It may be a style of motor which does not lend itself to speed control. If it is
a shaded pole motor, I understand that they run synchronous to the mains
frequency (with some slight slippage). How do you know if it is this type? Well
I have always known them to be the type used in small fans where there is a
laminated iron core with a winding on it, as well as one or two pairs of very
heavy copper shorted turns going through holes in the core.