15
14-2. Heater Break Alarm
The heater break alarm function outputs an alarm at the time of break of one of a plurality of heaters composing the heat source. The function
effectively prevents defects arising from the adverse effect of insufficient power.
14-2-1. Outline of Working
This function compares the quotient of output current of the PAC27 divided by output voltage with the set value, and outputs an alarm if the
current value is lower. Even if an alarm has been output once, it will be released when current value has recovered or output voltage turns 0.
In case self-hold is needed, a self-hold circuit has to be made up externally.
14-2-2. How to Set
1) Preparations before setting
Set VR of the H.B. Set at the maximum position and VR of the Heater Set at the minimum position. Put the PAC27 in operation and energize
the heater. Wait for the temperature of the heater to be fully stabilized. Confirm that the HB lamp remains off at the time.
2) Setting of Heater Set - Setting of heater capacity
Turn VR of the Heater Set clockwise slowly and gradually while the PAC27 output voltage is at 50% or above, and set it at the position on which
the HB lamp lights.
In the event VR of the Heater Set is turned excessively, turn it counterclockwise to put off the HB lamp and then carry out the setting again.
3) Setting of H.B. ALM Set - Setting of HB alarm activation point
After completing the setting of the Heater Set, set VR of the H.B. ALM Set at a position corresponding to the percentage of the heater current
value at the time of heater breakage.
Example 1 - When 5 heaters are used and breakage in one of them is to be detected:
Set at a mid point between the current % at the time of breakage of one heater, 4/5 x 100 = 80% and ordinary current, 100%, at
85%.
Example 2 - When one heater is used:
Set at mid point between current at the time of break, 0, and ordinary current, 100%, i.e., at 50%.
14-2-3. Notes on Setting
1) Setting should be performed when the PAC27 output is as high as possible (50% or higher). Setting with a Lower output (less than 50%) will
increase an adverse reading of detection error, which in turn leads to false activation.
2) For the H.B. alarm, a lower setting, less than 90%, is recommended for the following three reasons:
It may occur that contrary to your intention of establishing a setting at 95%, an actual setting as high as nearly 100% is set due to a variation
of mechanical accuracy concerning the H.B. setting device.
This function is designed on the assumption that voltage and current have similar waveforms. Depending on the types of loads, however, voltage
has a waveform which is different from or out of proportion to that of current. If this is the case, the accuracy of detection will decline and
which in turn leads to false activation of the alarm.
Even in the case of constant resistance heaters (made of nickel chrome), resistance values change according to heater temperature and the
change is hard to distinguish from that change in resistance which is generated when one of the heaters burns out.
In the event the number of heaters is large (5 or more), setting a lower than calculated level (a mean value between those at normal time and
that which is generated when one of the heaters burns out) has the effect of preventing false activation of the H.B. alarm, although it may be
impossible to detect a breakage of one of a plurality of heaters. (Detection is not possible when heater current falls by 10% or less.)
3) Detection of breakage in one of a plurality of heaters is possible only for constant resistance heaters. In some cases, such breakage may not be
detected regarding variable resistance heaters. If current falls below a set value during operation in the case of a constant resistance heater, a
lower value than the percentage of the current at the point of time when the current has fallen should be set.
Example: Two heaters are employed. Heater resistance at the start is so high that the current at the start amounts to only 70% of its normal
value. This means the current at the time of its fall is 70% and so a lower value than 70%, for example, 60% is set. Since the set
value is in the middle of the starting current, 70 %, and the current at the time of breakage of one heater, 50%, detection of breakage
of one of the two heaters is possible. If three similar heaters are used, the setting of a value exactly at the mid-point of current at
the time of breakage of a heater, 67%, and the starting current, 70%, is not possible. Thus, breakage of one of the three heaters is
undetectable.
4) A light load or a transformer load may lead to false activation of the H.B. alarm. With a light load which causes a heater current to be 10% or
less of the rated value, a heater breakage may not be detected. With a transformer load, voltage waveforms may become dissimilar or out of
proportion to those of the current and the accuracy of detection lowers accordingly. To prevent false activation, the H.B. alarm should be set at
50% as a standard value if the apparatus is used with a light load (less than 30% of the rated value) or a transformer load. (Detection of
breakage of one of a plurality of heaters is not possible.)
