depends on the motor type but *as the motor will be AC rather than DC the control method is a little different to just adding a resistor like you can do with a DC motor. Basically it doesnt just convert the reduction in power to heat like a resistor would do so dont worry about it or any heat generated to much. It is a more complicated answer if needs be but what you have planned wont cause any problems. :)
Thought about it just a little more and iff anyone is planning on running any AC motor for extended periods at below 30-40% speed then there is a potential cooling problem for the motor but that means 10 minutes plus (at least). There is derating factors etc but to tech for here.