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

Specification

• Autotransformer as per IEC 61558
• Insulation class E

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 =115°C

Insulation class E is prescribed.

Criterion for design

The autotransformer has to be designed for the temperature rise <75°K at 40°C 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.

3. Checking of the input data follows this.
• 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 core family.
• The primary Circuit is set to 4 for the star connection of the windings. The Overvoltage = 1.06 means that the prescribed temperature rise of 75°K must not be exceeded at the 6% overvoltage of the primary voltage.
• Note that the core can be provided with one gap per limb if the core assembly is set to value 2. In order to get the value of the no-load input current approx. 50% of the autotransformer nominal current the core is constructed with a defined gap of 6.25 mm (Gap = 250 x 0.025mm= 6.25mm).
• The inputs Time1 = 0.166 minutes / Load1 = 1.8 / Time2 = 10 minutes / Load2 = 0 describe the motor starting operation.
4. 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.
5. Connect up to the Rale design server.
6. Load up your input data file.

Now select the three-phase core family and (optional) the core from which a suitable core is to be searched by the computer program.

7. 
8. Click on OK.
9. 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:


10. Checking of the design data follows this.
• We now check the winding data and the filling factor (78.4<100%).
• The maximum temperature of the windings is 40°C+67°K = 107°C < 115°C
• No-Load temperature rise is 209.9°K: NO-LOAD OPERATION is not allowed!
• The windings' wire size calculated by the program is not optimized for production. In the test program, only one-wire size with the diameter 3.55mm should be employed.
11. This is followed by checking of the output voltage at the nominal input voltage of 243V: Uin = 1.06

12. Checking of the design data follows this.
• We now check the winding data and the filling factor (80.9<100%).
• The maximum temperature of the windings is 40°C+66.1°K = 106.1°C < 115°C
13. 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.
14. 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 = µ0 * Icc^2 * W * s / (4 * h * q) = 3.8e+6 << 1.18e+8 (N/m^2)

Where:

• µ0 = 12.56 e-7 H/m
• Icc = 1900A
• W = 72 turns
• s = 0.08m stack
• h = 0.18m height of the winding
• q = 9.61 e-6 Cu-cross section of the wire

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: