In fact, simply connect a potentiometer and
you have infinitely variable speed control. No Triacs, no frequency inverters and no whining noises!
Triacs are part of the thyristor family and are closely related to silicon-controlled rectifiers(SCR), unlike SCRs, which are unidirectional devices(can conduct current only in one direction), Triacs are
bidirectional and so current can flow in either direction.
Indeed, Triacs can be very sensitive to high
values of dv/dt between MT1 and MT2, so a phase shift between current and voltage(as in the case of an inductive load) leads to sudden voltage step that can make the device turn on in an unwanted manner.