Ametek ZF2 SCR Series, Instruction, Operation And Maintenance Manual

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ZF2/ZF3 SCR Power Controller Operation
Rev 3 Ametek HDR Power Systems 15
4.8 Unbalance Alarm
The circuit contains three True RMS current converters that output three dc voltages that represent the
rms ac currents. The logic circuit then selects the highest of the three load currents and computes a
percentage of that current as determined by the DIP switch setting. The available settings are 97.5,
95, 92.5, 90, 87.5, 75 and 50%. The default setting is 87.5%. The two lower currents are then
compared to this alarm setpoint. If either, or both are below this alarm value, then the green LED
associated with that line will go out and after a short delay, the red LED and relay will energize.
If the lower two currents are above the selected setpoint, then all three green LEDs will be on,
indicating a normal condition, and the red alarm LED and relay will not be energized.
Since current must be present to determine whether the three currents are equal, the alarm must be
disabled when the controller approaches very low power levels. When the highest current falls below
15 to 20% of the current transformer’s rating, the alarm is disabled, the amber “Insufficient Current”
LED lights, the red LED and relay are disabled and the status of the green LEDs is indeterminate.
Selecting an appropriate percentage of load unbalance using the DIP switches is the only setting most
users will ever need to make and might best be illustrated by the following example.
Five diagnostic indicators are provided on the Unbalance Alarm to help determine the status of the
load unbalance board. There are three green LEDs, one amber LED and one red LED. The three
green LEDs indicate the three phase currents are above the alarm setpoint, the amber LED indicates
that the line currents are insufficient to determine a load failure and the red LED indicates the alarm
condition. Each LED is labeled on the board.
The relay output is on terminals 58 (N.O.), 59 (Common) and 60 (N.C.) as shown in figure 4.7. The
relay is energized on alarm.
EXAMPLE
Assume a 3-phase, 120 amp controller is being used to control 12 heating
elements of equal size with 4 elements connected in parallel of each leg and the
controller is set at approximately 100 amps per leg. Each of the four elements on
each leg draw approximately 25 amps each so that the loss of one element will
drop the current to 75 amps or 75% of full current. The easiest way to select the
alarm setpoint is to pick the percentage that is half way between 100% and 75%.
In this case 87.5% would be appropriate.
To estimate the exact function of the circuit, assume that the exact currents on
the three legs are 100 amps, 99.5 amps, and 99 amps (using a precise, true RMS
meter). The circuit will select the highest current, 100 amps, and compute 87.5%
of that value, or 87.5% amps. If one element of the 99.5 amp leg opens, the
current on that leg will fall to about 74.6 amps. Since this will be below the 87.5
amps alarm setpoint, the circuit will signal an alarm condition. The actual current
on the low leg that will trigger an alarm can always be calculated by measuring
the three leg currents and multiplying the highest current by the percentage
selected by the DIP switch. The circuit constantly recalculates the allowable
current unbalance. If the command signal drops so that the highest current is 50
amps, the allowable low current would be 87.5% of 50 amps, or 43.75 amps. If
one of the four heating elements fails, the current on that leg will fall to about 37.5
amps (75% of 50 amps) and the alarm will be activated.