1. Three phase Current source Inverter
Mahamud hasan Sajib , Ishtiak Al Mamoon, Nafees Moonsur
Abstract:
A current source inverter is designed for
switching pulse generation. The CSI
(current source inverter) designed in this
paper is an auto sequential commutated
inverter (ASCI) type. For the
implementation of the CSI, there have
three main stages, Chopper design,
Inverter switching arrangement and
control circuit for keeping current
constant. It produces square wave.Here,
shift registers are used for pulse shifting.
The main zest of this paper is CSI is
such a machine that takes DC input and
produces a variable AC output. And the
application of CSI are controlling of the
speed of A/C machine, synchronous
motor starting in gas turbine, in
induction heating and lagging of VAR
generation at any properties.
Key words: current source inverter, 555
timer circuit, shift register, Chopper
circuit, ASCI
1.1 Introduction
A current source inverter circuit having a
plurality of branches, each branch
consisting of a pair of thyristors adapted
to be switched on alternating for power
supply from a D. C. power source and
diodes connected in series to the
thyristors. To each connection point
between a thyristor and a diode, there is
connected one end of one of a plurality
of commutating capacitors, while the
other ends of the capacitors are
connected in common. An auxiliary
charging source is provided between the
common connection of the commutating
capacitors and one of the terminals of
the D. C. source for performing an
auxiliary charging for each commutating
capacitor through a respective thyristor.
Between one of the terminals of the D.
C. power source and the common
connection of the commutating
capacitors, a series circuit consisting of a
diode and an auxiliary D. C. source is
provided constituting a by-pass for the
charging voltage of the commutating
capacitor when the associated thyristor is
in the ON state
1.2 Objective of this paper:
The object of the invention is to provide
a new current source inverter, providing
a high AC impedance DC current source
through an inductor connected in series
to a variable DC electric source so that
DC electric power is supplied to both
sides of a main DC bus of the main
inverter, providing a low AC impedance
DC voltage source by supplying the DC
electric power to both sides of an
auxiliary DC bus of the auxiliary
inverter through a rectifying circuit
consisting of diodes at both sides of the
auxiliary DC electric source bus from
the respective connecting points of the
main inverter, auxiliary inverter and the
three motor terminals and by
connecting a capacitor in parallel
between both of these sides supplied
with positive and negative DC voltage at
both sides of the auxiliary DC
bus,respectively,under normal operation,
and connecting a snubber circuit
comprising inductors, resistance and
diodes which are added in series or
parallel to respective semiconductor
elements for safe switching operation
there of.
1.3 Material and Methods:

2. Ideal current source is an electrical or
electronic device that delivers or absorbs
electric current. A current source is the
dual of a voltage source. In circuit
theory, an ideal current source is a
circuit element where the current
through it is independent of the voltage
across it. It is a mathematical model,
which real devices can only approach in
performance The voltage across an ideal
current source is completely determined
by the circuit it is connected to. When
connected to a short circuit, there is zero
voltage and thus zero power delivered.
When connected to a load resistance, the
voltage across the source approaches
infinity as the load resistance approaches
infinity (an open circuit). Thus, an ideal
current source can supply unlimited
power forever and so represents an
unlimited source of energy. Connecting
an ideal open circuit to an ideal non-zero
current source is not valid in circuit
analysis as the circuit equation would be
paradoxical, e.g., 5 = 0.No real current
source is ideal (no unlimited energy
sources exist) and all have a finite
internal resistance (none can supply
unlimited voltage). However, the
internal resistance of a physical current
source is effectively modeled in circuit
analysis by combining a non-zero
resistance in parallel with an ideal
current source.
300 Hz
squar
Pulse shifting for production 6
t i i l b i 4 bit hift
Pulse width reduction by using Monostable Multivibrator ( 74LS121)
6 kHz pulse
genera[mer
(555) IC
2 input AND Gate (7408)
Pulse Transformer stage
Gate
f
O
R
Gate
f
Gate
f
Gate
f
Gate
f
Gate
Th i
P PPu Pu P P
Eq 1.1 Thyriostor Equation
Temperature changes will cause the
above circuit to change the output
current since VBE is sensitive to
temperature. This can be compensated
for by including a standard diode D (of
the same semiconductor material as the
transistor) in series with the zener diode
as shown in the image on the left. The
diode drop (VD) tracks the VBE changes
due to temperature and thus suppresses
temperature dependence of the CCS.
1.4 Design a current source inverter:
A current source inverter requires forced
commutation if it supplies a non-leading
power factor load. Auto sequential
commuted inverter (ASCI) is the most
popular among all the types of forced
commuted current source inverters. The
three phase bridge configuration shown
in fig-3 is used almost exclusively in
practice. The thyristor T1 and T6 are the
principle switching devices of the
inverter. Where each of them conduct in
sequence ideally for 120° to establish a
six-stepped current wave in the output
line .
Fig 1.1 -Block diagram for gate pulse generation
The series diodes and the delta capacitor
bank of equal values which are

