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3 SPECIFICATIONS
3.2.7 Heater disconnection detection function (supported only by Q64TCTTBW,
Q64TCRTBW)
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(1) Heater disconnection detection function
(a) This is a function to check if there is a heater wire breakage using the
standard heater current value (load current value detected by the current
sensor (CT)), when the transistor output is on.
(b) The heater disconnection detection function compares the current values of
standard heater and heater disconnection alert, and determines that there is a
heater wire breakage when the current value of standard heater is below that
of heater disconnection alert.
However, when the transistor turned-on time is less than 0.5 seconds, the
heater disconnection detection is not performed.
(2) Heater disconnection compensation function
(a) Heater disconnection compensation
When the heater voltage drops, the heater current decreases as well.
The Q64TCTTBW, Q64TCRTBW heater disconnection detection measures
the heater current and determines the heater disconnection.
Therefore, when the heater voltage drops, there are possibilities that a false
alarm may be set due to the voltage change.
Therefore, the Q64TCTTBW, Q64TCRTBW compensates for a drop in heater
current (heater breakage compensation) so that the drop in heater current
does not activate the break detection.
(b) Heater disconnection compensation method
The heater disconnection compensation calculates "the heater current for
each channel" - "standard current," and the largest positive value is set as the
compensation value.
When there is no positive values, the value with the smallest negative value is
used as the compensation value.
The heater current for each channel is compensated with the compensation
value and a heater disconnection is detected when the compensated value
exceeds the specified heater disconnection detection setting value.
Example 1: When the difference from the standard current at each channel is:
Channel 1: -2%, Channel 2: 5%, Channel 3: -1%, Channel 4: -17%, the
compensation value becomes 5%.
The heater disconnection detection is performed from the values after a 5%
compensation: Channel 1: -7%, Channel 2: 0%, Channel 3: -6%, Channel 4: -
22%.
Thus, when the heater disconnection detection setting value is at 80%, only
channel 4 is detected as disconnected.
Heater
disconnection
Channel No.
detection setting
value
1
2
80%
3
4
Difference from the
Compensation
standard current
value
-2%
5%
5%
-1%
-17%
MELSEC-Q
Difference from the
standard current
Disconnected
after compensation
-7%
No
0%
No
-6%
No
-22%
Yes
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