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Inrush current, input surge current, or switch-on surge is the maximal instantaneous input
current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...
drawn by an electrical device when first turned on. Alternating-current
electric motor An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate f ...
s and
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
s may draw several times their normal full-load current when first energized, for a few cycles of the input waveform. Power converters also often have inrush currents much higher than their steady-state currents, due to the charging current of the input
capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...
. The selection of over-current-protection devices such as fuses and
circuit breaker A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the ris ...
s is made more complicated when high inrush currents must be tolerated. The over-current protection must react quickly to overload or short-circuit faults but must not interrupt the circuit when the (usually harmless) inrush current flows.


Capacitors

A discharged or partially charged capacitor appears as a short circuit to the source when the source voltage is higher than the potential of the capacitor. A fully discharged capacitor will take approximately 5 ''RC'' time cycles to fully charge; during the charging portion of the cycle, instantaneous current can exceed load current by a substantial multiple. Instantaneous current declines to load current as the capacitor reaches full charge. In the case of open circuit, the capacitor will be charged to the peak AC voltage (one cannot actually charge a capacitor with AC line power – this refers to unidirectional alternating voltage output from a rectifier). In the case of charging a capacitor from a linear DC voltage, like that from a battery, the capacitor will still appear as a short circuit; it will draw current from the source limited only by the internal resistance of the source and ESR of the capacitor. In this case, charging current will be continuous and decline exponentially to the load current. For open circuit, the capacitor will be charged to the DC voltage. Safeguarding against the filter capacitor’s charging period’s initial current inrush flow is crucial for the performance of the device. Temporarily introducing a high resistance between the input power and rectifier can increase the resistance of the powerup, leading to reducing the inrush current. Using an inrush current limiter for this purpose helps, as it can provide the initial resistance needed.


Transformers

When a
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
is first energized, a transient current up to 10 to 15 times larger than the rated transformer current can flow for several cycles. Toroidal transformers, using less copper for the same power handling, can have up to 60 times inrush to running current. Worst-case inrush happens when the primary winding is connected at an instant around the zero crossing of the primary voltage (which for a pure inductance would be the current maximum in the AC cycle) and if the polarity of the voltage half-cycle has the same polarity as the remanence in the iron core has (the
magnetic remanence Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. Colloquially, when a magnet is "magnetized", it has remanence. ...
was left high from a preceding half cycle). Unless the windings and core are sized to normally never exceed 50% of saturation (and in an efficient transformer they never are, such a construction would be overly heavy and inefficient), then during such a start-up the core will be saturated. This can also be expressed as the remnant magnetism in normal operation is nearly as high as the saturation magnetism at the "knee" of the
hysteresis Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
loop. Once the core saturates, however, the winding inductance appears greatly reduced, and only the resistance of the primary-side windings and the impedance of the power line are limiting the current. As saturation occurs for part half-cycles only, harmonic-rich waveforms can be generated and can cause problems to other equipment. For large transformers with low winding resistance and high inductance, these inrush currents can last for several seconds until the transient has died away (decay time proportional to ''X''L/''R'') and the regular AC equilibrium is established. To avoid magnetic inrush, only for transformers with an air gap in the core, the inductive load needs to be synchronously connected near a supply voltage peak, in contrast with the zero-voltage switching, which is desirable to minimize sharp-edged current transients with resistive loads such as high-power heaters. But for toroidal transformers only a premagnetising procedure before switching on allows to start those transformers without any inrush-current peak. Inrush current can be divided in three categories: :Energization inrush current result of re-energization of transformer. The residual flux in this case can be zero or depending on energization timing. :Recovery inrush current flow when transformer voltage is restored after having been reduced by system disturbance. :Sympathetic inrush current flow when multiple transformer connected in same line and one of them energized.


