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Introduction to Silicon Controlled Rectifier (SCR) Power Controllers

Click the SCR of choice below to see Features, Specifications and How To Order:

"A" Series--
Single Phase 15 through 70 Amp

"B" Series--
Single Phase 60 through 1200 Amp
"C" Series--
Three Phase  15 through 70 Amp
(2 leg--Zero Cross)
"D" Series--
Three Phase 60 through 1200 Amp
(2 leg--Zero Cross)
"E" Series--
Three Phase 60 through 1200 Amp
(3 leg--Phase Angle Fire)

SCR Power Controllers provide a relatively economical means of power control. SCR power controllers cost less and are more efficient than saturable core reactors and variable transformers. Compared to contactors, SCR power controllers offer a much finer degree of control and do not suffer from the maintenance problems of mechanical devices.

Features and Benefits of SCR's
High reliability
Because the SCR power controller is a solid-state device, it provides virtually limitless, trouble free operation with a minimum of maintenance.
Infinite resolution
Power, current or voltage can be controlled from zero to 100% with infinite resolution.
Extremely fast response
The SCR controller can toggle load power on and off rapidly, providing the means to respond quickly to command, load, and power supply changes.
Selectable control parameters
The SCR power controller can control the average load voltage, the RMS value of the load voltage, the RMS or the average load current or load power. It can also provide useful features such as current and voltage limiting.
 The SCR
The SCR has two states, On and Off, and allows current to flow in only one direction. An SCR unit is composed of two SCRs arranged to control AC power. SCRs can remain in the off state even though the applied potential may be several thousand volts; in the on state, they can pass several thousand amperes. When a small signal is applied the SCR will turn on in 10-100 microseconds. Once turned on it will remain on until the current through it is reduced below a very low value called the holding current.
Basically, an SCR power controller consists of the following:
Semiconductor power devices (SCR's and Diodes)
A control circuit normally referred to as the firing circuit
A means to dissipate the generated heat
Protective circuits (fuses and transient suppressors)

Special Features to
Improve Performance
There are some additional features offered on some of the SCRs sold by Tempco. These include the following:
Soft Start and Missing Cycle Detection
Phase-angle control units, designed to gradually increase power, preventing surges
Current Limiting
Phase-angle control units, designed to protect all attached equipment from power surges
Allows achievement of a linear relation between the command signal and the desired output
Over-Current Trip
Prevents the SCR from turning on if the preset current limit was exceeded in the last AC cycle
Shorted SCR Detection
Energizes a relay in the event of an SCR short
Zero-cross control units reduce the variations in power demand, helping stabilize the power supply
Zero-cross control units prevent SCR induced DC to protect inline transformers from saturating

Basic Control Modes
The power delivered to a load may be regulated or proportioned by SCR power controllers using either the zero-cross voltage switching (integral cycle) or the phase-angle control mode. Each control mode has its own specific advantages and disadvantages and each application should be reviewed to determine the most compatible mode of control.

Distributive Zero-Cross Control
The term zero-cross or synchronous operation of SCR's is derived from the fact that the SCR's are turned on only when the instantaneous value of the AC sinusoidal waveform is zero. Zero-cross controllers can provide two rather distinctively different types of control: time proportioning control, and distributive control.
The Distributive Control Technique combines power pulses of short duration to obtain the exact power level proportional to the command signal or set point.

Phase-Angle Control
In phase-angle control the SCR unit is turned on at a certain phase angle of the AC power supply that provides the correct percentage of power. Power is regulated by advancing or delaying the point at which the SCR is turned on within each half cycle. Below is an example of this for 50% power output.

Phase-angle control provides a very fine resolution of power and is used to control fast responding loads such as tungsten-filament lamps or loads in which the resistance changes as a function of temperature. Phase-angle control is required if the load is transformer-coupled or inductive.
Phase-angle controllers are typically more expensive than zero-cross controllers because the phase-angle circuit requires more sophistication than a zero-cross circuit. Phase-angle control of three-phase power requires SCR's in all three legs and is appreciably more expensive than zero-cross control, which only requires SCR's in two of the three legs.