Biyani's Think Tank Concept based notes
Industrial Electronics (B.Tech)
Apoorva Gupta Lecturer Deptt. of Electrical and Electronics Biyani Institute of Engineering and Technology, Jaipur
2 Published by :
Think Tanks Biyani Group of Colleges
Concept & Copyright :
Biyani Shikshan Samiti Sector-3, Vidhyadhar Nagar, Jaipur-302 023 (Rajasthan) Ph : 0141-2338371, 2338591-95 Fax : 0141-2338007 E-mail :
[email protected] Website :www.gurukpo.com; www.biyanicolleges.org
Edition : 2013 Price :
While every effort is taken to avoid errors or omissions in this Publication, any mistake or omission that may have crept in is not intentional. It may be taken note of that neither the publisher nor the author will be responsible for any damage or loss of any kind arising to anyone in any manner on account of such errors and omissions.
Leaser Type Setted by : Biyani College Printing Department
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Industrial Electronics
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Preface
I
am glad to present this book, especially designed to serve the needs of the
students. The book has been written keeping in mind the general weakness in understanding the fundamental concepts of the topics. The book is self-explanatory and adopts the “Teach Yourself” style. It is based on question-answer pattern. The language of book is quite easy and understandable based on scientific approach. Any further improvement in the contents of the book by making corrections, omission and inclusion is keen to be achieved based on suggestions from the readers for which the author shall be obliged. I acknowledge special thanks to Mr. Rajeev Biyani, Chairman & Dr. Sanjay Biyani, Director (Acad.) Biyani Group of Colleges, who are the backbones and main concept provider and also have been constant source of motivation throughout this Endeavour. They played an active role in coordinating the various stages of this Endeavour and spearheaded the publishing work. I look forward to receiving valuable suggestions from professors of various educational institutions, other faculty members and students for improvement of the quality of the book. The reader may feel free to send in their comments and suggestions to the under mentioned address.
Note:
A feedback form is enclosed along with think tank. Kindly fill the feedback form and submit it at the time of submitting to books of library, else NOC from Library will not be given.
Author
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4
Syllabus UNIT 1: SEMICONDUCTOR POWER DEVICES - Basic characteristics & working of Power Diodes, Diac, SCR, Triac, Power Transistor, MOSFETs, IGBT, and GTO. UNIT 2: RECTIFIERS & INVERTERS - Working principles of single and three phase bridge rectifiers, Voltage and current source inverters. UNIT 3: POWER SUPPLIES: Principle of operation of choppers. Step up, Step d own and reversible choppers. High frequency electronic ballast, Switch Mode Power Supply: Fly back converter, forward/buck converter, Boost converter and buck-boost converter. Uninterruptible Power Supply. UNIT 4: MOTOR CONTROL:Introduction to speed control of DC motors using phase controlled converters and choppers, Basic idea of speed control of three phase induction motors using voltage and frequency control methods. UNIT 5:Stepper Motors: Variable reluctance, Permanent mag net and hybrid stepper motors. Induction and dielectric heating control. .
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Unit 1
Semiconductor Power Devices Q.1
Explain the operating principle of GTO
Ans. Gate turn off thyristor is turned on applying a positive pulse it consists of four layers, pnpn, as like conventional thyristors. Functions except for turnoff are the same as those of conventional thyristors, therefore, we mainly describe the turn-off operation here. In the on-state of a GTO thyristor , the central base regions are filled with holes supplied from the anode and electrons supplied from the cathode. If reverse bias is applied to make the gate negative in respect to the cathode, part of holes in the p-base layer are extracted through the gate, suppressing the injection of electrons from the cathode. More hole current is extracted through the gate in response to this suppression, further suppressing the electron injection. In the course of this process, the cathode emitter junction (J3) is put into a reverse-bias state entirely, GTO thyristor is turned off. Fig. 1 illustrates the turn-off operation, using a two-transistor model
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Industrial Electronics Q.2
7
Explain Anode short GTO thyristor.
