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Synchronous motor
 
 
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A synchronous electric motor is an AC motor
AC motor

An AC motor is an electric motor that is driven by an alternating current. It consists of two basic parts, an outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field....
 distinguished by a rotor
Rotor (electric)

The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis....
 spinning with coils passing magnets at the same rate as the alternating current
Alternating current

In alternating current the movement of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again....
 and resulting magnetic field
Magnetic field

A magnetism field is a vector field which can exert a magnetic force on moving electric charges and on magnetic dipoles . When placed in a magnetic field, magnetic dipoles tend to align their axes parallel to the magnetic field....
 which drives it. Another way of saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor
Induction motor

An induction motor is a type of asynchronous AC motor where power is supplied to the rotating device by means of Electromagnetic induction. Other commonly used name is squirrel cage motor due to the fact that the rotor bars with short circuit rings resemble a squirrel cage ....
, which must slip in order to produce torque
Torque

Torque is the tendency of a force to rotate an object about an axis . Just as a force is a push or a pull, a torque can be thought of as a twist....
. Synchronous motor is like an induction motor except the rotor is excited by a DC field.






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Encyclopedia


A synchronous electric motor is an AC motor
AC motor

An AC motor is an electric motor that is driven by an alternating current. It consists of two basic parts, an outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field....
 distinguished by a rotor
Rotor (electric)

The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis....
 spinning with coils passing magnets at the same rate as the alternating current
Alternating current

In alternating current the movement of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again....
 and resulting magnetic field
Magnetic field

A magnetism field is a vector field which can exert a magnetic force on moving electric charges and on magnetic dipoles . When placed in a magnetic field, magnetic dipoles tend to align their axes parallel to the magnetic field....
 which drives it. Another way of saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor
Induction motor

An induction motor is a type of asynchronous AC motor where power is supplied to the rotating device by means of Electromagnetic induction. Other commonly used name is squirrel cage motor due to the fact that the rotor bars with short circuit rings resemble a squirrel cage ....
, which must slip in order to produce torque
Torque

Torque is the tendency of a force to rotate an object about an axis . Just as a force is a push or a pull, a torque can be thought of as a twist....
. Synchronous motor is like an induction motor except the rotor is excited by a DC field. Slip rings and brushes are used to conduct current to rotor.

Uses


Sometimes a synchronous motor is used, not to drive a load, but to improve the power factor on the local grid it's connected to. It does this by providing reactive power to, or consuming reactive power from the grid. In this case the synchronous motor is called a Synchronous condenser
Synchronous condenser

In electrical engineering, a synchronous condenser is a synchronous motor that is not attached to any driven equipment. Its field is controlled by a voltage regulator to either generate or absorb AC power#Real,_reactive,_and_apparent_power as needed to support a system?s voltage or to maintain the system power factor at a specified level....
.

Electrical power plants almost always use synchronous generators because it's very important to keep the frequency constant at which the generator is connected.

Low power applications include positioning machines, where high precision is required, and robot
Robot

A robot is a virtual or mechanical artificial agent. In practice, it is usually an Electromechanics which, by its appearance or movements, conveys a sense that it has Intention or Agency of its own....
 actuators.

Mains synchronous motors are used for electric clocks.

Advantages

Synchronous motors have the following advantages over non-synchronous motors:
  • Speed is independent of the load, provided an adequate field current is applied.
  • Accurate control in speed and position using open loop controls, eg. stepper motors.
  • They will hold their position when a DC current is applied to both the stator and the rotor windings.
  • Their power factor
    Power factor

    The power factor of an alternating current electric power system is defined as the ratio of the AC power flowing to the load to the AC power , and is a number between 0 and 1 ....
     can be adjusted to unity by using a proper field current relative to the load. Also, a "capacitive" power factor, (current phase leads voltage phase), can be obtained by increasing this current slightly, which can help achieve a better power factor correction for the whole installation.
  • Their construction allows for increased electrical efficiency when a low speed is required (as in ball mill
    Ball mill

    A ball mill is a type of grinder used to grind materials into extremely fine powder for use in paints, pyrotechnics, and ceramics....
    s and similar apparatus).


Examples

  • brushless permanent magnet DC motor
    Brushless DC electric motor

    A brushless DC motor is a synchronous electric motor which is powered by direct-current electricity and which has an electronically controlled commutation system, instead of a mechanical commutation system based on Brush es....
    .
  • stepper motor
    Stepper motor

    A stepper motor is a Brushless DC electric motor, synchronous electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely, without any feedback mechanism ....
    .
  • Slow speed AC synchronous motor
    AC motor

    An AC motor is an electric motor that is driven by an alternating current. It consists of two basic parts, an outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field....
    .
  • Switched reluctance motor
    Reluctance motor

    A reluctance motor is a type of synchronous electric motor which induces non-permanent magnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenon of magnetic reluctance....
    .


See also