GB2155565A

GB2155565A - Electromagnetic clutch with rotational speed detector

Info

Publication number: GB2155565A
Application number: GB08503069A
Authority: GB
Inventors: Per Carlo Boffelli
Original assignee: Baruffaldi Frizioni SpA
Current assignee: Baruffaldi Frizioni SpA
Priority date: 1984-02-07
Filing date: 1985-02-07
Publication date: 1985-09-25
Also published as: SE8500520L; DE3504193A1; FR2559222A1; GB8503069D0; SE8500520D0

Abstract

An electromagnetic clutch provided with electromagnetic means for measuring the rotational speed of the clutch rotor, is comprising at least one coil 17 which is integral with the fixed electromagnet 7a of the clutch and a device (a gap or a permanent magnet 19) integral with the rotor such that on rotation of the rotor an electromagnetic signal is generated which can be detected by the coil, with further electrical and electronic devices to activate mechanical means of protection and/or indication if the detected speed differs from normal operating values specified for the part driven by the rotor. The clutch may drive auxiliary apparatus, such as an air- conditioning compressor, in a car or a power take-off of a tractor. <IMAGE>

Description

SPECIFICATION Electromagnetic clutch with rotational speed detector This invention relates to electrogmagnetic clutches suitable for coupling rotatable parts to a drive component and concerns clutches for use with a belt or similar transmission in motor vehicles or the like.

Electromagnetic clutches are used in numerous applications for linking driven parts to a drive component, using a belt transmission or the like.

An example of such an application is in drives (engines) for motor vehicles, in which a belt links the drive shaft to various parts, such as the alternator, fan, power steering, air conditioning compressors and suchlike, many of which are fitted with electromagnetic clutches for engaging and disengaging the driven part with the drive belt.

In such an application breakage, damage to or simply loosening of the transmission belt or belts can cause stoppage of some or all of the connected parts, sometimes making impossible further operation of the entire vehicle within the requisite safety standards.

A problem then arises of displaying functional abnormalities of the transmission to enable action to be taken before the transmission belt is damaged irreparably or the part itself is damaged.

To this effect it is desirable to monitor the rotational speed of such parts as are connected to the transmission belt, or at least of one part, in order to register any deceleration, but well-known apparatus offered for this purpose is expensive and complex to install, and thus subject to difficult operating conditions inside the engine compartment, has limited reliability.

The purpose of this invention is to provide an electromagnetic clutch which contains suitable means for detecting the rotational speed of the driven component and for providing an alarm signal and/or automatic intervention of a protective system both of the driven part and the transmission, with a high degree of reliability.

According to the invention there is provided an electromagnetic clutch with a rotational speed detector device, suitable for coupling rotatable parts to a drive component by means of a belt transmission or the like in motor vehicles or the like, comprising a fixed annular electromagnet, a rotor coupled to the said transmission and connectable to a said rotatable part by means of a magnetizable anchor means and detector means for detecting the rotational drag of the said rotor, which detector means comprise at least one sensor means on said fixed electromagnet and generator means on said rotor to generate an electromagnetic signal which can be detected by the sensor means to provide an output signal dependent upon the rotational speed of said rotor.

This is achieved according to the invention with an electromagnetic clutch with a rotational speed measuring device, suitable for coupling driven parts to a drive component by means of a belt transmission or the like in motor-driven vehicles or the like, the clutch being provided with a fixed annular electromagnet, a rotor linked up to the said transmission and a magnetizable anchor for connecting the said parts to the rotor, electromagnetic devices being provided for detecting the rotational drag of the said rotor and comprising at least one sensor component solid with the fixed electromagnet and a device solid with the said rotor which generates a signal representative of the speed of the rotor that can be detected by the sensor, other electrical and electronic devices being provided for activation of automatic protection and/or signalling devices in the event of the detected value differing from normal operating values, including that relating to the speed of the drive which controls the said transmission.

One embodiment of the invention comprises the sensor component in the form of a coil mounted on the outer side surface of the electromagnet, with a corresponding recess on the inner side surface of the rotor, such that when the clutch is in operation the said coil detects changes in the magnetic flux produced by the electromagnet each time the said recess passes in front of it, as a result of changes in the air gap due to the recess, producing an electrical signal having a frequency equivalent to the rotational speed of the said rotor.

