Speed control of ac and dc motors?

To control the sped in an ac motor you have to vary the frequency.





The formula for synchronous speed is 120 X Frequency / number of poles.





The speed can be controlled in a dc motor by varying the field strength.





THis can be achieved by having a Variable resistor in series with the field.. The more resisrance the greater the speed.Speed control of ac and dc motors?
In DC Motors, Speed can be varied either by Armature Control - by varying the armature voltage or by Field Control - by changing the field current.





In AC Motors, Speed can be varied by changing the frequency applied.