Ametek SHZF2 SCR Series, Инструкция по эксплуатации и техническому обслуживанию

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SHZF2/SHZF3 SCR Power Controller Option Setup
Rev 2 Ametek HDR Power Systems 19
CHAPTER 5 – Options Setup
5.1 Option UB (Unbalance Alarm)
The easiest way to select an appropriate DIP switch percentage is to calculate the percentage load
drop that will result if one element opens, then pick a percentage half way between that percentage
and 100%. The following table illustrates that calculation.
Table 5.1 Unbalance DIP Switch Settings
Number of
Equal
Elements
% Current Drop if
one Leg Opens % Switch Used
1 100% 50%
2 50% 75%
3
33.3% 75 or 87.5%
4 25% 87.5%
5 20% 90%
6
16.7% 90 or 92.5%
7 14.3% 92.5%
8 12.5% 92.5%
9 11.1% 95%
10 10% 95%
Notice in the above chart that the drop in current and the needed alarm setpoint approaches 100% as
the number of elements increase. You must be aware that there are a number of factors that could
cause a current unbalance, or perceived unbalance, apart from the actual loss of one heating element.
Those factors may be actual current unbalances caused by the physical realities of the application, or
errors in measurement.
Actual current unbalances always exist. Since 3-phase power controllers turn on each leg for identical
amounts of time, the voltage output unbalance will track any line voltage unbalance. Better controllers
compensate for average line voltage fluctuations, but not voltage unbalances. If there is a 5%
unbalance in line voltages, the voltage output of the controller will have a 5% unbalance; and since
current is directly proportional to voltage (with identical load impedances), the output currents will have
a 5% unbalance. This factor alone is one of the most significant when one considers how many
heating elements to monitor with one unbalance alarm circuit. The line voltages may be fairly well
balanced at one part of the day when the alarm is originally tested, but you must take into
consideration the maximum line voltage unbalance over a period of days, weeks and months.
The actual resistances of all of the heating elements must be considered. How well matched are the
resistances when new? How much change will occur with age and when new elements are combined
with old at a later time? How much change is created if equal elements are operated with equal
voltages, but at different temperatures (due to unequal thermal load)?
The above factors are dominant, but some additional error will always be introduced by any measuring
circuit. The absolute accuracy and linearity of the current transformers and the measuring electronics
used will introduce some error.
Under fixed conditions, all types of errors can be trimmed out using the three gain potentiometers, P1,
P2 and P3. The output of the three true RMS converters can be measured using a precise DC digital
meter at test points TP1, TP2 and TP3 referenced to circuit common (terminal 64). Since the current
is represented by a DC voltage at this point, any precision DC meter can be used. When a 5 amp
current transformer is operating at full load on each input, the voltage at each test point will be about
3.01 Vdc. If there is any difference in this voltage, it can be trimmed out by increasing the gain of the
two lower signals to match the highest signal. The potentiometers allow an increase of up to 16% on