Download Activity 8C

E3065/08/1
RECTIFIER AND CHOPPER
UNIT 8
RECTIFIER AND CHOPPER
OBJECTIVES
General objective : To understand the concept of a rectifier.
Specific objectives : At the end of the unit you should be able to:
 Identify the power of an uncontrolled rectifier, semi-controlled rectifier, controlled
rectifier and chopper.
 Identify the uncontrolled rectifier and chopper circuit.
E3065/08/2
RECTIFIER AND CHOPPER
INPUT
8.1
INTRODUCTION OF RECTIFIER
The process of converting alternating current (or alternating voltage) into
pulsating direct current (or pulsating direct voltage) is known as rectification.
Rectification is accomplished with the help of diodes. Circuits which provide
rectification are called rectifier circuits. Rectifier circuits can provide either half-wave
rectification or full-wave rectification.
8.2
PRINCIPLE OF RECTIFIER
Assume a half-wave rectifier output is to be used to supply current to a load.
The output of the rectifier gives the expected half-cycle of sinusoidal output once
every cycle except that conduction of the rectifier diode is not allowed to begin at the
start of the cycle but after an angular measure of θ radians has occurred. The resulting
current waveform is shown in Fig. 8.2(a).
If the angle θ can be varied form 0 to  /2 radiants (or even from 0 to 
radians) then the mean value of current taken by the load can be varied as can the rms
current to be derived.
E3065/08/3
RECTIFIER AND CHOPPER
Fig 8.2(a) : Control of current to a load by variation of a firing angle θ.
E3065/08/4
RECTIFIER AND CHOPPER
Activity 8A
TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT
INPUT…!
8.1
Describe briefly a rectifier.
8.2
Draw the control of current to a load by variation of a firing angle θ.
Hii !!!!!…..Good Luck and
Try your best ….
E3065/08/5
RECTIFIER AND CHOPPER
Feedback To Activity 8A
8.1
The process of converting alternating current (or alternating voltage) into pulsating
direct current (or pulsating direct voltage) is known as rectification.
8.2
For this diagram, you should refer to input 8.2(a).
E3065/08/6
RECTIFIER AND CHOPPER
INPUT
8.3
SEMI-CONTROLLED RECTIFIER
For control of electric power or semi control power conditioning, the conversion
of electric power from one form to another is necessary and the switching characteristic
of the power device permit these conversions. The static power converter may be
considered as a switching matrix. The power electronics semi-control rectifier circuits
can classified into two types:
i. Diode rectifiers
ii. AC - DC converters (controlled rectifiers)
8.4
HALF-WAVE RECTIFICATION
The result of half-wave rectification is illustrated in Fig 8.4 (a), and the circuit
which performs the rectification is drawn in Fig 8.4 (b). The ground symbol in 8.4 (c)
is the reference point for voltages referred to in the discussion which follows.
(a)
(b)
Fig. 8.4 : Rectifying a 20 Vp-p sinusoidal waveform yields a + 9.3 Vp pulsating dc waveform.
E3065/08/7
RECTIFIER AND CHOPPER
Fig. 8.4 (c) : Half-wave rectifier circuit
8.5
FULL-WAVE RECTIFICATION
Full wave rectification can be provided with two diodes and a center-tapped
transformer as shown in Fig. 8.5 (a) , or it can be accomplished with four diodes and a
nontapped transformer (see Fig. 8.5 (b) ).
Figure 8.5.1(a) shows the direction and path of current flow for the ½ cycle
when the polarity of the transformer is as marked. Notice that only D1 is conducting
and that only the top half of the transformer is providing power. This is because D2 is
reverse-biased.
During the second ½ cycle (see 8.5.1 (b) ), the polarities of the transformer
windings are reversed. Therefore, D1 is now reverse-biased and D2 allows the current
to flow in the indicated direction and path. Notice that current through R1 is in the
same direction for each ½ cycle.
