Inrush current
When power is applied to electrical equipment, a temporary surge of high current may occur.
Large-capacity capacitors and incandescent filament lamps, for example, can experience a current much larger than the steady-state current when the power is turned on.
This phenomenon is known as inrush current.
The mechanisms that cause inrush current differ slightly between filaments and capacitors.
In the case of large-capacity capacitors
At the moment of power-up, the voltage rapidly changes from 0V→…V, which can be considered as a high-frequency event.
The reactance of a large-capacity capacitor becomes very low, allowing a large inrush current to flow瞬時 (instantaneously).
In the case of a heating element
When it is cold, its resistance is low, causing a momentary surge of large current at startup.
(Example)
・It is said that the filament of an incandescent bulb can experience a surge of current that is 8~20 times higher than normal during the first 1/100th of a second after startup.
・It is said that each time a fluorescent light is turned ON/OFF, the inrush current causes a reduction in its lifespan by approximately one hour.
Back electromotive force
In the case of inductive loads (coil components), a large voltage is applied in the opposite direction to the change in current when the power is turned on or off. This is known as back electromotive force (back EMF).
The magnitude of back EMF can vary depending on the shape and size of the electromagnetic components, but in AC equipment, it can reach up to five times the steady-state voltage, while in DC equipment, it may reach up to fifteen times.
Back EMF can propagate through the power supply line, potentially affecting other equipment.
(Example)
Even operating a small switch (relay) like the one below can cause a surge of 250V in the power supply line.
If a large inductive load is connected to the same power supply line, a significant back electromotive force is generated at the moment of driving or interrupting the inductive load.
This can lead to malfunction or damage to other equipment.