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synchronous machine

The synchronous machine is a rotating field machine, which is therefore operated with a three-phase alternating current (see Module 7) in the stator circuit. The rotor can be excited either permanently (with permanent magnets) or by a direct current circuit. This type of machine is used in power stations to generate electricity, but as it is highly efficient, it is also increasingly used in modern drive technology in smaller power classes. However, a major disadvantage is that the synchronous machine must always be controlled by power electronics, as it cannot be started up on a simple three-phase network.

Advantages:

  • Robust and maintenance-free.
  • High control dynamics.
  • High power density with low construction volume.
  • Overload capacity.

Disadvantages:

  • Complex converter technology.
  • Complex control system.
  • Controlled operation only possible with great effort or not possible at all.

1 Structure

  • Three stator windings offset by 120° from each other.
  • Laminated stator design, as it is subjected to an alternating magnetic field.
  • The exciter winding is located in the rotor.
  • The rotor is partially unlaminated.
Figure 1: Diagram of a cross-section of an externally excited synchronous machine.

Figure 2: Structure of a separately excited synchronous machine. The stator windings, offset by 120° from each other, are illustrated here in colour.

2 Field gradients

Figure 3: Direction of the rotating field vector. The rotor rotates synchronously with the sinusoidal rotating fields of the stator, which are offset by 120 degrees.

In a synchronous machine, the magnetic coupling between the stator and rotor generates the torque. In motor operation, the rotor field follows the stator field. The mechanical speed must therefore always be equal to the electrical rotational frequency. Between the stator and the rotor is the load-dependent pole angle \(\vartheta \). The torque is greatest at \(\vartheta =90^\circ \). If, during motor operation, the pole wheel angle exceeds \(90^\circ \) due to excessive load, the torque generated decreases and the machine „stalls“– that is, it comes to an abrupt halt.

  • The stator flux is created by the superposition of the fluxes of the three windings.
  • The current flow is generated by the excitation winding.

PIC

Figure 4: Torque characteristic curve as a function of pole wheel angle.

3 Equivalent circuit diagram

In mains operation, the terminal voltage \(U_1\) and the frequency \(\omega \) are fixed. This is a common operating condition for synchronous machines.

PIC

Figure 5: Complex single-phase ESB of a full-pole generator. For example, in grid operation.
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