How should one
design a motor starting
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)|
|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|
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)||
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
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.
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.
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
Now select the three-phase core family
and (optional) 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 core.
On completion of the design work, the following design data will be available and can be printed on the three pages:
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)
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|