Polyphase versus single-phase power

Polyphase power systems have a number of advantages over more simple single-phase systems. Indeed, it is not always true that simplicity is necessarily associated with the single-phase approach. The advantages are well known because of the long exploitation of the three-phase format in utility and industrial power systems. Because of shortcomings in power-handling capability, reliability, and cost, solid-state power techniques were slow to make their appearance in polyphase systems. That era is over. Bipolar power transistors, power MOSFETs, and thyristors have evolved to the point where respectable power levels can be implemented. At the same time, logic devices and techniques make waveform synthesis feasible with surprisingly few 1C modules. All in all, polyphase power systems can be said to possess the following advantages over the single-phase format:

- Three-phase induction motors are superior to single-phase ac motors in nearly all respects.

- Two-phase motors are excellent for use in servo systems. They are cheaper and more maintenance free than are dc motors. And, unlike single-phase induction motors, they are self-starting.

- For airborne and space applications, three-phase transformers provide weight reduction over single-phase units working at the same power level.

- Rectified polyphase voltage is easier to filter than the rectified voltage from a single-phase source.

- A three-phase inverter or power supply maintains power balance in a three-phase source. By contrast, single-phase inverters or power supplies contrib­ute to the imbalance of power in a three-phase power source.

- Three-phase transformer windings can be connected so that much of the third-harmonic energy in a square or quasi-square wave is attenuated. Such electrical or electronic filtering reduces the need to rely upon bulky physical filters. This is of considerable importance in the operation of motors; harmonics can cause temperature rise and rough operation. The cancellation of the third harmonic also removes higher-order odd harmonics that often cause electrical interference with other systems.

- The manner of connecting the windings of three-phase transformers also provides the designer and user with additional flexibility in the selection of output voltage and available current.

- Logic circuitry for generating the polyphase format can be readily manipulated to provide various performance features. These include dead-time intervals, stepped shapes to simulate sine waves, changing phase sequence for reversing induction motors, harmonic manipulation, frequency variation for speed control of induction and synchronous motors, and output-voltage control. Such logic circuitry also facilitates feedback applications for maintaining constant speed, torque, or horsepower. (2520)

Дата добавления: 2015-07-20