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There is no cheap method to control speed of AC motors. The speed eq. of AC motor is :
S = 120 * F / P
Where F = AC signal Frequency & P = No. of Poles in motor
Thus speed depends on F and P
But P is constant for given motor
Thus speed can only be controlled by varying the frequency.
For 3-phase motor, you require a 3-phase variable frequency PWM generator (you can use microcontroller). This PWM signal needs to be linearly amplified to get approximate AC output for motor which can either be done using MOSFETs or IGBT (more prefered).
All these are minimum requirements of AC drives which won't come cheap.