Electrical generator or motor structure – Dynamoelectric – Rotary
Patent
1990-12-03
1992-03-24
Stephan, Steven L.
Electrical generator or motor structure
Dynamoelectric
Rotary
310 47, 310 88, H02K 926, H02K 714
Patent
active
050991604
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a powered hand tool for machining and, more particularly, to a powered hand tool for machining magnetizable materials.
The invention is based on a powered hand tool having an enclosed ventilated drive motor, particularly an electric drive motor, in which air passes from the ambient atmosphere into the interior of the powered hand tool to cool the motor during operation. Such hand tools are well known, e.g. the angle grinder. To cool the windings of the electric motors of these powered hand tools, ambient air is as a rule blown through cooling-air ducts passing through the stator winding, a fan wheel mounted on the rotor shaft providing the necessary forced convection.
In enclosed-ventilated motors, the solid particles, occurring in particular in grinding machines, in the cooling-air flow create problems. These solid particles can cause damage on their way through the interior of the motor. Thus, for example, EP-A-0,261,306 teaches a protective means for the end windings of the armature of the electric motor having ribs which project above the end windings and deflect away the stray solid particles, before they can strike the end windings.
However, with the device described there, metal particles drawn in with the drawn-in air cannot be prevented from penetrating into the interior of an electric motor and settling there.
Experience shows that the metal particles settle at the locations where turbulence occurs in the flow of the drawn-in air on account of cross-sectional changes in the cooling-air channel and where there are at the same time powerful magnetic forces. The magnetic flux in the area of the annular gap between the rotor and the stator of an electric machine is particularly strong. It has been found in tests that the metal particles carried along with the drawn-in air mostly collect in this annular gap. The force of attraction of the magnetic field prevailing in the annular gap is so great there that even improved seals or a narrower gap between rotor and stator cannot reliably prevent dust laden with magnetisable particles from penetrating into the annular gap between rotor and stator.
If sufficient steel/stone dust has collected in the annular gap between rotor and stator, failure of the machine occurs through seizure of the rotor on the stator or through clogging of the bearing.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a powered hand tool of the above-described kind having an enclosed ventilated drive motor, preferably an electric drive motor, which is cooled by a drawn-in cooling air flow through at least one air duct and in which dust, particularly steel and/or stone dust, formed by action of the tool, does not penetrate into the interior of the drive motor, especially the gap between the rotor and the stator.
According to the invention at least one permanent magnet is fixed to the rotor shaft and positioned in the air duct.
The invention also resides in an air-cooled drive motor having a rotor shaft, a cooling air flow and at least one air duct through which the cooling air is drawn, in which at least one permanent magnet is provided in the vicinity of the air duct.
Compared with the prior art, the powered hand tool according to the invention, has the advantage of reliably preventing the penetration of grinding dust laden with magnetizable material into the annular gap between rotor and stator.
The simplicity of the invention can be considered to be a further advantage. Thus, it is sufficient to arrange a permanent magnet in the area of the cooling-air duct, which permanent magnet can attract the grinding dust drawn in with the cooling air and laden with magnetisable material.
Several embodiments of the invention are possible. First, the drive motor is advantageously an enclosed electric motor, which has a stator spaced from the rotor by an annular gap extending longitudinally along the rotor shaft. A preferred means for providing a cooling-air flow is, advantageously, a fan wheel attached to the ro
REFERENCES:
patent: 2769105 (1956-10-01), Altschwager et al.
patent: 3959677 (1976-05-01), Grieb
patent: 4385276 (1983-05-01), Bitzel
patent: 4399378 (1983-08-01), Krumm et al.
patent: 4564778 (1986-01-01), Yoshida
Schwarze Wilhelm
Stirm Helmut
Strozel Reinhold
LaBalle C.
Robert & Bosch GmbH
Stephan Steven L.
Striker Michael J.
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