MODULE III

Choppers and cyclo converters:

To produce quality goods in any industry, the processes necessarily require the use of
variable speed drives. Variable speed d.c and a.c drives are being increasingly used in all
industries. In many cases conversion of the dc source voltage to different levels is required
for the armature of the dc motor to obtain speed control. Generally the following techniques
are available for obtaining the variable dc voltage from a fixed dc voltage.
(1) Line commutated converters: conversion of AC supply to variable dc supply using
controlled rectifiers.
(2) AC link chopper: (INVERTER RECTIFIER)- in this dc is first converted to ac
by an inverter. The obtained ac is then stepped up or down by a transformer and
then rectified back to dc by a rectifier.
(3) Dc chopper: (DC to DC power converter)- a dc chopper is a static device (switch)
used to obtain variable dc voltage from a source of constant dc voltage.the dc
chopper offers greater efficiency, faster response, lower maintenance, small size,
smooth control and for many applications, lower cost, than motor-generator sets
or gas tubes approach.

Ac link chopper
DC choppers can be classified as
I according to input/output voltage levels
(a) step- down chopper (output voltage < input voltage)
(b) step- up chopper (output voltage > input voltage)
 Basic chopper configuration
II according to the direction of output voltage and current
(a) Class A type chopper
(b) Class B type chopper
(c) Class C type chopper
(d) Class D type chopper
(e) Class E type chopper
The voltage and current directions for above classes are shown in figure

Chopper configurations
III according to circuit operation
(a) First quadrant chopper : The output voltage and current must be
+ve (type A)
(b) Two quadrant chopper: the output voltage is +ve and current can
be +ve or ve. (class C) or the output current is +ve and the
voltage can be +ve or ve. (class D)
(c ) Four quadrant chopper : The output voltage and current must be
-ve (class E).
IV according to commutation method
 (a) Voltage commutated chopper
(b) Current commutated chopper
(c) Load commutated chopper
(d) Impulse commutated chopper

VOLTAGE STEP DOWN CHOPPER

Power circuit configuration and working principle of step down chopper

A chopper is a high speed on/off semiconductor switch. In general dc chopper consists

of power semi conductor devices (SCR, BJT, PMOSFET, IGBT, GTO, MCT etc. which
works as a switch.), input dc power supply, elements (R,L,C) and output load as in figure
below. The average output voltage across the load is controlled by varying on period and off
period (duty cycle) of the switch.

Voltage step down chopper

In this the chopper is represented by a switch S 1, which may be turned on or off as

desired. During the period Ton chopper is on and load voltage is equal to source voltage V s.
during the interval Toff chopper is off, load current flows through the free wheeling diode
F.D(S2). as a result, load terminals are short circuited by FD and load voltage is therefore zero
during Toff in this manner, a chopped dc voltage is produced at the load terminals. The load
current as shown in fig(b) is continuous. During Ton, load current rises whereas during Toff,
load current decays .

The input terminal s of the chopper are A1(+ve) and A2(-ve). The output terminals are
B1(+ve) and B2(-ve). The input dc voltage has a fixed value V1. The load resistance R is
connected to the output through an inductance L. the purpose of L is to smooth out
fluctuations in the output current caused by the switching process in the chopper. If L is large
the output voltage & current will be substantially dc with negligible ac ripples. The
inductance L performs the function of filtering off AC components is a power element. It
is usually iron cored and should be capable of carrying the full dc load current without
magnetic saturation.

Voltage step down chopper (a) circuit configuration during TON (S1 on, S2 off)
(b) circuit configuration during TOFF (S2 on, S1 off), (c) waveform of voltage across B1-B2
(d) waveform of current i2 ( e) waveform of current i1 , (f) waveform of freewheeling current i D

