there are a lot of different type of AC motors. some, like a sewing machine motor, can be controlled with a very simple speed control. some cannot be controlled with a speed control because the speed is dependant on the frequency.
you have to step out of time for a second.
if a motor spins at 1,700 rpm and delivers 1/4 HP
the formula uses time as a component. if you change the frequency, the time will change. if you cut the frequency in half, then the speed is 1,700 rp(m*2) and will deliver 1/4 hp per minute/2 or 1/8 hp per minute at half speed.
you still get 1/4, but now it takes 2 minutes to deliver it.
on 3-phase motors, you can alter the power to each coil so that you get a constant torque. total different animal.
you can get a treadmill motor that is really a DC permanent magnet motor that is run with an AC power supply.
if you can find a treadmill, look for a small block near the motor, the motor would be connected to this. and usually, you can use a POT to change speed.
a stepper with 7 amps and 70 volts will deliver about 1/3 hp. and will get HOT enough to cook eggs.
but, steppers over more power at low speeds and are not designed for high speeds.