GB2140219A - Electric machine - Google Patents

Abstract

An electric machine having a housing (3), a rotor (1) which is formed at least partly from a non-magnetic material, and a stator (5) includes a magnetized layer (23) which is provided on the non-magnetic material of the rotor. The magnetization of the layer is in the form of a continuous pattern extending around the rotor periphery. The machine furthermore comprises a detector (25) which cooperates with the magnetized layer. <IMAGE>

Description

SPECIFICATION Electric machine The invention relates to an electric machine having a housing, a rotor and a stator, which machine includes a marker arranged to rotate with the rotor and a detector which is fixed with respect to the stator and is arranged to cooperate with the marker.

Such an electric machine is known from German Offenlegungsschrift 3,031,946. The known electric machine is constructed as a direct current motor which is used, for example, in a tape driving mechanism of a tape recorder. In such a context it it necessary for the audio tape to be moved at a constant speed. In order to provide a control signal for that purpose, the rotor in the known machine is provided with an optical marker which is scanned optically-electronically by the detector. The optical marker may consist of alternating bright and dark surfaces.

The optical marker must be provided on the rotor very accurately. High requirements are imposed notably on the dimensions and positioning of the said surfaces. As a result of this, the known construction has the disadvantage that its manufacturing cost is very high.

It is an object of the invention to provide an electric machine of the kind set forth in the opening paragraph which can be manufactured in a simpier and cheaper manner.

The invention is characterized in that the marker is in the form of a ring-shaped magnetizable layer which is provided on a nonmagnetic portion of the rotor and is magnetized Sc as to generate a magnetic flux pattern, the detector being arranged to detect the magnetic flux pattern.

The material of the non-magnetic portion of the rotor on which the magnetizable layer is provided is preferably a synthetic resin, for example, polyacetate or polyamide.

The magnetic flux pattern which is generated by the magnetizable layer may be used to provide information on the number of revolutions and the speed of the rotor. It may have a variety of forms and may be such that, for example, sinusoidal, rectangular or sawtooth signals are generated in the detector. The signals which are generated in the detector when the rotor is rotating can be transmitted to an electronic control system, for example a servo-system, by means of which deviations in the rotor speed can be corrected.

The magnetizable layer may also be used for storing magnetically coded information for, for example, establishing the direction of rotation of the rotor or indicating the stationary state of the rotor.

The layer may be magnetized, for example, by positioning the rotor adjacent a magnetic recording head and rotating it. When the speed of the rotor is correct a signal current, for example a sinusoidal alternating current, is passed through the magnetic head, so that the layer, while it moves past the magnetic head, is magnetized in a pattern which corresponds to the changing magnetic flux which is generated by the magnetic head as a result of the signal current.

The detector may be, for example, a conventional magnetic head as is used in playback apparatus, for example a tape recorder or a cassette recorder. A magnetic head consists in principle of a ring-shaped core which is not completely closed and consists of a soft-magnetic material around which a coil is wound. The ends of the coil are constructed as poleshoes and de-limit a narrow gap. As another example the detector may be formed by a Hall element. The detector is preferably connected to the housing of the electric machine.

The electric machine may be constructed as a dynamo or as a motor, for example a direct current motor. Such a motor is particularly suitable for use in apparatus for video and/or audio recording and/or reproduction. In such contexts, one of the most important requirements imposed upon the motor is that the rotor has a very constant speed of rotation. The use of an electric machine according to the invention enables the measurement of the speed to be realized in a simple and cheap manner.

Preferably the material of the magnetizable layer is a dispersed magnetic particle mixture coated on the non-magnetic material of the rotor.

Such a mixture may be applied in the form of a magnetic lacquer comprising a binder in which a magnetic material has been dispersed in finely divided form using a dispersion agent. The binder may be a polymeric substance, for example polyvinylchioride, polyvinyl acetate or polyester.

Suitable dispersion agents are, for example, lecithine and mono- or diesters of phosphoric acid and alcohols.