3. connected to each of the upper and lower
groups of the thyristor constitute the
commutating elements. The capacitor
store charge for commutation and the
series diodes tends to isolate them from
the load . In normal inverter operation
the upper group and the lower group
operates independently and there are
six commutations per cycle of
fundamental frequency. When T2 is
fired T6 is instantaneously turned off by
the reverse voltage. Similarly when T3
fired T1 will be turned off in similar
way.
1.5 Out put of Shift Register
The 300 Hz pulse from the circuits of
figure of square wave generator to the
CLK of the circuit of of fig 5-a we got
six shifted pulses from the output
terminal 1,2,3,4,5& 6. These pulses are
all 50Hz 60º (3.3 ms) ON time and 270º
(16.67 ms) OFF time. Wave shape is
shown in following figure
Fig 1.2 -Shift resistor wave shape
This stage is necessary to make 3 or 4
very narrow pulse to ensure the
triggering of each Thyristor. If one of the
narrow pulse failed to trigger the
Thyristor then next can do the job. To
perform this operation, the pulse from
74121 is fed to the one output of
7408(AND GATE) and Other input is 6
Khz square pulse. It can be generated by
adding pulse-1 and pulse-6 after the
AND operation by using 7432.
Figure 1.3 : High frequency square pulse
Each of the output of the Circuits on fig-
is simplified by a driver transistor SL100
and fed to the pulse Transformer. The
pulse Transformer has the turn Ratio 1:1.
Necessary circuit and wave shape is
shown in following figure.
Figure1.4: Trigger pulse out put
Current source inverters have several
distinct advantages over variable voltage
inverters. They provide protection
against short circuits in the output stage.
S is has relatively simple control circuits
and good efficiency..
Figure1.5 : Inverter output
Now the trigger pulses from all the
driver circuits are applied to gate to
cathode of the Thyristor of the inverter

4. circuits shown in. The output voltage
figure is above
In three phase inverter Thyristor should
be triggered simultaneously T6 T1 , T1
T2, T2 T3, T3 T4,T4 T5, T5 T6. But in
ASCI inverter T1 and T6 are the
principle switching devices where each
of them conducts in sequence. Six
switching pulse of same frequency but
60º shifted one another are necessary for
inverter. To produce six switching pulse
of 50Hz by using shift resister, 300 Hz
clock pulse have to use at shift resister .
300 Hz clock pulse was generated by
using timer (555) IC. The frequency of
the pulse of and ON/OFF period can be
changed by adjusting the value of R1
and C3. Frequency can also changed by
changing the value of V3.
1.6 Conclusion
In a current source inverter the inverter
is more interactive with the load. It does
not require any extra power circuits
components. The communication circuit
for the thyristors is simpler. The current
source inverter is a dual of a voltage
source inverter. One of the
most important problems of the ASCI
system is the high voltage stresses
applied to the thyristors, diodes and
motor terminals. A direct cause of the
peak voltage stress is the peak voltage
value at both sides of the capacitor,
which is a function of the load condition,
capacitance and leakage inductance of
the motor. The magnitude of the
capacitance affects the selection of the
upper limit operating frequency, which
is determined by the no-load condition,
that is, when the DC current is
minimum. On the other hand, the
capacitor value should be selected to be
as small as possible in order to raise the
upper limit of the operating frequency
since the maximum peak voltage is
determined by the maximum load
condition, that is, when the DC current is
maximum, but in this case the peak
voltage of the capacitor becomes high.
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