magnetic flux
》》 the starting of the motor requires the generation of a rotating magnetic flux similar to the rotating flux in three phase motor
》》 two perpendicular coil That Give current 90° out of phase can generate the necessary rotating magnetic field which start the motor
》》There are single phase Motors are built with  two perpendicular winding

》 the best gift is achieved by connecting
• A  resistance
• An inductance or
• A capacitor in series with the starting winding.

》 Most frequently used is a capacitor to generate the starting torque

》 Single phase induction motor are classified and name according to the method employed to make them self starting

(I).  split phase induction motor:- started by phase motor section through the use of auxiliary for starting winding
(II). Capacitor Motors:- standard 5 phase motor connection through the use of an auxilliary  and capacitor these are 

classified as:-
(a). Capacitor start induction run motor
(b). Capacitor run induction motor
(c). capacitor start capacitor run induction motor
(III). Shaded pole induction motor:- started by the motion of the magnetic field produced by means of shading coil around

☆ split phase induction motor☆

In this motor having two winding main winding and auxiliary winding ,hence these winding are 90° electrically each other and same 90° phase angle in these  winding. there are main winding having low resistance and high reactance and auxiliary winding have high resistance and low reactance .switch motor when attain 75% of its rated speed.
  》Working:-

Is lags by V because low reactance and high resistance and Im lag V by very large due to high reactance hence its starting torque proportional to sign thita . it disconnect after attain 75% speed from rated .

Centrifugal switch operation:-
                                                        It is connected in series with the head salary winding and mounted on the rotor shaft when the motor started and reaches 75% of synchronous speed centrifugal switch is mechanically  opened and in running condition auxiliary  winding remains  cut out from the circuit
Therefore motor run only with main winding

Characteristics:-

a.) Starting torque 1.5 times to 2 times the full load torque
b.) Starting current 6 to 8 time the full load current
c.) Low operating power factor 0.55 to .65
d.) Low efficiency 60 to 65%
e.) Power rating 1/20 to 1/2 kW
f.) Speed range 2875rpm to 700 rpm

Applications :-
                            It has low starting current and moderate starting torque. It is used for easily started load and typical applications  including fan, saw, washing machine, blower, centrifugal
pumps. etc.

Capacitor induction motor:-
1.) Capacitor start motor
2.) Capacitor run motor
3.) Capacitor start capacitor run motor

Capacitor start motor :-

                                           In this motor electrolytic capacitor used for starting the motor .in this  motor 90° phase angle for starting purpose the motor . Im' and Is' Im lags V' due to high reactance of main winding in there Is'lead V' due to capacitor  Im'and Is' angle is 90° . this motor having high starting torque .it is used in Lathes , drilling , fan, etc.

Cheracteristcs :-
a.) Starting torque 3.5 to 4.5 times the full load torque
b.) ∝30° to 80° from starting torque can be improved
c.) power factor very from 0.65 to 0 .75.
d.)Power rating 1/10 to 3/4 kW

Requirement of capacitor
》 Electrolytic capacitor are used as  starting capacitor value around 50 microfarad to a few hundred micro farad
》 Capacitor designed for extremely short time duty service and is guaranteed for not more then 29 period of operation / hr each period not axceed 3 seconds

Applications :-
This motor has high starting torque and therefore is used for hard start load such as pumps compressor refrigerator units machine tools etc.