Ans. The structure for Anode short GTO thyristor is shown in Fig. 3 below:
At the J1 junction, the anode is partially shorted due to the n+ layers, so that the reverse voltage of the GTO thyristor is as small as that of the J3 junction (around 15V normally). However, excess carriers are extracted from the gate and from the n+ layer during the turn-off, enabling high-speed switching. This type of thyristor is suitable for applications that require high-speed switching but do not need high reverse voltage, such as voltage source inverters.
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8 Q.3
Explain reverse conducting GTO thyristor.
Ans
The structure of reverse conducting GTO thyristor is shown in Fig. 4 below.
The reverse conducting GTO thyristor consists of a fast recovery diode part and the anode short GTO thyristor part, the former of which is connected in parallel to the latter. The thyristor is the same type as the one described above. This is suitable for application to voltage source inverters for example, where a GTO thyristor requires Flywheel diode. No additional diode is necessary if this GTO thyristor is used, reducing the system size and weight.
Q.4
Explain what do you mean by MOSFET?
Ans
Mosfet Metal (or poly-silicon doped heavily to act like a metal). Oxide (SiO2, Acts as an insulator.). Semiconductor (One can selectively change the carrier type to n-type or p-type.) Field Effect (Device is controlled by an electric field as opposed to current.) Transistor (Three terminal devices)
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(a) When VGS (Gate-source voltage) (b) When VGS (Gate-source voltage) is supplied
is
not
Figure 3: The Structure of an Enhancement Type MOSFET and its Operation
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supplied
10 (a) When VGS (Gate-source voltage) (b) When VGS (Gate-source voltage) is supplied
is
not
supplied
The advantages of the lateral MOSFET are: 1. Low gate signal power requirement. No gate current can flow into the gate after the small gate oxide capacitance has been charged. 2. Fast switching speeds because electrons can start to flow from drain to source as soon as the channel opens. The channel depth is proportional to the gate volage and pinches closed as soon as the gate voltage is removed, so there is no storage time effect as occurs in bipolar transistors. The major disadvantages are 1. High resistance channels. In normal operation, the source is electrically connected to the substrate. With no gate bias, the depletion region extends out from the Nadrain in a pseudo-hemispherical shape. The channel length L cannot be made shorter than the minimum depletion width required to support the rated voltage of the device. 2. Channel resistance may be decreased by creating wider channels but this is costly since it uses up valuable silicon real estate. It also slows down the switching speed of the device by increasing its gate capacitance.
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Unit 2
Rectifiers & Inverters Q.1 Discuss In Short The Parallel Operation Of Thyristors. Ans. When thyristors are connected in parallel, the load current is not shared equally ,the current unbalance increases the junction temperature of the SCR carrying the higher current and decreases its internal resistance this increases its current sharing and may damage the thyristor.A A small resistance connected in series will force equal current sharing.
Magnetically coupled inductors can assure equal current sharing during the transient period.
Q.2 Explain di/dt thyristor protection. Ans. A thyristor requires a minimum time to spread the current conduction uniformly throughout the junction.If the rate-of-rise of anode current is very fast compared to the spreading velocity of a turn-on process, a hot spot will
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12 occur.As a result of the excessive temperature, the device may fail.Therefore, in practical circuits the device must be protected against high di/dt. Let us consider the following circuit.
Dm will conduct when thyristor T1 is off.If T1 is fired when Dm is still conducting, di/dt can be very high.In practice, the di/dt is limited by adding a series inductor Ls.Then
Vs Q.3 Ans
Ls
di dt
di dt
Vs Ls
Explain dv/dt thyristor protection.
If the switch S is closed at t = 0, a step voltage will be applied across the 1 thyristor T .The dv/dt may be high enough to turn on the device.The dv/dt 1 can be limited by connecting a capacitor C across T .When the thyristor T is s 1 1 turned on, the discharge current of capacitor is limited by resistor R .The RC s circuit is known as a snubber circuit, and the voltage across the thyristor will rise exponentially.The circuit dv/dt can be found approximately from: The snubber circuit can be designed based on the known value of the dv/dt for a device.
