Overview

Toroidal transformers represent the state of the art compared to conventional laminated E-I core transformers due to higher efficiency and significant savings in weight and volume.  The power rating of toroids has increased steadily and with it the importance of mitigating their only shortcoming: Inrush Current Surges, causing annoying dips in line voltage, fuse blowing and nuisance tripping of circuit breakers.

The initial current surge in a toroidal transformer depends on:

1.     The magnitude and direction of the residual magnetic field in the core and,
2.     The initial angle of the sinusoidal voltage at the turn-on point

Bridgeport Magnetics Group has developed and filed for patent for a novel Inrush Mitigating device, which addresses both issues.
When turned on, the surge mitigator first applies a brief pulse of DC to the primary winding in order to magnetize the core to a known direction and field strength. After this a timing circuit engages a power relay to switch the transformer on at the peak of the voltage sine wave with the effect of virtually eliminating any inrush current surge. The relay thus handles only the normal load current when closing and zero current when breaking. It resets in 250ms after turn-off.
 

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