Another embodiment of the clutch comprises a coil solid with the electromagnet, and a corresponding permanent magnet on the said rotor, this coil registering a magnetic signal, independently of operation of the clutch, each time the permanent magnet passes in front of it during rotation, producing an electrical signal having a frequency equivalent to the rotational speed of the rotor.

In another embodiment of the invention, the sensor consists of the same electromagnet coil and there is a permanent magnet inside the said electromagnet, with a recess, aperture or similar in the rotor corresponding to the pole piece of the electromagnet, the said coil registering variations in magnetic flux each time this recess, aperture or similar passes in front of the permanent magnet.

In certain instances, to obtain signals of increased intensity, the said permanent magnet can also be replaced with an electromagnet, fed together with the clutch coil or independently of it.

As an option, there may also be a permanent magnet solid with the fixed electromagnet of the clutch, with a localized or annular coil connected to the fixed electromagnet, with a recess, aperture or similar on the rotor facing the permanent magnet.

In a further embodiment of the invention, there are one or more recesses on the outer side surface of the fixed electromagnet, with a similar number of corresponding recesses on the rotor, an electrical signal being produced that can be registered by this electromagnet coil or by an ancillary coil adjacent to it, each time the said recesses are facing. In the forms of construction described above, a permanent magnet may also be installed in the concavity of the rotor, affording a signal of higher intensity.

In the clutch according to the invention, a reflective optical sensor device can be inserted in the fixed electromagnet on the surface facing the rotor, with a reflective area on the rotor corresponding to the sensor device; this reflective optical sensor can, in this case, supply a signal detecting the rotation speed of the rotor.

In all of the above constructional forms this sensor device is connected to an electronic circuit to compare the said signal produced by the sensor device with a comparative reference value and to produce the intervention of alarm signals and/or protective devices if the said signal is found to be incompatible with this comparative value.

This reference value in the electronic circuit can be a fixed threshold value, corresponding to the minimum admissible rotational speed, or one or more pairs of values, defining the normal operating range of the parts to which the clutch is linked, or it may be a signal produced by a device detecting the revolutions of the said drive component, generating an alarm signal and/or activation of automatic protection devices if the roational speed of the rotor differs from the rotational speed appropriate to the said drive component, taking into account the transmission ratio, or a speed proportional to this, or if it is found to be incompatible.

A preferred form of construction for the said clutch is used to link ancillary parts in motor vehicles to the engine of same by means of a belt transmission or similar, the said parts comprising the cooling fan, power steering pump, compressor for the air conditioning unit, etc.

The clutch according to the invention can be used to link the air conditioning compressor and the said automatic protection devices controlling disengagement of the clutch itself in the event of shutdown of the compressor, thus protecting the transmission belt from slipping, and hence ensuring operation of the other parts connected up to it.

The device according to the invention, both for detecting absolute speed and relative speed in regard to the drive, can also be applied in other cases where it is neessary to safeguard a branched component when in abnormal conditions, e.g. in an agricultural machine where tools work in the ground, mechanical conveyors and tractor power takeoffs.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows the connection of ancillary driven parts to the drive shaft of a motor vehicle engine; Figure 2 shows one embodiment of a clutch according to the invention, in section on the line ll-ll of Fig. 3; Figure 3 is a sectional view on the line III 111 of Fig. 2; Figure 4 shows another embodiment of a clutch according to the invention taken on the line IV-IV of Fig. 5; Figure 5 is a sectional view on the line V-V of Fig 4; Figure 6 shows another embodiment of a clutch according to the invention, in section on the line VI-VI of Fig. 7; Figure 7 is a sectional view on the line VII VII of Fig. 6;; Figure 8 is a sectional view on the line VI Il- Vlil of Fig. 6; Figure 9 shows another embodiment of a clutch according to the invention, in section as per plan IX-IX of Fig. 10; Figure 10 is a sectional view on the line X X of Fig. 9; Figure 11 is a sectional view on the line XI Xl of Fig. 9; Figure 1 2 shows a further embodiment of the invention, in section on the line XII-XII of Fig. 13; Figure 1 3 is a sectional view on the line XIII-XIII of Fig. 12; Figure 14 is a sectional view on the line XIV-XIV of Fig. 12; Figure 1 5 is a diagram of an electronic control device; Figure 1 6 is a diagram of an electronic comparator and control device; ; Figure 1 7 shows parts fitted with a clutch according to the invention; Figure 18 is a sectional view on the line XVIII-XVIII of Fig. 17; Figure 1 9 is a sectional view on the line IXX-IXX of Fig. 17; Figures 20, 21 show two different angle positions of the anchor and rotor; Figure 22 is a control circuit diagram.

