- Phase fired controllers
Phase control (PFC), also called phase cutting, is a method of
pulse width modulation(PWM) for power limiting, applied to AC voltages. It works by modulating a thyristor, SCR, triac, thyratron, or other such gated diode-like devices into and out of conduction at a predetermined phase of the applied waveform.
Phase fired control is often used to control the amount of
voltage, current or power that a power supplyfeeds to its load. It does this in much the same way that a pulse width modulated (PWM) supply would pulse on and off to create an average value at its output. If the supply has a DC output, its time base is of no importance in deciding when to pulse the supply on or off, as the value that will be pulsed on and off is continuous.
PFC differs from PWM in that it addresses supplies that output a modulated waveform, such as the sinusoidal
ACwaveform that the national grid outputs. Here, it becomes important for the supply to pulse on and off at the correct position in the modulation cycle for a known value to be achieved; for example, the controller could turn on at the peak of a waveform or at its base if the cycle's time base were not taken into consideration.
Phase fired controllers take their name from that fact that they trigger a pulse of output at a certain phase of the input's modulation cycle. In essence, a PFC is a PWM controller that can synchronise itself with the modulation present at the input.
Most phase fired controllers use thyristors or other solid state switching devices as their control elements. Thyristor based controllers may utilise Gate Turn Off (GTO) thyristors, allowing the controller to not only decide when to pulse the output on but also when to turn it off, rather than having to wait for the waveform to pass within the element's Zero Cross Point.
Output reduction by bucking
A phase fired controller, like a buck topology
switched-mode power supply, is only able to deliver an output maximum equal to that which is present at its input, minus any losses occurring in the control elements themselves. Provided the modulation during each cycle is predictable or repetitive, as it is on the national grid's AC mains, to obtain an output lower than its input, a phase fired control simply switches off for a given phase angle of the input's modulation cycle. By triggering the device into conduction at a phase angle greater than 0 degrees, a point after the modulation cycle starts, a fraction of the total energy within each cycle is present at the output.
'Boosting' by derating
To achieve a 'boost' like effect, the PFC designs must be derated such that maximum present at the input is higher than the nominal output requirements. When the supply is first turned on or operating under nominal conditions, the controller will continually be delivering less than 100% of its input. When a boost is required, the controller delivers a percentage closer to 100% of the maximum input available.
Derating of mains powered, phase fired controllers is important as they are often used to control resistive loads, such as heating elements. Over time, the resistance of heating elements can increase. To account for this, a phase fired control must be able to provide some degree of extra voltage to draw the same heating current through the element. The only way of achieving this is to purposely design the supply to require less than 100% of the input's modulation cycle when the elements are first put in place, progressively opening the supply up towards delivering 100% of the input modulation cycle as the elements age.
Previously, extremely expensive and heavy multi-tapped
transformers were used as the supplies for such elements, with the corresponding winding tap being connected to the element to produce the desired temperature. This limited the temperature resolution to the number of tap combinations available. They often find their way into controllers designed for equipment such as electric ovens and furnaces. These applications are basic, consisting of what amounts to a resistor across a power supply's output.
In modern, usually high power, equipment, the transformer is replaced with phase fired controllers connecting the load directly to the mains, resulting in a substantially cheaper and lighter system. However, the method is usually limited to use in equipment that would be unrealistic without it. This is because removal of the mains transformer means that the load is in direct galvanic contact with the input. For industrial ovens and furnaces the input is often the national grid AC, which is itself galvanically referenced to the Earth. With the controller's output referenced to the Earth, a user need only be in contact with the Earth and one of the output terminals to risk receiving an electrical shock. With many high power pieces of equipment running from three phase 415 V, high current capable inputs and having the entirety of any metallic housing or framework present Earthed (grounded), this is a serious risk that must be assessed with care.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Thyristor — The thyristor is a solid state semiconductor device with four layers of alternating N and P type material. They act as bistable switches, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward … Wikipedia
Rectifier — A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers … Wikipedia
Mathematics and Physical Sciences — ▪ 2003 Introduction Mathematics Mathematics in 2002 was marked by two discoveries in number theory. The first may have practical implications; the second satisfied a 150 year old curiosity. Computer scientist Manindra Agrawal of the… … Universalium
Korean Air Lines Flight 007 — Flight 007 redirects here. For other uses, see Flight 7 (disambiguation). Korean Air Lines Flight 007 Artist s rendition of HL7442, the KAL 747 lost during Flight 007 Occurrence summary … Wikipedia
STS-62 — Infobox Space mission mission name = STS 62 insignia = Sts 62 patch.png shuttle = Columbia launch pad = 39 B launch = March 4, 1994, 8:53:01 a.m. EST landing = March 18, 1994, 8:10:42 a.m. EST, KSC Runway 33 duration = 13 days, 23 hours, 16… … Wikipedia
Mars Express — CG image of Mars Express Operator ESA Mission type Orbiter + Lander Satellite of … Wikipedia
2010 Polish Air Force Tu-154 crash — Polish Air Force Tu 154 crash Part of the fuselage near Smolensk Accident summary Date 10 April … Wikipedia
navigation — navigational, adj. /nav i gay sheuhn/, n. 1. the act or process of navigating. 2. the art or science of plotting, ascertaining, or directing the course of a ship, aircraft, or guided missile. [1520 30; < L navigation (s. of navigatio) a voyage.… … Universalium
F-16 Fighting Falcon — infobox Aircraft name= F 16 Fighting Falcon caption=A USAF F 16 over Iraq after refueling, 2008 type= Multirole fighter national origin = United States manufacturer= General Dynamics Lockheed Martin first flight= 2 February avyear|1974… … Wikipedia
Flight controller — Flight controllers are personnel who aid in the operations of a space flight, working in Mission Control Centers such as NASA s Mission Control Center, or ESA s Operations Center. Flight controllers sit at computer consoles and use telemetry to… … Wikipedia