Motion Control Brushless Motors
AC (Alternating Current) Motors act as Brushless Motors with the windings located in the stator and fixed magnets in the rotor. They have the advantages of no brush maintenance, less electrical noise, greater thermal efficiency and more precise control by adding more windings to the stator.
Whereas the DC Motor uses brushes to physically connect with the rotor to change the electromagnetic polarities, an AC motor relies on the natural shape of the supply waveform to control its polarities.
AC Motors are classified by their type of rotor and the number of phases used. For the rotor type, they can be a Synchronous motor where the rotor turns at the supply frequency, or Induction motors which turn slightly slower. In terms of phases, a motor can use a single phase or up to three phases.
A common single-phase AC motor is the induction motor which generally provides high starting torque by using a special start up winding in conjunction with a starting capacitor and a centrifugal switch. The capacitor and special winding are temporarily connected to the power source when starting and once the motor reaches speed, they are disconnected by the centrifugal switch.
Three phase AC induction motors are used in higher power applications and use the phase differences between the three phases of the supply to create a smoother rotating electromagnetic field in the motor. The construction of the two motors is depicted below with colour coordinated electromagnets and corresponding waveforms.