Rs
Vs I TD
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The value of R is found from the discharge current I . s TD The load can form a series circuit with the snubber network as shown below.
dv dt
0.632Vs
0.632Vs RsCs
We can show that the damping ratio of the second order circuit will be: where L is the stray inductance. s To limit the peak voltage overshoot across the T , a damping ratio in the 1 range of 0.5 to 1 is used.The L is typically high, and R should be high and C s s
Rs o
R 2
Cs Ls
L
should be small to retain the desired value of damping ratio.A large value of R will reduce the discharge current, and a low value of Cs will reduce the s snubber losses. Q.4 Ans
What do you mean by three phase dual converter? The four quadrant operation is generally required in variable-speed drives application for this we have to imply a converter which can work in all four quadrants the three-phase dual converters are extensively used for there purposes. In this when first group is working as inverter than the second group is working as rectifier and when first group is working as rectifier then second group is working as inverter
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14 The following figure1 shows three-phase dual converters where two threephase converters are connected back-to-back and the waveforms for input voltages, output voltages, and the voltage across the inductor.
fig1 Three phase dual converter circuit and waveform Due to instantaneous voltage differences between the output voltages of converters, a circulating current flows through the converters. This circulating current is limited by a reactor. The two converters are controlled in such a way that if α1 is the delay angle of converter 1, the delay angle of converter 2 is α2 = π- α1. Q.5
Discuss the effects of source and load impedance.
Ans
The source inductance should be as small as possible to limit the transient voltage. Also source inductance may cause commutation problem for the chopper. Usually an input filter is used to overcome the problem of source
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Industrial Electronics
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inductance. The load ripple current is inversely proportional to load inductance and chopping frequency. Peak load current depends on load inductance. To limit the load ripple current, a smoothing inductor is connected in series with the load as shown in figure1
i0 Chopper V
+ L O A D
FWD
v0
Figure1
The different modes of the operation are discussed below
MODE 1 OPERATION
T1
LS +
IL
+ VC
_C
iC
VS L
D1
_
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L O A D
16 Thyristor T1 is fired at t = 0. The supply voltage comes across the load. Load current IL flows through T1 and load.At the same time capacitor discharges through T1, D1, L1, & ‘C’ and the capacitor reverses its voltage.This reverse voltage on capacitor is held constant by diode D1.
Capacitor Discharge Current C sin t L 1 Where LC & Capacitor Voltage iC t
V
VC t
V cos
t
MODE 2 OPERATION
IL +
_
LS VC
VS
IL C
+
T2
L O A D
_
Thyristor T2 is now fired to commutate thyristor T1.When T2 is ON capacitor voltage reverse biases T1 and turns if off. The capacitor discharges through the load from –V to 0.Discharge time is known as circuit turn-off time. Q.6 Explain Power factor improvement techniques. Ans. The power factor of phase-controlled converters depends on delay angle, and it is generally low. These converters inject harmonics into the supply. Forced
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commutation can improve the input power factor and reduce the harmonics levels with advancement of devices (GTO or IGBT), the forced commutation can be implemented in practical systems. The basic techniques of forced commutation which are: Extinction Angle Control Symmetrical Angle Control Pulse-Width Modulation Single-Phase Sinusoidal Pulse-Width Modulation Three-Phase PWM Control Extinction Angle Control:In this switch s1 is turned on at ωt =0 and is turned off by forced commutation at ωt =π –β. switch s2 is turned on at ωt = π and is turned off by forced commutation at ωt =2π –β. The following figure1 shows circuit diagram and waveform for Extinction Angle Control for single phase forced commutated semiconverter
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fig1 The following figure2 shows circuit diagram and waveform for Extinction Angle Control for single phase forced commutate full converter
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Industrial Electronics
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Fig2 Symmetrical Angle Control:In this switch s1 is turned on at ωt =( π –β)/2 and is turned off at ωt =( π +β)/2 by forced commutation .Switch s2 is turned on at ωt = ( 3π –β)/2 and is turned off by forced commutation at ωt =( 3π +β)/2 .The output voltage is controlled by varying conduction angle β.