As can be seen from Fig. 1, various ancillary parts of a motor vehicle engine are linked up to a driving pulley 1, activated by the drive shaft, by means of a transmission belt 2. These parts can be, for example, an alternator 3, a cooling fan 4, a power steering pump 5 and an air conditioner compressor 6.

Some of these parts are fitted with an electromagnetic clutch for engaging, for example, the air conditioning compressor, the power steering pump and the engine cooling fan.

The use of a clutch according to the invention can provide detection of operating faults in the transmission, i.e. detecting shutdown or deceleration of a pulley, hence providing an alarm signal or automatic disengagement command.

One embodiment of a clutch according to the invention is illustrated in Figs. 2 and 3.

An electromagnet 7 is fixedly secured to a fixed structure on the vehicle 8, such as, for example, the body of the compressor or the like, by means of screws 9. A rotor 10, constituting the pulley for the transmission belt, rotates around the electromagnet 7. The rotor 10 is supported by a bearing 11 and can be coupled to a shaft 1 2 of the driven parts by means of a coupling means or anchor 13, which is held against rotor 10 by the action of the magnetic field of the electromagnet 7 and linked to shaft 1 2 by means of elastic strips 14 and flange 1 5.

A recess 1 6 is provided in the rotor and a corresponding recess 1 7 in electromagnet 7, as can be seen more clearly in Fig. 3. To detect the rotational speed of the rotor 10, a measuring coil 18 is provided in the recess 1 7. Discontinuity of the air gap due to the recess 1 6 produces a variation in the magnetic flux produced by coil 7a of electromagnet 7 and when recess 1 6 passes in front of the measuring coil 18, this causes an electrical signal of the same frequency as the rotational speed of the rotor 10, which can therefore be used to measure this rotational speed.

Hence, when the clutch is engaged, i.e.

when coil 7a is energised, an indication of the rotational speed of rotor 10 can be obtained, and a signal indicative of any shutdown in the parts connected to the clutch, due to rupture or deceleration of the transmission belt 2 or to stoppage of the part itself, due to seizure, for example. The signal thus produced can then be used, for example, to activate automatic disengagement of the clutch or an alarm signal.

Figs. 4 and 5 show another embodiment of the invention in which a permanent magnet 1 9 is provided in recess 1 6.

When the magnet 1 9 passes circuit 18, it generates a current in the latter even when electromagnet 7 is not energised, such that the rotational speed of rotor 10 can be detected in any conditions.

Hence it is sufficient for only one of the parts driven by belt 2 to be fitted with the clutch according to the invention, to obtain an indication of a disturbance or fault in the belt transmission itself in all instances.

Figs. 6, 7 and 8 show another form of construction of the clutch in which the magnet 1 9 is positioned on the front, or side wall, of rotor 10 and coil 1 8 faces it.

Further forms of construction are possible; and Figs. 12, 1 3 and 14 show an option with a permanent magnet 1 9a mounted inside the fixed electromagnet 7 and in front of it there is a detecting coil 1 8a mounted near rotor 1 0, which, as illustrated in the figure, can be annular, similar to coil 7a, albeit of reduced dimensions, or a localized coil, as coil 1 8 illustrated in Fig. 7.

On rotor 10, there is a recess 16, throughhole or machining which gives rise to a localized discontinuity in the air gap; this recess or similar can be at the front, as illustrated, or on the periphery to provide an optimum signal detectable by coil 1 8a in relation to the form of electromagnet 7.

This construction avoids the need to fix a permanent magnet to rotor 10, which is subject to heavy dynamic stresses in view of the high rotational speed, and it can be linked up to the fixed electromagnet 7 affording greater constructional stability.