(a) During one half-cycle, D1 conducts and D2 is cutoff (reverse-biased)
E3065/08/8
RECTIFIER AND CHOPPER
(b) During the other half-cycle, D2 conducts and D1 is cutoff
Fig, 8.5 : Full-wave rectifier with center-tapped transformer
Figure 8.5(b) shows a full-wave, bridge rectifier circuit. Notice that this circuit
provides twice as much dc voltage as does the previous full-wave circuit when both
circuits use the same transformer. The bridge rectifier circuit does not use the center tap
of the transformer and it requires four diodes.
During ½ cycle, two of the diodes in Fig. 8.5(c ) conduct and allow the full
secondary voltage to force current through load resistor R1. the remaining two diodes
are reverse-biased and thus prevent the diode bridge from short-circuiting the
transformer secondary.
Fig, 8.5(c ) : Full-wave, bridge rectifier
E3065/08/9
RECTIFIER AND CHOPPER
Activity 8B
TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT
INPUT…!
8.3
Draw a Half-wave rectifier circuit.
8.4
Draw a Full-wave rectifier with center-tapped transformer.
Hii !!!!!…..Good Luck ..
E3065/08/10
RECTIFIER AND CHOPPER
Feedback To Activity 8B
8.3
For this section, you should refer to diagram half-wave rectifier circuit in figure 8.4(c).
8.4
The answer for full-wave rectifier with center-tapped transformer.
(a) During one half-cycle, D1 conducts and D2 is cutoff (reverse-biased)
(b) During the other half-cycle, D2 conducts and D1 is cutoff
E3065/08/11
RECTIFIER AND CHOPPER
INPUT
8.6
INTRODUCTION OF CHOPPER.
A dc chopper is the equipment that can be used as a dc transformer to step up or
step down a fixed dc voltage. The chopper can also be used for switching- mode
voltage regulators and for transferring energy between two dc resources. However,
harmonics are generated at the input and load side of the chopper, and these harmonics
can be reduced by input and output filters.
8.7
PRINCIPLE OF CHOPPER.
A chopper can operate on either fixed frequency chopper or variable
frequency. A variable-frequency chopper generates harmonics of variable frequencies
and a filter design. A fixed – frequency chopper is normally used. A chopper circuit
uses a fast turn off as a switch and requires commutation circuitry to turn it off. The
circuits are the outcome of meeting certain criteria: (1) reduction of minimum on-time
limit, (2) high frequency of operation, and (3) reliable operation.
8.8
TYPE AND BASIC OPERATION OF CHOPPER FUNCTION CIRCUIT
The development of alternative switching (e.g., power transistors, GTO s), the
applications for type and circuit of choppers are limited to high power levels and
especially, to traction motor control. Some of chopper type and circuit used by traction
equipment manufactures are discussed in this section.
E3065/08/12
RECTIFIER AND CHOPPER
8.8.1
IMPLUSE-COMMUTATED CHOPPERS
The impulse-commutated chopper is a very common circuit with two
thyristors as shown in figure 8.8(a) and is also known as a classical chopper.
At the beginning of operation, thyristor T2 is fired and this causes the
commutation capacitor C to charge through the voltage Vc , which should be
supply voltage Vs in the fist cycle. The plate A becomes positive with respect to
plate B. The circuit operation can be divided into five modes, and the
equivalent circuits under steady-state conditions are shown in Fig. 8.8(b). We
shall assume that the load current remains constant at a peak value Im during the
commutation process. We shall also redefine the time origin, t = 0, at the
beginning of each mode. Mode 1 begins with T1 is fired. The load is connected
to the supply. The commutation capacitor C reverses also its charge through the
resonant reversing circuit formed by T1, D1, and Lm.
Fig. 8.8(a): Impulse – commutated chopper.
E3065/08/13
RECTIFIER AND CHOPPER
Fig. 8.8(b): Mode equivalent circuit.
