How do you change AC motor speed?

A change in frequency of the motor will cause a change in the speed of the AC conveyor motor. This frequency change is brought about by a device called a Variable Frequency Drive. It is used to vary the frequency of the motor from 0 to 100%. I have no idea what the second answer is talking about.

ANSWER: considering that no mention of type of motor is involved the frequency change theory is quite invalid since 50 or 60 Hz AC line runs clocks from banks to wake up calls and while it is true that the frequency is not accurate from second to second it is however very accurate from minutes to minutes.