Motor Classifications
Direct Current (DC) Motors
Direct Current (DC) Motors are motors in which the rotor is activated by a steady current. The force causing the rotation in a DC motor is caused by the interaction of the rotor current with the stator's magnetic field. The speed of the motor's rotation is a function of the amount of current flowing through the stator.
Alternate Current (AC) Motors
Alternate Current (AC) Motors are motors in which a variable current flows through the stator. The current variations can be in a series of smooth sinusoidal cycles or can occur in abrupt digital transitions. The rotation of an AC motor is caused by sequencing the current cycles among the individual stator windings. Depending upon the material used to make the rotor, the rotor will be drawn toward an activated winding by either magnetic attraction or magnetic reluctance. The speed of an AC motor's rotation is dependent upon the speed at which the current changes are sequenced through the stator windings.
Synchronous Motors
Synchronous Motors are Alternating Current (AC) motors in which the rotor maintains the same frequency and phase as the rotating stator field.
Asynchronous Motors
Asynchronous Motors are Alternating Current (AC) motors designed such that the rotor movement is not synchronized with the moving stator field. An asynchronous motor's rotating stator field induces a current in the rotor windings. This induced current, in turn, generates a force, drawing the rotor toward the stator.
In an asynchronous motor, torque is only generated when the rotor is not in phase with the stator. If the rotor were to become aligned with the stator field, the torque would disappear, thus, causing the rotor to pause. The stator field basically pulls the rotor. This lag between the stator and the rotor is commonly referred to as a "slip."