A "transformer inrush current limiter" is a device or mechanism used to mitigate the effects of inrush currents when energizing a transformer. Inrush current refers to the temporary surge of current that flows through a transformer's primary winding when it is initially connected to a power source. This surge of current is caused by the magnetization of the transformer's core and the charging of the magnetizing inductance.
Inrush currents can be problematic because they can lead to several issues, one of which is core saturation. Core saturation occurs when the magnetic flux within the transformer's core reaches its maximum limit, causing a significant increase in the core's magnetizing current and potentially leading to increased losses, overheating, and distortion of the output waveform.
A transformer inrush current limiter aims to prevent core saturation by limiting the magnitude of the inrush current while still allowing the transformer to reach steady-state operation. There are a few common methods used to achieve this:
Series Inductors: Placing a series inductor in the primary winding circuit of the transformer can limit the rate at which current rises during energization. This gradual increase in current reduces the likelihood of core saturation. The inductor's impedance restricts the initial current surge and then gradually allows the current to reach its steady-state value.
NTC Thermistors: Negative Temperature Coefficient (NTC) thermistors are temperature-sensitive resistors whose resistance decreases as their temperature increases. Placing NTC thermistors in series with the primary winding effectively creates a higher impedance during startup due to the thermistors' initial resistance. As the transformer heats up during operation, the thermistor's resistance decreases, allowing the full load current to flow.
Electronic Current Limiters: These are more advanced solutions that involve electronic circuits and control mechanisms. They use components like transistors, thyristors, or integrated circuits to actively monitor and control the current flow during startup. These devices can provide precise control over the inrush current and are often adjustable based on the transformer's characteristics.
Pre-insertion Resistors: These are resistors temporarily inserted in series with the transformer's primary winding during startup. They provide a higher impedance path for the initial current surge. Once the transformer reaches its steady-state operating conditions, the resistors are bypassed.
The primary goal of all these methods is to control the rate at which the current rises during transformer energization. By doing so, they prevent rapid flux buildup in the core, which in turn prevents core saturation and the associated problems. The specific method chosen depends on factors such as the transformer's size, design, and the level of precision required in current control.