For your convenience you will find some suggestions on the following page to make it easier for you to choose the right control transformer.

To be able to select the correct total power factor cos φ in the nomograms, the following quantities must be known or must already have been determined by you.

- Active power P
_{total} in W (e.g.: 350 W)
- Reactive power Q
_{total} in var (e.g.: 550 var)

We assume that the values P_{total} and Q_{total} determined by you are dimensioned in such a way that safe switching is guaranteed even with several contactors.

Now draw a vertical line on the x-axis at the determined effective power. Draw another line in the horizontal direction at the reactive power.

At the point of intersection of the two lines, read off the power factor cos φ = 0.54. From this point of intersection, you extend the line in vertical direction upwards until you intersect the line of the total apparent power factor total S_{total} = 0.54.

From this intersection point, draw a line to the right until you intersect the y axis. At the intersection of the y-axis, you can read the total apparent power requirement of the control transformer (S_{total} = 652 VA).

The following data is now known:

cos φ = 0.54 - > rounded off 0.50

S_{total} = 652 VA KB

From the table "Short-term power and open-circuit voltages" in the element "STE", select from the column cos φ=0.5 the control transformer STE-350 with a short-term power of 840 VA KB and a continuous power of 350 VA DB.