Brakes – Operators – Electric
Reexamination Certificate
1999-11-03
2001-07-03
Oberleitner, Robert J. (Department: 3613)
Brakes
Operators
Electric
Reexamination Certificate
active
06253885
ABSTRACT:
FIELD OF THE INVENTION
The invention comprises an electromagnetic brake for complementing the braking action of friction wheel brakes of an automotive vehicle.
BACKGROUND ART
It is known design practice to complement the braking action of friction wheel brakes of an automotive vehicle by using eddy current retarders. These convert the kinetic energy of the vehicle into electrical energy in the form of eddy currents. The electrical energy is dissipated in the form of heat.
Such eddy current brakes include a stator comprising either a permanent magnet or an electromagnet to create a magnetic flux field. The stator is stationary and a rotor is connected drivably to a torque delivery element, such as the drive shaft for automotive vehicle traction wheels.
In the case of an electromagnetic stator, the stator poles have electromagnetic windings that are excited with a voltage created by an onboard electrical power source, such as a vehicle battery alternator system.
An example of an eddy current brake with a permanent magnet stator has been described by H. Sakamoto et al in a publication entitled “Design of Permanent Magnet Type Compact ECB Retarder”, SAE Technical Paper 973228, dated Nov. 17-19, 1997. An electromagnetic eddy current retarder is described by J. Bigeon et al in a paper entitled “Analysis of an Electromagnetic Brake”, published in a journal entitled Electric Machines And Power Systems, 10:285-297, 1985.
Permanent magnet eddy current brakes have stator poles that are solid and, of necessity, are formed of magnetic materials with rare earth ingredients such as Neodymium Ferrous Boron (NdFeB). Such materials are both costly and difficult to maintain and handle due to their corrosion characteristics. Although an electromagnetic eddy current brake is less costly because of the lack of expensive materials, such as Neodymium Ferrous Boron, they too typically are of substantial weight because of the solid design of the stator.
Both permanent magnet eddy current brakes and electromagnetic eddy current brakes comprise a drive shaft and a rotor drum that rotate together in a magnetic field established by the stator windings. The braking force generated at the rotor is created by interaction of rotor eddy currents and the stator magnetic field. The eddy currents are formed principally at or near the outer surface of the rotor drum, depending on the speed of rotor rotation.
The kinetic energy involved in this braking action generates heat. Cooling fins usually are added to the outer surface of the rotor drum to dissipate the heat produced by the eddy currents.
Aside from the weight and cost disadvantages of presently known eddy current brakes, the design of known brakes requires complex assembly procedures because of the number of parts involved in the assembly. These parts include seals and steel closure plates that separate the eddy current brake from harsh environments to prevent rust and corrosion.
SUMMARY OF THE INVENTION
An objective of the invention is to provide a stator for an eddy current brake that is electrically energized and is of reduced weight. A further object of the invention is to provide a stator for an eddy current brake that is adaptable for commercial manufacturing and assembly operations with reduced cost.
The eddy current brake of the invention includes a steel stator with magnetic poles that are energized by electric windings to create the necessary flux field. A rotor in the form of a steel rotor drum is mounted within the stator poles so that a small air gap exists between the pole faces of the stator and the periphery of the rotor drum.
The poles are cast or otherwise formed as an integral part of the overall stator assembly using high permeability steel. The geometry of the poles can be strategically controlled to reduce substantially the mass of the stator while maintaining optimum braking torque specifications for a particular design. In this way, there is minimal degradation of output braking torque while total stator weight is minimized. Further, the height of the field poles can be designed to provide an optimum air gap to maintain the required flux density.
The design characteristics of the stator make it possible to reduce the field pole weight by about 40 to 50 percent compared to prior art stators for an eddy current brake of this kind.
The stator of the invention requires electric current excitation. This is provided by multi-turn copper windings, which are shaped so that they can be prefabricated and assembled over the pole pieces with a simple assembly procedure. The coils can be automatically wound on a nonferrous bobbin using a standard winding machine and assembled to the pole piece with automated equipment.
In order to increase the braking efficiency of the eddy current brake of the invention, the design includes magnetic field pole caps, which retain the stator field windings in place following assembly. The caps are designed to reduce field pole magnetic saturation as the rotor rotates at high speed relative to the stator. This is done as the caps increases the effective pole face area. The pole caps can be attached to the face of the stator poles using simple fastener devices, such as through bolts.
The eddy current brake of the invention can be mounted in the driveline with simple brackets that connect the stator to the housing of a vehicle differential and axle assembly for an automotive vehicle.
The rotor for the eddy current brake of the invention is fixed to a drive shaft in the vehicle driveline by a flange and supporting arms. This design allows for a convenient mounting procedure for the rotor and the axle drive shaft. Alignment and concentricity of the rotor is established by properly locating the holes for mounting bolts that secure the rotor to the driveline axle housing.
A protective shield is used to protect the stator windings from exposure to dirt and road debris and other harsh elements in the environment. The shield is made of steel so that it can provide additional braking torque by containing leakage of flux produced by the stator windings.
The stator design has strategically placed cavities or hollow portions at the cores of the field poles. This significantly reduces the weight of the stator without adversely affecting the overall braking capacity. Further, the hollow field pole design of the invention reduces the self-inductance of the field windings because of the reduced mass of the field poles.
The permeability of the stator is reduced because of the cavities or hollow portions of the stator. It thus is possible to achieve a faster system response to a driver command for braking. The reduction in permeability of the material of the poles can be expressed as a change in inductance per unit length of the pole (e.g., Henrys per cm).
The increase in the effective pole areas created by the caps on the stator poles reduces the tendency of the flux field to be concentrated in a dense fashion at the end faces of the poles. In this way, the caps improve the braking efficiency.
REFERENCES:
patent: 2679015 (1954-05-01), Carter
patent: 3235758 (1966-02-01), Dinsmore
patent: 4491753 (1985-01-01), Kumatani
patent: 5303802 (1994-04-01), Kuwahara
patent: 6148967 (2000-11-01), Huynh
patent: 560157 (1923-09-01), None
patent: 686759 (1965-03-01), None
patent: 07154958 (1995-06-01), None
“Analysis of an Electromagnetic Brake”, by J. Bigeon et al, Electric Machines and Power Systems, 10:286-297, 1985.
“Design of Permanent Magnet Type Compact ECB Retarder”, by Harou Sakamoto et al, SAE Technical Paper No. 973228, pp. 19-25, Nov. 17-19, 1997.
Tsai Hong
Xiang Youqing
Nguyen Xuan Lan
Oberleitner Robert J.
Shelton Larry I.
Visteon Global Technologies Inc.
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