Capacitor start capacitor run 

characteristics 

a ) starting torque 3 to 3.5 time the full load
b.) 60% efficiency opprox.
c.) Power factor .09
d.) Power rating 1/8 to 3/4 HP
e.) Pull out torque up to 260% of rated torque

Applications :-
》It combines the advantage of capacitor start and permanent capacitors and used for hard to start loads such compressor ,pump conveyors etc.
》 These motors are often employed requiring a quite operating motor such as in hospitals and  studios

Shadded pole motor theory of operations :-

 Small motor requiring only small starting torque may be made self starting by using low resistance copper bands called shadding bands. The emf induced in the shadding band cause a current to flow in the band . This current oppose the change of flux to which it is due and thus there is a time lag between the fluxes in the shadded and unshadded portion of the pole . The flux in the shadded portion of tge pole lags the flux in the rottor portion . the result is like a rotating field moving in the direction from the unshadded to the shadded portion of the pole and a starting torque is produced

characteristics :-
a.) Low starting torque 0.4 to 0.8 times the full load torque
b.) Efficiency 35% approximate
c.) Power factor 0.45 approx.
d.) Pull out torque 15% of full load torque
e.) Power rating 1/300 to 20HP ( 2.5 W to 40W)

Applications :- Because their poor starting torque low power factor and poor efficiency such motor are only suitable for low power applications such as for toys, portable fan, electric clock, hair drier, humidifier , photocopy machine etc.

MCQ on Refrigerator & AC 51-105

51. One ton refrigeration corresponds to
(A) 50 kcal/ min✔
(B) 50 kcal/ hr
(C) 80 kcal/ min
(D) 80 kcal/ hr

52. The process, generally used in winter air-conditioning to warm and humidity the air, is called
(A) Humidification
(B) Dehumidification
(C) Heating and humidification✔
(D) Cooling and dehumidification

53. The leaks in a refrigeration system using Freon are detected by
(A) Halide torch which on detection produces greenish flame lighting✔
(B) Sulphur sticks which on detection gives white smoke
(C) Using reagents
(D) Smelling

54. The reduced ambient air cooling system has
(A) One cooling turbine and one heat exchanger
(B) One cooling turbine and two heat exchangers
(C) Two cooling turbines and one heat exchanger✔
(D) Two cooling turbines and two heat exchangers

55. In vapour compression cycle, the condition of refrigerant is saturated liquid
(A) After passing through the condenser✔
(B) Before passing through the condenser
(C) After passing through the expansion throttle valve
(D) Before entering the expansion valve

56. Which of the following refrigerant has the maximum ozone depletion potential in the stratosphere?
(A) Ammonia
(B) Carbon dioxide
(C) Sulphur dioxide
(D) Fluorine✔

57. If the evaporator temperature of a plant is lowered, keeping the condenser temperature constant, the h.p. of compressor required will be
(A) Same
(B) More✔
(C) Less
(D) More/less depending on rating

58. Hydrogen is used in Electrolux refrigeration system so as to _________ the rate of evaporation of the liquid ammonia passing through the evaporator.
(A) Equalize
(B) Reduce
(C) Increase✔
(D) None of these

59. Pick up the wrong statement. A refrigerant should have
(A) Tow specific heat of liquid
(B) High boiling point✔
(C) High latent heat of vaporisation
(D) Higher critical temperature

60. The pressure at the inlet of a refrigerant compressor is called
(A) Suction pressure✔
(B) Discharge pressure
(C) Critical pressure
(D) Back pressure

61. Condensing temperature in a refrigerator is the temperature
(A) Of cooling medium
(B) Of freezing zone
(C) Of evaporator
(D) At which refrigerant gas becomes liquid✔

62. In aircraft, air refrigeration Cycle is used because of
(A) Low weight per tonne of refrigeration✔
(B) High heat transfer rate
(C) Low temperature at high altitudes
(D) Higher coefficient of performance

63. Highest pressure encountered in a refrigeration system should be
(A) Critical pressure of refrigerant
(B) Much below critical pressure✔
(C) Much above critical pressure
(D) Near critical pressure

64. The refrigerant used for absorption refrigerators working on heat from solar collectors is a mixture of water and
(A) Carbon dioxide
(B) Sulphur dioxide
(C) Lithium bromide✔
(D) R-12

65. One ton of the refrigeration is
(A) The standard unit used in refrigeration problems
(B) The cooling effect produced by melting 1 ton of ice
(C) The refrigeration effect to freeze 1 ton of water at 0°C into ice at 0°C in 24 hours✔
(D) The refrigeration effect to produce 1 ton of ice at NTP conditions

