HOW DOES AN AC SYNCHRONOUS MOTOR WORKS?
The AC Synchronous Motor maintains its fixed rotation speed in synchronization with the frequency of the electric power source (AC). Therefore, the power source frequency is critical to the efficient operation of the AC Synchronous Motor. The fixed speed of the AC Synchronous Motor is determined by the pole spacing in the stator units. It is possible to increase and decrease rotation output by attaching a gearhead.
A unique characteristic of the AC Synchronous Motors is they are not self-starting. A synchronous motor has no starting torque. It has torque only when running at synchronous speed. Either a squirrel-cage winding is added to the rotor to cause it to start, or a DC motor is used to bring the rotor to near synchronous speed, at which time the AC is then applied. Once up to speed, the rotor snaps into step with the rotating magnetic field and will continue to rotate at synchronous speed between no-load and full-load.
Once up to speed, the synchronous motor will keep in synch with the AC power source and develop torque. The torque developed when the motor snaps into synch speed is called pull-in torque. If the load on the motor is increased to the point where the rotor is pulled out of synch (excessive torque), the motor will stop. The maximum torque a motor can develop before synchronization is lost is called pull-out torque. The pull-out torque is generally 1.5 times the continuously rated torque.
AC Synchronous Motors have the ability to stop within a few degrees of rotation. Under a stall condition, overheating is eliminated because the starting and running currents are nearly identical (i.e. in sync). The direction of rotation of an AC Synchronous Motor can be reversed with the use of a phase shifting network and single-pole, three-position switch.
Construction of synchronous motors are similar to alternators with rotating fields. Synchronous motors are often used to drive DC generators. The basic construction of low-speed synchronous motors is the same as that of stepping motors. More information about the construction of an AC Synchronous Motor - as well as motor characteristics including starting, stopping, and stalling - are further explained in the following technical document.
AC SYNCHRONOUS MOTOR APPLICATIONS:
Some typical applications for single-phase AC Synchronous Motors include:
• Valve Operations
• Metering Pumps
• Cryogenic Pumps
• Dampers
• Table Lifts
• Simple Position & Process Controls • Stirring
• Linear Actuators
• Edge Guides
• Variable Transformers
• Conveyor Systems
• Remote Control of Switches
