Geno Peter / International Journal of Engineering Science and Technology
Vol. 2(9), 2010, 4222-4227
DESIGN OF SINGLE PHASE FULLY
CONTROLLED CONVERTER USING COSINE
WAVE CROSSING CONTROL WITH VARIOUS
PROTECTIONS
GENO PETER .P
Assistant Professor, Department of EEE ,Loyola Institute of Science and Technology, Loyola Nagar,
Thovalai, Tamil Nadu, 629 302,India
Abstract:
The single phase fully controlled converter is used to convert single phase A.C supply to D.C supply. Such
converter finds application in dc motor loads for motoring and electrical braking of the motor. There are two
types of control schemes to control the firing of thyristors, they are Cosine wave crossing control and Ramp
comparator control. In this paper, cosine wave crossing control is used for the control circuit. The advantage of
this scheme is that the output voltage is proportional to the control voltage ie., the output voltage is independent
of the variation of input voltage. The various protections such as over current, short circuit, under voltage
protections etc are included. The main objective of this project is to design an efficient, simple, robust and
economical control circuit thereby making the fully controlled converter. The fully controlled converter uses
four thyristors . It is a two quadrant converter was voltage polarity can reverse, but current direction cannot
reverse because of unidirectional nature of thyristors. In this paper, I have presented the control circuit for the
thyristors along with the protection circuits to control Dc Motors of rating 220V,5.8 A and 1500rpm .
Keywords: Electrical braking; Cosine wave crossing control; Motoring.
1. Schemes For Generating Gating Pulses
There are numerous approaches of designing gate pulses and the required delay angle. The purpose of this
section is not to give an exhaustive treatment of generating gate pulses but rather mention broad schemes mostly
commonly used in the firing pulse generator. They are


Cosine wave crossing control
Ramp comparator control
2. Cosine Wave Crossing Control
A linear output voltage control is obtained by this method. This scheme also provides automatic negative
feedback to the change in supply voltage. A control voltage (Ec) generates a firing pulse at the crossing of the
control voltage and a cosine voltage derived from supply voltage.
The phase angle (α) is given by
α= cos-1[EC/ Emax]
The output voltage of the converter is
Vo= Vomax * cos α= Vomax *cos[cos-1[EC/ Emax]]
Eq. (1)
Vomax =2VM /π
Eq. (2)
= Vomax*[ EC/ Emax ]
Eq. (3)
Emax = VM /K
Eq. (4)
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Geno Peter / International Journal of Engineering Science and Technology
Vol. 2(9), 2010, 4222-4227
Vo = K1* EC
Eq. (5)
K1=[2*K/π]
Eq. (6)
Vo α EC
Hence the output voltage is not effected by supply voltage and it remains constant as long as control voltage
(EC) is constant.
.
2.1. Operation of the Control Circuit
A (230/6V) step down transformer is given as input to the inverting terminal of the cosine wave generator .The
cosine wave generator shifts the sine wave by 90 degrees and converts into a cosine wave as shown in fig 1and
fig 2
Fig. 1 Cosine Wave Crossing Control Firing Circuit
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Fig 2 Control Circuit Waveforms
This cosine wave is fed as input to the inverting terminal of the comparator. The comparator converts the cosine
wave into a square wave. The negative portion of the square wave is clipped off by the voltage limiter. This
square wave is fed as input to the monostable multivibrator. The monostable gives two output each of 10 ms
duration delay for each halfcycle this output is fed as input to the AND gate, where the monostable output is
compared with the carrier frequency pulses of 10 KHz generated using an astable multivibrator, thus the pulses
required for triggering the thyristors were obtained. This is fed as input to the transistor which acts as a current
amplifier. Thus amplified gate pulses are obtained as output to trigger the thyristors.
3.1.1. Design for monostable multivibrator
Time delay, T= 1.1 R*C
Eq. (7)
Required time delay, T= 10ms
C=0.1µF
R=90KΩ
Design for carrier frequency generator
Frequency ,F=(1/T) =(1/(0.693(Ra+2Rb))*C)
Here Ra=4.7KΩ
Rb=4.7KΩ
C=0.01 µF
Hence we get frequency, F=10 Khz
Eq. (8)
3.1.2. Design of Driver Circuit
RG and R3 form a potential divider. RG limits the gate current whereas R3 limits the voltage across the gate and
cathode. RG is generally ≥100Ω in small SCR and R3< 1000 Ω.
