Inverter topologies: Voltage-source or current-source
In very basic terms, a variable-frequency drive (VFD) consists of three sections, moving from the drive’s input to output. A rectifier (or converter) changes ac input to dc, followed by a dc link that serves as an energy storage circuit, and then an inverter switches dc back to variable frequency ac output. Among different ways to categorize VFDs, configuration of the inverter section is an important one—namely, current-source inverter (CSI) and voltage-source inverter (VSI).
One distinguishing characteristic is the energy storage section between converter and inverter. VSI drives use capacitive energy storage, while CSI drives use inductive energy storage in their respective dc links for voltage and current. Another topology of current-source drives is the load-commutated inverter (LCI), which also employs a dc link inductor, but relies on commutation by the connected motor (or load) via switching direct current to the motor windings. This contrasts with a standard CSI drive where a line-commuted rectifier and self-commutated inverter are typical.
VSI drives work with both induction and synchronous motors, some CSI drives also work with induction and synchronous motors, but LCI drives are limited to only synchronous motors.
According to TM GE Automation Systems (TMEIC GE), voltage-source inverter is the only choice for drives above a certain power rating, compared to older technology current-source inverter drives. “In addition, any drive load that requires high torques and high response, such as a steel mill drive, cannot use current-source because of its much slower response due to the inductive source,” says Tim Russell, senior system engineer. “CSI drives are best suited for pumps and fans.”
LCI drives are intended for very large power output, and in that sense are an exception to the overall power limit of current sourcing. LCI drives are advantageous for ratings up to 50,000 hp (37,500 kW) or even higher and for control of synchronous motors, explains Rick Hoadley—principal consulting applications engineer, MV drives—at ABB Inc. “LCI drives operate at a slightly leading power factor, which allows devices in their inverter section to be load commutated,” he says. “This eliminates induction motors, which can’t run with a leading power factor.” LCI drives are available from ABB and Siemens.
Power-switching devices constitute another difference between CSI and VSI drives. Whether a power device is current- or voltage-switched determines its applicability to the type of drive. These power semiconductors range from the venerable silicon-controlled rectifier (SCR) and gate turn-off (GTO) thyristor to newer symmetrical gate-commutated thyristor (SGCT) and injection-enhanced gate transistor (IEGT).
TM GE Automation Systems provides the following attributes and trade-offs among some of these devices:
Current- switched devices—SGCT and integrated gate-commutated thyristors (IGCT)—require many more parts in firing/gate control than voltage-switched devices, such as IEGT and insulated-gate bipolar transistors (IGBT), which are available in LV and MV versions.
Voltage-switched devices—IGBT and IEGT—have much lower switching losses than current-switched devices.
Conduction losses are nearly equal for equivalent voltage- and current-rated devices: SGCT, IGCT vs. IGBT, IEGT.
Voltage-switched devices allow higher switching rates and provide better output waveforms.
Frank J. Bartos, P.E., is Control Engineering consulting editor. Reach him at email@example.com.