Cut Sheet

Volume 10—Enclosed Control CA08100012E—November 2012 www.eaton.com V10-T17-75
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17.8
Technical Data and Specifications
Control Power Transformer Selection
Control Power Transformer Selection Procedure
The following steps will assure that the secondary voltage
delivered by your transformer will be either 85%, 90% or 95%
of the nameplate secondary voltage under maximum inrush
conditions, at rated input voltage. A typical selection example
based on these steps follows.
Step 1
Calculate the total SEALED
(steady state) VA load of
your control circuit. This is
done by adding the continuous
VA requirements of the
maximum number of
components that will be
energized at any given time,
including non-inductive as
well as inductive components.
For Sealed VA data see
Page V10-T17-76.
Step 2
Calculate the PEAK INRUSH
VA of your control circuit.
First, analyze the sequence of
operation of all components.
Then add together the inrush
VA ratings of the components
that will be energized
simultaneously. Next,
determine the peak—or
maximum simultaneous—
inrush VA load that the
transformer will “see.”
The VA requirements for
indicating lights, timers
and other non-inductive
components which do not
have an inrush VA also should
be included since they will
present a load to the
transformer at the time of
maximum inrush.
Step 3
Calculate the TRANSFORMER
SELECTION INRUSH VA.
Use the following formula:
Selection Inrush =
Note: Transformer selection
inrush VA also can be determined
by adding the inrush VA and sealed
VA arithmetically, but this usually
results in an oversized transformer.
Step 4
Determine the correct
transformer NAMEPLATE
VA RATING. Refer to the
Inrush/Regulation Data table:
If the line supply to the
transformer is fairly stable
(does not vary more than
5%), use the 90% secondary
voltage column—the 90%
column is most commonly
used. If the line supply
voltage varies up to 10%, use
the 95% voltage regulation
column. To determine the
correct VA transformer rating,
go down the column until you
arrive at the inrush VA rating
closest to, but not less than,
the Transformer Selection
Inrush VA calculated in
Step 3. The left hand column
of the table will give the
corresponding transformer
NAMEPLATE VA RATING.
(VA Sealed)
2
+ (VA Inrush)
2
Inrush/Regulation Data
Note: When evaluating supply-
line stability, remember that
supply-line voltage drop
frequently is associated with
motor-starting inrush current.
When motors and motor controls
are connected to a common
feeder, the controls will
experience a momentary voltage
dip when the motor starts. This
reduces the control transformer
voltage supplied to the motor
starting contactor and may cause
the contactor to chatter or drop out.
Selection Example
Steps 1 and 2
By following Steps 1 and 2
described in column one at
left, analysis of the control
circuits shows the following
sealed VA and inrush VA data:
Example Data
Step 3
Following Step 3 in column
two at left, the Transformer
Selection Inrush VA is
calculated at 1773 VA.
Transformer Selection VA
Steps 4 and 5
Following Steps 3 and 4 at left,
the Inrush/Regulation Data
Chart is then consulted to
find the correction nameplate
VA size of the transformer.
Under the 90% Secondary
Voltage column, we find that
a 320 VA transformer will
deliver 2224 VA, amply
covering circuit demands of
1800 Inrush VA. Checking
this selection against the
requirements of 311 VA sealed,
we confirm that a 320 VA
transformer will be sufficient.
Transformer
VA Rating
55°C
Inrush VA—40% Power Factor
At 95%
Secondary
Voltage
At 90%
Secondary
Voltage
At 85%
Secondary
Voltage
60 137 185 227
95 242 329 409
105 294 407 512
180 592 842 1071
225 929 1312 1663
275 1271 1801 2288
320 1581 2224 2816
380 2124 3048 3895
550 3196 4604 5896
850 5500 7914 10,141
1100 8382 12,067 15,477
1500 11,100 16,066 21,032
2000 21,820 24,356 41,100
3000 29,123 32,770 59,997
5000 74,595 111,000 145,000
7500 104,000 162,000 219,000
10,000 111,000 166,000 237,000
Qty. Description Sealed VA Inrush VA
3 Three-pole Size 1 contactors 60 309
2 Three-pole Size 3 contactors 99 780
4 Relays 88 620
2 Electronic timers 36 36
4 Indicating lights 28 28
Totals 311 1773
3,240,250
=
1800
=
(VA Sealed)
2
+ (VA Inrush)
2
=
(311)
2
+ (1773)
2
=