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For
the majority of standard industrial control
circuit applications, the following application
data can be used to properly size a control
circuit transformer.
INRUSH
VA - You will need to know the TOTAL
inrush current of all of the components
that will be energized together, to properly
size the transformer. Inrush VA is the product
of the load voltage multiplied by the current
that is required during the circuit start
up. Total inrush is calculated by adding
the inrush requirements of all connected
devices (contactors, timers, solenoids,
relays, lights, etc.) that will be energized
together.
SEALED
VA - You will also need to know the
TOTAL running current. Sealed VA is the
product of the load voltage multiplied by
the current required to operate the connected
circuit after circuit start up is complete.
Sealed VA is calculated by adding all of
the Sealed VA requirements of all of the
components that will be running at one time.
PRIMARY
VOLTAGE - is the voltage available from
the electrical distribution source and it's
operational frequency.
SECONDARY
VOLTAGE - is the voltage required by
the load.
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The
accepted industry standard formula used to calculate
INRUSH VA is as follows:
After
the INRUSH VA has been calculated, the chart below
can be used to complete the sizing process.
-Refer
to the Regulation Data Chart. If the primary voltage
is basically stable and does not vary by more
than 5% from nomimal voltage, the 90% SECONDARY
VOLTAGE column should be used. If the primary
voltage varies between 5% and 10%, then the 95%
SECONDARY VOLTAGE column should be used.
-After
determining the proper secondary voltage column,
read down until a value equal to or greater than
the CALCULATED INRUSH VA is found. In no case
should a figure less than the CALCULATED INRUSH
VA be used.
-Read
left to the TRANSFORMER VA RATING column to determine
the proper transformer for this application. As
a final check, make sure that the TRANSFORMER
VA RATING is equal to or greater than the TOTAL
SEALED VA requirements calculated earlier. If
not, then select a transformer with a VA rating
equal to or greater than the TOTAL SEALED VA.
-Now
that the proper VA rating of the transformer has
been determined, a transformer can be selected
by using the proper primary voltage, secondary
voltage, and VA rating of the given transformer.
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