How should one
design a three-phaserectifier/safety transformer for 24Vdc, 80Adc
supplywith RC load,
as per IEC 38 and IEC 61558 ?
Technical specification relevant only to design
Electrical data and diagram
||3 x 230V per phase , +6%, -10%, sinusoidal
|Min. output voltage
||20.4Vdc, at -10% input voltage
|Nominal output voltage
||24Vdc, at nominal input voltage
||28.8Vdc, at +6% input voltage
|Nominal output current
||80Adc, RC load
Ambient and operating conditions:
|Mode of operation
||Non-inherently short-circuit proof
- 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. Normally, we try to provide effective protection both
for the transformer and for the rectifier. Very often, we arrive at a combined
protection solution consisting of three fuses matched to the rectifier and a
thermal cutout. For these reasons, short-circuit and overload are not design
criteria. The design criterion for purposes of IEC61558 is only temperature
|Max. winding temperature in
test q max (° C)
|Max. winding 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 above-prescribed limit values for voltages and for the ripple of a 24V
dc supply are laid down by IEC 38. A rectifier/transformer with RC load, for
which the minimum and maximum output voltages are prescribed, is designed in
accordance with the regulation criterion (Criterion = 1).
Rectifier/transformers with RC load can be designed ONLY in accordance
with the regulation criterion. For that reason, we have to select a
regulation solution such that the prescribed limit values for output voltage
and temperature of the windings do not exceed q
Regulation (voltage increase)
For purposes of designing the transformer in accordance with the
regulation criterion (voltage increase), we have to input the value for the
increase in the secondary voltages. This is calculated as follows, on the basis
of the DC voltage information:
The transformer is calculated for an input voltage of 207Vac. In this context,
the output voltage under load - in the hot state - must not fall short of
20.4Vdc. The maximum no-load voltage must not exceed the level of 28.8 at
243.8Vac input voltage.
At the input voltage of 207Vac, the no-load voltage must be below the values of
28.8*207/243.8=24.59Vdc. This corresponds to a DC voltage increase of
100*(24.5-20.4)/20.4 = <20%. The secondary voltage increase should be <10%
in accordance with the following table.
Regulation of secondary voltage
Regulation of the DC voltage of a single-phase shunt rectifier
with RC load
Regulation of the DC voltage of a triple-phase shunt rectifier
with RC load
The first design calculation was performed with 10% regulation. At that
level, the temperature of the windings was too high in operation at 243.8V. Only
at 3.5% regulation who were satisfactory results obtained.
Output voltage ripple
The program calculates the magnitude of the required capacity on the part of
the smoothing condensers for the prescribed DC ripple.
5% ripple is prescribed in accordance with IEC 38
Our calculation was performed with a single-chamber bobbin unit. With a
double-chamber bobbin unit, the transformer is somewhat larger. For this
purpose, for however, we can use smoothing condensers of three to five times
smaller capacity, and thus achieve the same ripple. In order to find the optimum
solution, we have to perform one calculation with a single-chamber bobbin unit
and one calculation with a double-chamber bobbin unit.
Induction and Fe quality
The choice of induction is selected at a voltage below the input voltage of
minus 10% between 1.2 and 1.4.
The rectifier/transformer is very often installed together with the rectifier in
a case or in a cabinet. For that reason, the loss output of the transformer
should be sized low and the grain-related Fe quality should be used: M6X, ORSI
Connection of transformer windings
The primary winding is connected in a star circuit. The secondary is used in
"Delta", so that the current through the winding is less and the application of
parallel-connected flat wires can be avoided.
Procedure for design
- 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 output voltage of 21Vdc at 80Adc is ensured at the
under-voltage for the 207V input voltage. Temperature is designed
simultaneous with the over-value of 243.8V for input voltage of 207V.
- Regulation = 3-5% <10% was selected such that the maximum
temperature of the windings does not exceed the value of
q nominal and the output voltage is within the limits
prescribed by IEC 38.
- Rac/Rdc = 1.25 was increased such that the program uses no
parallel-connected flat wires.
- Winding space = 0.8 should be used for automatic selection of the
core (Selection=0) with thick wires.
- 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 triple-phase core family.
- Save your input data file. In this specimen design calculation,
we saved the input data in input data file CAL0006E.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 three-phase core family from which a suitable core
is to be searched for your application. Ensure that the marked core is
- Click on OK.
- Start your design work. In the system for automatic selection of the
core from your prescribed three-phase core family, the program will offer
you an adequately sized core for your application. Click on OK in order to
accept the core.
On completion of the design work, the following design data will be available
and can be printed on three pages:
- This is followed by checking of the design data.
- Firstly, we check the DC output voltages in no-load mode for input
voltage of 244.3Vac: 27.1Vdc < 28.8Vdc
- We then check the winding data and the filling factor (82.0<100%).
- The maximum temperature of the windings is 40°C+66.9°K = 106.9°C <
This is followed by checking of the output voltage for an input voltage of
U in = 1.
On the third page of the winding datasheet we should check the output
voltages for an input voltage of 207V: 21Vdc > 20.4Vdc.
- If the design data is not satisfactory, then there are two
ways by which we can implement the desired correction:
- 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, CAL0006E.TK2, is supplied
together with this document. Copy it into the directory in which your Rale
demo program is installed.
Tips & Tricks
Temperature in nominal operating mode is too high
- Reduce your regulation and increase your induction.
- Employ a better iron quality
- Increase your cooling surface area. In the case of 3UI cores, the base
angles are very effective for this purpose. You can reduce the windings'
temperature rise by approximately 10%-15%.
For the prescribed DC ripple of 5%, the program has calculated the value of
For the same output voltage ripple, a 2-chamber transformer requires only
We selected regulation = 3.5%, so that the overtemperature would not
be too high. In the case of a double-chamber transformer which has a higher
scatter inductance, a higher degree of regulation is permissible. In our case,
regulation = 5% is perfectly suitable.