Metek TWRF1 SCR Series, Instruction, Operation And Maintenance Manual

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TWRF1 SCR Power Controller Description
Rev 2 Ametek HDR Power Systems 3
CHAPTER 1 - Description
1.1 General Description
The TWRF1 is a Single-phase, phase-fired power controller, and can operate with a wide variety of
input signals and line voltages up to 575 Vac and currents ranging from 30 through 225 amperes. The
TWRF1’s output voltage is proportional to its input signal, with the output voltage regulated to +1%
with a +10% line voltage change. The TWRF1 features pulse-train firing which provides high immunity
to possible SCR firing disturbances, and phase-lock loop circuitry which ensures line frequency
synchronization. Other features are input isolation from ground, soft-start, and automatic restart
circuitry. A customer provided potentiometer controls to the Command signal. A variety of other
external control methods are available, more detailed description of the features mentioned above is
provided in Chapter 3 of this manual.
1.2 Application
Phase-Fired SCR Power Controllers provide infinitely variable firing angles for precise control of
single-phase power to resistive or inductive loads. This precise control is essential for transformer
coupled loads or loads with great resistance changes due to temperature or start-up. Phase-Fired
units may also be used on straight resistive loads as well, but the lower power factor and higher
harmonics may present a problem to other equipment mounted nearby. A zero fired model would be a
better choice.
1.3 Operation
The TWRF1 accomplishes power control by the switching action of a pair of inverse-parallel power
SCRs. The switching, or gating, of the SCRs is controlled by a digital firing control circuit synchronized
“in phase” with line frequency (either 50 or 60 Hz). A soft-start (approximately a 1 cycle ramp) feature
is standard that allows the Power Controller to fire into the primary of a transformer or to control power
to heating elements which have a large resistance change during start-up. The output may be adjusted
by the on-board multi-turn potentiometers or by a voltage or current signal from a process controller.
ZERO and SPAN controls allow the user to calibrate the SCR unit's output to the process command
signal. Terminals are available for connection of a remote manual potentiometer.
An overcurrent trip is included. This circuit protects from output current overloads by serving as a fast-
gate shutdown (similar to an electronic fuse.) Trip point is adjustable from 30% to 200% of the SCR
Power Control’s current transformer rating. When the peak current level exceeds the preset trip level,
SCRs are gated off, and the load current is shut down within 1/2 cycle. A form C relay output is
provided.
The overcurrent trip includes a Form C relay output and provisions for a remote overcurrent reset push
button. The relay output terminals for the TWRF1 are 35 and 36 for the N.C. and 36 and 37 for the
N.O. The remote reset PB connects to terminals 33 and 34. Reference Figure 3.7.
1.4 Selecting the Proper Rated Power Controller
Use the Table below to select the proper sized TWRF1. You will need the incoming line voltage and
the transformer leakage impedance. For voltages other than 480V, ratio the incoming voltage using
the 480V chart provided below. For example, a 240V system the kVA would be half that on the chart
for any given kVA. The kVA is based upon the one cycle surge ratings of the SCRs installed in each
of the power controls.