Inrush current
Inrush current, input surge current, or switch-on surge is the maximal instantaneous input
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 periods to fully charge; during the charging period, instantaneous current can exceed steady-state current by a substantial multiple. Instantaneous current declines to steady-state 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, so this refers to a varying but unidirectional voltage; e.g., the 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 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
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
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
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) thermistors 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.[2]
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
Switch-off spike
When a
See also
References
- ISBN 1119065089, pages 8–73.
- ^ NTC thermistors Archived 2008-07-10 at the Wayback Machine at Temperatures.com.
- ^ "Electrical Engineer". 1896.
External links
- IEC 61000–4–30, Electromagnetic Compatibility (EMC) – Testing and measurement techniques – Power quality measurement methods, Published by The International Electrotechnical Commission, 2003.