66. The dry bulb temperature during sensible heating of air
(A) Remains constant
(B) Increases✔
(C) Decreases
(D) None of these

67. Vertical lines on pressure-enthalpy chart show constant
(A) Pressure lines
(B) Temperature lines
(C) Total heat lines✔
(D) Entropy lines

68. During heating and dehumidification process, dry bulb temperature
(A) Remains constant
(B) Increases✔
(C) Decreases
(D) None of these

69. The evolution of heat of solution takes place in ammonia absorption plant when
(A) Ammonia vapour goes into solution✔
(B) Ammonia vapour is driven out of solution
(C) Lithium bromide mixes with ammonia
(D) Weak solution mixes with strong solution

70. In a pressure enthalpy chart, the space to the left of the saturated liquid line represents
(A) Wet vapour region
(B) Superheated vapour region
(C) Sub-cooled liquid region✔
(D) None of these

71. In vapour compression cycle, the condition of refrigerant is high pressure saturated liquid
(A) After passing through the condenser
(B) Before passing through the condenser
(C) After passing through the expansion or throttle valve
(D) Before entering the expansion valve✔

72. In a bootstrap air evaporative cooling system, the evaporator is provided
(A) Between the combustion chamber and the first heat exchanger
(B) Between the first heat exchanger and the secondary compressor
(C) Between the secondary compressor and the second heat exchanger
(D) Between the second heat exchanger and the cooling turbine✔

73. The COP of a domestic refrigerator
(A) Is less than 1
(B) Is more than 1✔
(C) Is equal to 1
(D) Depends upon the make

74. The air cooling system mostly used in transport type aircrafts is
(A) Simple air cooling system
(B) Simple evaporative air cooling system
(C) Bootstrap air cooling system✔
(D) All of these

75. The higher temperature in vapour compression cycle occurs at
(A) Receiver
(B) Expansion valve
(C) Evaporator
(D) Compressor discharge✔

76. Dry bulb temperature is the temperature of air recorded by a thermometer, when
(A) It is not affected by the moisture present in the air✔
(B) Its bulb is surrounded by a wet cloth exposed to the air
(C) The moisture present in it begins to condense
(D) None of the above

77. In refrigerators, the temperature difference between the evaporating refrigerant and the medium being cooled should be
(A) High, of the order of 25°
(B) As low as possible (3 to 11°C)✔
(C) Zero
(D) Any value

78. The evaporator changes the low pressure liquid refrigerant from the expansion valve into
(A) High pressure liquid refrigerant
(B) Low pressure liquid and vapour refrigerant
(C) Low pressure vapour refrigerant✔
(D) None of these

79. Choose the correct statement
(A) A refrigerant should have low latent heat
(B) If operating temperature of system is low, then refrigerant with low boiling point should be used✔
(C) Pre-cooling and sub-cooling bf refrigerant are same
(D) Superheat and sensible heat of a refrigerant are same

80. Carbon dioxide is
(A) Colourless
(B) Odourless
(C) Non-flammable
(D) All of these✔

81. Reducing suction pressure in refrigeration cycle
(A) Lowers evaporation temperature
(B) Increases power required per ton of refrigeration
(C) Lowers compressor capacity because vapour is lighter
(D) All of the above✔

82. The coefficient of performance of a domestic refrigerator is ________ as compared to a domestic air-conditioner.
(A) Same
(B) Less✔
(C) More
(D) None of these

83. If a gas is to be liquefied, its temperature must be
(A) Increased to a value above its critical temperature
(B) Reduced to a value below its critical temperature
(C) Equal to critical temperature✔
(D) None of the above

84. The capacity of a domestic refrigerator is in the range of
(A) 0.1 to 0.3 TR✔
(B) 1 to 3 TR
(C) 3 to 5 TR
(D) 5 to 7 TR

