3. Introduction
• A power electronic converter is useful only
when its output voltage is controllable
• There are different techniques available for
controlling the output of different converters
like time ratio control for DC choppers, PWM
control for inverters etc.
• For phase controlled rectifiers the technique is
control of switching angle
9-Nov-12 EE-321N, Lec-14 3
4. Contd...
• Moreover, the previously described techniques
(Unit-2) are not suitable if the converter has more
than one thyristor (which is generally the case)
• The schemes employed for switching angle control
are:
1. Cosine wave crossing control
2. Ramp comparator control
3. Digital firing scheme
4. Equidistant pulse firing scheme
9-Nov-12 EE-321N, Lec-14 4
5. 1. Cosine Control
• This method employs a cosine wave obtained
from the input supply thru an integrator.
• The cosine wave is compared against a dc
reference signal and correspondingly trigger
pulses are obtained
• The basic principle is similar to that of a dual
converter in which output voltage ratio is equal
to cosine of the firing angle
• Based on this the rectifier can be thought of as a
amplifier with linear transfer characteristic
9-Nov-12 EE-321N, Lec-14 5
11. Operation
• The input supply is synchronized and stepped
down through a synchronizing t/f to obtain v1
• Then it is fed to integrator to obtain the cosine
signal v2
• This cosine wave is compared against the dc
control voltage which is limited to ±Ecm
• v3 is the o/p of comparator 1 which is
obtained when Ec is higher than v2
9-Nov-12 EE-321N, Lec-14 11
12. Operation...contd
• Similarly, output v4 is obtained from
comparator 2.
• The signals v3 & v4 make the clock pulse
generators to produce output pulses which in
turn set the respective F/Fs
• The o/ps vi & vj of the F/Fs can be used to
trigger SCRs in a single phase converter
• In practice, F/F outputs are ANDed with high
frequency carrier wave (3-5 kHz) to obtain
pulse train
9-Nov-12 EE-321N, Lec-14 12
13. Relation between Output Voltage
& Control Voltage
From the waveforms it can be observed that
firing angle is governed by the intersection of
cosine wave and control signal & can be
expressed as:
9-Nov-12 EE-321N, Lec-14 13
2m c
1 c
2m
cos
cos
V E
E
V
14. Contd...
The output of a single phase full converter is
given as:
Which can be written as
9-Nov-12 EE-321N, Lec-14 14
m
0
2
cos
V
V
1 c
m m
0 c c c
2m 2m
2 2
cos cos
E
V V
V E k E
V V
15. Contd...
This shows that cosine control scheme provides
a linear transfer characteristic which improves
the closed loop response of the converter
9-Nov-12 EE-321N, Lec-14 15
16. Remark 1
• This scheme is sensitive to line supply variations.
For 3 ph converters adjustments should be made
to equalize all the line voltages otherwise firing
angles will be unequal
• But this scheme has a “self-regulating” property
because any change in the input voltage will lead
to corresponding change in the cosine wave
leading to mean dc voltage being constant
9-Nov-12 EE-321N, Lec-14 16
17. Remark 2
• The size of the control scheme circuit
increases as the number of phases/pulses
increases because firing pulse is produced
individually for each thyristor
• This can be overcome by using a cosine time
multiplexing instead of individual timing
control
• Cosine control scheme also applies to dual and
cycloconverters
9-Nov-12 EE-321N, Lec-14 17
18. Further Reading
1. Dubey, G. K., et. al. , “Thyristorized Power
Controllers”, New-Age International, 1986.
2. Pelley, B. R., “Thyristor Phase-Controlled
Converters & Cycloconverters”. Wiley-
Interscience, 1971.
3. Arora, O. P., “Power Electronics Laboratory:
Theory, Practice & Organization ”, Narosa
Publishing House, 2007.
9-Nov-12 EE-321N, Lec-14 18