8.8.2. IMPULSE-COMMUTATED THREE-THYRISTOR CHOPPERS
The problem of undercharging can be remedied by replacing diode D1
with thyristor T3, as shown in Fig. 8.8(c). In good chopper, the commutation
time, tc, should ideally be independent of the load current. tc could be made less
dependent on the load current by adding an antiparallel diode Df across the main
thyristor as shown in Fig. 8.8(c) by dashed lines. A modified version of the
circuit is shown in Fig. 8.8(d)., where the charge reversal of the capacitor is
E3065/08/14
RECTIFIER AND CHOPPER
done independently of main thyristor T1 by firing T3 . There are four possible
modes and their equivalent circuits are shown in Fig. 8.8(e).
Fig. 8.8(c) : Impulse – commutated three-thyristor chopper.
Fig. 8.8(d): Impulse-commutated chopper with independent charge reversal.
E3065/08/15
RECTIFIER AND CHOPPER
Fig. 8.8(e): Equivalent circuits.
E3065/08/16
RECTIFIER AND CHOPPER
8.8.3.
RESONANT PULSE CHOPPERS
A resonant pulse chopper is shown in Fig. 8.8(f). As soon as the supply
is switched on, the capacitor is charged to a voltage Vc through Lm, D1, and
load. The circuit operation can be divided into six modes and the equivalent
circuits are shown in Fig. 8.8(g).
Fig. 8.8(f): Resonant pulse chopper.
E3065/08/17
RECTIFIER AND CHOPPER
Fig 8.8(g): Equivalent circuit for modes.
E3065/08/18
RECTIFIER AND CHOPPER
Activity 8C
TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT
INPUT…!
8.5
Describe briefly the chopper.
8.6
Draw the circuit and explain briefly the operation of a chopper type impulse
commutated.
Hii !!!!!…..Good Luck and
Try your best ….
E3065/08/19
RECTIFIER AND CHOPPER
Feedback To Activity 8C
8.5
The chopper is the equipment that can be used as a dc transformer to step up or step
down a fixed dc voltage. The chopper can also be used for switching- mode voltage
regulators and for transfered energy between two dc resources.
8.6
Impulse-commutated choppers circuit operation can be divided into five modes, and
the equivalent circuits under steady-state conditions are shown in Fig. 8.6(a), (b) and
(c),. We shall assume that the load current remains constant at a peak value Im during
the commutation process. We shall also redefine the time origin, t = 0, at the beginning
of each mode. Mode 1 begins with T1 is fired. The load is connected to the supply.
The commutation capacitor C reverses also its charge through the resonant reversing
circuit formed by T1, D1, and Lm.
E3065/08/20
RECTIFIER AND CHOPPER
Fig. 8.6(a): Impulse – commutated three-thyristor chopper.
Fig. 8.6(b): Impulse-commutated chopper with independent charge reversal.
Fig. 8.6(c ): Equivalent circuits.
E3065/08/21
RECTIFIER AND CHOPPER
INPUT
8.9
SKETCHING CURRENT AND VOLTAGE WAVE
There are no fixed rules for designing or sketching of copper circuit and the
design varies with the types of circuit used. The designer has a wide range of choice
and values of LmC components are influenced by the designer’s choice of peak resonant
reversal current, and peak allowable voltage of the circuit. The voltage and current
ratings LmC components and devices is left to the designer based on the considerations
of price, availability, and safety margin. In general, the following steps are involved in
the design:
a.
b.
c.
d.
Identify the modes of operations for the copper circuit.
Determine the equivalent circuits for the various modes.
Determine the currents and voltages for modes and their waveforms.
Evaluate the values of commutation components LmC that would satisfy the
devices.
A chopper with a highly inductive load is shown in Fig. 8.9(a). The load current ripple
is negligible (I=0). If the average load current is Ia, the peak load current is
Im=Ia + I= Ia The input current, which is of pulsed shape as shown in Fig 8.9(b).
E3065/08/22
RECTIFIER AND CHOPPER
Fig. 8.9(a) : Input current waveform of chopper
The wave forms for currents and voltages are shown in figure 8.9(b). In the
following analysis, we shall redefine the time origin t=0 at the beginning of each mode.
Fig. 8.9(b) : Chopper waveforms
E3065/08/23
RECTIFIER AND CHOPPER
Fig 8.9(c): Equivalent circuit for modes.
