Current Source Inverters

Current source inverters convert a constant DC current into an AC current with variable frequency. They are robust converters used in very high power motor drives and induction heating.

Core Concept

A massive DC link inductor maintains a constant input current. This makes the inverter inherently immune to short circuit faults across the output. The current is routed through thyristors or GTOs to the load.

The output current waveform is a square wave or stepped wave. The output voltage waveform depends entirely on the load impedance. Filter capacitors are required at the AC output to assist with commutation and filter harmonics.

Auto sequential commutated inverters use diodes and capacitors to turn off the main thyristors. The load phase current has a 120 degree conduction period. This creates a highly reliable but slow switching system.

Key Formulas

Fundamental RMS current I1rms = (sqrt(6) / pi) * Idc. Output voltage is determined by load Z and current. Peak reverse voltage depends on the commutation capacitor.

Given: Idc = 100 A, three phase CSI.
Formula: I1rms = (sqrt(6) / pi) * Idc.
Steps:
1. sqrt(6) = 2.449.
2. I1rms = (2.449 / 3.1416) * 100 = 77.97 A.
Final Answer: Fundamental RMS current = 77.97 A.

Exam Tip: In a CSI, the output current is independent of the load, but the output voltage depends on the load impedance. A large DC link inductor is mandatory.

Performance Parameters

• Inherent short circuit protection capability.
• Needs a large and heavy DC link inductor.
• Output capacitors are necessary for load commutation.
• Slower dynamic response compared to voltage source inverters.
• Capable of four quadrant operation without extra circuitry.

Quick Revision

• Converts stiff DC current to AC current.
• Highly reliable for heavy industrial loads.
• Load voltage adapts to the current and impedance.
• Thyristors or GTOs are commonly used.
• Regenerative braking is naturally supported.
• Exam trap: Assuming the output voltage is a square wave instead of the current.