85. The lowest thermal diffusivity is of
(A) Iron
(B) Lead
(C) Aluminium
(D) Rubber✔

86. In a vapour compression cycle, the refrigerant immediately after expansion valve is
(A) Liquid
(B) Sub-cooled liquid
(C) Saturated liquid
(D) Wet vapour✔

87. Which of the following statement is correct for ammonia as a refrigerant?
(A) It is toxic to mucous membranes.
(B) It requires large displacement per TR compared to fluoro carbons.
(C) It reacts with copper and its alloys.
(D) All of these✔

88. Critical pressure of a liquid is the pressure
(A) Above which liquid will remain liquid✔
(B) Above which liquid becomes gas
(C) Above which liquid becomes vapour
(D) Above which liquid becomes solid

89. The optimum effective temperature for human comfort is
(A) Higher in winter than in summer
(B) Lower in winter than in summer✔
(C) Same in winter and summer
(D) Not dependent on season

90. Formation of frost on evaporator in refrigerator
(A) Results in loss of heat due to poor heat transfer✔
(B) Increases heat transfer rate
(C) Is immaterial
(D) Can be avoided by proper design

91. A one tonne refrigerating machine means that
(A) One tonne is the total mass of machine
(B) One tonne refrigerant is used
(C) One tonne of water can be converted into ice
(D) One tonne of ice when melts from and at 0° C in 24 hours, the refrigeration effect is equivalent to 210 kJ/min✔

92. The suction pipe diameter of refrigerating unit compressor in comparison to delivery side is
(A) Bigger✔
(B) Smaller
(C) Equal
(D) Smaller/bigger depending on capacity

93. The coefficient of performance (C.O.P.) of a refrigerator working as a heat pump is given by
(A) (C.O.P.)P = (C.O.P.)R + 2
(B) (C.O.P.)P = (C.O.P.)R + 1✔
(C) (C.O.P)P = (C.O.P)R - 1
(D) (C.O.P)P = (C.O.P)R

94. Presence of moisture in a refrigerant affects the working of
(A) Compressor
(B) Condenser
(C) Evaporator
(D) Expansion valve✔

95. During heating and humidification, the final relative humidity of air
(A) Can be lower or higher than that of the entering air✔
(B) Is lower than that of the entering air
(C) Is higher than that of the entering air
(D) None of the above

96. Which of the following cycles uses air as the refrigerant?
(A) Ericson
(B) Stirling
(C) Carnot
(D) Bell Coleman✔

97. The curved lines on a psychrometric chart indicates
(A) Dry bulb temperature
(B) Wet bulb temperature
(C) Dew point temperature
(D) Relative humidity✔

98. On the pressure-enthalpy diagram, condensation and desuperheating is represented by a horizontal line because the process
(A) Involves no change in volume
(B) Takes place at constant temperature
(C) Takes place at constant entropy
(D) Takes place at constant pressure✔

99. When the temperature of the surrounding is higher than the temperature of the body, then the heat loss by convection from the body to the surrounding will be
(A) Positive
(B) Negative✔
(C) Zero
(D) None of these

100. The general rule for rating refrigeration systems (excepting for CO₂ system) is to approximate following h.p. per ton of refrigeration
(A) 0.1 to 0.5 h.p. per ton of refrigeration
(B) 0.5 to 0.8 h.p. per ton of refrigeration
(C) 1 to 2 h.p. per ton of refrigeration✔
(D) 2 to 5 h.p. per ton of refrigeration

101. The atmospheric air at dry bulb temperature of 15°C enters a heating coil maintained at 40°C. The air leaves the heating coil at 25°C. The bypass factor of the heating coil is
(A) 0.376
(B) 0.4
(C) 0.6✔
(D) 0.67

102. The change in evaporator temperature in a refrigeration cycle, as compared to change in condenser temperature, influences the value of C.O.P.
(A) More✔
(B) Less
(C) Equally
(D) Unpredictable

