Inrush current limiter

A transformer relay ( relay or TSRL short transformer ) or a Transformatorsanfteinschalter, Trafosanfteinschalter, is an electronic assembly which avoids the higher inrush current when switching on of a transformer with an iron core. It also serves as a relay for switching on and off of the transformer for this purpose and having a control input.

Basics

When using a transformer relays, Transformatorsanfteinschalters flows when switching a transformer no increased inrush current. This is achieved by performing the switching for a short time before a bias of the transformer core by means of small, unipolar voltage time and then is turned fully anti-phase at the appropriate time. The magnetization of the iron core is brought to the biasing to the normal operating point of reversal of the induction B on the hysteresis curve, and then fully enabled.

Cause of the increased inrush current of a transformer is the remanence ( residual magnetism ) and to unsuitable switch-on: Is this the beginning of a voltage half-wave and take the Spannunganstiegssrichtung Moreover, together with the direction of the residual magnetization, gets the iron core to magnetic saturation, so that during the voltage half-wave the current increases sharply. ( The voltage-time area of a half wave in this case the iron core not magnetize! ) The resulting asymmetrical magnetization is reduced only over a number of full waves of the copper resistance of the primary winding and the AC - internal resistance.

The inrush current is limited in the case of iron saturation only by the internal line resistance and the resistance of the transformer primary winding. The inrush current is low loss transformers therefore particularly large. Transformers with low standby currents and low iron loss reserve also a particularly high remanence after switching off, which in turn is partly responsible for the high inrush current. The inrush current can reach up to more than 80 times the rated current. The current surge can be controlled only with contracts and in rated current to large backups that often cause tripping of the parent or guard the transformer can not protect without transformer switching relays or inrush current limiters.

Design and function

Transformer relays consist of a semiconductor switch ( triac ) with control and a mechanical relay, which bridges the semiconductor switch according to the switch.

Conventional and well-known soft -start devices to dim just a bipolar phase gating of small to large current flow angles, starting always continue on until the load with the full mains voltage is operated. Just idling transformers with low losses have with this dimming trouble because can not symmetrical lock on the holding current through the low idle currents the actuator, Triac, thyristors, then to one side, forming a voltage time - overweight ( voltage-time = time- integrated voltage curve the magnetic flux results ) and the transformer so that in the saturation drive. However, the transformer relays, however, only works with unipolar voltage time and forces so that from the beginning veritable no-load currents, as the next chart shows.

The bias and coercive field of the transformer core is to Einschaltbeginn by pulses of only one polarity (see picture: voltage curve at the beginning ) brought in one direction. After a predetermined number of pulses or at detecting the onset of saturation of the transformer at the beginning of oppositely directed voltage half-wave is fully turned on ( sinusoidal voltage waveform in the picture above ). Both of which are coordinated by a controller which detects the zero crossings of the mains voltage.

The accompanying chart describes the TSR -up peak and also shows the magnetic flux. The measured transformer has to start a remanence of -0.95 T. The individual unipolar voltage time transport the magnetic flux step by step to the end point of the hysteresis curve. Is then fully turned on and then the inrush current not only limited, but a current surge is even completely avoided. Noteworthy is the behavior of the magnetic flux, as indicated in the graph of flux density as after reaching the maximum positive remanence of the magnetic flux of the next voltage-time area is moved to the end point of the hysteresis curve, then the magnetic flux falls in the break back to the positive maximum remanence back. Thus, the iron core will respond within the remanence - limits first integrating the voltage time surfaces by the magnetic flux is increased after each unipolar voltage-time area, from the maximum remanence hears the integration effect on, and the core now acts as a magnetic spring. The current is switched on by the full mainly the active current of the R- load, which is in phase with the mains voltage. In the first half-waves after finishing turning rises above the voltage zero crossing nor the slightly increased load reactive power, because the transformer was a little too biased.

The transformer core is characterized initially biased in a direction to be below when switching the voltage in the opposite direction fully Exceptional dynamic range, without going into saturation. The time required for proper biasing voltage time width has only once to the transformer type, the core of the air gap or residual air gap to be adjusted, for which purpose a potentiometer, trimmer, is used. The method operates entirely independently of the load.

After completion of power-up cycle, the triac is bypassed with a relay contact to prevent its loss of heat during the further operation and to increase the short circuit strength.

A switch-on of the same transformers as used in the above picture, but turned on with a switch vertex, can be seen in the picture. In the zero-crossing switched the current peak would be even greater.

If a toroidal transformer switched with a peak switch triggers a backup under certain circumstances, as can be seen in the picture. The Scheiteleinschaltverfahren is only suitable for transformers with little or Nullremanenz, as is the case only for transformers with a substantial air gap in the iron core.

Application

Typical properties of fields of transformer switching relays are:

• It is frequent switching on and off is required.
• It is called for increased reliability without accidentally triggering security (security of supply ).
• On the primary side of the transformer is to be secured to the rated current.
• Upstream, for example, building side security elements should not trip when switching the transformer.
• Save cable cross section at longer lines to the transformer while maintaining the selectivity of the hedges.
• It is an energy-saving transformer with high efficiency and thus inevitably high inrush current can be used.
• A transformer is located in a closed case without cooling air supply and may therefore produce very little heat and therefore has a high inrush current without its limitation.
• With the special design "slow dimming " may be a transformer relays several switching power supplies together, even those with power factor correction, gently turn so that the 16 A B- building - protection does not trip. Otherwise usually triggers already more than two together switched mode power supplies of 50 watts these hedge from. The attenuation of the current peaks amounts to more than a factor of 10 computers, monitors, laptops, industrial controls can behind a selective protection, which is arranged, for example, for a " UPS ", are gently turned on,

Examples are isolating transformers in endoscopy trucks or at traffic lights with LED indicators. In many vehicles with power feeds, such as those of broadcasters and the Agency for Technical Relief, transformer switching relays are located in front of the isolation transformers. Low-voltage heaters that are fed through transformers can be switched on and off with the transformer relays in front of the transformer, in special design without bypass relay, a quick beat, so that the heat energy can be exactly dosed.

Transformer switching relays are particularly suitable for ring core and C-core transformers, since they have high magnetic flux density near the onset of saturation. These core designs using textured core sheet in which let the white districts by the magnetic flux particularly easy to align. Toroidal transformers also have no technologically conditioned air gap and therefore a high remanence in the iron core and therefore advised when turned on without inrush particularly fast in the saturation of the entire core.

Transformer switching relay can also be three-phase transformers without inrush off or overclock in rapid succession.

Pros and Cons

Transformer relays have against such inrush current limiters, which consist of a thermistor, the advantage that they are immediately ready for the next shift, that is, require no cooling phase, and that they avoid the inrush entirely. Compared with other electronic soft starters, they have the advantage of a shorter start-up phase and the precise involvement also idling transformers.

Transformer switching relay can also dominate the loss of only one half-wave of the supply system by switch it off immediately and at the right time again without causing an inrush current. ( The half -wave failure test is a criterion for electromedical equipment. ) Transformer switching relays survive if properly hedge a short circuit. About transformer relays operated transformers can be secured to the rated current and are therefore better protected against overload with less lag fuses. The temperature load of fuses, such as occurs in inrush is avoided, so that their reliability and service life increases.

The disadvantage of transformer relays compared to, for example, thermistors is the higher - cost components, and hence the price. They are insufficiently used in high volume applications for private consumers. In other applications, can be reasonable and can outweigh the benefits of the higher price; so for example, isolation transformers in medical devices and traffic light systems.