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) |
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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
Feedback/Feedforward
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
Sync-Guard
Zero-cross control units reduce the variations in power demand, helping
stabilize the power supply
Trans-Guard
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.
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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.
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