Choppers and Cycloconverters

1. Choppers
Mrs Shimi S L
Assistant Professor, NITTTR,
Chandigarh, India

2. Introduction
• Chopper is a static device.
• A variable dc voltage is obtained from a
constant dc voltage source.
• Also known as dc-to-dc converter.
• Widely used for motor control, smart grid, ect.
• Also used in regenerative braking.
• Thyristor converter offers greater efficiency,
faster response, lower maintenance, smaller
size and smooth control.

3. Chopper Controlled DC Motor

5. Block Diagram of DC-DC Converters

6. Choppers are of Two Types
 Step-down choppers.
 Step-up choppers.
 In step down chopper output voltage is less
than input voltage.
 In step up chopper output voltage is more
than input voltage.

7. Classification of Chopper
Based on input/output voltage levels
• Step-down Chopper (Buck converter)
• Step-up Chopper (Boost Converter)
• Buck Boost Converter
Direction of output voltage and current
• Class A
• Class B
• Class C
• Class D

8. Based on Circuit operation
• First (Single) Quadrant Chopper
• Two-quadrant Chopper
• Four Quadrant Chopper
Based on Commutation Method
• Voltage Commuted Chopper
• Current Commuted Chopper
• Load commuted Chopper
• Impulse Commuted Chopper

9. Classes of chopper (A,B,C,D and E)

10. Control Strategies
• The output dc voltage can be varied by the
following methods.
• (a) Time-ratio control, and
– Pulse width modulation control or constant
frequency operation.
– Variable frequency control.
• (b) Current limit control

11. Pulse Width Modulation
or
Constant Frequency Operation
• tON is varied keeping chopping frequency ‘f’ &
chopping period ‘T’ constant.
• Output voltage is varied by varying the ON
time tON

12. V0
V
V
V0
t
t
tON
tON tOFF
tOFF
T

13. Variable Frequency Control
• Chopping frequency ‘f’ is varied keeping either
tON or tOFF constant.
• To obtain full output voltage range, frequency
has to be varied over a wide range.
• This method produces harmonics in the output
and for large tOFF load current may become
discontinuous

15. Disadvantages of Variable Frequency
Control Strategy
(a) The frequency has to be varied over a wide range for the
control of output voltage in frequency modulation. Filter
design for such wide frequency variation is, therefore, quite
difficult.
(b) For the control of a duty ratio, frequency variation would be
wide. As such, there is a possibly of interference with systems
using certain frequencies, such as signaling and telephone
line, in frequency modulation technique.
(c) The large OFF time in frequency modulation technique, may
make the load current discontinuous, which is undesirable.
Thus, the constant frequency system using PWM is the preferred
scheme for dc-dc converters (choppers).

16. Current Limit Control

17. Stepping Down a DC Voltage
• A simple approach that shows the evolution

18. Pulse-Width Modulation in DC-DC Converters

19. Step-Down DC-DC Converter: Waveforms

20. Step-Down DC-DC Converter: Waveforms

21. Step-Up DC-DC Converter

22. Step-Up DC-DC Converter : Waveform

23. Step-Down/Up DC-DC Converter
• The output voltage can be higher or lower than the
input voltage

24. Step-Down/Up DC-DC Converter : Waveform

25. Classification Of Choppers
• Choppers are classified as
–Class A Chopper
–Class B Chopper
–Class C Chopper
–Class D Chopper
–Class E Chopper

26. Class A Chopper
V
Chopper
FWD
+

v0
v0
i0
i0
L
O
A
D
V

27. • When chopper is ON, supply voltage V is
connected across the load.
• When chopper is OFF, vO = 0 and the load
current continues to flow in the same direction
through the FWD.
• The average values of output voltage and
current are always positive.
• Class A Chopper is a first quadrant chopper .

28. • Class A Chopper is a step-down chopper in
which power always flows form source to load.
• It is used to control the speed of dc motor.
• The output current equations obtained in step
down chopper with R-L load can be used to
study the performance of Class A Chopper.

