Synchronous motors are a type of alternating current (AC) electric motor that run at synchronous speed, which refers to the speed of the rotating magnetic field within the motor. Also, synchronous motors are characterized by having zero slip, meaning that they do not have a difference in speed between the two rotating members within the motor. This is one of the key differences between synchronous motors and induction motors because induction motors require slip to properly function.
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Synchronous motors can be single or poly-phase motors, but are most commonly 3 phase motors. Three-phase synchronous motors have a continuous series of three overlapping AC cycles. An often utilized type of electrical motor, synchronous motors are beneficial for a wide range of applications and industries including: industrial, used to power equipment used in distribution centers, manufacturing facilities and material handling and transport; defense, for use in military equipment such as weapon and aircraft handling elevators; mining, for use in equipment such as vertical and diagonal mine shaft lifts; transportation, for use in vehicle propulsion such as for small residential vehicles or large commercial vehicles such as trains; and marine, for use in shipboard elevators, automated cargo unloading, shipping vessel launch equipment and more.
Synchronous motors are characterized by their rotor with attached field winding, slip rings and a stator carrying an armature winding. The armature winding is able to develop a magnetic field inside the motor as it is excited by the rotation of the rotor and its field winding. There are two main types of synchronous motors: a direct current (DC) excited motor and a non-excited motor. A DC excited synchronous motors operates by means of receiving direct current through slip rings that are attached to the motor's rotor. Through receiving the direct current, the synchronous motor becomes excited, which means that it becomes magnetized and a magnetic field develops. This type of synchronous motors is typically made in sizes that exceed one horsepower (HP). Non-excited motors can be further categorized into two main types of designs: reluctance and hysteresis. A reluctance-design non-excited synchronous motor begins operation very similar to an induction motor, but shortly after changes to operate normally at synchronous speed as a synchronous motor. A hysteresis-design synchronous motor provides the equipment being powered with a precise and constant speed, whereas the speed of a reluctance synchronous motor depends of the power supply frequency. Both designs of non-excited synchronous motors are built in sub-fractional sizes, meaning sizes that are below one HP.