When S1 is ON, the voltage V1 is applied in reverse across power diode. Therefore S2 must
stay off as S1 remains ON. when S1 ON, a current build up in the load resistance which is
labelled as i2. The growth of i2 is exponential because of inductance L. the switch S1 is kept
on for a time interval TON and then turned off. At the instant when S1 off, i 2 has a finite value.
(Ip1). This peak current cannot instantly fall to zero, because of the presence of L. the decay of
i2 causes an induced voltage L(di/dt) to appear across the inductance. Due to this voltage the
diode gets forward biased and current flow to continue. The term freewheeling is
commonly used to describe the flow of current in this manner without being caused by
the voltage source but solely due to the stored energy in the inductor. The purpose of
diode S2 is to provide the freewheeling path when S1 is turned off. The on and off periods of
S1 shows that during TON, the output current i2 is the same as i1. The freewheeling diode
current labelled as iD. is same as i2 during TOFF.. waveforms are as in figure.
The second chopper switching cycle commences when S1 is turned ON again at the
end of first TOFF. So the current again starts to build up as before. There is already an initial
current (Iv1), and therefore the second peak Ip2 will be larger than Ip1. Consequently the valley
magnitude Iv2 at the end of the second cycle will also be larger than I v1. In this way as the
switching progresses, both the peak and valley magnitude progressively increases. However,
the difference between the successive cycles become less and less. After several cycles, the
difference between successive cycles become negligibly small, and we say that the circuit
conditions have reached steady state. This means that the peak current is effectively the same
in successive cycles. A similar statement is true for the valley currents.
Voltage relationship.
Figure( c) shows the waveform of the voltage vB1B2, at the output terminals B1-B2 of the
chopper. This is a train of rectangular pulses of duration T ON. This voltage consist of dc
component and ac component. The ac component is the ripple voltage. The purpose of
using inductance L is to absorb the ripple voltage across it and present only the dc component
to the load resistor R. The magnitude of the output dc voltage at the load terminals will be
given by the average height of the waveform of figure (c ).
From figure, the average load voltage Vo is given by
Ton Ton
V2 V2 .V1 DV1
Ton Toff T
Ton on - time , Toff off - time
T Ton Toff chopping period
Ton
D duty cycle
T
( a or D)
Thus load voltage can be controlled by varying duty cycle .
TON
V2 .V1 V1 D
T

D being the switching duty cycle of the chopper, defined as the ratio of ON time to total
cycle time. Therefore the voltage conversion ratio a of the chopper, defined as the ratio
of output to input voltage will be
V2
a D
V1
 The switching duty cycle can be varied ideally in the range 0-1 by variation of the ON
time. It is therefore possible to operate the chopper with any desired voltage conversion ratio
below unity and to vary the ratio according to requirements by adjusting the duty cycle.

The equation shows that load voltage is independent of load current . V 2 can also be
written as
V2 f .Ton .V1
1
f chopping frequency
T

current relationships.
If the chopper has been working for a sufficient number of cycles at constant duty cycle and
load conditions, repetitive conditions will prevail from cycle to cycle. After repetitive
conditions can be assumed, the load current waveform will be as shown in fig( c) below. The
path of load current during TON and TOFF are as shown in figure (a) and (b).

Current waveform under repetitive conditions

During TON, the load current rises from the valley magnitude labelled I v in figure (c )
to peak magnitude Ip. during TOFF, the current decays from Ip to Iv. This sequence is repeated
during successive cycles.
Figure below shows graphically the growth of load current during the progress of switching.
the individual chopper cycles are labelled as 1,2,.n in the figure.
 Growth of load current as switching progresses

We shall assume that the current is zero initially at the commencement of the first
cycle. The initial current at commencement of T ON in the nth cycle is labelled as valley
current Iv(n). At the end on TON in this cycle, the current reaches the peak magnitude labelled
Ip(n). for convenience we shall first take our reference zero of time (t=0) at the commencement
of TON. The loop equation during TON will be
di
L Ri V
dt
With i = Iv(n). at t=0. The solution of this gives, during TON
V
i I v ( n ) e t / 1 (1 e t / ) where L/R is the time constant of the output circuit
R

V1
i I v ( n ) e t / (1 e t / ) where L/R is the time constant of the output circuit
R

At the end of the ON period (t= TON ) i = Ip(n) with

Choice of filter inductance and/frequency.

We can use the equation

V11 e 0.5T /
I pp
R 1 e 0.5T /

To determine the inductance value and/or choose the chopper frequency so as to limit the
maximum value of the peak-to- peak ripple current as required. The above equation gives

1 a I pp R
e 0.5T / where a
1 a V1

This may be written as

T 1 a
2 ln where T 1/f, with f as the chopping frequency and the load circuit ti me constant
1 a
L/R, making theses substituti on, we get
R
f L
2 ln 1 a / 1 a

An important practical implication of this equation is that the ripple current can be kept
below a specified limit, either by increasing the value of L or by proportionately increasing
the chopper frequency. This explains why chopper converters should be designed to work at
high switching frequencies to minimize filter inductance requirements.

