Electrical generator or motor structure – Dynamoelectric – Rotary
Patent
1986-04-11
1988-06-14
Duggan, Donovan F.
Electrical generator or motor structure
Dynamoelectric
Rotary
H02K 2112
Patent
active
047514167
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a synchronous servomotor of the kind which comprises a stator with a stator core provided with slots and a winding placed therein, and a rotor with a rotor core and field poles in the form of permanent magnets attached to the rotor core and located in the air gap between the stator core and the rotor core.
Synchronous servomotors of this kind give rise to a problem generally referred to as reluctance torque or reluctance cogging, which is caused by magnetic interaction between the permanent magnetic poles on the one hand and the slots and interlying teeth in the stator core on the other. This magnetic interaction, which is completely independent of the current flowing in the stator winding and is therefore present even when no current passes therethrough, gives rise to a varying or pendulous torque on the rotor when the rotor is rotated in the stator, this torque variation being superposed on the driving torque generated by the current in the stator winding. In actual fact, when no current flows through the stator winding, the rotor exhibits a number of preference positions which the rotor strives to take, this number being dependent on the number of poles of the motor. This phenomenon causes the motor to run unevenly and results in deviations from desired motor speeds, the absolute magnitude of the effect on the speed of the motor being inversely proportional to motor speed and therewith being particularly noticeable at low speeds. Consequently, the reluctance torque or reluctance cogging constitutes a particularly serious problem in servomotors which must be capable of being driven very precisely with an extremely wide range of speeds, and also at very low speeds.
Endeavours have been made to overcome this problem, by using a smooth stator core without winding slots and by placing the stator winding on the smooth inner surface of the stator core, the winding being wound with the aid of a special winding technique. With this solution, however, the stator winding takes up a substantial part of the air gap between the stator core and the rotor core, which means that the air gap must be made larger, resulting in poor use of machine dimensions. Another known method of reducing the reluctance torque is to design the motor with skewed slots and/or field poles. Although both of these solutions provide an acceptable reduction of the reluctance torque they are encumbered with other drawbacks. For example, when using permanent magnets the use of skewed poles is mechanically difficult and relatively expensive to put into effect. The use of skewed slots for the stator winding also gives rise to certain difficulties in conjunction with the manufacture of the stator winding, particularly since it is desirable that this can be done mechanically.
Consequently, an object of the present invention is to provide a synchronous servomotor of the kind described in the introduction in which the reluctance torque or reluctance cogging is greatly reduced in a simpler and, from the aspect of manufacturer, a far less expensive manner than has hitherto been possible.
The characterizing features of the servomotor according to the invention are set forth in the following claims.
The invention will now be described in more detail with reference to the accompanying drawings, in which:
FIGS. 1-6 are schematic diagrams illustrating the basic principle of the invention and advantageous embodiments thereof;
FIG. 7 illustrates schematically an advantageous embodiment of the permanent magnet poles of a servomotor according to the invention;
FIG. 8 is a schematic axial sectional view of a 4-pole servomotor constructed in accordance with the invention;
FIG. 9 is a schematic sectional view taken on the line IX--IX in FIG. 8.
The fundamental concept of the invention resides in the displacement of certain poles relative to positions which correspond to uniform pole distribution by an angle corresponding to a fraction of the pitch of the stator core slots instead of distributing the poles of the motor uniformly aroun
REFERENCES:
patent: 3860843 (1975-01-01), Kawasaki
patent: 4280072 (1981-07-01), Gotou
patent: 4438362 (1984-03-01), Brown
patent: 4504755 (1985-03-01), Semones
AB ELMO
Duggan Donovan F.
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