CSI

1. CURRENT SOURCE INVERTERS (CSI)
• In the CSIs, the input current is constant.
• The amplitude of output current from the CSI is independent of
the load, but the magnitude of output voltage and its waveform
is dependent upon the nature of load.
• A CSI converts the input DC current to an AC current, and the
frequency of the AC current depends upon the frequency of
Switching Devices (rate of triggering the SCRs).
• The amplitude of the AC output current can be adjusted by
controlling the magnitude of the DC input current.

2. • Since CSI is a constant current system, it is used typically to
supply high power factor loads where impedance will be
remain constant or decreases at harmonic frequencies in
order to prevent problems either on switching or with
harmonics voltage.
• An VSI requires feedback diode whereas a CSI does not
require any feedback diode. The commutation circuit of CSI
is very simple as it contains only capacitors.
• Because power semiconductors in a CSI have to withstand
reverse voltage, devices like power transistors, power
MOSFETS, and power BJTs cannot be used in CSIs.
• THYRISTORS are the Best Power Semiconductor
Switches used in CSI Drives

3. Applications of CSI
• Speed control of AC motors
• Lagging VAR compensation
• Solar photovoltaic utility systems
• Synchronous motor starting

4. Concept of Current Source

6. Single-Phase Full-Bridge Inverter (CSI)
with Pure C Load
I
T1
T2
T3
T4
V0

+
LOAD
Vin
Current source

+
V

7. Principle of Operation with Waveforms

8. T/2
Current input to CSI
I
0
t
T1T2 T3T4 T1T2 T3T4 T1T2 T3T4 T1T2 T3T4
i0
I
0
-I T t
t
t
2T
T
T/2
0
0
Output current
Output voltage
Input voltage
3T/2
f = 1/T
f = 2/T
0
in

9. Voltage Shape and its Value at t = T/2
T/4 ≤ t ≤ T/2

11. T/2 ≤ t ≤ 3T/4

13. 3T/4 ≤ t ≤ T

15. T ≤ t ≤ 5T/4

17. Practical Circuit
C1

+
LOAD
IS

+
C2
T2
D2
T3
D3
T4
D4
T1
D1
VC2
VC1
i0

18. A basic thyristor-based single-phase CSI is shown. Capacitors
are used here as a commutating device. Working of this
inverter can be explained using following points:
• Initially, T1 and T2 are conducting, and the capacitors are
charged with the polarities, as shown. The positive load
current is flowing through T1, D1, load, D2, and T2.
• To turn off the conducting thyristors T1 and T2, the other two
thyristors T3 and T4, are triggered.
• When these thyristors (T3 and T4) start conducting, the
capacitor voltage VC1 and VC2 appears as reverse voltage
across the thyristors T1 and T2, respectively. So, thyristors T1
and T2 get turned off.

19. • Now, the load current flows through T3, C1, D1, load, D2, C2,
and T4. So, the polarity of the capacitors reverses due to the
charging current in the reverse direction, that is, the right plate
of capacitors C1 and C2 become positive, and the left plate
becomes negative. When the capacitor voltage becomes equal
to the load voltage, the load current becomes zero, and the
diodes D1 and D2 get turned off.
• The load current now flows in the reverse direction through T3,
D3, load, D4, and T4.
• Similarly, when the thyristors T1 and T2 are triggered, the
capacitor voltage VC1, VC2 turn off T2 and T4 in a similar
fashion. In this way, the cycle repeats and output AC voltage of
the desired frequency is obtained.

20. Conclusions
• The slope of Current is (IS/C)
• The Peak Value of Voltage from Zero axis as reference is
VP = (IST/4C)
• The Peak-to-Peak value of the Voltage in the circuit is
VP-P = 2VP = (IST/2C)
• For Pure Capacitive Load in this Full-Bridge Inverter
(CSI) each pair of Thyristors will equally conduct for a
period of T/2

21. In CSI Output Current shape is Square, But output
VOLTAGE shape depends upon the nature of load
(a) R Load - Square Shape
(b) Pure C Load - Triangular Shape
(c) R-C Load - Exponential (Rise and Fall) Shape

22. Three - Phase CSI
• A basic three-phase CSI, feeding a star-connected resistive
load, is shown. Diodes are used in series with each switch to
handle reverse voltage, which should be avoided to appear
across the power semiconductor-based switches.
• The switches are turned on in the sequence of their
numbers, with an interval of 60°, and each switch conducts
for 120° duration.

23. IS
S1 S3 S5
S4 S6 S2
D1 D3 D5
D4 D6 D2
3-phase star
connected load

24. • At a time two switches conduct, that is, one from the
upper group (T1, T3, T5) and other from the lower group
(T4, T6, T2).
• The load current of a particular phase is +IS when upper
switch of the corresponding arm conducts, and it is -IS
when the lower switch conducts.

25. T1
T2
I
ic
ib
Is
ia
II III IV V VI I II III
T1,
T6
T3,
T2
T4,
T3
T4,
T5
T6,
T5
T1,
T6
T1,
T2
T3,
T2
0 60 120 180 240 300 360 420 480 540
t
t
t
Is
+Is
Is
Is
Is

26. Advantages of CSI
• It has inherent short-circuit fault protection because the
input is a controlled-current source, so the current does
not rise to a dangerously high level during the fault, so, it
is more reliable.
• It requires a simple commutation circuit, that is, only
capacitors.
• It does not require any feedback diode.
• It may use a converter-grade SCR, which is cheaper than
inverter-grade SCR.
• It has inherent four-quadrant operation capability due to
converter–inverter combination.

27. Disadvantages of CSI
• It suffers from stability problems at light loads.
• It suffers from operating problems at high
frequency.
• Its response is sluggish.