Inrush Currents | The Use of Static Voltage Stabilisers and Frequency Converters with Inductive Loads
Inductive loads, such as motors, transformers, or inductors, are components in electrical circuits that have the property of inductance. Inductance is the ability of a component to store energy in its magnetic field when current flows through it. This is similar to the way a capacitor stores energy in its electric field.
When you switch on an inductive load, there’s a phenomenon known as “inrush current” or “switch-on surge”. Here’s why this happens:
Initial state: Before the switch is closed, there’s no current flowing in the circuit. The inductor’s voltage is zero, and it’s not storing any energy.
Switch Closure: When the switch is closed, the voltage across the inductor changes instantaneously from zero to the supply voltage. However, due to the property of inductance, the current through the inductor can’t change instantaneously. This is due to “Lenz’s Law”, which states that the current change in an inductor will always act in a direction to oppose the cause of the change. Therefore, the inductor initially acts like a short circuit, and a large current (the inrush current, or switch-on surge) begins to flow.
Steady State: Over time, the inductor current increases as the magnetic field builds up. Once the field is fully established, the inductor behaves like a normal circuit component with a certain resistance, and the current settles down to its normal steady-state value. This transition period is called the “Transient Response”.
The switch-on surge for Static Voltage Stabilisers and Static Frequency Converters can cause various problems, such as blowing fuses, tripping circuit breakers, damaging components, or causing voltage drops that affect other parts of the system.
For this reason, when installing / selecting a Static Voltage Stabilizer or Frequency Converter for an inductive load it’s often necessary to take measures to limit the inrush current. There are several ways to do this, such as using inrush current limiters (ICLs), soft-start circuits, or negative temperature coefficient (NTC) thermistors. Alternatively ensure the Voltage Stabilizer / Converter is suitably rated to accommodate the inrush current.
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