Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

IN THE LITERATURE ON VARIous types of electronic power supplies for adjustable-speed drive motors, two words that arise frequently are converter and inverter. One can sometimes be confused for the other. The use of "converter" to describe several unrelated pieces of equipment, performing different functions, adds to the confusion.

Inverter, however, means only one thing: "A machine, device, or system that changes direct-current power to alternating-current power." That's the IEEE definition. In any type of drive system including an a-c motor, then, the inverter is the output stage of the power supply, to which the motor is directly connected. That's the origin of the a-c voltage supplying the motor at variable frequency. The d-c input to the inverter comes from some kind of rectification apparatus taking its input from the a-c line.

In some drives, the d-c power comes from a simple diode rectifier bridge that we call the rectifier. To "rectify" means to "straighten out," which describes the process of changing an oscillating sinusoidal voltage waveform into a straight line-a constant value.

Often, though, that rectification involves more complex semiconductor circuitry. To distinguish that from the diode assembly, the term converter is used instead, meaning a circuit "that changes alternating-current power to direct-current power." In other words, converter and inverter actions are just the reverse of one another.

In electrical technology, a converter may also be something entirely different. It may convert one a-c frequency to another, for example (the "frequency converter"). It may provide three-phase power from a single-phase a-c source (the "phase converter"). At one time, "converter" also described a vacuum tube combining the functions of an oscillator and mixer in certain types of radio receivers. Today, understanding the function of the device will tell you what term properly describes it.

By Richard L. Nailen, P.E., EA Engineering Editor