29. Output current
Thyristor
gate pulse
Output voltage
ig
i0
v0
t
t
t
tON
T
CH ON
FWD Conducts

30. Class B Chopper
V
Chopper
+

v0
v0
i0
i0
L
E
R
D

31. • 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.

32. • 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.

33. Output current
D
conducts Chopper
conducts
Thyristor
gate pulse
Output voltage
ig
i0
v0
t
t
t
Imin
Imax
T
tONtOFF

34. Class C Chopper
V
Chopper
+

v0
D1
D2
CH2
CH1
v0
i0
i0
L
E
R

35. • Class C Chopper is a combination of Class A
and Class B Choppers.
• For first quadrant operation, CH1 is ON or D2
conducts.
• For second quadrant operation, CH2 is ON or
D1 conducts.
• When CH1 is ON, the load current is positive.
• The output voltage is equal to ‘V’ & the load
receives power from the source.
• When CH1 is turned OFF, energy stored in
inductance L forces current to flow through
the diode D2 and the output voltage is zero.

36. • 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.
• On turning OFF 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.

37. • Average output voltage is positive
• Average output current can take both positive
and negative values.
• 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.

38. Gate pulse
of CH2
Gate pulse
of CH1
Output current
Output voltage
ig1
ig2
i0
V0
t
t
t
t
D1 D1D2 D2CH1 CH2 CH1 CH2
ON ON ON ON

39. Class D Chopper
V
+ v0
D2
D1 CH2
CH1
v0
i0
L ER i0

40. • Class D is 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.
• Output voltage vO = - V .

41. • Average load voltage is positive if chopper ON
time is more than the OFF time
• 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.

42. Gate pulse
of CH2
Gate pulse
of CH1
Output current
Output voltage
Average v0
ig1
ig2
i0
v0
V
t
t
t
t
CH ,CH
ON
1 2 D1,D2 Conducting

43. Gate pulse
of CH2
Gate pulse
of CH1
Output current
Output voltage
Average v0
ig1
ig2
i0
v0
V
t
t
t
t
CH
CH
1
2
D , D1 2

44. Class E Chopper
V
v0
i0
L ER
CH2 CH4D2 D4
D1 D3
CH1 CH3
+ 

45. Four Quadrant Operation
v0
i0
CH - CH ON
CH - D Conducts
1 4
4 2
D D2 3- Conducts
CH - D Conducts4 2
CH - CH ON
CH - D Conducts
3 2
2 4
CH - D Conducts
D - D Conducts
2 4
1 4

46. • 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 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.

47. • 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.

48. Commutation in DC-DC Choppers
i) Voltage commutation
ii) Current commutation
iii) Load commutation

49. Voltage Commutation

50. Different Modes of Voltage Commutation Chopper

51. Current commutation

52. Different Modes of Current Commutation Chopper

53. Cycloconverters

54. Types of cycloconverters
• Step-down cycloconverters ( fo<fs )
• Step-up cycloconverters ( fs<fo )
Applications
• Speed control of high power ac drives
• Induction heating
• Static VAR generation
• Power supply in aircraft or shipboards

55. Equivalent circuit of Cycloconverters

57. Single phase to single phase
step up cycloconverter
Mid-point Cycloconverter Wave form

58. P1 -> Forward biased ,ON
N2 -> Forward biased ,OFF
P2,N1 -> Reverse biased
P1 -> Forward biased ,OFF
N2 -> Forward biased ,ON
P2,N1 -> Reverse biased

59. Bridge Type Cycloconverter Wave form
Single phase to single phase
step up cycloconverter

60. P1,P2 -> Forward biased ,ON
N,N2 -> Forward biased ,OFF
P3,P4,N3,N4 -> Reverse biased

61. P1,P2 -> Forward biased ,OFF
N,N2 -> Forward biased ,ON
P3,P4,N3,N4 -> Reverse biased

62. RL Load Wave form
P1 P2 P1 P2 N2 N1 N2 N1

63. Resistive Load Wave form

64. Three phase to single phase
cycloconverter

65. • Gating circuitry is suitably designed to introduce
progressive firing angle dealy.
• Vary progressively the firing angle of the thyristors, firing
angle at M = 90°
at N < 90°
• At O angle is further reduced than it is at N and so on.
In this way , a small delay in firing angle is introduced at
O, P, G, R and S.
• At S firing angle = 0°,
o The mean output voltage, given
Vo=Vdocos∝
and is maximum at S.
• At M,
V= 0 as α = 90°

