This transformer speed controller has two speeds settable, trickle and boost.
The speed is selected via a 2 way connection so you can switch with a thermostat, (changeover type) or humidistat or a variety of other sensors with the aid of a contactor.
Thus a fan can be made to run at a slow speed until the temp/humidity raises above a pre-set level at which point it runs at full/high speed.
This is then a variac/transformer version of various electronic fan controllers.
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