103. Most thermostatic expansion valves are set for a superheat of
(A) 5°C✔
(B) 10°C
(C) 15°C
(D) 20°C

104. In a refrigeration system, heat absorbed in comparison to heat rejected is
(A) More✔
(B) Less
(C) Same
(D) More for small capacity and less for high capacity

105. A thermostatic expansion valve in a refrigeration system
(A) Ensures the evaporator completely filled with refrigerant of the load✔
(B) Is suitable only for constant load systems
(C) Maintains different temperatures in evaporator in proportion to load
(D) None of the above

MCQ on Refrigerator & AC 1-50

1. Freon group of refrigerants are
(A) Inflammable
(B) Toxic
(C) Non-inflammable and toxic
(D) Nontoxic and non-inflammable✔

2. The boiling point of ammonia is
(A) -10.5°C
(B) -30°C
(C) -33.3°C✔
(D) -77.7°C

3. For obtaining high COP, the pressure range of compressor should be
(A) High
(B) Low✔
(C) Optimum
(D) Any value

4. A reversible engine has ideal thermal efficiency of 30%. When it is used as a refrigerating machine with all other conditions unchanged, the coefficient of performance will be
(A) 1.33
(B) 2.33✔
(C) 3.33
(D) 4.33

5. Cooling water is required for following equipment in ammonia absorption plant
(A) Condenser
(B) Evaporator
(C) Absorber
(D) Condenser, absorber and separator (rectifier)✔

6. The freezing point of sulphur dioxide is
(A) -56.6°C
(B) -75.2°C✔
(C) -77.7°C
(D) -135.8°C

7. Mass flow ratio of NH₃ in comparison to Freon-12 for same refrigeration load and same temperature limits is of the order of
(A) 1: 1
(B) 1: 9✔
(C) 9: 1
(D) 1: 3

8. In a refrigeration system, the expansion device is connected between the
(A) Compressor and condenser
(B) Condenser and receiver
(C) Receiver and evaporator✔
(D) Evaporator and compressor

9. The vapour compression refrigerator employs the following cycle
(A) Rankine
(B) Carnot
(C) Reversed Rankine
(D) Reversed Carnot✔

10. In actual air-conditioning applications for R-12 and R-22, and operating at a condenser temperature of 40° C and an evaporator temperature of 5° C, the heat rejection factor is about
(A) 1
(B) 1.25✔
(C) 2.15
(D) 5.12

11. Rating of a domestic refrigerator is of the order of
(A) 0.1 ton✔
(B) 5 tons
(C) 10 tons
(D) 40 tons

12. A human body feels comfortable when the heat produced by the metabolism of human body is equal to the
(A) Heat dissipated to the surroundings
(B) Heat stored in the human body
(C) Sum of (A) and (B)✔
(D) Difference of (A) and (B)

13. The bank of tubes at the back of domestic refrigerator is
(A) Condenser tubes✔
(B) Evaporator tubes
(C) Refrigerant cooling tubes
(D) Capillary tubes

14. In a lithium bromide absorption refrigeration system
(A) Lithium bromide is used as a refrigerant and water as an absorbent
(B) Water is used as a refrigerant and lithium bromide as an absorbent✔
(C) Ammonia is used as a refrigerant and lithium bromide as an absorbent
(D) None of the above

15. The condition of refrigerant after passing through the condenser in a vapour compression system is
(A) Saturated liquid✔
(B) Wet vapour
(C) Dry saturated vapour
(D) Superheated vapour

16. In a refrigeration cycle, the flow of refrigerant is controlled by
(A) Compressor
(B) Condenser
(C) Evaporator
(D) Expansion valve✔

17. The colour of the flame of halide torch, in case of leakage of Freon refrigerant, will change to
(A) Bright green✔
(B) Yellow
(C) Red
(D) Orange

