Common Power Controller Terms

3 wire wye
A three phase load in which one end from each load in tied together.
4 wire wye
A three phase load in which one end from each load in tied together and then the common point is used as a neutral.
The positive power terminal of an SCR or Diode.
The average value of a particular parameter such as voltage or current.
The negative power terminal of a SCR or Diode.
Current Limiting
A means to limit the maximum amount of current applied to the load.
Command Signal
An input variable applied to an SCR power controller to adjust its output.
The ability of a material to conduct electricity. Conductance is the inverse of resistance.
Current Feedback
A means to maintain the applied current to the load. The applied current is maintained as a linear function of the command signal regardless of line voltage and load changes.
CT - Current Transformer
A device used for sensing current. Its output is an electrically isolated signal proportional to the measured current.
The rate of rise of applied current to an SCR as the unit turns on.
A semiconductor which allows current to pass in one direction only.
Distributive Control
A means of controlling the on/off time of the SCR's to obtain a desired power output to the load.
Refers to the maximum rate of rise of applied voltage across an “off” SCR that will NOT cause a false turn-on. Usually stated as volts/second.
Forward Drop
The voltage drop across a semiconductor when that device is conducting current in its normal forward direction.
The signal terminal of an SCR. The terminal used to turn on an SCR.
Device used to transfer heat away from either an SCR or Diode.
Refers to a particular arrangement of SCR's and Diodes in an SCR power controller. Three SCR's and three Diodes.
Amperes squared times seconds. Refers to the subcycle current characteristics of either an SCR or a fast clearing fuse.
Induction heating
Is a method of providing fast, consistent heat for manufacturing applications which involve bonding or changing the properties of metals or other electrically-conductive materials. The process relies on induced electrical currents within the material to produce heat.
Refers to the manner in which the SCR's are connected in three-phase applications.
Refers to the manner in which the SCR's are connected in three-phase applications.
Inrush Current
That current which surges into a low impedance load or that which is drawn by a transformer during saturation.
Light Emitting Diode. Often used as status indicators.
Linear control
The output to the load is proportional to the command signal provided.
Metal Oxide Varistor. A device used to protect SCR's and Diodes from voltage transients.
Optically coupled
There is no physical connection from input to output. The gap is traversed with light which provides better isolation and noise immunity.
Over Current Trip
A detection and shutdown circuit which interrupts the operation of the SCR's if the SCR current has exceed a preset value.
Phase angle
In phase-angle control, each SCR of the back-to-back pair is turned on for a variable portion of the half-cycle that it conducts. Power is regulated by advancing or delaying the point at which the SCR is turned ON within each half cycle. Light dimmers are an example of phase-angle control.
PIV (peak inverse voltage)
The maximum reverse voltage that the SCR can block without damage.
RMS voltage and/or RMS current
RMS stands for Root Mean Squared, which describes how to calculate the RMS value of a waveform. The RMS value of a waveform gives you an idea of how much work can be done by the waveform, the DC equivalent.
Scott-T transformer
A special transformer that has a three phase primary and two single phase secondaries.
SCR (silicon controlled rectifier)
Typically used in a back to back configuration in order to control both the positive and negative half cycles of an AC line. Click here for a more thorough description of an SCR.
Shorted SCR detection
This provides a means to indicate when an SCR fails in the "ON" state.
Single phase
An AC source with only one voltage waveform.
The output voltage is ramped from zero to the desired output at a known rate. This feature prevents inrush currents when controlling variable resistance loads or inductive coupled loads.
Surge current or inrush current
The inrush current into transformers or loads that have a low resistance when cold.
Three phase
An AC source with three voltage waveforms, 120 degrees apart from each other.
Zero Cross
In zero-cross control, the load power is turned on or off only when the instantaneous value of the sinusoidal waveform is zero. Load power is controlled by switching the SCRs "on" for a number of complete electrical cycles, and then "off" for a number of complete electrical cycles.