Suitable magnetizable particles are inter alia CrO2 particles, Fe particles and y-Fe203 particles.' Auxiliary substances, for example antistatic, agents such as soot, and lubricants such as fatty acid, paraffin and graphite may be incorporated in the magnetic layer.

The use of such a lacquer can facilitate mechanisation of the manufacture of the machine.

The magnetizable layer may be manufactured, for example, by providing the material thereof together with an organic solvent in a uniform layer on the rotor, after which the desired information is recorded in the magnetic layer when the layer has dried.

Preferably the rotor is in the form of a disc on the peripheral surface of which the magnetizable layer is provided.

It is often desired that the dimensions, notably the axial dimensions, of electric machines be small. This is the case, for example, with electric motors which are used in playback apparatuses. It is known per se to construct such motors with a rotor which comprises a disc-shaped support in which coils are incorporated. If such a disc-shaped carrier is present and the magnetizable layer is provided on the peripheral surface thereof, the detector may be placed opposite to said peripheral surface, as a result of which the axial dimensions of the motor can be kept small.

It is highly desirable that the surface on which the magnetizable layer is provided during manufacture exhibits no irregularities. It has been found in practice that it is simpler to make the peripheral surface of a rotor disc smooth and uniform than the side faces of such a rotor disc.

An embodiment of the invention will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawing, in which Figure 1 is a perspective view of the embodiment in a partly disassembled condition.

Figure 2 is a simple block diagram of a speed control device for the embodiment shown in Figure 1.

The rotor 1 shown in Figure 1 forms a part of a controllable electric.motor which is not shown in its entirety. The rotor 1 is surrounded by a housing 3. A stator in the form of an annular permanent magnet 5 is present in the housing 3. The rotor 1 is in the form of a disc and comprises five coils 7 which are embedded in the non-magnetic material of which the major portion of the disc is made. In this embodiment the non-magnetic material is acryl butadiene styrene mixed with polycarbonate.

The wire ends 9 of the coils are connected to segments 11 of a commutator 13. The rotor 1 and the commutator 13 are connected to a motor shaft 15. In the operating condition the coils are energized in known manner by means of two brushes 17 and 19 which make contact with the successive segments 1 The brushes 17 and 1 9 are connected to a current source (not shown).

An annular magnetizable layer 23 is provided on the peripheral surface 21 of the rotor 1. The layer 23 consists of a magnetic lacquer of polyvinyl acetate and CrO2 particles and is magnetized so that a continuous flux pattern is present around the peripheral surface 21. A magnetic reproducing head 25, shown diagrammatically, is connected to the housing 3 opposite to the peripheral surface 21 and serves as a detector. The magnetic head 25 is connected electrically to an electronic control system 27 as is shown diagrammatically in the block diagram of Figure 2.

The control of the speed of the motor will be explained with reference to the block diagram.

When the motor is connected to a current source 29, an electric current flows through the coils 7 and the rotor 1 starts moving. A signal is continuously induced in the magnetic head 25 by the magnetic flux pattern generated by the magnetic lacquer 23. The control system 27 reacts to this signal by controlling the electric current supply to the coils 7 in such manner that the rotor 1 rotates at a uniform previously adjusted speed.

Claims (4)

1. An electric machine having a housing, a rotor and a stator, which machine includes a marker arranged to rotate with the rotor and a detector which is fixed with respect to the stator and is arranged to cooperate with the marker, characterized in that the marker is in the form of a ring-shaped magnetizable layer which is provided on a non-magnetic portion of the rotor and is magnetized so as to generate a magnetic flux pattern, the detector being arranged to detect the magnetic flux pattern.

2. An electric machine as claimed in Claim 1, characterized in that the material of the magnetizable layer is a dispersed magnetic particle mixture coated on the non-magnetic material of the rotor.

3. An electric machine as claimed in Claim 1 or 2, characterized in that the rotor is in the form of a disc on the peripheral surface of which the magnetizable layer is provided.

4. An electric machine substantially as described herein with reference to Fig. 1 or to Figs.

1 and 2 of the drawing.