Q. Frequency-controlled induction-motor drives ?

The converters employed for variable-frequency drives can be classified as:

• Voltage-source inverter (which is the only one considered here)

• Current-source inverters

• Cycloconverters, which allow a variable-frequency ac supply with voltage-source or current-source characteristics obtained from a fixed-frequency voltage source.

Acommon symbol for the self-commutated semiconductor switch is shown in Figure. The control signal (either voltage or current) is denoted by CS, and the diode gives the direction in which the switch can conduct current. GTOs, power transistors, and MOSFETs are classified as self-commutated semiconductor devices because they can be turned off by their respective control signals: GTOs by a gate pulse, a power transistor by a base drive, and a MOSFET by a gate-to-source voltage. A thyristor, on the other hand, is a naturally commutated device that cannot be turned off by its gate signal. A thyristor combined with a forced commutation circuit behaves like a self-commutated semiconductor device, however. The self-commutation capability makes its turnoff independent of the polarity of the source voltage, the load voltage, or the nature of load. Self-commutated semiconductor switches are suitable for applications for converters fed from a dc source, such as inverters and choppers.