Phase-control facts

Power systems using thrusters often exert control of load power by control of the time of firing during the ac cycle. This implies delaying conduction of the thruster so that only a selectable portion of the ac cycle is available for the load. The non-sinusoidal voltage and current wave applied to the load has operational, measurement, analytic, and harmonic interference ramifications not encountered with straight dc or with ac sine-wave power. However, such phase-control circuits are relatively simple, efficient, and very convenient. Efficiency is high because turn on is extremely fast, and turn off is reasonably fast. Also, conductive losses tend to be low because of the volt or so dropped across these devices. Manufacturers provide a wide variety of thyristors, from signal-level types to giant devices capable of handling kilovolts and kilo-amperes simultaneously. A nice thing about using thyristors for phase-controlled power is the self commutating feature, wherein turn off automatically obtains when the current wave goes through zero. Phase-controlled power can be accomplished either on a half-wave or on a full-wave basis. A single SCR (silicon-controlled rectifier) yields a half-wave circuit, whereas triacs enable full-wave operation from a single device.

Several relationships pertaining to phase-count oiled power are often overlooked or become tr& sources of confusion. As might be suspected, a half-wave circuit can only deliver half the load power forthcoming from a full-wave circuit. However, it follows that the RMS (root mean square) current capability of a half wave controller is then 70.7 percent, not 50 percent of a full-wave system I same token, the maximum RMS voltage that a half-wave circuit can deliver to the load is 70.7 percent that of a full-wave controller; the full-wave technique can deliver just about the same RMS load voltage as the RMS line voltage.

There is often needless worry that sufficient control range will be attained by a phase-control system. Such apprehension is generally unfounded for the following reasons: in a full-wave control circuit, a conduction angle of 30 degrees corresponds to only 3 percent of full-load power; a conduction angle of 150 degrees provides 97 percent of full-load power. This means that 94 percent of full power control is avail­able from a phase adjustment of only 120 degrees. Effort to produce wider phase adjustment obviously leads to greatly diminishing returns. Numbers for a half-wave control situation are different, but likewise lead to the conclusion that phase-control throughout the 30- to 150-degree range suffices for practical power control. (2250)

Дата добавления: 2015-07-20