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.
  • Registered Users Member
    Employee Employee

    In reply to Leonardo David:

    You might also find this article interesting on The Grid. 

    Induction and Synchronous Motors: Similarities and Differences




  • Registered Users Member

    Synchronous motor

    Synchronous motor is an AC motor that generates a torque by a DC-powered excitation magnetic field that interacts with a rotating magnetic field of the armature to rotate at a synchronous speed.

    The structure of the synchronous motor is basically the same as that of the synchronous generator, and the rotor is also divided into a salient pole and a hidden pole. But most synchronous motors are salient. The installation form is also divided into horizontal and vertical.

    Induction motor

    An induction motor, also known as an "asynchronous motor", is a device that places a rotor in a rotating magnetic field and obtains a rotational moment under the action of a rotating magnetic field, thereby rotating the rotor.

    The stator is the part of the motor that does not rotate. The main task is to generate a rotating magnetic field. The rotating magnetic field is not achieved mechanically. Instead, it is connected to a pair of electromagnets by alternating current, so that its magnetic pole properties change cyclically, so it is equivalent to a rotating magnetic field.



    Working principle

    The rotating magnetic field generated by the stator (the rotational speed is the synchronous rotational speed n1) and the relative motion of the rotor winding, the rotor winding cutting magnetic induction line generates an induced electromotive force, thereby generating an induced current in the rotor winding. The induced current in the rotor winding interacts with the magnetic field to generate electromagnetic torque that causes the rotor to rotate. Since the induced current gradually decreases as the rotor speed gradually approaches the synchronous speed, the generated electromagnetic torque also decreases accordingly. When the asynchronous motor operates in the motor state, the rotor speed is less than the synchronous speed.

    Difference between synchronous motor and induction motor

    The synchronous motor and induction motor are the most widely used types of AC motor. The difference between the two types is that the synchronous motor rotates at a rate locked to the line frequency since it does not rely on current induction to produce the rotor's magnetic field. By contrast, the induction motor requires slip: the rotor must rotate slightly slower than the AC alternations in order to induce current in the rotor winding.

    Small synchronous motors are used in timing applications such as in synchronous clocks, timers in appliances, tape recorders and precision servomechanisms in which the motor must operate at a precise speed; speed accuracy is that of the power line frequency, which is carefully controlled in large interconnected grid systems.

    Synchronous motors are available in self-excited sub-fractional horsepower sizes to high power industrial sizes.

    In the fractional horsepower range, most synchronous motors are used where precise constant speed is required. These machines are commonly used in analog electric clocks, timers and other devices where correct time is required. In higher power industrial sizes, the synchronous motor provides two important functions. First, it is a highly efficient means of converting AC energy to work. Second, it can operate at leading or unity power factor and thereby provide power-factor correction.

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