Reasons for inrush current in inductive loads

AN441

4/16

Doc ID 3579 Rev 3

Figure 3.

Magnetic field H versus induction B (continuous rating)

According to Equation 2, at start up at zero voltage the current is higher (longer time to

integrate voltage and so higher induction reached in the 1st cycle) and so there is a higher

risk of reaching magnetic core saturation.

Also a second phenomenon can increase risk of saturation. This phenomenon is due to

point where H equals 0. If a positive voltage is applied from a point where there is a positive

remanent induction, the induction will start to increase from a higher initial value, so will

reach saturation faster (refer to appendix 1 for further explanation on this phenomenon).

To avoid this phenomenon in circuits controlled by an AC switch, device switch on has to be

implemented on the reverse polarity according to previous switch off. Figure 4 shows two

different test results carried out on a 200 VA 230 V to 12 V transformer. Curve A shows the

current waveform, recorded after a previous identical current waveform. The particularity of

this waveform is that the first half-cycle conduction is in the same polarity as the previous

one. In this case the transformer reaches saturation very rapidly and the transformer

behaves like a short circuit. The peak current is limited only by the series resistance of the

transformer.

Curve B shows the same recording but here with the first half-cycle conduction in reverse

polarity compared to the last one. These two curves clearly show that saturation is reached

in case A due to previous conduction. Then load current can be approximately eight times

higher than if care is taken to always trigger the device for an integer value of full-cycle

periods.

B

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B(t)

H

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