18. For air conditioning the operation theater in a hospital, the percentage of outside air in the air supplied is
(A) Zero
(B) 20
(C) 50
(D) 100✔

19. In vapour compression cycle using NH₃ as refrigerant, initial charge is filled at
(A) Suction of compressor
(B) Delivery of compressor
(C) High pressure side close to receiver✔
(D) Low pressure side near receiver

20. The temperature of air recorded by a thermometer, when it is not affected by the moisture present in the air, is called
(A) Wet bulb temperature
(B) Dry bulb temperature✔
(C) Dew point temperature
(D) None of these

21. Absorption system normally uses the following refrigerant
(A) Freon-11
(B) Freon-22
(C) CO2
(D) Ammonia✔

22. Which of the following statement is correct?
(A) In vapour absorption refrigerator, the compression of refrigerant is avoided.
(B) Sub-cooling can be achieved by circulating more quantity of cooling water through the condenser.
(C) In vapour compression refrigeration, the vapour is drawn in the compressor cylinder during its suction stroke and is compressed adiabatically during the compression stroke.
(D) All of the above✔

23. Allowable pressure on high pressure side or ammonia absorption system is of the order of
(A) Atmospheric pressure
(B) Slightly above atmospheric pressure
(C) 24 bars
(D) 56 bars✔

24. The C.O.P. of a Carnot refrigerator in winter will be _________ as compared to C.O.P. in summer.
(A) Same
(B) Lower
(C) Higher✔
(D) None of these

25. Chaperon equation is a relation between
(A) Temperature, pressure and enthalpy
(B) Specific volume and enthalpy
(C) Temperature and enthalpy
(D) Temperature, pressure, specific volume and enthalpy✔

26. During humidification process, __________ increases.
(A) Wet bulb temperature
(B) Relative humidity✔
(C) Dry bulb temperature
(D) Specific humidity

27. Where does the lowest temperature occur in a vapour compression cycle?
(A) Condenser
(B) Evaporator✔
(C) Compressor
(D) Expansion valve

28. The ratio of actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume of saturated air at the same temperature and pressure, is called
(A) Humidity ratio
(B) Relative humidity✔
(C) Absolute humidity
(D) Degree of saturation

29. Under cooling in a refrigeration cycle
(A) Increases C.O.P✔
(B) Decreases C.O.P
(C) C.O.P remains unaltered
(D) Other factors decide C.O.P

30. In a domestic vapour compression refrigerator, the refrigerant commonly used is
(A) CO₂
(B) Ammonia
(C) R-12✔
(D) All of these

31. The COP of a vapour compression plant in comparison to vapour absorption plant is
(A) More✔
(B) Less
(C) Same
(D) More/less depending on size of plant

32. The fluids used in Electrolux refrigerator are
(A) Water and hydrogen
(B) Ammonia and hydrogen
(C) Ammonia, water and hydrogen✔
(D) None of these

33. Domestic refrigerator working on vapour compression cycle uses the following type of expansion device
(A) Electrically operated throttling valve
(B) Manually operated valve
(C) Thermostatic valve
(D) Capillary tube✔

34. The condition of refrigerant after passing through the expansion or throttle valve, in a vapour compression system is
(A) High pressure saturated liquid
(B) Wet vapour
(C) Very wet vapour✔
(D) Dry vapour

35. An important characteristic of absorption system of refrigeration is
(A) Noisy operation
(B) Quiet operation✔
(C) Cooling below 0°C
(D) Very little power consumption

36. The centrifugal compressors are generally used for refrigerants that require
(A) Small displacements and low condensing pressures
(B) Large displacements and high condensing pressures
(C) Small displacements and high condensing pressures
(D) Large displacements and low condensing pressures✔

37. Rick up the incorrect statement
(A) Lithium bromide used in vapour absorption cycle is non volatile
(B) Lithium bromide plant can't operate below 0°C
(C) A separator is used in lithium bromide plant to remove the unwanted water vapour by condensing✔
(D) Concentration of solution coming out of lithium bromide generator is more in comparison to that entering the generator

