Fixed speeds motor
Alternating-current electric motors run at speeds closely determined by the number of poles in the motor and the frequency of the alternating current supply. This is unlike the steam engine, which can be made to run over a range of speeds by adjusting the timing and duration of valves admitting steam to the cylinder.
AC motors can be made with several sets of poles, which can be chosen to give one of several different speeds (say, 720 rpm/12 Hz or 1,800 rpm/30 Hz mechanical for a 60 Hz electrical motor). The number of different speeds available is limited by the expense of providing multiple sets of windings. If many different speeds or continuously variable speeds are required, other methods are required.
Direct-current motors allow for changes of speed by adjusting the shunt field current. Another way of changing speed of a direct current motor is to change the voltage applied to the armature.
An adjustable speed drive might consist of an electric motor and controller that is used to adjust the motor's operating speed. The combination of a constant-speed motor and a continuously adjustable mechanical speed-changing device might also be called an adjustable speed drive. Power electronics based variable frequency drives are rapidly making older technology redundant.