Synchronous AC motors. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Electric motors. Synchronous AC motors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electric motors

Synchronous motor is a design in which (unlike an asynchronous motor) the rotational speed is constant under various loads.

Engine Composition

Synchronous motors are used to drive constant speed machines (pumps, compressors, fans). The stator of a synchronous electric motor contains a winding connected to a three-phase current network and forming a rotating magnetic field. The motor rotor consists of a core with an excitation winding.

The excitation winding is connected to a direct current source through slip rings. The excitation winding current creates a magnetic field that magnetizes the rotor. The rotors of synchronous machines can be salient pole (with pronounced poles) and implicit pole (with implicit poles).

On fig. 16.13, but the core 1 of a salient-pole rotor with protruding poles is shown. Excitation coils 2 are placed on the poles. In fig. 16.13, b shows an implicit-pole rotor, which is a ferromagnetic cylinder 1. On the surface of the rotor in the axial directions, grooves are milled into which the excitation winding 2 is laid.

AC synchronous motors
Rice. 16.13. Types of cores of synchronous electric motors: a - the core of a salient-pole rotor; b - implicit pole rotor

Principle of operation

Consider the principle of operation of a synchronous motor according to fig. 16.14.

AC synchronous motors
Rice. 16.14. Synchronous motor operation model

We represent the rotating magnetic field of the stator as magnet 1. We represent the magnetized rotor as magnet 2. Let's turn magnet 1 through an angle a. The north magnetic pole of magnet 1 will attract the south pole of magnet 2, and the south pole of magnet 1 will attract the north pole of magnet 2. Magnet 2 will rotate by the same angle α. We will rotate magnet 1. Magnet 2 will rotate together with magnet 1, and the rotational frequencies of both magnets will be the same, synchronous: n₂ = n₁.

A synchronous motor with no excitation winding on its rotor is called a synchronous reluctance motor.

The rotor of a synchronous reluctance motor is made of ferromagnetic material and must have pronounced poles. The rotating magnetic field of the stator magnetizes the rotor. The salient-pole rotor has unequal magnetic resistances along the longitudinal and transverse axes of the pole. The lines of force of the stator magnetic field are bent, trying to follow a path with less magnetic resistance.

The deformation of the magnetic field will cause, due to the elastic properties of the lines of force, a reactive moment that rotates the rotor synchronously with the stator field. If a braking moment is applied to the rotating rotor, the axis of the magnetic field of the rotor will turn by an angle relative to the axis of the magnetic field of the stator.

With increasing load, this angle increases. If the load exceeds a certain allowable value, the engine will stop, fall out of synchronism. Synchronous motors have no starting torque. This is due to the fact that the electromagnetic torque acting on the stationary rotor changes its direction twice in the period T of the alternating current. Due to its inertia, the rotor does not have time to move off and develop the required number of revolutions. Currently, asynchronous starting of a synchronous motor is used. An additional short-circuited winding is placed in the grooves of the rotor poles.

The rotating magnetic field of the stator induces eddy currents in the short-circuited starting winding. When these currents interact with the magnetic field of the stator, an asynchronous electromagnetic torque is formed, causing the rotor to rotate. When the rotor speed approaches the stator field speed, the motor is pulled into synchronism and rotates at synchronous speed. The short-circuited winding does not move relative to the field, eddy currents are not induced in it, and the asynchronous starting torque becomes zero.

Author: Koryakin-Chernyak S.L.

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