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Introduction to SCR
Power Controllers
Power Delivered to a
load may be regulated using silicon controlled rectifiers in a phase or integral
cycle control configuration. With the former, a transport lag not exceeding 8.3 msec
occurs. With integral cycle control, transport lag, a function of base frequency, is
never less than 8.3 msec.
The significance of the transport
lag depends on the load and its thermal time constants. A lamp could not tolerate a
long lag controller, because it has a short thermal time constant and would have
objectionable flicker. A furnace, with its relatively long thermal time constant,
could accept either system and respond with similar performance.
Phase Control
provides true continuous regulation of power to a load. the power is supplied every
half cycle in a typical single-phase system, and three times/cycle in three-phase
operation. Because of the frequency at which power is supplied, this control is
applicable to loads with short thermal time constants where precise temperature regulation
is a prime factor.
RFI is a function of load
parameters. High inductance gives a low rate of current rise and decreases the
amount of RFI generated.
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,
inductive, or capacitive.
Phase-angle controllers are
typically more expensive than zero-cross controllers because the phase-angle circuit
requires more sophistication than a zero-cross circuit.
Features for Phase Angle Control
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Over Current Trip
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Current Limiting
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Soft-start ramp
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Phase loss, low voltage detection
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Time delay for power outage conditions in
conjunction w/soft start prevents nuisance fuse blowing
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Specifications
| Control Signals |
4-20 ma, 0-5VDC, potentiometer |
| Control Modes |
Zero Crossover, phase angle |
| Load Configuration |
Zero Crossover 2 leg, zero crossover
three leg |
| Phase Angle |
3 SCR/3 Diode, 6 SCR (all three leg
control) single
phase zero crossover or phase angle |
| Line Voltage |
120, 208, 240, 380, 415, 440/480,
575/600 single phase or three phase |
| Load Current |
10 to 1000 amps single or 3 phase |
| Ambient temperature |
32 deg. F - 120 deg. F |
| Cooling |
Convection cooled (no fans) |
The integral cycle (zero
crossover) type of control is used for either single phase or three phase resistive loads
where switching load voltage in full ON and OFF integral cycles is not objectionable.
The integral cycle zero crossover control switch ON/OFF at zero crossover of the
voltage/current waveform thus practically limiting the generation of RFI. A further
disadvantage is that voltage and current meters pulse making load monitoring difficult.
Since phase control provides continuous
voltage to the load (not ON/OFF control), it can also provide a continuous current
feedback which can be used for current limiting or current control of the load current.
Adjustable Current Limiter-phase angle
control
Restricts output
current to a pre-set adjustable value. A doughnut type of current transformer is
provided on the current limiting circuit. This is factory set for maximum rated
current of the SCR Controller but can be adjusted to limit the current to the load
circuit. This feature is often required for heating applications, D.C. Power
Supplies and motor controls.
Soft-start ramp-phase angle control
Provides a gradual
increase of load voltage/current for heating elements which have a low cold resistance or
to prevent load inrush (saturation).
SCR POWER CONTROLLERS
The TEMP, INC. Series of High
Performance Three Phase and Single Phase SCR Power Controllers are completely solid state,
utilizing silicon Controlled Rectifiers (Thyristors) and magnetic amplifier firing
circuits. Designed and built to cope with rugged industrial environments and
for precision control of power in a variety of applications including electric furnaces,
kilns, temperature chambers, lamp control, DC power supplies, high performance
replacements
for saturable core reactors, etc.
Single Phase Connections
Three Phase Connections
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Temp, Inc.
Instrumentation Specialists