Internal pole machine

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Section through a salient pole machine ( )
Section through a full pole machine ( )

The internal pole machine is a type of single-phase and three-phase synchronous machine. In machines of this type, the excitation is housed in the rotor . The excitation can be realized by coils or permanent magnets that are built into the rotor. The counterpart to the internal pole machine is the external pole machine .

Design of the internal pole machine depending on the rated speed:

The design of the rotor of the internal pole machine depends on the rotor speed and results from the mains frequency at which the synchronous machine is operated. The rotor speed results as follows:

(The 60 is used to relate the calculated speed to one minute)

For a machine with the number of pole pairs that is operated on a 50 Hz network, the result is a speed of 1000 min −1 . With an increasing number of pole pairs there is a lower speed and vice versa.


There are two types of construction:

  1. Salient pole rotors have a large diameter, with a short length and space for many distinct magnetic poles (pole pairs) arranged in pairs. They are suitable for speeds up to 1000 min -1 . They are unsuitable for speeds beyond this, as high centrifugal forces act on the machine due to the large diameter .
  2. Full pole rotors are used at high speeds, i.e. a few pole pairs. The rotor is made of robust chrome-nickel steel with a small diameter, but all the greater length. The excitation winding is placed in the slots of the rotor.


Klaus Fuest and Peter Döring: Electrical machines and drives 6th edition, Vieweg Wiesbaden 2004, ISBN 3-528-54076-1