Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-01-03
2003-01-28
Tamai, Karl (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S077000, C310S254100
Reexamination Certificate
active
06512318
ABSTRACT:
This invention relates to a self-starting brushless electric motor of the type which comprises reluctance poles (ferromagnetic salient poles) at least on one of the two relatively moving motor parts and one or more permanent-magnetic poles in the pole system.
Self-starting brushless electric motors can be supplied with direct current pulses of a single polarity or with alternating current polarity. When motors with moderate shaft power are supplied electronically, direct-current pulse supply uses the least number of electronic switches and thus gives the lowest system costs for motor and supply electronics. On the other hand, for higher power, when the number of electronic switches in the supply electronics in the motor must anyway be increased, it may be advantageous to supply the motor with alternating current polarity so that electric power will be supplied to the motor during both half-periods, thus achieving more uniform torque development and reducing the electrical conduction losses in the winding.
A single-strand brushless motor has only one winding supplied from a single external current source and provided on one of two or more parts which are rotatable or otherwise movable in relation to each other. Such a motor can be self-starting, i.e. develop a driving torque when at standstill in a predetermined direction, the preferential starting direction, only if this starting direction is inherent in the design of the motor. Self-starting in the preferential direction may be built into the motor by providing asymmetry in the soft-magnetic iron core, e.g. through the use of asymmetrical salient poles and/or asymmetrical permanent-magnet poles, or by providing auxiliary windings with no connection to external current sources, e.g. short-circuited current paths as in known shaded-pole motors. Such current paths can only conduct current under the influence of a varying magnetic field linked to these current paths. In order for current to flow in such current paths when the motor is stationary, the winding connected to an external current source must be supplied with a pulsed or alternating current.
It can be shown theoretically that motors that are not provided with auxiliary windings but are nevertheless able to exert torque in any rotor position even when the motor winding is deenergized must always contain a permanent-magnet pole.
In the following the description is limited to motors for rotary movement which have a first part, in the following called the stator, provided with a winding, and a second part, in the following called the rotor, arranged inside the stator and rotatable in relation thereto. It will, however, be appreciated that these two parts may exchange places, that the air gap separating the stator from the rotor need not be cylindrical but may equally well be flat or conical, and that the relative movement between the parts of the motor need not be rotary but may equally well be linear or a combination of rotary and linear, i.e. occur simultaneously about and along an axis of rotation.
The function of the motor may be described as comprising work cycles which are repeated a given number of times for each revolution. At extremely low speed, e.g. when starting up from standstill, the work cycle for a motor designed to be supplied with DC pulses of one polarity consists of one part when the winding carries current and another part when the winding is currentless. For a motor designed to be supplied with current pulses of alternating polarity the work cycle consists of one part when the winding is supplied with current of one polarity, followed by a currentless part and thereafter a part when the winding is supplied with current of opposite polarity followed by another currentless part of the work cycle.
In the currentless state the rotor must reach a starting position, i.e. a position in which the winding, if supplied with current, gives rise to a driving torque, namely a torque in the preferential direction of the motor, that is sufficiently high to overcome any frictional torque or the like in the motor and/or in the object driven by the motor. The torque generated in the motor through permanent-magnetic forces must maintain its direction and be of sufficient strength until the rotor reaches a position in which the winding can be energized. It will be understood that the demand for torque development in currentless state means that the motor must include at least one permanent-magnet pole.
Motors operating in accordance with the principles described and exhibiting magnetic asymmetry in the pole system are known through WO90/02437 and WO92/12567. An object of the present invention is to obtain improvements in motors of the type represented by the motors in the aforesaid publications.
This object is achieved by means of the arrangement of magnetically active stator and rotor elements (poles).
Besides the opportunity of realizing constructionally alternative embodiments, the invention also offers the opportunity of increasing the force generated by the motor—torque in a rotating motor and linearly acting force or “tractive force” in a linear motor—in one or more respects:
Increasing the torque generated by permanent-magnet poles that pulls the rotor of the currentless motor to the nearest starting position. Such improvement is advantageous in applications where high frictional torque may appear in the driven object, for example in shaft seals.
Increasing torque appearing in a motor whose rotor is stationary in a starting position and whose winding is supplied with the highest current available. Such improvement is also advantageous in the situations mentioned in the preceding paragraph.
Increasing, at least in certain embodiments, the air-gap power of the motor for given heat losses, thereby giving a smaller and economically more favourable motor for a given purpose, which may be a great advantage when low motor weight is of importance for certain types of applications, e.g. in hand-held tools or other hand-held objects, but is also an economic advantage in general, provided an unavoidable cost increase in the supply electronics does not cancel the effect.
The magnetically active elements in the motor of relevance to the invention are as follows:
Coils on the stator
In principle the coils form a single current circuit and may be connected in series and/or in parallel. When the supply electronics consist of several units operating in parallel, these may be connected each to its own coil or group of coils, as if they formed a single electrical circuit. Instead of supplying the winding with alternating current polarity a two-part winding can be used, the two winding halves being supplied with a single current polarity, but the winding halves having magnetically opposite directions.
Ferromagnetic salient poles (reluctance poles)
In most of the motors shown according to the invention, ferromagnetic salient poles, in the following also called reluctance poles, are to be found on the stator, alone or together with permanent-magnet poles.
There may also be reluctance poles on the rotor, but preferably not mixed with permanent-magnet poles. A mixture of these pole types on the rotor can be contemplated but is normally not meaningful.
The reluctance poles on both stator and rotor may be magnetically asymmetrical. For magnetically asymmetrical stator poles the asymmetry should be directed in the opposite direction to the preferential direction of motion of the motor, whereas on the rotor the asymmetry should be in the same direction as the preferential direction of motion.
Alternatively or in addition, the reluctance poles on both stator and rotor may, however, show a certain magnetic asymmetry in the opposite direction to that described above without this making the motor inoperable.
Permanent-magnet poles
Motors with only reluctance poles on the rotor must always be provided with a permanent-magnet pole on the stator. The permanent-magnet poles on the stator preferably are magnetically balanced, i.e. equal in number and size of both polarities.
In certain cases it is an
Schaer Roland
Török Vilmos
Wissmach Walter
Browdy and Neimark , P.L.L.C.
Tamai Karl
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