Motors

When an
electric motor An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate f ...
, AC or DC, is first energized, the rotor is not moving, and a current equivalent to the stalled current will flow, reducing as the motor picks up speed and develops a
back EMF Counter-electromotive force (counter EMF, CEMF, back EMF),Graf, "counterelectromotive force", Dictionary of Electronics is the electromotive force (EMF) manifesting as a voltage that opposes the change in current which induced it. CEMF is the EMF c ...
to oppose the supply. AC induction motors behave as transformers with a shorted secondary until the rotor begins to move, while brushed motors present essentially the winding resistance. The duration of the starting transient is less if the mechanical load on the motor is relieved until it has picked up speed. For high-power motors, the winding configuration may be changed ( wye at start and then delta) during start-up to reduce the current drawn.


Heaters and filament lamps

Metals have a positive temperature coefficient of resistance; they have lower resistance when cold. Any electrical load that contains a substantial component of metallic resistive heating elements, such as an electric
kiln A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes. Kilns have been used for millennia to turn objects made from clay int ...
or a bank of tungsten-filament
incandescent bulb An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxid ...
s, will draw a high current until the metallic element reaches operating temperature. For example, wall switches intended to control incandescent lamps will have a "T" rating, indicating that they can safely control circuits with the large inrush currents of incandescent lamps. The inrush may be as much as 14 times the steady-state current and may persist for a few milliseconds for smaller lamps up to several seconds for lamps of 500 watts or more. (Non-graphitized) carbon-filament lamps, rarely used now, have a negative temperature coefficient and draw more current as they warm up; an "inrush" current is not found with these types.


Protection

A resistor in series with the line can be used to limit the current charging input capacitors. However, this approach is not very efficient, especially in high-power devices, since the resistor will have a voltage drop and dissipate some power. Inrush current can also be reduced by inrush current limiters. Negative-temperature-coefficient (NTC)
thermistor A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''. Thermistors are divided based on their conduction ...
s are commonly used in switching power supplies, motor drives and audio equipment to prevent damage caused by inrush current. A thermistor is a thermally-sensitive resistor with a resistance that changes significantly and predictably as a result of temperature changes. The resistance of an NTC thermistor decreases as its temperature increases. As the inrush current limiter self-heats, the current begins to flow through it and warm it. Its resistance begins to drop, and a relatively small current flow charges the input capacitors. After the capacitors in the power supply become charged, the self-heated inrush current limiter offers little resistance in the circuit, with a low voltage drop with respect to the total voltage drop of the circuit. A disadvantage is that immediately after the device is switched off, the NTC resistor is still hot and has a low resistance. It cannot limit the inrush current unless it cools for more than 1 minute to get a higher resistance. Another disadvantage is that the NTC thermistor is not short-circuit-proof. Another way to avoid the transformer inrush current is a "transformer switching relay". This does not need time for cool down. It can also deal with power-line half-wave voltage dips and is short-circuit-proof. This technique is important for IEC 61000-4-11 tests. Another option, particularly for
high-voltage High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, ''high voltage'' refers to voltage above a certain threshold. Equipment and Electrical conductor, conductors that carry high ...
circuits, is to use a pre-charge circuit. The circuit would support a current-limited precharge mode during the charging of capacitors and then switch to an unlimited mode for normal operation when the voltage on the load is 90% of full charge.


Switch-off spike

When a
transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
,
electric motor An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate f ...
,
electromagnet An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in ...
, or other inductive load is switched off, the inductor increases the voltage across the switch or breaker and cause extended arcing. When a transformer is switched off on its primary side, inductive kick produces a
voltage spike In electrical engineering, spikes are fast, short duration electrical transients in voltage (voltage spikes), current (current spikes), or transferred energy (energy spikes) in an electrical circuit. Fast, short duration electrical transients ( ...
on the secondary that can damage insulation and connected loads.


See also

*
Ripple (electrical) Ripple (specifically ripple voltage) in electronics is the residual periodic variation of the DC voltage within a power supply which has been derived from an alternating current (AC) source. This ripple is due to incomplete suppression of the al ...


References

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External link

*IEC 61000–4–30, Electromagnetic Compatibility (EMC) – Testing and measurement techniques – Power quality measurement methods, Published by The International Electrotechnical Commission, 2003. Electrical parameters