Ametek SHZF2 SCR Series, Инструкция по эксплуатации и техническому обслуживанию

Страница: 28 / 48

Theory SHZF2/SHZF3 SCR Power Controller
22 Ametek HDR Power Systems Rev 2
For 50% power, the time required to describe the operation of the variable-time-base would be the
period of two cycles – one cycle on and one cycle off. Hence the time-base of the
SHZF2/SHZF3would be the period of two cycles for 50% power and four cycles for 75% power. The
time-base varies for other power levels as well, so its period is always that of a whole number of
cycles. This theoretical relationship is shown in Table 6.1. It may not appear precisely as indicated
when the output is viewed with an oscilloscope, as other factors such as voltage feedback may be
occurring.
An important consideration is that, for discrete cycles of output, the “off” time is the minimum possible
for any specific power demand (see above). This reduces thermal shock to the load and thus extends
load element life. Of equal importance, variable-time-base firing provides excellent control resolution
and fast response, and the circuit design facilitates accurate voltage regulation. The lowest shock
factor the load is achieved at 50% output (one cycle on, one cycle off).
6.4 Pulse-Train Firing
The output of the SHZF2/SHZF3 to the SCRs consists of a pulse-train of approximately 17 kHz. This
means that for every cycle of controlled output, the SCRs would receive approximately 142 trigger
pulses, which will ensure constant gating during the “on” period of the SHZF2/SHZF3’s output. This
also ensures that the SCR is gated on in case of a mid-cycle turnoff.
6.5 Phase-Lock Loop
When the output of the oscillator is locked to the input signal, it is said to be “locked” in phase. The
control voltage is such that the frequency of the voltage controlled oscillator is exactly equal to the
frequency of the input signal. For each cycle of input there is one cycle of oscillator output. By this
method, the SHZF2/SHZF3’s output will be held at 50 Hz or 60 Hz, depending on configuration. The
input signal is filtered, then compared to the oscillator, improving noise immunity over conventional
sync circuits.
6.6 SPAN and ZERO Controls
The ZERO and SPAN potentiometers are screwdriver adjustable. The center position of the ZERO
control corresponds to zero bias and zero power output. The approximate zero setting can be found
by rotating the control over its range, stopping near the midpoint of rotation. The precise zero setting
can be found by observing the output level while rotating the control near the midpoint of rotation.
From the midpoint to extreme CCW (counterclockwise) is the zero or negative region. Operating the
control in this region, one can zero the output from the SHZF2/SHZF3 at any output level of a
temperature controller or other external control device. From midpoint to extreme CW is the positive
control region, whose extremes correspond to 0% and 100% power output, respectively, as manually
adjusted.
The SPAN control is used as a final adjustment to set the SHZF2/SHZF3 output to precisely 100%,
when the principal control device (external controller, external “manual” control or ZERO
potentiometer) is set to demand 100% output.
6.7 Voltage-Squared Feedback
Voltage-squared feedback simulates power feedback. It is used to help maintain constant power
delivered to the load when the line voltage changes and the load resistance remains constant.
6.8 Shut-down
The shutdown circuit provides a quick SCR shutdown signal. This is accomplished via a customer
provided contact across terminal 3 and terminal 4. This removes the input signal to the comparator
that tells the circuit when to gate the SCRs on. It also instantaneously turns off the SCR gating pulses.