Mode 1 begins when main thyristor T1 is fired and the supply is connected to the
load. This mode is valid for t = kT.
Mode 2 begins when commutation thyristor T2 is fired. The commutation
capacitor reverses its charge through C, Lm, and T2.
E3065/08/24
RECTIFIER AND CHOPPER
Mode 3 begins when T2 is self-commutated and the capacitor discharges due to
resonant oscillation through diode D1 and T1. Assuming that the capacitor current rises
linearly from 0 to Im and the current of thyristor T1 falls from Im to 0 in time tx.
Mode 4 begins when current through T1 falls to zero. The capacitor continues to
discharge through the load at a rate determined by the peak load current.
Mode 5 begins when the freewheeling diode Dm starts conducting and the load
current decays through Dm. the energy stored in commutation inductance Lm and source
inductance Ls is transferred to capacitor C.
Mode 6 begins when the overcharging is complete and diode D1 turns off. The
load current continues to decay until the main thyristor is refired in the next cycle. In
the stedy-state condition Vc = Vx.
8.10
DEFINITION OF MARK SPACE RATIO (TIME RATIO CONTROL)
The sequence of events within the frequency counter is controlled by the time
ratio base, which must provide the timing for the following events: resetting the counter
, opening the count gate, closing the count gate, and storing the counted frequency in
the latch. The resetting of the counter and storing the count are not critical events as
long as they occur before and after the gate period, respectively. The opening and
closing of the count gate, on the other hand, determine the accuracy of the frequency
counter and are very critical in its timing.
Since the accuracy of the frequency counter depends directly on the accuracy of
the time ratio base signal, the time base is driven from a accurate crystal controlled (e.g;
oscillator). This element of the time base is typically a temperature compensated crystal
oscillator operating at several megahertz. A crystal oven could be used to supply a
similar accuracy, except that the oven require the application of power to provide the
correct frequency and is available for use immediately after power-on. Fig. 8.10(a)
shows a simplified diagram of temperature-compensated crystal oscillator.
E3065/08/25
RECTIFIER AND CHOPPER
Fig, 8.10(a) : Block diagram of a temperature-compensated crystal oscillator
8.11
COMPARING STEP-UP AND STEP-DOWN CHOPPER DOWN.
A chopper can be considered as dc equivalent to an ac transformer with a
continuously variable turns ratio. Like a transformer, it can be used to step-down or
step-up a dc voltage source.
8.11.1 PRINCIPLE OF STEP-UP OPERATION
A chopper can be used to step-up a dc voltage and an arrangement for
step-up operation is shown in Fig. 8.11(a). When switch SW is closed for time
t1, the inductor current rises and energy is stored in the indicator, L. if switch is
opened for time t2, the energy stored in the inductor is transferred to load
through diode D1 and the inductor current falls. Assuming a continuous current
flow, the waveform for the inductor current is shown in Fig. 8.11(b). For values
of k tending to unity, the output voltage becomes very large and is very
sensitive to changes in k, as shown in Fig. 8.11(c).
(a) Step-up arrangement
E3065/08/26
RECTIFIER AND CHOPPER
(b) Current waveform
(c) Output voltage
Fig. 8.11: Arrangement for step-up operation
8.11.2 PRINCIPLE OF STEP-DOWN OPERATION
The principle of operation can be explained by Fig. 8.11(d). When
switch SW is closed for time t1 , the output voltage Vs appears across the load.
If the switch remains off for a time t2 , the voltage across the load is zero. The
waveforms for the output voltage and load current are also shown in Fig.
8.11(e). The chopper switch can be implemented by using a (1) power BJT, (2)
power MOSFET, (3) GTO, or (4) forced-commutated thyristor. The practical
devices have a finite voltage drop ranging from 0.5 to 5 V, and for the sake of
simplicity we shall neglect the voltage drops of these power semiconductor
devices.
E3065/08/27
RECTIFIER AND CHOPPER
(d) Circuit
(e) Waveforms
Fig. 8.11: Step-down chopper with resistive load
E3065/08/28
RECTIFIER AND CHOPPER
Activity 8D
TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT
INPUT…!