38. During dehumidification process, the relative humidity
(A) Remains constant
(B) Increases
(C) Decreases✔
(D) None of these

39. The refrigerant widely used in domestic refrigerators is
(A) Ammonia
(B) Carbon dioxide
(C) Sulphur dioxide
(D) R-12✔

40. The moisture in a refrigerant is removed by
(A) Evaporator
(B) Safety relief valve
(C) Dehumidifier
(D) Driers✔

41. During sensible cooling of air ________ decreases.
(A) Wet bulb temperature
(B) Relative humidity
(C) Dry bulb temperature✔
(D) Specific humidity

42. At lower temperatures and pressures, the latent heat of vaporisation of a refrigerant
(A) Decreases
(B) Increases✔
(C) Remain same
(D) Depends on other factors

43. The wet bulb depression is zero when relative humidity is
(A) Zero
(B) 0.5
(C) 0.75
(D) 1.0✔

44. The C.O.P of a refrigeration cycle with increase in evaporator temperature, keeping condenser temperature constant, will
(A) Increase✔
(B) Decrease
(C) Remain unaffected
(D) May increase or decrease depending on the type of refrigerant used

45. During humidification process, dry bulb temperature
(A) Remains constant✔
(B) Increases
(C) Decreases
(D) None of these

46. The vapour pressure of refrigerant should be
(A) Lower than atmospheric pressure
(B) Higher than atmospheric pressure✔
(C) Equal to atmospheric pressure
(D) Could be anything

47. Which of the following statement is wrong?
(A) The performance of the vapour compression refrigerator varies considerably with both vaporising and condensing temperatures.
(B) In vapour compression cycle, the useful part of the heat transfer is at the condenser.
(C) In ammonia-hydrogen (Electrolux) refrigerator, no compressor, pump or fan is required.
(D) The effect of under-cooling the liquid refrigerant is to decrease the coefficient of performance.✔

48. In a vapour compression system, the condition of refrigerant before passing through the condenser is
(A) Saturated liquid
(B) Wet vapour
(C) Dry saturated vapour
(D) Superheated vapour✔

49. For proper refrigeration in a cabinet, if the temperature and vapour pressure difference between cabinet and atmosphere is high, then
(A) Bigger cabinet should be used
(B) Smaller cabinet should be used
(C) Perfectly tight vapour seal should be used✔
(D) Refrigerant with lower evaporation temperature should be used

50. During sensible heating of air _________ decreases.
(A) Wet bulb temperature
(B) Relative humidity✔
(C) Dry bulb temperature
(D) Specific humidity

Application of magnetic materials

☆  Electrical motor and generator ☆

》 As the electric motor Electromegtets  in the spinning stator to turn . There is an electrically slippring  on the stator that directs the power to a different magnet section of the stator to achieve rotation Quick

☆Magnetic Storage ☆

》 Magnetic storage ifs magnetic recording are terms from engineering referrinv to the storage of data on a magnetized medium . Magnetic storage uses different patterns of magnetizing in a magnetizable material to store dtat and is a form of non volatile . The information access using one and more real wire head

☆Magnetic Bearing☆

》A magnetic bearing is a bearing which support a load using magnetic leviation . Magnetic bearing support moving machinery without physical contact for axample they can leviate a rotating shaft and permit leviate motion with very low friction load no mechanical wear .

☆Magnetic Resonance Image MRI☆

》A magnetic resonance algorithm is a type of magnetic resonance imaging scan that uses a magnetic field and pulses of ratio wave energy to provide picture of blood vessels inside the body . In many case MRI can provide information that can not be obtained from a x-rays ultrasound and computed tomography scan.