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20 The following figure3 shows waveform for Symmetrical Angle Control
Fig3 Pulse-Width Modulation Control In the PWM the converter switches are turned on and off several times during a half cycle and the output voltage is controlled by varying the width of the pulses. The following figure4 shows waveform for Pulse-Width Modulation Control
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Fig4 Single-Phase Sinusoidal Pulse-Width Modulation In this the pulse width are generated by comparing a triangular reference voltage Vr of amplitude Ar and frequency Fr with a carrier half sinusoidal voltage Vc of variable amplitude Ac and frequency 2Fs.The following figure5 shows waveform for Single-Phase Sinusoidal Pulse-Width Modulation
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22
Fig5
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Industrial Electronics
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Unit 3
Power Supplies Q.1 What do you mean by chopper ? Name its different classifications. Ans. A Chopper is a static device using it a variable dc voltage is obtained from a constant dc voltage source. It is also known as dc-to-dc converter and is widely used for motor control and in regenerative braking. Choppers are of Two Types:1) Step-down choppers. In this output voltage is less than input voltage 2) Step-up choppers. In step up chopper output voltage is more than input voltage. We can classify choppers as Class A Chopper Class B Chopper Class C Chopper Class D Chopper Class E Chopper
Q.2 Explain different classes of choppers in detail. Ans. The different classes of choppers are as follows 1.Class A chopper Class A Chopper is a first quadrant chopper .In these when the chopper is ON, supply voltage V is connected across the load and when the chopper is OFF, vO = 0 and the load current continues to flow in the same direction through the freewheeling diode ,the average values of output voltage and current are always positive. Class A Chopper is a step-down chopper in which power always flows form source to load.
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24 The equivalent circuit diagram for the Class A or first quadrant chopper is shown in fig(1.a)and fig(1.b)respectively,
i0
+
v0
Chopper V FWD
Fig 1.a diagram of Class A chopper
L O A D
v0 V
Fig 1.b
i0 Circuit First quadrant operation
To study the performance of Class A Chopper the output current waveform obtained in step down chopper with R-L load can be used
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Industrial Electronics
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ig
Thyristor gate pulse t
i0
Output current CH ON t FWD Conducts
v0
Output voltage
tON
t T
Fig(1.c)waveform for Class A chopper
Class B Chopper:
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26 In this average output voltage is positive and average output current is negative, therefore Class B Chopper operates in second quadrant. Class B Chopper is a step-up chopper in this chopper, power flows from load to source. When chopper is ON, E drives a current through L and R in a direction opposite to that shown in figure. During the ON period of the chopper, the inductance L stores energy. When Chopper is OFF, diode D conducts, and part of the energy stored in inductor L is returned to the supply. Class B Chopper is used for regenerative braking of dc motor. The equivalent circuit diagram for the Class B or second quadrant chopper is shown in fig (2.a) and fig (2.b) respectively,
D
i0
v0
+ R L v0
V Chopper
E
i0
Fig 2.a fig2.b Circuit diagram of Class B chopper Second quadrant operation The waveform for Class B chopper is shown in figure 2.c
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Industrial Electronics
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ig
Thyristor gate pulse t
i0 tOFF
tON T Output current
Imax Imin v0
t
D conducts Chopper conducts
Output voltage
t Fig2.c waveform for Class B chopper Class C chopper: The Class C Chopper is a combination of Class A and Class B Choppers, for first quadrant operation, CH1 is ON or D2 conducts and for second quadrant operation, CH2 is ON or D1 conducts. When CH1 is ON, the load current is positive and the output voltage is equal to ‘V’ & the load receives power from the source and when CH1 is turned OFF, energy stored in inductance L forces current to flow through the diode D2 and the output voltage is zero and the Current continues to flow in positive direction. When CH2 is triggered, the voltage E forces current to flow in opposite direction through L and CH2, the output voltage is zero now on turning OFF
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28 CH2 , the energy stored in the inductance drives current through diode D1 and the supply output voltage is V, the input current becomes negative and power flows from load to source. Average output voltage is positive while average output current can take both positive and negative values. Some point to keep in mind regarding Class C chopper are :• Choppers CH1 & CH2 should not be turned ON simultaneously as it would result in short circuiting the supply. •
Class C Chopper can be used both for dc motor control and regenerative braking of dc motor.