8.7
Explain the steps involved in chopper circuit design.
8.8
Explain briefly what is the difference between chopper step-up and chopper step-down.
E3065/08/29
RECTIFIER AND CHOPPER
Feedback To Activity 8D
8.7
The following steps are involved in the design:
i.
ii.
iii.
iv.
8.8
Identify the modes of operations for the copper circuit.
Determine the equivalent circuits for the various modes.
Determine the currents and voltages for modes and their waveforms.
Evaluate the values of commutation components LmC that would satisfy the
devices.
The different are:
Chopper Step-Up
When switch SW is closed for time t1, the inductor current rises and energy is
stored in the indicator, L. if switch is opened for time t2, the energy stored in the
inductor is transferred to load through diode D1 and the inductor current falls
Figure 8.8(a): Step-up arrangement
E3065/08/30
RECTIFIER AND CHOPPER
Chopper Step-Down
When switch SW is closed for time t1 , the output voltage Vs appears across the
load. If the switch remains off for a time t2 , the voltage across the load is zero.
Figure 8.8(b): Step-down arrangement
E3065/08/31
RECTIFIER AND CHOPPER
SELF-ASSESSMENT
Question 8-1
a.
Briefly describe a rectifier.
b.
Briefly describe the principle behind a rectifier.
c.
Draw the control of current to a load by variation of a firing angle θ.
d.
Draw the Half-wave rectifier circuit and waveform.
e.
Draw the Full-wave rectifier with center-tapped transformer.
Question 8-2
a.
Briefly describe the type of choppers.
b.
Briefly describe the principle behind a choppers.
c.
Draw the control of current to a load by variation of circuit and waveform of one type
of coppers.
d.
Draw the circuit and waveform for step-up chopper.
e.
Draw the circuit and waveform for step-down chopper.
E3065/08/32
RECTIFIER AND CHOPPER
Feedback To Self-Assessment
Answer 8-1
a.
The process of converting alternating current (or alternating voltage) into pulsating
direct current (or pulsating direct voltage) is known as rectification.
b.
Current to a load by variation of a firing angle θ.
c.
Half-wave rectifier circuit.
(a)
(b)
E3065/08/33
RECTIFIER AND CHOPPER
Rectifying a 20 Vp-p
d.
( c)
sinusoidal waveform yields a + 9.3 Vp pulsating dc waveform.
Full-wave rectifier with center-tapped transformer
(a) During one half-cycle, D1 conducts and D2 is cutoff (reverse-biased)
(b) During the other half-cycle, D2 conducts and D1 is cutoff
E3065/08/34
RECTIFIER AND CHOPPER
e.
Full-wave, bridge rectifier
Answer 8-2
a.
Type of Choppers;
i. Impluse-commutated choppers
ii. Impulse-commutated three-thyristor choppers
iii. Resonant pulse choppers
b.
Principle of choppers;
i.
Impluse-commutated Choppers
The impulse-commutated chopper is a very common circuit with two
thyristors as shown in figure 8.2(a) attached, and is also known as a classical
chopper. At the beginning of operation, thyristor T2 is fired and this causes the
commutation capacitor C to charge through the voltage Vc , which should be
supply voltage Vs in the fist cycle. The plate A becomes positive with respect to
plate B. The circuit operation can be divided into five modes, and the
equivalent circuits under steady-state conditions are shown in Fig. input 8.8(b).
E3065/08/35
RECTIFIER AND CHOPPER
We shall assume that the load current remains constant at a peak value Im
during the commutation process. We shall also redefine the time origin, t = 0,
at the beginning of each mode. Mode 1 begins with T1 is fired. The load is
connected to the supply. The commutation capacitor C reverses also its charge
through the resonant reversing circuit formed by T1, D1, and Lm.
Fig. 8.2(a): Impulse – commutated chopper.
ii.
