Electrical generator or motor structure – Dynamoelectric – Linear
Patent
1997-03-27
1998-04-28
Stephan, Steven L.
Electrical generator or motor structure
Dynamoelectric
Linear
310 14, H02K 4103
Patent
active
057448795
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a synchronous linear motor of a type including a primary section having rotor grooves for receiving a single-phase or multi-phase rotor winding, and a secondary section including a sequence of permanent magnets, each of the magnets having two pole pairs acting as north and south poles, with the length of the secondary section in the moving direction exceeding the length of the primary section. A synchronous linear motor of this type is known from U.S. Pat. No. 4,908,533.
High quality synchronous motors for applications as actuating motors should generate forces rather uniformly and without malfunctioning. In rotating synchronous motors, periodic variations in force ("force waviness") occur which are mainly caused by grooves disposed on the stator. In order to compensate for the force waviness as well as for all other effects on the drive shaft torque caused by the grooves, the rotor and stator poles are usually beveled across the width of a groove.
There is also known from U.S. Pat. No. 4,908,533 for synchronous linear motors, that the force waviness can be reduced by beveling the poles across the width of a groove of the wound primary section. Since in top plan view, the edges of the faces of the primary section are parallel to the grooves in the primary section, both the front and rear faces of the poles are beveled in the conventional beveling operation of the grooves. The force waviness can also be reduced by a method known from EP 0 334 645 A1, wherein the core of the primary section of a synchronous linear motor is formed as a ferromagnetic plate and coils are placed in the air gap of the linear motor in such a way that the face regions of the plate project beyond the air gap coils and form a step along the longitudinal median line of the linear motor.
In contrast to rotating synchronous motors which continue indefinitely when viewed along the circumference, the synchronous linear motor is special in that it has a beginning and an end. At the transition points at the beginning and the end of a synchronous linear motor, there are generated periodic motor end forces in the moving direction which adversely effect the operation of the linear motor. The motor end forces are generated because the linear motor covers the magnetic poles differently, depending on the motor position. As a result, there exist preferred positions where the stored magnetic energy of the linear motor is particularly large. Additional forces are then required in order to move the linear motor away from these preferred positions. The force with which the linear motor pulls itself into the preferred positions of the magnetic poles, is called "pole force." The pole force can reach up to approximately 25% of the rated motor force. There exists a preferred position above each magnetic pole. Consequently, the pole force has the same periodicity as the magnetic poles and interferes with the motor force; this phenomenon is referred to as "pole waviness." Since the pole force does not depend on the motor current, it represents a passive force which is also present in the absence of current. The pole force does not perform any work, because it operates alternating in the moving direction and opposite to the moving direction of the linear motor. During operation, the pole force is added to the force generated by the motor current. The pole force has nothing in common with the groove force which is the force describing the interaction between the edges of the magnetic poles and the stator grooves.
In the conventional synchronous linear motor, the aforedescribed "pole waviness" causes an imprecise movement which is particularly undesirable if such motors are used as precision actuators.
SUMMARY OF THE INVENTION
Therefore, it is the object of the invention to improve the motion accuracy of a synchronous linear motor of the abovedescribed type.
The object of the invention is attained by providing the primary section with front and rear end faces which are free of grooves and free of a windi
REFERENCES:
patent: 4868431 (1989-09-01), Karita et al.
patent: 4908533 (1990-03-01), Karita et al.
patent: 5166563 (1992-11-01), Bassine
Feiereisen Henry M.
Jones Judson H.
Krauss-Maffei AG
Stephan Steven L.
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