Inrush current

As inrush current refers to the electric current that flows immediately after the power of an electric consumer. It differs from the rated current and must be so considered in the design of electrical equipment, such as switches, fuses, relays, as it can be a multiple of the rated current flowing later. However, it is the increased inrush current with suitable ( auxiliary ) circuits ( Inrush, also called Sanfteinschalter or " soft starter " ) limit or completely avoided.

Transformers, switching power supplies, electric motors, light bulbs and filaments (eg, the heating wires of electron tubes ) typically have a high inrush current; this can be more than 10 times the operating current. The duration of the increased inrush current can be between a few milliseconds to several seconds, depending on the type of electrical load.

Electrical Fuses and circuit breaker must be sized so that they cause on the one hand with permanent overcurrents, on the other hand tolerate an excessive inrush current. The response to short current pulses is referred to as trigger features and must be tailored to the inrush current of the connected devices. For example, the melting fuse wire a sluggish has a sufficiently high heat capacity, which prevents its temperature reaches the critical value during the inrush current. As a measure for triggering the fuse is considered the value of the integral of the square of current over time. This value corresponds to the energy which is absorbed by the fuse during the Einschaltes. Miniature circuit breaker ( short trip) are also specified in terms of their magnetic tripping threshold; to avoid undesired tripping of the maximum inrush current must not exceed this value.

Causes of excessive inrush

Inductive components

An inductor such as a coil attached to a DC voltage source, the inrush current initially rises slowly as the induced counter-voltage in the applied voltage, according to Lenz's Law counteracts. For large inductors, it may take seconds to the rated current is reached. DC solenoids and relays, therefore, always have inrush currents which are less than the operating current.

When AC voltage, however, an increased current to flow shortly after power-up; in ideal coils can this be up to twice the rated current. When the coil includes an iron core, but much higher switching are possible:

• The core of the transformer can be a short time after power is magnetically saturated, in particular where it is biased ( remanence ). In the saturation of the reactive resistance of the coil is small, and it can flow a multiple of the rated current. Cause and countermeasures are explained in detail in switching on the transformer.
• Sagittarius and pull magnets, which are operated with AC voltage, cause high inrush currents, because at the moment of the suit there is a large air gap and therefore the inductance and reactance are small.

Motor

Electric motors ( both DC and AC motors) have a high inrush current, because of the acceleration of the rotating flywheel to nominal speed more power and thus more electricity than for keeping the speed is necessary. This portion of the inrush current is called the starting current. Increased current will flow until it reaches steady state speed; depending on the size of the engine, flywheel and load this time can range from fractions of a second and the number of seconds.

With a direct current motor, the starting current can be from the applied voltage and the resistance of the windings in accordance with Ohm's law be calculated:

In a running motor, however, a voltage is induced that is proportional to the speed and the applied voltage is opposite to the terminal; the power is then the ratio of the difference of the two voltages and the resistance:

The high starting current has increased torque result, which speeds up the engine. The current decreases with increasing speed, until the steady state speed is reached.

For induction motors, the inductive resistance of the winding is greatly reduced at a standstill because of the rotor ( squirrel cage ) is similar to a short-circuited secondary winding of a transformer. The inductive resistance increases until the armature reaches the driving speed, i.e., when the rotor speed has reached nearly that of the rotating field. The squirrel-cage induction motors can be designed so that the starting current is less, but then their already low starting torque, ( that can be generated by the motor starting torque ) decreases further, the losses increase.

In addition, most electric motors have for AC voltage (eg induction motors) with a conditional by the iron core current ( possible magnetic saturation, see section " Inductive Components" ). This portion of the inrush current is in contrast to the starting current of short duration (a few line cycles ). A typical induction motor has depending on the design a magnetically induced Einschaltstromüberhöhung from 4-12 times and a starting current is approximately 4-8 times its rated current.

Capacitor

Capacitors are usually not loaded yet at the moment of switching on and act - in DC - for a short time like a short circuit. In AC inrush depends on the instantaneous voltage at the time at which the switch is closed. Optimal capacitor is discharged at a zero crossing switch because the inrush current of the capacitor is then minimal.

Examples are capacitors for power factor correction, including fluorescent lamps. When charging the capacitors after the rectifier primary switched mode power supplies and frequency converter zero voltage switching is not sufficient as a rule, to avoid a high inrush current. Here, soft start devices which gradually recharge the not yet loaded charging capacitor, smoothing capacitor, the peak voltage of the power grid are.

For capacitive power- soft starters, electronic Einschaltstrombegrenzungs circuits or hot conductors are used.

Consumers with PTC behavior

Bulbs and heating coils (eg heater, cathode heating of electron tubes ) are metallic PTC, they conduct especially well when they are cold. Heating them through the current flowing through it, so you ohmic resistance so that the current only drops to the nominal value increases. The higher the working temperature and the purity of the metals of the filaments, the higher the inrush current; Therefore, the inrush current is particularly high for halogen lamps and projector lamps and can exceed 15 times the rated current.

The starting current of incandescent lamps and heating coils can hardly be reduced if they are turned on in the minimum voltage of the AC voltage, because the process of heating comprises several periods of alternating voltage ( see picture above).

Radiators and electric cookers, however, have no increased inrush because their heating elements are made of alloys, such as constantan, whose resistance depends only slightly on the temperature and the working temperatures are lower than for incandescent lamps.

Countermeasures

For electric motors and transformers special devices exist to limit the inrush current, they are called among other things, the article soft start. In single-phase AC motors and AC motors creates a phase control remedy ( soft start ) for three-phase motors, a frequency converter. For smaller transformers and capacitive loads (switching power supplies ) is often an inrush current limiter with thermistor (NTC ) upstream. Also the heating coils of electron tubes older tube sets with series heater were thermistor ( URDOX resistor) upstream to limit the inrush current - but here they prevented from burning through the heating coils. Transformers can also be switched on with a transformer relays with no inrush current. Switching power supplies can be switched on with a special design of the transformer relays without inrush current.

Powerful light bulbs that need to be switched very frequently, for example, in stage lighting or light organs are often operated preheated, therefore as strong dimmed that they only just not yet visible light (Pre Heat ). By this measure the inrush current is clearly reduced, which not only prevents current surges in the mains, but also increases the life of the bulbs. The Pre Heat reduces the high inrush current of up to 15 times the rated current to about 1.5 to 4 times and reduces the elapsed time when dimming to full light output.