I'm looking for an explanation of synchronous vs. asychronous motors. What is this all about? Aren't these both AC induction motors?
On October 9, 2003, DA wrote:
> Aren't these both AC induction motors?
In a word, no.
I suggest looking up "electric motors" on HowStuffWorks.com, and then under "More Information" checking out the spectacular "Motion Control Tutorial" on Motorola's site. The direct link is
The fundamental difference between the two is that if you command a synchronous motor to move 1/16th of a turn, it moves exactly 1/16th of a turn. With an AC induction motor, you only get a rough idea of what the rotor is doing. The amount of movement you get depends on lots of factors, like load. So synchronous means that what the rotor does is "synchronized" with what it is commanded to do. Asynchronous means that the rotor does approximately what it is commanded to do.
Synchronous motors are thus considerably more complicated to build and control. That's why you see a correspondingly higher price. But they can do nifty things, like precision positioning and high torque at zero speed.
Sage Automation, Inc.
Yes, they are both AC induction motors but there is a difference. The speed of synchronous motors is locked to the supply frequency.
The speed of asynchronous motors varies slightly with load. At no load, the motor will run at almost synchronous speed. The more the motor is loaded, the more the motor will "slip" behind synchronous speed. Slip may be 5% at full load.
Most induction motors are asynchronous.
Synchronous motors are usually either very small (electric clocks, stereo turntables) or very large (megawatts).
NO!! A synchronous motor IS NOT an Induction motor.
By definition, a synchronous motor operates at line frequency. This is accomplished by either permanaent magnets on the rotor, or by a winding on the rotor that is supplied (externally) by a DC current, thus creating a rotating electro-magnet.
In contrast, an induction machine operates at a speed below line frequency. The rotor has windings that are not supplied by any external source. Most large induction motors have a squirrel cage winding on the rotor. The magnetic field of the rotor is induced by the difference in frequencies of the stationary winding (stator, operating at line frequency) and the rotor (operating at the slip frequency).
ALL induction motors are asynchronous.
J. C. Upton, P.E.
Synchronous motors are like the kind used in clocks and timers. Then turn at a rate that is exactly in step with the frequency of the applied power. Asynchronous motors are the higher power motors like you find in appliances. They are designed to run at a speed slightly slower than the applied power frequency. The degree of "slip" depends on the load on the motor. The more torque that is called for, the greater the slip. With no load, these motors approach, but never quite equal, the synchronous frequency.
In a synchronous motor, stator and rotor are syncronous, whereas in an asyncronous motor not (magnetic fields in rotor and stator are rotating in different frequencies).
In a sync motor, rotor is seperately exited. In an async motor, generation of current in the rotor is maintained by *induction*. Therefore, an aysnc motor is an induction motor, a sync motor is not.
The difference is that synchronous machines' have their rotor leads brought out of the machine via slip rings, and the motor "field" is created by applying a DC current to the rotor leads. The magnetic field produced by the rotor poles locks in with the rotating field from the stator windings, and the shaft and stator field rotate in sychronism.
In an asynchronous machine, there is no electrical connection to the rotor windings. Instead, they are short circuited. As the stator field rotates, magnetic flux from it induces currents to flow in the rotor, (rather than by conduction, as in a synchronous machine,) thereby being an "induction" machine. It is asynchronous because its operating speed (as a motor)is slightly less than the stator's rotating field speed (synchronous speed). This difference in speed is referred to as slip.
The speed of a synchronous motor is fixed by the frequency of the AC system. The rotor has DC poles which lock into the frequency of the rotating magnetic field created by the stator windings. Small motors such as clock motors may use permanent magnets in the stator. Large motors have wound salient poles or more often a wound rotor. Increased torque results in a change in the physical angle between the stator field and the rotor field but not the speed. Synchronous motors have virtually no torque at zero speed and sometimes have separate start systems. Synchronous machines can be used to provide VARs.
An induction motor runs a little slower than the applied frequency. The rotor excitation is AC. It is usually achieved by tranformer action from the stator (hence induction motor?)so does not necessarily require any separate excitation system, slip rings etc so is less expensive to build. That is why it is the workhorse of industry. As torque increases the slip increases. Induction motors typically have high starting torques but have zero torque at synchronous speed. Induction motors represent reactive loads so they absorb VAR's.
Although induction mtors are generally used for industrial drives, there are cases where a large synchronous machine may be selected to provide constant speed or better efficiency or as a VAR source for power factor improvement.
Sychronous motor - Rotation angle of rotor is ALWAYS sychronised with the alignment (rotation when motor is turning) of the stator fields. The rotor is synchronized with the stator fields. This is true regardless of how the magnetic fields in the rotor are created.
Asynchronous motor. Rotation angle of the rotor SLIPS in phase with respect to the rotation of the stator fields. The rotor is NOT sychronized (i.e. is asychronous) with the stator fields.
Today, when used with the proper drive amplifier, feedback devices and control system, both sync and async motors can be used for torque, velocity and position control. Synchronous motors of permanent magnet construction generally provide the highest bandwidth response.