☆Eddy Current ☆

》 Eddy current brake like a conventional friction brake , is responsible for slowing object, as a train or a roller coaster . However unlike electromechanical brakes, which apply mechanical pressures on two separate objects eddy current brake slow an object by creating eddy current through electromagnetic induction which create resistance and turn it  either heat or electricity

Types of magnetism

☆ Diamagnetism
☆ Paramagnetsim
☆ Feromagnetsim
☆ Antiferromagnetsim

■ Diamagnetsim

• Diamagnetsim is a fundamental property of all matter, although it is usually very week. It is due to non co-operative behaviour of obtaining electrons when exposed to an applied magnetic field . Diamagnetic substance are compund of atoms which have no net magnetic moment . however when exposed to a filed .If we plot M vs H we see

■ Paramagnetism

• This class of material some of the atoms are ions in the material have a net magnetic moment due to unpaired electron in partially filled orbit . Once of the most important atoms with unpaired electrons in iron . However the individual magnetic moment do not interact magnetically, and like diamagnetism , the magnetization is zero when the field is removed . In the presence of the field there is now a partial alignment of the atomic magnetic moment in the direction of the filed resulting in a net positive magnetism and positive susceptibility

■ Ferromagnetism

When tou think magnetic materials, you probably think iron nickel or magnetic . Unlike paramagnetic materials, the atomic.moment is that materials exhibit very strong interactions. These interaction produce by electronic exchange force and result in a parallel or anti parallel alignment of atomic moment . Exchange fore are very large equivalent to a filed on the order of 1000Tesla or approximately a 100million times the strength of the earth field.

■ Antiferromagnetism
• The material moment of atoms or molecules usually to the spin of electron align in a regular pattern with neighbouring spins pointing in opposite direction . This is like ferromagnetism ,a manifestation of ordered magnetsim . Generally antiferromagnetic order may exist at sufficiently low temperature ., vanishing at and above a certain temp.The tempreature above the Neel temperature the material is typically paramagnetic.

☆Applications

•  Eletro motor ange generator
•  Magnetic storage
• Magnetic bearing
• Magnetic separator
• Medical appliance
• Magnetic Resonance Imaging MRI
• For security purpose
•Eddy current brake

Magnetic material and its properties

Magneticmaterial and its properties

》It is a common characteristics of materials . Magnetic properties of a material involve concept based on the magtnetic dipole moment . Soke of the material have the ability to create internal dipole moment . for this reason this kind of material present cpecial type of properties.

• Attracting other magnetic material.
• Inducing pole in this material.
• The polarity of teo pole are opposite
• When magnetizing occuring then some parameter of material is changed ( current I, Magnetic flux)

☆Magnetic moment :

• A vector quantity associated with the magnetic properties of electric current loops or, more generally magnets . It is equal to the amount of current flowing through the loop multiplied by the area encomlossed  by the loop, and its direction is established by the Right hand rule for rotations

☆Magnetic field or intensity ☆

• A field of force associated with changing electric fields, as when electric charge are in motion . Magnetic field exert defective force on moving electric charges.
• Most magnet have magnetic field as a result of the spinning motion of the electrons orbiting the atoms of which they are composed.
electromagnet create such field from electric current moving through coils

   ☆Magnetic Susceptibility ☆

• In electromagnetisim the magnetic susceptibility Xm is demensionless proportionally constant that indicate degree of magnetization of a material in responsee to an applied magnetic field

M=xH

                 ☆Hystersis☆

• In addition to the curie temperature and saturation magnetization ferromagnetism can retain a memory of an applied field once it is removed
 this behaviour is called Hystersis and a plot of the variation of magnetization with magnetic field is called hystersis loop

              ☆Curie Temperature☆

• Even though electronic exchange  force  in ferromagnets are very large , thermal energy eventually overcome the exchange and produce a rendomizing effect . This occur at a particular temperature called curie temperature Tc . Below the curie temperature the ferromagnet is ordered and above it, disordered. The saturation magnetization goes to zero at the curie temp. A typical plot of magnetization vs temp. for magnetic is shown below