•
Class C Chopper can be used as a step-up or step-down chopper.
The equivalent circuit diagram for the Class C or quadrant figure of chopper is shown in fig (3.a) and fig (3.b) respectively,
CH1
D1 i0
+
v0
R
V CH2
D2
L v0
Chopper E
Fig (3.a) Circuit diagram of Class C chopper
i0
Fig (3.b) First and Second quadrant operation
The waveform for Class C chopper is shown in figure 3.c
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Industrial Electronics
ig1
29
Gate pulse of CH1 t
ig2
Gate pulse of CH2 t
i0 Output current t D1 CH1 D2 CH2 D1 CH1 D2 CH2 ON ON ON ON
V0
Output voltage
t Fig(3.c) Waveform for Class C chopper
Class D Chopper Class D is also a two quadrant chopper. When both CH1 and CH2 are triggered simultaneously, the output voltage vO = V and output current flows through the load. When CH1 and CH2 are turned OFF, the load current continues to flow in the same direction through load, D1 and D2, due to the energy stored in the inductor L. The Output voltage vO = - V
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30 The average load voltage is positive if chopper ON time is more than the OFF time i.e. tON > tOFF and average output voltage becomes negative if tON < tOFF . Hence the direction of load current is always positive but load voltage can be positive or negative. The equivalent circuit diagram for the Class D or quadrant operation is shown in fig (4.a) and fig (4.b) respectively,
v0 CH1
D2 R i0
L
E
V + D1
Fig (4.a) Circuit diagram of Class D chopper
i0
v0 CH2
Fig (4.b) First and Fourth quadrant operation
The waveform for Class D chopper is shown in figure 4.c and 4.d
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ig1
Gate pulse of CH1 t
ig2
Gate pulse of CH2 t
i0 Output current
v0
CH1,CH2 ON
t D1,D2 Conducting Output voltage
V Average v0
Fig(4.c) Waveform for Class D chopper
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t
32
ig1
Gate pulse of CH1 t
ig2
Gate pulse of CH2 t
i0 Output current
v0
CH1,CH2 ON
t D1,D2 Conducting Output voltage
V Average v0
t
Fig(4.d) Waveform for Class D chopper
Class E Chopper Class E is a four quadrant chopper. When CH1 and CH4 are triggered, output current io flows in positive direction through CH1 and CH4, and with output voltage vO = V, this gives the first quadrant operation. When both CH1 and
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CH4 are OFF, the energy stored in the inductor L drives iO through D2 and D3 in the same direction, but output voltage vO = -V. Therefore the chopper operates in the fourth quadrant. When CH2 and CH3 are triggered, the load current io flows in opposite direction & output voltage vO = -V. Since both iO and vO are negative, the chopper operates in third quadrant. When both CH2 and CH3 are OFF, the load current iO continues to flow in the same direction D1 and D4 and the output voltage vO = V. Therefore the chopper operates in second quadrant as vO is positive but iO is negative. The equivalent circuit diagram for the Class D or quadrant operation is shown in fig (5.a) and fig (5.b) respectively,
CH1
i0
V
+ CH2
CH3
D1 R
L
v0 D2
D3
E
CH4
Fig(5.a)Circuit diagram of Class E chopper
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D4
34
v0 CH2 - D4 Conducts D1 - D4 Conducts
CH1 - CH4 ON CH4 - D2 Conducts i0
CH3 - CH2 ON CH2 - D4 Conducts
D2 - D3 Conducts CH4 - D2 Conducts
Fig(5.b)Fourth quadrant operation
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Unit 4
Motor Control Q.1 Explain the construction and working principle of DC motor ? Ans. The Construction is very similar to a DC generatorThe dc machine can operate bath as a generator and a motor.When the dc machine operates as a motor, the input to the machine is electrical power and the output is mechanical power. In fact, the dc machine is used more as a motor.DC motors can provide a wide range of accurate speed and torque control. Principle of operation – when a current-carrying conductor is placed in magnetic field, it experiences a mechanical force., F = Bli
Q.2 Ans
Give different methods of speed control of DC motor ? The speed contro can be achieved by the following methods • Armature Voltage Control, Vt •
Field resistance control,
•
Armature resistance Control, Ra
•
Speed increases as Vt increases, Ra increases and field flux
decreases
