Power electronics digital notes b. Tech III year

SCR
3 in a forward direction and through 
SCR
4 in a reverse 
direction. This will cause 
SCR
4 to become non-conductive just the same 
SCR
1. This entire 
sequence is referred to as commutation and typically in a modern inverter would occur in a 
period of time less than 50 microseconds. During this interval, chokes 1and 2 must have 
sufficient transient impedance to prevent a significant increase in current from the DC source.
Diodes 1, 2, 3 and 4 serve two functions. The first is to return any stored energy that may be 
"kicked back" from the load to the source. They also serve to prevent the choke from generating 
a high transient voltage immediately after commutation.
Figure: 5.14 Circuit diagram of parallel commutated inverter
 

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Three Phase DC-AC Converters
Three phase inverters are normally used for high power applications. The advantages of a three phase 
inverter are: 
• The frequency of the output voltage waveform depends on the switching rate of the swtiches and hence 
can be varied over a wide range. 
• The direction of rotation of the motor can be reversed by changing the outpu
t phase sequence of the 
inverter. 
• The ac output voltage can be controlled by varying the dc link voltage.
The general configuration of a three phase DC-AC inverter is shown in 
Figure 
Two types of control 
signals can be applied to the switches: 
• 180° con
duction 
• 120° conduction
Figure: 5.15 Circuit diagram of three phase bridge inverter
180-Degree Conduction with Star Connected Resistive Load
The configuration of the three phase inverter with star connected resistive load is shown in 
Figure. 
The 
following convention is followed: 

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
• A current leaving a node point