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I n d u s t r i a l C o n t r o l T r a n s f o r m e r s 6
SELECTING INDUSTRIAL CONTROL TRANSFORMERS
To make the proper transformer selection, the load must be completely analyzed, which involves every electrically energized component
in the control circuit.
All electromagnetic control devices have two current requirements; the first to energize the coil; the second to maintain the contact for a
definite period of time. The initial energizing of the coil, which takes 5 to 20 milliseconds, requires many times more current than normal.
This is referred to as volt-ampere inrush, which is immediately followed by the sealed volt-amperes—the amount of current required to
hold the contact in the circuit.
Five step selection
1.Determine the voltage and frequency of supply circuit: Example: 460 Volts, 60 Hz.
2. Determine the total inrush VA of the control circuits from the manufacturer’s data or the contactor data table. Do not neglect the
current requirements of indicating lights and timing devices that do not have an inrush VA but are energized at the same time as the
other components in the circuit. Their total VA should be added to the total inrush VA.
3.Refer to the regulation data chart. If the supply circuit voltage (Step 1) is reasonably stable and fluctuates no more than ± 5%, refer
to the 90% Secondary Voltage column. If it fluctuates as much as ± 10%, refer to the 95% Secondary Voltage column. Go down the
column you have selected until you arrive at the inrush VA closest to, but not less than, the inrush VA of your control circuit.
4.Read to the far left side of the chart and you have selected the continuous nominal VA rating of the transformer needed. The
secondary voltage that will be delivered under inrush conditions will be either 85%, 90%, or 95% of the rated secondary voltage
depending on the column selected from the regulation data chart. The total sealed VA of the control circuit must not exceed the
nominal VA rating of the transformer selected from the manufacturer’s data or the contactor’s data table.
5.Refer to the specification tables on the following pages to select a transformer according to the required continuous nominal VA
and primary/secondary voltages.
Table 1 Inrush VA
Inrush VA @ 20% and 40% Power Factor
85% Secondary Voltage 90% Secondary Voltage 95% Secondary Voltage
Normal VA
Rating 20% P.F. 40% P.F. 20% P.F. 40% P.F. 20% P.F. 40% P.F.
50 362 224 289 179 217 134
75 579 354 462 283 345 211
100 839 522 664 413 489 304
150 1326 842 1003 637 679 431
250 3447 2281 2462 1629 1477 977
300 3894 2618 2812 1890 1731 1163
350 5418 3689 3870 2635 2322 1581
500 6496 4575 4691 3304 2887 2033
750 8377 5811 5913 4102 3449 2393
1000 11329 9005 7789 6191 4248 3377
1500 25519 18803 18013 13273 10508 7742
2000 28178 21600 19372 14850 10566 8100
3000 34797 28391 24562 20041 14328 11690
5000 138500 84542 100000 61058 61550 37574
① Data is most current at time of printing. Contact individual manufacturer for updates.