What is the meaning of synchronous speed in motors?

Also, what does it mean when a motor is asynchronous?

  • In a motor, synchronous speed is the speed at which the magnetic field rotates. Depending on motor design, the actual mechanical speed may be equivalent (synchronous motor) or slightly smaller (asynchronous motor).

    The synchronous speed is a function of:

    • The electrical frequency used, typically 60 Hz or 50 Hz.
    • The number of poles the motor has.

    The formula for calculating synchronous speed is:

    • Sync. Speed (in RPM) = 120 x Frequency (Hz) / # poles

    For example, consider a motor with 4 poles operating at 60 Hz:

    • Sync. RPM = 120 x 60 / 4 = 1,800 RPM

    If that same motor was used in a country with a 50 Hz electric supply:

    • Sync. RPM = 120 x 50 / 4 = 1,500 RPM

    Synchronous motors have a DC winding or a permanent magnet in their rotor, and rotate at exactly the synchronous speed. Electric power systems use synchronous generators to ensure that the entire grid has the same frequency.

    On the other hand, asynchronous motors generate their rotor field by induction, and rotate slightly below synchronous speed. For example, an asynchronous motor with 4 poles and connected to a 60-Hz supply might rotate at 1750 RPM instead of 1800 RPM. Asynchronous motors are also called induction motors, and there are two main types:

    • Squirrel-cage motors, which have rods of aluminum or copper embedded in the rotor (simulating a cage). Current is induced in these rods, making the rotor spin due to the interaction between the current and the magnetic field.
    • Wound-rotor motors, which have windings in the rotor to which external resistances can be connected, allowing the motor's speed to be controlled by increasing or decreasing resistance.
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    Als Antwort auf Leonardo David:

    You might also find this article interesting on The Grid. 

    Induction and Synchronous Motors: Similarities and Differences

     

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