Q.3 Draw torque speed characteristics and give the equation for speed control. Ans. Torque-speed characteristics equation:For Armature circuit—
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36
Va
Raia
L
dia dt
ea
In steady state –
Va
Ra I a
Ea
Therefore speed is given by,
Ra kT
2
Te
Va kT
Three possible methods of speed control: Armature resistance Ra Field flux F Armature voltage Va Torque-speed characteristics of DC motor
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Unit 5
Stepper Motors Q.1 What is stepper motor ? Ans. A Stepper Motor or a step motor is a brushless, synchronous motor which divides a full rotation into a number of steps. Unlike a brushless DC motor which rotates continuously when a fixed DC voltage is applied to it, a step motor rotates in discrete step angles.A stepper (or, stepping) motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. Q.2 Explain how stepper motor works. Ans. Stepper motors work on the principle of electromagnetism. There is a soft iron or magnetic rotor shaft surrounded by the electromagnetic stators. The rotor and stator have poles which may be teethed or not depending upon the type of stepper. When the stators are energized the rotor moves to align itself along with the stator (in case of a permanent magnet type stepper) or moves to have a minimum gap with the stator (in case of a variable reluctance stepper). This way the stators are energized in a sequence to rotate the stepper motor. Q.3 Explain Variable reluctance stepper motor ? Ans. The variable reluctance stepper has a toothed non-magnetic soft iron rotor. When the stator coil is energized the rotor moves to have a minimum gap between the stator and its teeth. The teeth of the rotor are designed so that when they are aligned with one stator they get misaligned with the next stator. Now when the next stator is energized, the rotor moves to align its teeth with the next stator. This way energizing stators in a fixed sequence completes the rotation of the step motor.The resolution of a variable reluctance stepper can be increased by increasing the number of teeth in the rotor and by increasing the number of phases. Q.4
Give the classification on the basis of phase for step motor ?
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38 Ans. The step motors are mostly two phase motors. 1. Unipolar 2. Bipolar.
The process of movement generation is as under
rotor follows the stator field one step invert magnetic field in 1 coil for the next step invert magnetic field in the other coil A stepper motor always needs electronic control. step angle depends on number of poles -> fixed by construction Q.5 Explain Unipolar stepper motor ? Ans. The unipolar motor have five, six or eight leads. In the designs where the common of two poles are separate but centre tapped, motor have six leads. If the centre taps of the two poles are internally short, the motor has five leads. Eight lead unipolar facilitates both series and parallel connection whereas five lead and six lead motors have series connection of stator coils. The unipolar motor simplifies the operation because in operating them there is no need to reverse the current in the driving circuit. These are also called bifilar motors. In bipolar stepper there is single winding per pole. The direction of current need to be changed by the driving circuit so the driving circuit of the bipolar stepper becomes complex. These are also called unifilar motors. Q.6
Explain Permanent Magnet Stepper motor ?
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Ans. The rotor and stator poles of a permanent magnet stepper are not teethed. Instead the rotor have alternative north and south poles parallel to the axis of the rotor shaft.When a stator is energized, it develops electromagnetic poles. The magnetic rotor aligns along the magnetic field of the stator. The other stator is then energized in the sequence so that the rotor moves and aligns itself to the new magnetic field. This way energizing the stators in a fixed sequence rotates the stepper motor by fixed angles.The resolution of a permanent magnet stepper can be increased by increasing number of poles in the rotor or increasing the number of phases.
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