Automation Canada - Machine Safety - 19

inrush current, the lower the additional effort in terms of system design and maintenance.
But first, let's define what exactly AC inrush current is and why it occurs.
WHAT IS AC INRUSH CURRENT FOR A POWER SUPPLY?
In short, the inrush current is the power surge at the input of a power supply. It occurs during a short time period, when the input capacitors charge
up aſter the input voltage is applied. The inrush current is higher than the nominal or operational current of the power supply.
Therefore, it needs to be limited to ensure a stable operation and to avoid tripping fuses or circuit breakers. There are various technological
solutions to achieve a low inrush current.
HOW CAN AC INRUSH CURRENT BE LIMITED?
In the following, we will explain the four most common methods, that are used to limit the inrush current in industrial power supplies.
Graphic: This rating shows 4 methods to limit the inrush current in a power supply.
1. INRUSH CURRENT LIMITATION USING NTC THERMISTORS
NTC is short for " Negative Temperature Coefficient " . NTC thermistors are the easiest and the least expensive way of limiting the inrush current.
This solution is based on thermally sensitive semiconductor resistors which show a decrease in resistance as temperature increases.
When power is first applied, the NTC thermistor is cold, so it has a high impedance and effectively limits the charging current to the input
capacitors. The NTC thermistor heats up very rapidly due to its own losses, so the impedance drops to a low level, allowing all the required current
to flow.
This solution also has some disadvantages. The effect of temperature sensitive resistors on the inrush current strongly depends on the ambient
temperature.
If it is too cold (e.g. below zero °C), start-up problems can occur. If it is too hot, the inrush current limitation of the NTC thermistors may not work
correctly.
CANADIAN AUTOMATION
VOLUME 4, ISSUE 2
19

Automation Canada - Machine Safety

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