AN441

Reasons for inrush current in inductive loads

Doc ID 3579 Rev 3

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These two equations show that, when the inductive load is switched on in full-wave, the

transient current depends on the Triac first turn-on delay with respect to the mains voltage

zero point. Figure 2 shows the load current curve for triggering at zero voltage and triggering

at the peak mains voltage. This figure comes from PSPICE simulation for a 150 ohm and

5 H load switched on at a voltage of 230 V rms at 50 Hz.

Triggering at zero voltage brings the highest inrush current which can be up to twice the

peak current reached in case of triggering at peak voltage.

Figure 2.

Inrush current difference according to Triac first turn-on delay

Due to this high peak current, two problems may occur at AC switch level.

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case the component can be damaged.

●

AC switch temperature may exceed the maximum allowed junction temperature (this

will not lead necessarily to device failure but electrical parameters are not anymore

guaranteed if working temperature is above max allowed value).

Inrush currents have also to be checked to fit electromagnetic compatibility standards.

Actually, IEC 61000-3-3 standard make it mandatory to limit inrush currents of appliances

connected to the power network to reduce the flickering effect on lighting.

It should also be noted that reducing inrush current helps to increase the reliability of the

load and other switches or breakers used in series with the load.

1.2

Magnetic core saturation due to remanent induction

In transient operation, the induction can follow a different path and reach the saturation

value BS for which the magnetic field H increases very rapidly even for a low induction

variation (see Figure 3). At saturation level the magnetic material permeability decreases

drastically, down to air permeability. This leads to a lower inductance value. The load current

is then mainly limited by the load resistance, and can increase substantially. Saturation then

leads to a high increase of the coil current.

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Turn-on at zero crossing

Turn-on at peak mains voltage

Mains voltage