Topic: Inrush Current/Design1

Designing three phase, low inrush current universal autotransformer for 3x208V/120V, 50/60Hz, 9A

Input parameters

Input Voltage 3 x 380/400/480/690V, +-6%, star connection, 50/60HzHz, sine wave
  Wire Cu, round, single insulated
  Layer insulation No
  Final insulation Yes
Output Nominal output voltage 3 x 208, star connection with 3 x 120V between phases and neutral line
  Nominal output current 3 x 9A
  Wire Cu, round, single insulated
  Layer insulation No
Core Steel M45, not annealed
  Assembly Gaped with 5mil between E and I for limiting the remanence in the core.
Bobbin Type Single section
Design Insulation class B, max. operating temperature 120C,
  Ambient temperature 40C

About “low” inrush current

Note that you can not prescribe the value of the inrush current. But using the following rules you can easy limit it under the value you need:

  • If you use cold rolled or grain oriented steel set the induction between 0.9 and 1.3.
  • Use high temperature rise to increase the resistance of windings.
  • Unfortunately the remanence cannot be calculated in an alternated stacked core and vary between 20% and 70% of the operating induction. If you want to get a small and enough accurate amount of the calculated inrush current then use the gaped core assembly to reduce the remanence. Normally 3-5mil gap is big enough to reduce the remanence less than 5% of the operating induction and increase the accuracy of the inrush current calculation. The control parameter of the gap size is the amount of the no-load current. A good no-load current is smaller than 25% of the nominal primary current.
  • If you design a transformer set the primary outside. Reduce the primary wire cross section and increase the secondary wire cross section so that the temperature rise and the output voltage stay under the value you need. Due to the fact that the primary resistance is higher the inrush current will be smaller.

If you use cold rolled or grain oriented steel and you set the nominal operating induction at 0.8-0.9T then the inrush current will be very small and can be neglected. Normally, in order to save the material costs the used operating induction is 1.1-1.3T

If the relationship between the peak value of the inrush current and the peak value of the nominal current of an inverter is less than 8 then the transformer can be connected to the inverter and the inrush current is low enough.

If the relationship between the rms value of the inrush current within the first period and nominal value of a slow fuse current is less than 15 then the slow fuse can be used on the primary side of transformers. If you don’t want to use the slow fuse you can use the calculated rms inrush current (view the last design page) during the first period and calculate I2t.

With the calculated I2t you can select any fuse from catalogue.

Diagnose Input and Circuit CORE/BOBBIN/STEEL/CASE Ceneral Data