Overview

• NEEDS BULKY IRON-CORED DC LINK INDUCTANCE

• PROBLEMS OF COST, SIZE, WEIGHT AND LOSS

• NEEDS DEVICES WITH SYMMETRIC BLOCKING CAPABILITY

• LOAD IMPEDANCE DOES NOT AFFECT THE LOAD CURRENT WAVE

• FOR PWM OPERATION, ONE SIDE SHOULD BE CAPACITIVE AND OTHER SIDE INDUCTIVE. FOR PHASE CONTROL, BOTH SIDES CAN BE INDUCTIVE

• SLOW DYNAMIC RESPONSE

• CONVERTER SYSTEM IS INHERENTLY REGENERATIVE WITH 4-QUADRANT DRIVE CAPABILITY

• INABILITY TO OPERATE AT NO-LOAD CONDITION

• PARALLEL CONVERTER OPERTION IS DIFFICULT

• MORE RUGGED AND RELIABLE OPERATION NO SHOOT-THROUGH PROBLEM

• SIMPLER FAULT PROTECTION

Figure 5.1: General features of current-fed converters.

The PWM current-fed inverter requires a dc current source as indicated in the figure. The current source can be generated from a variable dc voltage source (via a dc-dc converter from dc voltage or phase-controlled rectifier from ac voltage) by feedback control of dc current. The salient features of current-fed converters (inverters or rectifiers) are summarized in this figure. The phase-controlled converters discussed in Chapter 3 also fall in this class. The power devices that are reverse blocking, such as thyristors, GTOs, IGCTs, or IGBTs, with a series blocking diode can be used. The other features will be evident later.

Figure 5.2: General power circuit of current-fed converter systems.

The figure shows a general power circuit for a three-phase current-fed inverter that uses reverse or symmetric blocking GTO devices. The inverter operates in square-wave mode to generate six-step (or six-pulse) current wave to the load. The load is shown as an equivalent circuit of induction or synchronous motor, which is represented...