Inrush current
Encyclopedia
Incandescent light bulb
The incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...
s have high inrush currents until their filaments warm up and their resistance increases. Alternating current electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
s and transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s may draw several times their normal full-load current when first energized, for a few cycles of the input waveform. Power converter
Power converter
A power converter is an electrical or electro-mechanical device for converting electrical energy. It may be converting AC to or from DC, or the voltage or frequency, or some combination of these.Amongst the many devices that are used for this purpose are;...
s also feature high inrush currents relative to their steady state currents. This is typically the charging current of the input capacitance
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
. The selection of overcurrent protection devices such as fuses
Fuse (electrical)
In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...
and circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
s is made more complicated when high inrush currents must be tolerated. The overcurrent protection must react quickly to overload or short circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
but must not interrupt the circuit when the (usually harmless) inrush current flows.
Transformers
When a transformerTransformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
is first energized a transient current up to 10 to 50 times larger than the rated transformer current can flow for several cycles. This happens when the primary winding is connected around the zero-crossing of the primary voltage. For large transformers, inrush current can last for several seconds. Toroid transformers can have up to 80 times larger inrush, because the remnant magnetism is nearly as high as the saturation magnetism at the "knee" of the hysteresis loop. This is caused because the transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
will always have some residual flux density
Flux density
-Formal Statement:The flux density is simply defined as the amount of flux passing through a unit-area. -Mathematical Statement:The flux density would essentially be the number of field lines passing through a defined unit-area...
and when the transformer is re-energized the incoming flux will add to the already existing flux which will cause the transformer to move into saturation. Then only the resistance of the primary side windings and the power line are limiting the current.
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 limiter
Inrush current limiter
An inrush current limiter is a component used to limit inrush current to avoid gradual damage to components and avoid tripping the supply's fuse or circuit breaker. Negative temperature coefficient thermistors and fixed resistors are often used for this. Less often, other components are also used...
s. 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
.
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 deal also 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
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
circuits, is to use a pre-charge
Pre-charge
Pre-charge of the powerline voltages in a high voltage DC application is a preliminary mode which current-limits the power source such that a controlled rise time of the system voltage during power up is achieved....
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.