Technical specification relevant only to design
Electrical data and diagram
|Input voltages sinusoidal||3 x 400V (phase-phase)|
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:|
|Mode of operation||Duty cycle, 10 seconds on, 10 minutes off|
|Test conditions||Non-inherently short-circuit proof, protected by a thermal fuse|
- Autotransformer as per IEC 61558
- Insulation class E
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%.
|Max. winding temperature in nominal operating mode q nominal (°C)||100||115||120||140||165|
Insulation classMax winding temperature in nominal operating mode =115°C
Insulation class E is prescribed.
Criterion for designThe 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 unitAn autotransformer is constructed exclusively with single-chamber bobbin units.
Induction and Fe-qualityA motor starting autotransformer is designed exclusively with cold-rolled steel M45, M50 or 530-50 at the induction approx. 1.6T.
No-load currentIn 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 currentThe 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
- 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 functions key F1. There is extensive description for each input field.
- 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.
Now select the three-phase core family and (optional) the core from which a suitable core is to be searched by the computer program.
On completion of the design work, the following design data will be available and can be printed on the three pages:
- 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.
- 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
- 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.
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)
- 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:
|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|
|Wire size||Only one wire size|
|Taps||The taps can be recalculated by hand or in the test mode|