Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
1998-01-29
2001-04-24
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C318S132000, C318S434000, C318S721000, C318S724000
Reexamination Certificate
active
06222331
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
The present invention relates generally to electrical machines (which include both motors and generators), and more particularly to a system (apparatus and method) for providing power to the machine components and a control and operating system for the same. More particularly, this relates to such a machine and system which does not use brush commutation, and which provides optimized performance of the machine, and also enables the overall design of the machine itself to be improved for better performance.
One of the more common electrical machines used for various industrial applications is the conventional commutator controlled DC motor having a stator with at least one set (pair) of oppositely positioned fixed north and south poles, and a rotor positioned between the poles. The rotor is wound with coils that extend parallel to the axis of rotation of the rotor and are positioned at circumferentially spaced locations around the rotor.
Electric power is supplied to the coils of the rotor through the commutator which is interconnected with the rotor and rotates with the rotor. An outside DC power source is connected to the commutator rotor through stationary brushes positioned on opposite sides of the commutator and in electrical contact therewith. The commutator is made up of segments which are in turn connected electrically to the coils in the rotor. Thus, with rotation of the rotor, power is fed from the two brushes to the commutator segments in a predetermined sequence so that the electric current flows through each coil in one direction during one portion of the rotational path of travel of the rotor, and then in the opposite direction during the other portion of the rotational path of the rotor. The commutator is arranged so that the magnetic fields created by the coils interact with the field created by stator to cause the rotation of the rotor.
Such commutator controlled DC motors have several desirable features for various industrial applications. One significant advantage is that this motor can develop very high torque at startup thus providing a significant advantage over other types of electric motors, such as synchronous motors operating from alternating current. Another advantage of such commutator controlled DC motors for some applications is that the rotational speed of the rotor can be easily changed to match certain operating requirements. For example, the motor can develop very high torque at slow speed, and yet under other circumstances, it can operate at much higher speeds.
However, the commutator controlled DC motors have certain drawbacks, and some of these relate to the problems associated with the commutator and the brushes. It is necessary that the commutator segments be spaced at least a short distance from one another to avoid arcing between adjacent commutator segments.
However, the brushes must constantly be creating and breaking electrical contact with the rotating commutator segments. This creates maintenance problems. Also, higher costs are involved due to the commutator and brush rigging assembly and other factors. Another drawback is that because of the possible sparking, these are not used in environments where sparking would be hazardous. The practical consequence is a widespread desire to avoid the mechanical commutator apparatus altogether.
Another drawback related to the brush/commutator control system is that the positioning of the switch-over points is usually a compromise in accordance with the operating conditions of the motor. Ideally, if the motor is operating at a given desired power output and at a given speed, the brushes can be placed at optimized locations so that the switch over of the current supplied to the coils is perfectly timed to optimize the efficiency and other performance characteristics of the rotor. However, if the motor is heavily loaded so that it is required to develop high torque, the rotor will “lag”. The reason for this is that when the rotor is developing high torque, the entire magnetic field between the north and south poles of the stator will bend or distort in a direction opposite to the rotation of the rotor. Thus, in terms of the operating effect of the magnetic fields created, the north and the south poles of the stator actually shift. Also the rotational speed of the motor can affect the optimized position of the brushes.
Ideally, the commutator brushes should be shifted accordingly so that the switch-over point becomes optimized for that mode of operation. Various attempts have been made in the prior art to mechanically shift the brushes or compensate for this situation in other ways, but in large part these attempts have not been successful and/or commercially feasible. Accordingly, quite often the commutator and brushes are placed at a compromise position to accommodate for the various operating conditions that are expected for the motor.
However, beyond this, in a commutator controlled DC motor, the entire machine design must be compromised in certain respects because of the inherent operating characteristics of the motor. Due to the fact that there is the potential for sparking between the brush and the commutator segments, there is a limit to the level of voltage that can be applied to this type of motor. Also, the air gap between the armature and the rotor has to be greater than a certain minimum distance, because of the induced voltages caused by the commutation of a coil (short circuit) in the coils tends to cause sparking between the commutator segments. One of the disadvantages in having this larger gap is that it increases the reluctance of the magnetic path across the path across the air gap, and thus decreases the torque that can be developed by the motor. This exacerbates the matter by this causing other complications, such as greater distortion of the field when the motor is operating under a heavy load.
DC generators are substantially the same as (or at least closely similar to) DC motors, but operate in a reverse mode. In other words, in the generator the power input is generally through a rotating shaft, and the output is a DC current. In large part, the same advantages and disadvantages that are inherent in DC motors are also present in DC generators.
Given these disadvantages of such DC motors, one would be led to believe that a motor operating on alternating current would be more desirable, since these do not have the commutator and brush interaction (thus enabling higher voltage machines, with less power loss), and also permitting one to be better able to optimize the motor designs with minimum air gaps and maximum field strengths. To some extent this is true, and such motors have found wide commercial use for some applications. However, such motors operating off alternating current typically suffer from slip, severe lack of start up torque, and a difficulty of control in terms of variable speed.
In view of this, it is the object of the present invention to provide an improved control system for electric machines in general, and particularly with certain types of electric engines to which the present invention is well adapted to alleviate in large part problems such as described above. Also, the present invention in addition to providing an improved power and control system for electric motors and generators, further comprises, in combination, an electric motor having an optimized design to maximize the benefits provided by the present invention in permitting the motor or generator itself to be designed more effectively, and also to enable more efficient (and in some instances more versatile) operation.
SUMMARY OF THE INVENTION
The present invention relates to dynamo-electric machines, which can operate either as a motor or as a generator. The current flow in the coils of the machine can be optimized to accomplish efficient power transfer, and to alleviate many of the problem existing in the prior art.
The present invention comprises a dynamo-electric machine which comprises a field member and an armature assembly.
The fie
Global Electric Motor Co. Ltd.
Johnston Michael G.
Moore & Van Allen
Ro Bentsu
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