Inrush current limiter

INRUSH CURRENT LIMITER – OVERVIEW OF THE PRODUCT RANGE

Necessity of inrush current limiter

Most electronic devices are supplied with alternating voltage. Capacitors (capacitive load) are usually used here, which are responsible for a brief high inrush current. When switched on, the capacitor acts like a short circuit, which normally causes upstream fuses to be triggered. In principle, a high inrush current has no negative effects on the devices being operated. Nevertheless, it must be prevented that a fuse is triggered during switch-on, which switches off the appliance for a short time. Pulse currents can also 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. The inrush current can be reduced, but not avoided, by using specially calculated transformers. The reason why inrush current limiters sometimes have to be used in transformers is due to the magnetic memory of the iron core, the remanence of the induction.

The remanence remains present after switching off. The polarity of the remanence depends on the polarity of the last voltage half-wave before switching off. If, 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, a large inrush current is generated in the winding due to the iron saturation that then 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. The transformer thus loses its inductive resistance. The magnetization in the iron core is constantly transported from minus to plus and vice versa in time with the frequency (Hz) of the alternating voltage due to the size of the voltage time surface under the sine voltage half-wave and runs on the hysteresis curve up to the inflection points, which also correspond to the typical open-circuit peaks of the transformer.

In the case of iron saturation, the current flowing into the transformer is only limited by the resistance of the copper in the primary winding and the resistance of the mains supply line. The copper resistance in the primary coil is particularly low in low-loss transformers, but conversely their inrush current is particularly high. Transformers with small air gaps and therefore low iron losses have a particularly high remanence induction in the iron core. This means that a low-loss transformer has very high inrush currents, which must be reduced by limiting the inrush current. The high inrush current can be up to 50 times the rated current. Therefore, protection on the primary side only by a fuse is often not sufficient, as this must be designed to be particularly slow, but the secondary side is not protected against overload.

Design of the inrush current limiters from Breimer-Roth

Our inrush current limiters work with NTC technology (thermistors), relays and integrated bypass relays, depending on the power and existing mains. The function of the inrush current limiters can be explained as follows: At the moment the transformers are switched on, the inrush current practically causes a short circuit. The PTC thermistor used takes over the inrush current limitation. Almost the entire mains 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 thermistor heats up and its resistance drops. Our product range includes single-phase and three-phase inrush current limiters with different outputs.