66. • After point S, a small delay in firing angle is
further produced progressively at points T, U, V,
W, X and Y.
• At Y, the firing angle is again 90° and mean
output voltage is 0.
• Mean output voltage wave is obtained by joining
points pertaining to average voltage values.
fabricated output voltage is
shown by thick curve

67. Fabricated output voltage = Fundamental
frequency output voltage + several other
harmonic components
High frequency components
Can be eliminated by
Low Inductance

68. The Fig, shows half-cycle of fundamental frequency
output voltage (marked mean output voltage), there
are eight half cycles of supply frequency voltage,
i.e.
fo =1/8 fs
Where fs is the supply frequency.
• For obtaining positive half cycle of low-frequency
output voltage, firing angle is varied from 90° to 0°
and
90° to 0°.

69. • For obtaining one cycle (consisting of one positive half
cycle and one negative half cycle)of low frequency
output voltage,
Firing angle 90° to 0 to 90° for +ve half cycle and from
90° to 180° and back to 90° for –ve half cycle.
Magnitude of progressive change in firing angle
= (reduction factor in frequency)x 120°
Progressive step variation in firing angle
= 1/8 x 120°=15°

70. α = 90° at M
α = 90° - 15° = 75° at N
α = 75° - 15° = 60° at O
α = 60° - 15° = 45° at P
α = 45° - 15° = 30° at Q
α = 30° - 15° = 15° at R
α = 15° - 15° = 0° at S and so on till α = 90° at Y
From M to Y, there is one half cycle of low frequency
output voltage and eight half cycle of supply
frequency, Indicating,
fo =1/8 fs

71. Anti-Parallel connection of two three-phase half-
wave converters allows flow of current in both
directions
Positive converter group permits the flow
of current during positive half cycle
Negative converter group permits the flow
of current during negative half cycle

72. Fig.4 ( a) Voltage waveform (b) Current waveform

73. Three phase to single phase
cycloconverter
(a) Schematic Diagram (b) Wave form

74. Three phase to three phase
cycloconverter

77. Advantages and Disadvantages of Cyclo-converter
Advantages
1. In a cyclo-converter, ac power at one frequency is converted directly to a lower
frequency in a single conversion stage.
2. Cyclo-converter functions by means of phase commutation, without auxiliary forced
commutation circuits. The power circuit is more compact, eliminating circuit losses
associated with forced commutation.
3. Cyclo-converter is inherently capable of power transfer in either direction between
source and load. It can supply power to loads at any power factor, and is also capable of
regeneration over the complete speed range, down to standstill. This feature makes it
preferable for large reversing drives requiring rapid acceleration and deceleration, thus
suited for metal rolling application.
4. Commutation failure causes a short circuit of ac supply. But, if an individual fuse blows
off, a complete shutdown is not necessary, and cyclo-converter continues to function with
somewhat distorted waveforms. A balanced load is presented to the ac supply with
unbalanced output conditions.
5. Cyclo-converter delivers a high quality sinusoidal waveform at low output frequencies,
since it is fabricated from a large number of segments of the supply waveform. This is
often preferable for very low speed applications.
6. Cyclo-converter is extremely attractive for large power, low speed drives.

78. Disadvantages
1. Large number of thyristors is required in a cyclo-converter, and its control
circuitry becomes more complex. It is not justified to use it for small
installations, but is economical for units above 20 kVA.
2. For reasonable power output and efficiency, the output frequency is
limited to one-third of the input frequency.
3. The power factor is low particularly at reduced output voltages, as phase
control is used with high firing delay angle.