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MELSEC-Q
9 LOADING AND INSTALLATION
9.2 Calculating Heat Generation by PLC
The ambient temperature inside the board storing the PLC must be suppressed to a
PLC usable ambient temperature of 55°C.
For the design of radiation from the storing board, it is necessary to know the average
power consumption (heating value) of the devices and instruments stored in the board.
Here the method of obtaining the average power consumption of the PLC system is
described.
From the power consumption, calculate a rise in ambient temperature inside the board.
How to calculate average power consumption
The power consuming parts of the PLC are roughly classified into six
blocks as shown below.
(1) Power consumption of power supply module
The power conversion efficiency of the power supply module is approx. 70 %,
i.e., 30 % of the output power is consumed by heating. As a result, 3/7 of the
output power becomes the power consumption.
Therefore the calculation formula is as follows.
Wpw = (I
5
v 5) (W)
3
7
I
5V
: Current consumption of logic 5 VDC circuit of each module
(2) Power consumption of a total of 5 VDC logic section of each
module
The power consumption of the 5 VDC output circuit section of the power supply
module is the power consumption of each module (including the current
consumption of the base unit).
W
5V
= I
5V
5 (W)
(3)
A total of 24 VDC average power consumption of the output module
(power consumption for simultaneous ON points)
The average power of the external 24 VDC power is the total power consumption
of each module.
W
24V
= I
24V
24 (W)
(4) Average power consumption due to voltage drop in the output
section of the output module
(Power consumption for simultaneous ON points)
W
OUT
= I
OUT
Vdrop Number of outputs Simultaneous ON rate (W)
I
OUT
: Output current (Current in actual use) (A)
Vdrop : Voltage drop in each output module (V)
(5) Average power consumption of the input section of the input
module(Power consumption for simultaneous ON points)
W
IN
= I
IN
E Number of input points Simultaneous ON rate (W)
I
IN
: Input current (Effective value for AC) (A)
E : Input voltage (Voltage in actual use) (V)