1.
Current Fed Inverters with
Self-Commutated Devices
Powerpoint Presentation by
Asmeet Haldar
Department – Electrical
Year – 4th , 8th Sem
Roll – 27901620031
SUBJECT – Advanced Electric Drive
Ideal Institute of Engineering

2.
Introduction
• Current-fed inverters requires a “stiff” constant current source input -
thus are sometimes referred to as CSI (current source inverters or
current stiff inverters).
A large inductance can be used to change a variable voltage input to a
variable current input.
VSI-inverters and CSI-inverters are dual to each other.

3.
Inverter Operation Modes
Two inverter operation modes are established depending on the
thyristor firing angle:
1) Load-commutated inverter
Applies when /2<<.
2) Force-commutated inverter
Applies when <<3/2.

4.
Load-Commutated Inverter Mode
Consider =3/4. In this case vca < 0 => thyristor Q5 is turned off by the
load. This requires load to operate at leading power factor =>
motoring mode of a synchronous machine operating in over-
excitation. Vd = -Vd0cos
Force-Commutated Inverter Mode
Consider =5/4. In this case vca> 0 and so thyristor Q5 is not turned off
by the load. Thus some type of forced commutation is required in this
case. Lagging VAR is consumed by the load => motoring mode of an
induction motor. Vd=-Vd0cos

5.
Self-Commutated Devices
• Self-controlled symmetric blocking devices, e.g. GTO’s can be turned
on and off by gate current pulses. This allows the 6-step waveform to
be directly controlled.

6.
Self-Commutated Devices Disadvantages
A major disadvantage of this scheme is the potential for resonance
between the capacitors and the motor inductance. Care must be taken
to avoid impressing current harmonics into the motor/capacitor
network which will excite one of the system resonance frequencies.
This can be avoided by careful use of PWM. However, since the motor
parameters must be known to implement such an approach, this drive
is not popular for general-purpose applications