Necessity of inrush current limiters
Most electronic devices are supplied with AC voltage. In most cases capacitors (capacitive load) are used, which are responsible for a high inrush current for a short time. When switched on, the capacitor acts like a short circuit, which normally causes upstream fuses to trip. In general, a high inrush current does not have any negative effects on the operated equipment. Nevertheless, it must be prevented that a fuse is triggered during switch-on, which switches off the device for a short time. In addition, pulse currents can have a negative effect on the service life of various devices, including components in switching power supplies, such as capacitors.
Under certain conditions, a large inrush current also occurs when a transformer is switched on. By using specially calculated transformers, the inrush current can be reduced, but not avoided. The reason why inrush current limiters must sometimes be used with transformers is due to the magnetic memory of the iron core, the remanence of the induction.
The remanence remains after switching off. The polarity of the remanence depends on the polarity of the last voltage half-wave before switching off. When the transformer is switched on, the mains voltage half-wave with which the switch-on process begins has the same polarity as the remanence, so there is a large inrush current in the winding because of the iron saturation that occurs. The iron core of the transformer continues to be magnetized in the same direction as before, but the core cannot remagnetize further than saturation. Thus, the transformer loses its inductive resistance. The magnetization in the iron core is constantly transported from negative to positive and vice versa in time with the frequency (Hz) of the AC voltage due to the size of the voltage time surface under the sinusoidal voltage half-wave, running on the hysteresis curve up to the inflection points, which also correspond to the typical no-load peaks of the transformer.
In the case of iron saturation, the current flowing into the transformer is limited only by the resistance of the copper in the primary winding and the resistance of the power supply line. The copper resistance in the primary coil is particularly small in low-loss transformers, but their inrush current is, conversely, particularly high. Transformers with small air gaps and thus low iron losses have a particularly high remanence induction in the iron core. Thus, a low-loss transformer has very high inrush currents, which must be reduced. The high inrush current can be up to 50 times the rated current. Therefore, protection only on the primary side by a fuse is often not sufficient, since this must be designed to be particularly slow, but the secondary side is not protected against overload.
Construction of our inrush current limiters
Our inrush current limiters work, depending on the power and the existing network, with NTC – technology (thermistors), relays and integrated bypass relays. The function of the inrush current limiters can be explained as follows: At the moment of switching on the transformers, the inrush current practically causes a short circuit. The inserted hot conductor takes over the inrush current limiting. Almost the entire line voltage runs through the NTC resistor. At this moment, the transformer leaves the saturation range of the iron core and when the consumer loads the secondary side, the hot conductor heats up and its resistance decreases. In our production range we have single-phase and three-phase inrush current limiters with different power ratings.