The rotating speed of ac induction motors is determined by the frequency of ac applied to the stator, not the applied voltage. However, stator voltage must also drop to prevent excessive current flow through the stator at low frequencies. The V/Hz Profile tracks the proper stator voltage for any given input frequency.
Synchronous dc motors often top the field in high-performance motor controls. This group of motors includes brushed, brushless, wound-field, and permanent-magnet varieties. They are widely used simply because it's easier to regulate the operation of dc motors than that of ac motors. This is especially true if the application demands tight control of motor torque, velocity, or position.
Within specific limits, the torque of a dc motor is a linear function of input current. So it is relatively easy to get solid performance out of a dc motor with standard proportionalintegralderivative (PID) controllers.
In reality, the type of motor that works best in any given application depends on many other system variables. Ease of motor maintenance, failure modes, operating environments, motor temperature rise and cooling, and costs are just a few of the factorsto consider.
