How should one
design a low inrush current
universal control transformer
for 800VA continuous output power
as per IEC 61558 ?
Technical specification relevant only
Electrical data and diagram
|Input voltages sinusoidal
||210V, 230V, 250V, 380V,
|Nominal output voltage
|Max. no-load voltage
||253V at nominal input
|Nominal output current
Ambient and operating conditions:
|Mode of operation
- Safety transformer as per IEC 61558
- Insulation class E
A transformer with non-inherently short-circuit protection as
per IEC 61558 is equipped with a safety. Very often, we arrive at
a combined protection solution consisting of a primary-side fuse
and a thermal cutout. For this reason, short-circuit and overload
are not design criteria. The criterion for design for purposes of
IEC 61558 is only temperature q nominal
and the maximum permitted regulation (voltage increase),
in relation to nominal output voltage at 10% nominal input
temperature in test q max (° C)
temperature in nominal operating mode q nominal
Max winding temperature in nominal operating mode =115°C
Max winding temperature in test mode = 215°C
Insulation class E is prescribed.
The criterion for design
The computer program employs two design criteria:
- Regulation (voltage increase)<10%
- Temperature rise <75°K at 40°C ambient temperature
and insulation class E.
Since we do not know which of these criteria is more
important for design purposes, we enter Criterion=0 and
leave it to the computer program to decide.
Control transformers have to be made with a low internal
resistance and a low scatter impedance. For that reason, control
transformers are constructed exclusively with single-chamber
Induction and Fe-quality
Low Inrush-current transformers are operated at a low
induction (1.2T-1.5T). In the output range up to approximately
200VA, cold-rolled Fe quality 5.3W/kg is used at 1.5T, 50Hz. Over
and above that, we find cold-rolled Fe-quality 4.00W/kg at 1.5T,
50Hz and grain-oriented Fe-quality 1.11W/kg at 1.5T,50Hz.
The peak value of the inrush current in the first period
after switch-on at the zero-axis crossing point should not exceed
a factor of 15 x the peak value for nominal input current. This
can best be achieved if:
- The nominal induction is 1.2T-1.5T.
- The core is constructed with a defined gap of approx
0.05-0.10 mm, so as to reduce residual magnetism
- The primary winding is routed to the outside.
- If you are not yet acquainted with Rale design software,
please read the text "How should I design a small
transformer?". Keep a copy of this text within
convenient reach whenever performing design work.
- Fill in the design input mask as follows. If you need any
help, press function key F1. There is extensive
description for each input field.
- The Selection input field is set at 0. This
means that the program should search on-line for a
suitable core for this application, from your selected
- Save your input data file. In this specimen design
calculation, we saved the input data in input data file CAL0007E.TK1.
This input data file was supplied together with this
document. Copy it into the directory in which your Rale
demo program is installed.
- Connect up to the Rale design server.
- Load up your input data file.
- Now select the core family and the core from which a
suitable core is to be searched by the computer program.
- Click on OK.
- Start your design work. In the system for automatic
selection of the core from your prescribed core family,
the program will offer you an adequately sized core for
your application. Click on OK in order to accept the
On completion of the design work, the following design
data will be available and can be printed on the three pages:
- This is followed by checking of the design data.
- We now check the winding data and the filling factor
- The maximum temperature of the windings is 40°C+66.9°K
= 106.9°C < 115°C.
- Regulation = 3.73% < 10%.
- The inrush current factor is slightly more than 15 x :
With a reduction in induction, there would be no problem
in reducing the inrush current factor. In the test
program, however, it is possible to set a lower primary
winding wire size manually to increase the impedance of
the primary winding. However, so that the temperature of
the windings does not then exceed the prescribed limit
values, we have to increase the secondary winding wire
size at the same time.
- The primary windings' wire size, calculated by the
program are not optimized for production. In the test
program, only two wire sizes should be employed: 1.4mm to
240V and 0.85mm between 240 and 420V.
- This is followed by checking of the output voltage at the
nominal input voltage of 230V: U in = 1
- If the design data is not satisfactory, then there are
two ways by which we can implement the desired
- You can return to the input mask (function key F2),
correct the input data and redesign the transformer.
- Or you can access the test program (function key F5),
modify the designed transformer manually and redesign the
transformer by that means.
- On completion of the design work, you can print out the
design data on-line, or save it on your local PC and
print it out off-line. The output data file from this
design example, CAL0007E.TK2, is supplied together
with this document. Copy it into the directory in which
your Rale demo program is installed.
Tips & Tricks
Instantaneous output power
In accordance with IEC 61558, the instantaneous output power
of a control transformer is measured as follows:
The transformer is connected to the nominal input voltage. At the
output, there is selected an inductive load for which cosj =0.5, such that the output voltage is 95%
of the nominal output voltage. In this context, the output
voltage is measured as follows.
Instantaneous output power =
0.95*nominal output voltage* Imeasured
In our case the instantaneous output power is 2991VA.
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