Lightening Arrestorhttps://drive.google.com/open?id=0ByqOGBiWj3wMWjEyb0picENWMzg
Thursday, 14 September 2017
Wednesday, 13 September 2017
Protective Relays
Experiment No : 1 Date 14-09-2017
Objective :
(i) Study the principle of induction relays
(ii) Study construction & Working of different Induction relays
Objective :
(i) Study the principle of induction relays
(ii) Study construction & Working of different Induction relays
Electromagnetic
induction relays operate on the principle of induction motor and are widely used
for protective relaying purposes involving a.c. quantities. An induction relay
essentially consists of a pivoted aluminum disc placed in two alternating
magnetic fields of the same frequency but displaced in time and space. The
torque is produced in the disc by the interaction of one of the magnetic fields
with the currents induced in the disc by the other. To understand the
production of torque in an induction relay, refer to the elementary arrangement
shown in Fig. below (i). The two a.c. fluxes φ2 and φ1 differing in phase by an
angle α induce E.M.F.s’ in the disc and cause the circulation of eddy currents
I2 and I 1
respectively. These
currents lag behind their respective fluxes by 90 Referring to Fig. below (ii)
where the two a.c.
fluxes and induced currents are shown separately for clarity, let where
Ø1 = Ø1msinwt
Ø2 = Ø2msin (wt + Ɵ)
Fig.1 Principle of Working of Induction Relay
Image Courtsey : http://electricalbaba.com/principle-of-induction-type-relays/
Where Ɵ is the phase angle by which Ø2 leads Ø1. It may be assumed with negligible error that the paths in which the rotor currents flow have negligible self-inductance, and hence rotor currents are in phase with their voltages:
iØ1 ∝ d Ø1 / dt ∝ Ø1coswt
iØ2 ∝ d Ø2 / dt ∝ Ø2cos (wt+Ɵ)
As clear from the figure shown above, the two forces F1 and F2 are in opposition, and consequently we may write the equation for the net force (F) as follows:
F = (F2 – F1)
∝ (Ø2iØ1– Ø1iØ2)
Putting the values of the quantities into equation (1), we get
F ∝ Ø1mØ2m[sin (wt + Ɵ)coswt – cos (wt + Ɵ)sinwt ]
Therefore,
F ∝ Ø1mØ2mSinƟ
(i) Shaded-pole structure.
The general arrangement of shaded-pole structure is shown in Fig. Below It consists of a pivoted aluminum disc free to rotate in the air-gap of an electromagnet. One half of each pole of the magnet is surrounded by a copper band known as shading ring. The alternating flux φs in the shaded portion of the poles will, owing to the reaction of the current induced in the ring, lag behind the flux φu in the unshaded portion by an angle α. These two a.c. fluxes differing in phase will produce the necessary torque to rotate the disc. As proved earlier, the driving torque
(ii) Watthour-meter structure.
This structure gets its name from the fact that it is used in watthour meters. The general arrangement of this type of relay is shown in Fig. 21.8. It consists of a pivoted aluminium disc arranged to rotate freely between the poles of two electro magnets. The upper electromagnet carries two windings; the pirmary and the secondary. The primary winding carries the relay current I1 while the secondary winding is connected to the winding of the lower magnet. The primary current Induces e.m.f. in the secondary and so circulates a current I2 in it. The flux φ2 Induced in the lower magnet by the current in the secondary winding of the upper magnet will lag behind φ1 by an angle α.The two fluxes φ1 and φ2 differing in phase by α will produce a driving torque on the disc proportional to φ1φ2sin α. An important feature of this type of relay is that its operation can be controlled by opening or closing the secondary winding circuit. If this circuit is opened, no flux can be set by the lower magnet however great the value of current in the primary winding may be and consequently no torque will be produced. Therefore, the relay can be made inoperative by opening its secondary winding circuit.
Fig. aside shows the general arrangement of an induction cup structure. It most closely resembles an induction motor, except that the rotor iron is stationary, only the rotor conductor portion being free to rotate. The moving element is a hollow cylindrical rotor which turns on its axis. The rotating field is produced by two pairs of coils wound on four poles as shown. The rotating field induces currents in the cup to provide the necessary driving torque. If φ 1 and φ2 represent the fluxes produced by the respective pairs of poles, then torque produced is proportional to φ1 φ2 sin α where α is the phase difference between the two fluxes. A control spring and the back stop for closing of the contacts carried on an arm are attached to the spindle of the cup to prevent the continuous rotation. Induction cup structures are more efficient torque producers than either the shaded-pole or the Watt-hour meter structures. Therefore, this type of relay has very high speed and may have an operating time less
then 0·1 second.
(iii) Induction cup structure.
Fig. aside shows the general arrangement of an induction cup structure. It most closely resembles an induction motor, except that the rotor iron is stationary, only the rotor conductor portion being free to rotate. The moving element is a hollow cylindrical rotor which turns on its axis. The rotating field is produced by two pairs of coils wound on four poles as shown. The rotating field induces currents in the cup to provide the necessary driving torque. If φ 1 and φ2 represent the fluxes produced by the respective pairs of poles, then torque produced is proportional to φ1 φ2 sin α where α is the phase difference between the two fluxes. A control spring and the back stop for closing of the contacts carried on an arm are attached to the spindle of the cup to prevent the continuous rotation. Induction cup structures are more efficient torque producers than either the shaded-pole or the Watt-hour meter structures. Therefore, this type of relay has very high speed and may have an operating time less
then 0·1 second.
