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How should one design a motor starting
three-phase autotransformer
for 100kVA continuous motor power as per IEC 61558?

Technical specification relevant only to design

Electrical data and diagram

Input voltages sinusoidal 3 x 400V (phase-phase)
Frequency 50/60Hz
Nominal output voltage 3 x 200V (phase-phase), normally between 50% and 70% of the input voltage
Nominal output current 145A (100kVA)
Motor starting current 2 .5 x 145 = 362.5A
Starting time 10 seconds
Pause between two starts 10 minutes

Ambient and operating conditions:

Ambient temperature 40C
Mode of operation Duty cycle, 10 seconds on, 10 minutes off
Test conditions Non-inherently short-circuit proof, protected by a thermal fuse


Design criteria

IEC 61558
An autotransformer with non-inherently short-circuit protection as per IEC 61558 is equipped with a internal protection. Very often, we arrive at a combined protection solution consisting of a primary-side fuse (short circuit protection) and a thermal cut-out (overload protection). For this reason, short-circuit and overload is not design criteria. The criterion for design for purposes of IEC 61558 is only temperature q nominal at the overvoltage of 6%.

Insulation class






Max. winding temperature in nominal operating mode q nominal (C)






Insulation class
Max winding temperature in nominal operating mode =115C
Insulation class E is prescribed.

Criterion for design
The autotransformer has to be designed for the temperature rise <75K at 40C ambient temperature, the overvoltage of 6% and insulation class E. The mechanical stress of the windings in short-circuit operation is normally in the power range up to 100kVA not criterion of the design. The criteria of design is the temperature rise: Criterion=2

Bobbin unit
An autotransformer is constructed exclusively with single-chamber bobbin units.

Induction and Fe-quality
A motor starting autotransformer is designed exclusively with cold-rolled steel M45, M50 or 530-50 at the induction approx. 1.6T.

No-load current
In order to avoid high voltage spikes between the operation A and B the no-load current of the autotransformer has to be approx. 50% of the motor nominal current. This can best be achieved if the core is constructed with a defined gap. Note that under this condition the continuous no-load operation isn't normally allowed.

Input current
The output current during the operation A is not constant. Its rms-value should be entered in accordance with the following recommendation:

Load1 = (Imax/Inom+0.33*((Imax/Inom-1)^2))^0.5 = 1.8

Design procedure

  1. 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.
  2. Fill in the design input mask as follows. If you need any help, press functions key F1. There is extensive description for each input field.

  1. Checking of the input data follows this.
  1. Save your input data file. In this specimen design calculation, we saved the input data in input data file CAL0013E.TK1. This input data file was supplied together with this document. Copy it into the directory in which your Rale demo program is installed.
  2. Connect up to the Rale design server.
  3. Load up your input data file.
  4. Now select the three-phase core family and (optional) the core from which a suitable core is to be searched by the computer program.

  5. Click on OK.

  6. 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 core.

    On completion of the design work, the following design data will be available and can be printed on the three pages:

  7. Checking of the design data follows this.
  1. This is followed by checking of the output voltage at the nominal input voltage of 243V: Uin = 1.06
    1. Checking of the design data follows this.
    1. If the design data is not satisfactory, then there are two ways by which we can implement the desired correction:
  • 13. 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; CAL0013E.TK2 is supplied together with this document. Copy it into the directory in which your Rale demo program is installed.
  • Tips&Tricks

    Mechanical stress of the windings
    The following simple calculation has to show that the mechanical stress (N/m^2) is smaller than the critical value for Cu 1.18e+8 N/m^2 and it is no criteria for design of a motor starting autotransformer in the power range up to 100kVA of the nominal motor power. Because of this there is no reason to discuss about to high current density and how big it must be.

    Stress = 0 * Icc^2 * W * s / (4 * h * q) = 3.8e+6 << 1.18e+8 (N/m^2)


    Autotransformer for ventilator motor
    This autotransformer has one input voltage and between 4 and 8 output voltage s (taps) for control of the output power of the ventilator motor. The typical input for this autotransformer is:

    Input voltage Nominal net voltage
    Output voltage The tap near to 50% of the nominal motor voltage
    Output current (if not prescribed by customer) 80% of the nominal motor current
    Operation Continuos
    Wire size Only one wire size
    Taps The taps can be recalculated by hand or in the test mode