Impulse-commutated three-thyristor choppers
The problem of undercharging can be remedied by replacing diode D1
with thyristor T3. In good chopper, the commutation time, tc, should ideally be
independent of the load current. tc could be made less dependent on the load
current by adding an antiparallel diode Df across the main thyristor. A modified
version of the will charge reversal of the capacitor is done independently of
main thyristor T1 by firing T3 .
iii.
Resonant pulse choppers
A resonant pulse chopper as soon as the supply. It is switched on, the
capacitor charges to a voltage Vc through Lm, D1, and load. The circuit
operation can be divided into six modes and the equivalent circuits are shown in
explanation input at figure 8.8,(e) and (f): Resonant pulse chopper and
equivalent circuit for modes.
E3065/08/36
RECTIFIER AND CHOPPER
c.
Current and waveform of chopper
The voltage and current ratings LmC components and devices is left to the
designer based on the considerations of price, availability, and safety margin. In
general, the following steps are involved in the design:
i.
ii.
iii.
iv.
Identify the modes of operations for the copper circuit.
Determine the equivalent circuits for the various modes.
Determine the currents and voltages for modes and their waveforms.
Evaluate the values of commutation components LmC that would satisfy the
devices.
A chopper with a highly inductive load is shown in Fig. 8.9.1a. The load current ripple
is negligible (I=0). If the average load current is Ia, the peak load current is
Im=Ia + I= Ia The input current, which is of pulsed shape as shown in Fig 8.9.1b.
Fig. 8.9.1: Input current waveform of chopper
E3065/08/37
RECTIFIER AND CHOPPER
For the reference of the copper waveform, you can see figure 8.8.(g) and consider as
that figure in the explanation below.
Mode 1 begins when main thyristor T1 is fired and the supply is connected to the
load. This mode is valid for t = kT.
Mode 2 begins when commutation thyristor T2 is fired. The commutation
capacitor reverses its charge thgrough C, Lm, and T2.
Mode 3 begins when T2 is self-commutated and the capacitor discharges due to
resonant oscillation through diode D1 and T1. Assuming that the capacitor current rises
linearly from 0 to Im and the current of thyristor T1 falls from Im to 0 in time tx.
Mode 4 begins when current through T1 falls to zero. The capacitor continues to
discharge through the load at a rate determined by the peak load current.
Mode 5 begins when the freewheeling diode Dm starts conducting and the load
current decays through Dm. the energy stored in commutation inductance Lm and source
inductance Ls is transferred to capacitor C.
Mode 6 begins when the overcharging is complete and diode D1 turns off. The load
current continues to decay until the main thyristor is refired in the next cycle
d.
Principle of step-up operation
A chopper can be used to step-up a dc voltage and an arrangement for step-up
operation is shown in Fig. 8.11.1(a). When switch SW is closed for time t1, the
inductor current rises and energy is stored in the indicator, L. if switch is opened for
time t2, the energy stored in the inductor is transferred to load through diode D1 and the
inductor current falls. Assuming a continuous current flow, the waveform for the
inductor current is shown in Fig. 8.11.1(b). For values of k tending to unity, the output
voltage becomes very large and is very sensitive to changes in k, as shown in Fig.
8.11.1(c).
E3065/08/38
RECTIFIER AND CHOPPER
(b) Step-up arrangement
(b) Current waveform
(c) Output voltage
Fig. 8.11.1: Arrangement for step-up operation
E3065/08/39
RECTIFIER AND CHOPPER
e.
Principle of step-down operation
The principle of operation can be explained by Fig. 8.11.2(a). When
switch SW is closed for time t1 , the output voltage Vs appears across the load.
If the switch remains off for a time t2 , the voltage across the load is zero. The
waveforms for the output voltage and load current are also shown in Fig.
8.11.2(b). The chopper switch can be implemented by using a (1) power BJT,
(2) power MOSFET, (3) GTO, or (4) forced-commutated thyristor. The
practical devices have a finite voltage drop ranging from 0.5 to 5 V, and for the
sake of simplicity we shall neglect the voltage drops of these power
semiconductor devices.
(a) Circuit
(b) Waveforms
Fig. 8.11.2: Step-down chopper with resistive load