That looks like an SCR-based speed control for induction motors. They work just like a lamp dimmer, delaying the turn-on during each AC half-cycle, except that they use SCRs instead of a triac (because of the inductive load).
The turn-on of the SCRs are typically done by the rising voltage of the AC line charging a cap on each AC cycle, at a rate determined by the speed-pot (also like a lamp dimmer). There is probably nothing inside there that is low voltage.
To control it from an applied voltage, you could replace the pot/cap delay with some other form of delay circuit, but it would need to be able to trigger the SCRs and survive in a 230 volt environment.