Assembly and Installation
Chapter 3
35
CONSTANT VOLTAGE TRANSFORMER
In applications where the AC line is especially unstable and subject to unusual
variation, a constant voltage transformer can be used to stabilize the input
voltage to the System Power Supply as well as the input voltage to the user
devices.
A constant voltage transformer compensates for voltage changes at its input to
maintain a steady voltage at its output. If a constant voltage transformer is
required, it must be connected to the System Power Supply and all input devices
connected to the Mini-PLC-2/15 Controller. Output devices should be
connected on the same AC line, but not necessarily after the constant voltage
transformer.
The constant voltage transformer must have a sufficient power rating for its
load. The transformer power rating for the System Power Supply should be at
least 225VA (volt-ampere).
Determining the proper size transformer must be based on several factors:
1.
The user must determine the System Power Supply power requirement
from the nameplate or product specifications. The power requirement for
the 1771-P1 Power Supply is 75 VA.
2.
Determine total power requirement of inputs drawing power from this
transformer. Add worst case power requirement of output devices which
will also draw power from this transformer. When output devices are
connected, the transformer size is substantially increased.
3.
Add input device power requirement and output device power requirement
(only those connected to the transformer secondary). Multiply the Power
Supply VA rating by 3. Add this figure to the input/output device
requirement.
These calculations determine the proper transformer size, allowing ample power
to be delivered to the Power Supply throughout the entire AC cycle, and
provide the necessary power for I/O devices.
NOTE: If the output devices connected to the transformer are motors follow the
manufacturer’s transformer specifications. Some manufacturers recommend a
reserve capacity of 6 to 8 times the motor VA requirement to handle starting
current surges.
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