Voltage step up chopper- Basic principle of operation

The circuit configuration of the chopper converter used for stepping up a dc volatage is
shown in figure.
In this average output voltage Vo greater than input voltage Vs can be obtained by a
chopper called step-up chopper. In this chopper, a large inductor L in series with source
voltage Vs is essential as shown in fig(a). when the chopper CH is on, the closed current path
is as shown in fig(b) and inductor stores energy during T on period. When the chopper CH is
off,as the inductor current cannot die down instantaneously, this current is forced to flow
through the diode and load for a time Toff, fig( c). as the current tends to decrease, polarity of
the emf induced in L is reversed as shown in fig( c). As a result, voltage across the load,
given by V0 = Vs +L(di/dt), exceeds the source voltage Vs. In this manner, the circuit of
fig( a) acts as a step up chopper and the energy stored in L is released to the load.

(a) Step up chopper (b) L stores energy

(c) L.di/dt added to Vs. (d) voltage and current waveforms.

When CH is on, current through the inductance L would increase from I 1 to I2 as

shown in figure.(d). when CH is off, current would fall from I 2 to I1. With CH on Vs is applied
to L. i.e. vL = Vs. when CH is off, KVL for fig (c) gives vL V0 +Vs = 0, or vL =V0
-Vs. here vL = voltage across L. variation of source voltage v s, source current is, load voltage
v0 and load current i0 is sketched in figure (d). assuming linear variation of output current, the
energy input to inductor from the source, during the period Ton, is
Win = (voltage across L)(average current through L) Ton
I I
Vs . 1 2 Ton
2

During the time Toff, when chopper is off, the energy released by inductor to the load is
Woff = (voltage across L)(average current through L) Toff
I I
(V0 Vs ). 1 2 .Toff
2
Considering the system to be lossless, these two energies given by the above equations will
be equal.
I I I I
Vs . 1 2 Ton (V0 Vs ). 1 2 .Toff
2 2

Vs.Ton VoToff Vs .Toff

VoToff Vs (Ton Toff ) Vs .T

Vs.Ton VoToff Vs .Toff

T T 1
or Vo Vs Vs Vs
Toff T Ton 1

From the above equation, it is seen that average voltage across the load can be stepped up by
varying the duty cycle. If chopper is always off, =0 and V0 = Vs. If this chopper is always
on, =1, and V0 =(infinity) as shown in figure below. In practice, chopper is turned on and
off so that is variable and the required step up average output voltage, more than source
voltage is obtained.

(a) Variation of load voltage with duty cycle (b) regenerative braking of dc motor.

The principle of step up chopper can be employed for the regenerative braking of dc
motors. This is illustrated in figure (b) where motor armature voltage Ea represents Vs. voltage
Vo is the dc source voltage. When CH is on, L stores energy. When CH is off, L releases
energy. In case Ea/(1-) exceeds Vo,dc machine begins to work as a dc generator and armature
current Ia flows opposite to motoring mode.power now flows from dc machine to source Vo
causing regenerative braking of dc motor. Motor armature voltage E a is directly proportional
to field flux and motor speed. Therefore even at decreasing motor speeds, regenerative
braking can be made to take place provided duty cycle and field flux are so adjusted that E a/
(1-) is more than the fixed source voltage Vo.
Two quadrant and four quadrant choppers (Analysis not required).

Generation of timing pulses for a single phase chopper.

The block schematic of a circuit scheme for generating the timing pulses for a single
phase chopper is shown in figure (a). the related waveforms are shown in fig (b).

Timing pulse generation for a single phase chopper

The switching frequency of the chopper is set by programming the circuit block
labelled clock oscillator. to output pulses at this frequency. If there is a need to vary the
chopper frequency then we may use a voltage controlled oscillator for our clock and adjust
the frequency as required, by adjusting the reference voltage to it, as is shown in figure. The
output of the clock generator is used to synchronise the frequency of the ramp generator. The
ramp voltage output is compared with an adjustable control voltage using a comparator chip.
The comparator output serves as the timing pulses for the chopper. The ON period of the
chopper will be the pulse width of the timing pulses from the comparator.
The output pulse width, and therefore the duty cycle of the chopper could be varied by
varying the control voltage input to the comparator, as shown in the figure. The operation of
the circuit is further made evident by the relevant waveforms sketched in fig(b).
If the power semi conductor switch of the chopper is a latching device such as a GTO,
what is mainly needed to turn it ON will be a pulse of short duration at each leading edge of
the timing pulse train. For turn off switching, it will need another pulse at each trailing edge
of the timing pulse train. These can be obtained by using two monostable multi vibrator
chips- one triggered by the leading edges and the other triggered by the trailing edges.

Voltage and current commutation.

Basic Principle of Cyclo converters: single phase and three phase. (Analysis not required).