Control Transformers — Overview

Type T (9070T) transformers are designed with low impedance windings for excellent voltage regulation and can accommodate the high inrush current associated with contactors, starters, solenoids, and relays. Type T transformers are manufactured using the most advanced insulating materials and are the best choice if size and cost are of concern.

Type TF (9070TF) transformers include factory-installed primary and secondary fuse blocks. Type TF transformers consist of two primary fuse blocks and one secondary fuse block. The primary includes rejection-style clips to increase the AIC ratings for the fuses. Since the fuse blocks are mounted on the top of the transformer, Type TF transformers are interchangeable with Type T transformers except for their increased height.

Refer to the following for additional information, selection, and wiring diagrams.

• Control Transformers Product Range

• Square D™ Industrial Control Transformers Catalog

Selection Guide

1. Determine the inrush and sealed VA of each coil in the control circuit and the VA of all other components.

2. Total the sealed VA of all operating coils and the VA of all other loads. (This determines the minimum VA size required for the circuit.)

3. Total the inrush VA of all coils that are starting at the same time and all loads and coils that are running.

4. Locate a value in the VA column of the Regulation Chart for Type T table shown below, that is equal to or greater than the value calculated in step 2.

5. In the VA row selected in step 4, find the inrush value under the appropriate voltage regulation column. If this value is greater than the calculated value from step 3, this is the correct transformer VA rating.
   
   If the inrush value on the selected VA row is not greater than the calculated value from step 3, use the next higher transformer VA rating, that is, the rating on the next row.

If your supply voltage is stable and fluctuates less than 5%, Schneider Electric recommends you use the 90% secondary voltage column. If your supply voltage is not stable and fluctuates more than 10% we recommend you use the 95% secondary voltage column. We recommend that you never use the 85% secondary voltage column since magnetic devices lose life expectancy if they are continuously started at 85% of rated voltage.