Let RG =100 Ω and R3=500 Ω
The maximum voltage at the gate and cathode is
VGK= ((VC - VV )-0.7)/( R3 + RG )* R3 ` )= 4V
Eq. (9)
Maximum gate current
IGM =( VGK – 0.7)/ RG =33mA
Eq. (10)
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Geno Peter / International Journal of Engineering Science and Technology
Vol. 2(9), 2010, 4222-4227
3. Fully Controlled Converter
3.1. Design of Power Circuit
Form Factor=1.11 =(Iac/Idc )
Eq. (11)
Idc = 12/1.11 =10A
As the motor current is 5.8A, The thyristor rating of 12A, 600V is sufficient
Voltage rating of SCR= √2*230 =325V
Hence a PIV rating of 600V is sufficient.
So we use TYN612 thyristor, which is of 600V, 12 A
3.2. Design of Snubber Circuit
A snubber is a simple electrical circuit used to suppress electrical transients. Snubbers are frequently used with
an inductive load where sudden interruption of current flow would lead to a sharp rise in voltage across the
device creating the interruption. Frequently, a snubber can consist of just a small capacitor in series with a small
resistor. This combination can be used to suppress the rapid rise in voltage across thyristor, preventing
erroneous turn-on of the thyristor, it does this by limiting the rate of rise in voltage across the thyristor to a value
which will not trigger it. Snubbers are also often used to prevent arcing across the contacts of relays
dV / dt = Vm /(R*C)
Eq. (12)
where Vm is the peak voltage = 325V
dV / dt = 50V/µs
C=0.1µF
R= 60Ω
3.3. Operation for Motor Load
Fig 3 Fully Controlled Converter
When the single phase fully controlled bridge converter is connected with the motor load , during positive half
cycle thyristor T1 and T3 are forward biased and these two thyristors are fired simultaneously at ωt=α, the load is
connected to the supply through T1 and T3. Due to inductive load T1 and T3 will continue to conduct till ωt=π+α,
even though the input voltage is already negative as shown in the fig 3. During the negative half cycle of the
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Vol. 2(9), 2010, 4222-4227
input voltage the thyristors T2 and T4 at ωt=π+α, will apply supply voltage across thyristors T1 and T3 as reverse
blocking voltage. T1 and T3 will be turned off due to line or natural commutation and load current will be
transferred from T1 and T3 to T2 and T4 respectively. During the period from π+α , the input voltage and input
current is positive and the power flows from the supply to the load. The converter is said to be operated in
rectification mode. During the period from π to π+α input voltage is negative and the input current is positive ,
and there will be reverse power from the load to the supply. The converter is said to be operated in inversion
mode
4. Over Load Protection
Fig 4 Over Load Protection Circuit
The power converters may develop short circuits or faults and the resultant fault currents must be cleared
quickly. Fast acting fuses are normally used to protect the semiconductor devices. As the fault current increases
the fuse opens, and clears the fault current in a few milliseconds.
4.1. Design of Shunt resistance
The load current IL = 5A
Required Drop = 2.5 V
Hence R=0.5 Ω
As seen in fig 4 the required voltage drop is set by adjusting the preset pot. Under normal conditions i.e., when
the load current is less than 5A we get logic high at the output of the optocoupler. This is given to the clear pin
of the monostable multivibrator . When the load current increases IL >5A then the output of the optocoupler will
give logic low. Hence the generation of pulses is stopped. Thus providing overload protection
5. Conclusions
5.1. Control Obtained
The output voltage of the converter
Eq.(13)
Vo= (2VM /π)* cos α
The thyristor firing angle control was obtained from 30 degrees to 150 degrees, the output voltage control
obtained was
When α= 30°
Vo= (2VM /π)* cos 30°
Vo= 180V
When α= 150°
Vo= (2VM /π)* cos 150°
Vo= (-180V)
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Geno Peter / International Journal of Engineering Science and Technology
Vol. 2(9), 2010, 4222-4227
It was possible to vary the output voltage from +180V to -180V
The output voltage is not affected by supply voltage and it remains constant as long as control voltage (EC) is
constant
Acknowledgments
The author wishes to thank Mr. P.Jeba Peace, Nokia Siemens, India and Dr. M.T Nicholas, Principal, LITES for
their support.
References
[1]
[2]
[3]
Bimbhra,P.S. Power Electronics, Khanna Publishers, India, 1999 pp. 190-197.
Mohammad H. Rashid. (1994): Power Electronics-circuits device and Application, Prentice Hall of India Pvt Ltd, India.
Singh M.D. (1998). Power Electronics, Tata McGraw Hill Publishing Company Ltd, India.
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