GE 90-30 PLC Series Installation And Hardware Manual page 390

Step 2: Calculation for PLC Power Supplies
A basic rule for Series 90 power supplies is that they are 66% efficient. Another way of stating this
is that the power supply dissipates 1 watt of power in the form of heat for every 2 watts of power it
delivers to the PLC. Therefore, you can calculate the total power requirement for all of the
modules in the rack served by a particular power supply using the method in Step 1 above, then
divide that figure by 2 to arrive at the power supply dissipation value. You cannot simply use the
rating of the power supply (such as 30 watts) for this calculation because the application may not
require the full capacity of the power supply. If you are using the +24VDC output on the power
supply's terminal strip, you should calculate the power drawn, divide the value by 2, and add it to
the total for the power supply. Since each Series 90-30 rack has its own power supply, each rack
should be calculated on an individual basis.
Step 3: Output Calculations for Discrete Output Modules
Discrete solid state Output modules require two calculations, one for the module's signal-level
circuits, which was already done in Step 1, and one for the output circuits. (This output circuit
calculation is not required for the Relay Output modules.) Since the solid state output switching
devices in these modules will drop a measurable amount of voltage, their power dissipation can be
calculated. Note that the power dissipated by the output circuits comes from a separate power
source, so it is not included in the figure used to calculate PLC power supply dissipation in Step 2.
To calculate output circuit power dissipation:
Discrete Output Module Example:
The Series 90-30 PLC I/O Module Specifications Manual, GFK-0898, lists the following for the
IC693MDL340 16-Point Discrete 120VAC Output Module:
Use that value for all of the calculations for this module.
In this example, two of the Output module's output points drive solenoids that control the advance
and retract travel of a hydraulic cylinder. The solenoid manufacturer's data sheet shows that each
GFK-0356Q Appendix F Calculating Series 90-30 Heat Dissipation
In the Series 90-30 I/O Module Specifications Manual, GFK-0898, find the value for the
Output Voltage Drop for your particular module.
Obtain the required current value for each device (such as a relay, pilot light, solenoid,
etc.) connected to an output point on the module and estimate its percent of "on-time." To
obtain the current values, check the device manufacturer's documentation or an
electronics catalog. The percent of on-time can be estimated by someone familiar with
how the equipment operates or will operate.
Multiply the Output Voltage Drop times the current value times the estimated percent of
on-time to arrive at average power dissipation for that output.
Repeat for all outputs on the module. To save time, you could determine if several
outputs were similar in current draw and on-time so that you would only have to make
their calculation once.
Repeat these calculations for all Discrete Output modules in the rack.
Output Voltage Drop: 1.5 Volts maximum
F
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