Power Electronics Assignments
Power
Electronic
Assignment for Chapter:-1
1. Define PIV and SUF for
rectifier circuits.
2. Differentiate between
holding current and latching current for SCR.
3. Explain the construction
of UJT,and explain its characteristics.
4. A transformer with line
voltage ratio 440V/320V is used for a three phase full wave bridge rectifier.
Compute (1) load voltage (2) PIV.
5. The line voltage on six
phase side is 250V for a six phase half wave rectifier. Calculate (1) load
current (2) ripple frequency if the load resistance is 10 ohm.
6. Draw the circuit diagram
for three phase half wave rectifier and explain its working with the help of
necessary waveforms.
7. Draw the symbol of
IGBT,PUT,GTO,TRIAC.
8. Explain construction of
MCT.
9. Draw and explain
characteristics and construction of IGBT.
10.
State advantages of poly phase rectifier over single phase
rectifier.
11.
State different control methods for SCR and explain any one
method.
12.
Explain AC load
control using SCR.
13.
Write the name of the device and draw its symbol. 1)SITH
2)RCT.
14.
What is controlled rectifiers? Explain its need.
15.
Explain pulse control using UJT – full wave circuit.
16.
Explain three phase full wave bridge rectifier.
17.
Application of Polyphase Rectifiers.
18.
Define holding and latching current of SCR.
19.
Give the full name and draw the symbol of GTO,MOSFET,TRIAC,MCT.
20.
Why IGBT is most popular in industrial application?
21.
Give the classification of thyristor family.
22.
Explain workin5g of SCR by two transistor circuit analogy.
23.
Derive the expression for Edc and Idc for three phase half
wave rectifier.
24.
Derive the expression for Irms for three phase half wave rectifier.
25.
Derive the expression for
Idc for single phase half wave controlled rectifier.
Assignment for Chapter :-2
1. Why gate protection is
required in SCR?
2. What is thermal
resistance?
3. State condition for SCR
commutation.
4. Explain why commutation is
required for SCR.
5. What is function of freewheeling
diode?
6. Explain how can you select
heat sink for SCR.
7. Give and explain reasons
to protect SCR.
8. What is snubber circuit
and why it is useful?
9. State different methods of forced
communication of SCR and explain any one method.
10.
Draw the thermal equivalent circuit of SCR.
11.
Draw the full protection circuit for SCR combining different
protection and explain briefly.
12.
Show the importance of fast acting fuse.
13.
Explain the turn off
characteristics of SCR.
14.
Explain class A commutation method for SCR.
15.
Write short notes on class C and class D type communication.
16.
What is non linear surge absorber and how it can protect
against overvoltage.
17.
Explain power ratings of SCR.
18.
Explain thermal resistance with it’s equivalent circuit.
19.
Explain the need of protection of SCR.
20.
Explain surge current and I2t rating of SCR.
21.
State different methods of mounting of SCR. Explain any two.
22.
Explain power rating of SCR.
23.
Draw the wave shape of single phase half wave rectifier
i.
With R load
ii.
With RL load
iii.
With RL load and FW diode.
Assignment for Chapter:-3
1. Write applications of
chopper.
2. Explain John’s chopper.
3. Explain the working
principle of DC chopper.
4. Draw the circuit for class
D chopper and class E chopper.
5. Give the application of
chopper.
6. Explain the working
principle of DC chopper.
7. Draw the circuit of John’s
and Morgan chopper.
8. Explain constant frequency
control method for controlling output voltage of chopper.
9. Write in short on step up
chopper.
10.
Explain the working of step up chopper with the help of
circuit diagram.
11.
With the help of circuit diagram explain the working of class
B chopper.
12.
Describe Morgan’s chopper with the help of circuit diagram.
13.
State basic principle of DC chopper.
14.
Give classification of chopper and explain class A chopper.
15.
Draw the circuit of John’s and Morgan chopper.
16.
Explain use of chopper for speed control of slip ring
induction motor.
Assignment for ChapterChapter:-4
1. Give classification of
inverter.
2. State advantages of
PWM(Pulse Width Modulation)method.
3. State application of
cycloconverter.
4. Draw the waveform of
bridge type cycloconverter in which output frequency is one forth of the input
frequency.
5. What is cycloconverter?e
State different types of cycloconverter.
6. Tate various method of
pulse width modulation and explain sinusoidal pulse width modulation.
7. Draw and explain single
phase series inverter with its waveform.
8. Draw the circuit diagram
and wave form for single phase to single phase bridge cycloconverter.
9. What is advantages &
disadvantages of pulse width modulation.
10.
Explain single phase width modulation – SPWM.
11.
Explain the working principle of cycloconverter.
12.
Write the application of cycloconverter.
13.
Draw the circuit of single phase series inverter and explain
its working.
14.
Draw the circuit of center tapped transformer type
cycloconverter and explain its working when the load is purely resistive.
15.
Draw and explain single phase to single phase bridge type cycloconverter
for resistive load.
16.
Give classification of inverter.
17.
What are application of cycloconverter?
18.
State basic principle of inverter.
19.
State advantages and application of inverter.
20.
Draw the waveform of bridge type cycloconverter in which
output frequency is one fourth of the input frequency.
21.
Draw the circuit and waveform of single phase to single phase
bridge cycloconverter with pure resistive load.
22.
Start various methods of PWM and explain any one.
23.
Draw and explain single phase parallel inverter with its
waveform.
24.
List the active devices that can be used to build inverter.
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