Figs. 9, 10 and 11 show a particularly convenient form of construction of a clutch according to the invention, in which a permanent magnet 1 9a is positioned inside electromagnet 7 and a recess 1 6 or similar is provided on rotor 10 at the front, or a recess 16', as illustrated by a broken line in Figs. 9 and 10. The detecting coil in this case is the same coil 7a of electromagnet 7.

This affords a simplified construction, since disturbances in the electromagnetic field due to recess 1 6 passing in front of the pole pieces of electromagnet 7 in the area in which the field produced locally by permanent magnet 1 9a is directed, can be detected directly by coil 7a, to which an electronic control circuit is connected, with insertion where required of a decoupling condenser.

To obtain a signal of greater intensity where necessary, magnet 1 9a, inserted in fixed electromagnet 7, can be replaced by an electromagnet, fed together with electromagnet 7 or independently of it. In addition, the rotors illustrated in Figs. 4 to 7, fitted with permanent magnets, can be used with the construction illustrated in Figs. 9 to 14, with satisfactory results.

A further construction option, which is not illustrated in the figures, provides for one or more recesses on rotor 10 with a corresponding number on electromagnet 7; detection of the signal can then be made with an auxiliary annular or localized coil 1 8 or 1 8a, or with the same coil 7a of the electromagnet. For some applications it is also possible to insert various sensor devices in electromagnet 7, such as, for example, an optical reflection sensor, which can recognize an area on the rotor with particular reflective properties, such as, for example, a mirror surface, an unpainted section, a hole or the like; in this case a sensor device located in a protected position within the clutch can detect the rotational speed.

Fig. 1 5 shows a diagram of an electronic detector for the automatic disengagement control of the coil of electromagnet 7.

The signal produced by detector coil 18, 1 8a, on passage of magnet 1 9 or recess 16, is amplified in amplifier 20 and coupled to a comparator 21 which transforms it into a sawtooth signal and feeds it to a comparator 22.

In comparator 22 this signal is compared with a predetermined threshold signal, and if a value is found corresponding to too low a speed of rotor 10, a fault signal is generated which controls the opening of a power transistor 23 which directly feeds clutch coil 7a, or triggers an alarm or emergency signal.

A timer 24 is provided for integration of the fault signal produced by the comparator 22 in the event of the speed of rotor 10 falling below the minimum specified value, to maintain transistor 23 in conduction for a predetermined period, including when there is a fault signal produced by comparator 22, such that the clutch is disengaged only if the said signal is extended, thus affording normal engagement for start up.

Fig. 1 6 shows a block diagram of a device which affords an alarm signal in the event that the rotational speed of rotor 10 on one of the clutch systems of the connected parts on belt 2 is lower than that of the drive pulley (or that applicable with the existing transmission ratio).

This affords opportune display of slip in the transmission belt 2, due to deceleration, for example.

To this effect, a signal produced by a drive revolution detector 25 is compared with the signal produced by measuring coil 18. These signals are filtered and made compatable by filtering circuits 26 and 27 respectively, amplified by amplifiers 28 and 29, and then passed on to the summing amplifier 30.

The summing amplifier 30 carries out a comparison between the signals received, and provides an output signal to comparator 31 which controls the activation of an alarm signalling device when the difference between the number of revolutions of the engine and that of pulley 1 is higher than a minimum threshold value, indicating unexpected slip of belt 2.

Hence it is sufficient for just one of the clutches on the said ancillary parts to be constructed according to the invention, as in each case there is opportune indication of malfunction of the transmission belt.

By disengaging the clutch which gave rise to the alarm, it can be determined immediately whether the breakdown was due to unexpected shutdown or stoppage of the driven parts, as can occur following seizure of a compressor, or to deceleration or rupture of the belt. In the first instance, on disengagement of rotor 10, which is no longer restrained, regular rotation can resume, thus improving durability of the belt and operation of the other components; in the case of deceleration or rupture of the belt, however, there is no transmission and to resume operation repairs must be carried out.

In some cases, for example applying a clutch according to the invention to parts operating in a restricted speed range, the clutch control device can be used for controlling automatic disengagement outside the optimum speed range for correct operation, for example at too low or too high a speed.

It is also possible to create corresponding notches or discontinuities on the opposed anchor and rotor surfaces, such that, in the event of slippage between the rotor and the anchor, due to blockage of the driven component, there is a periodic variation in the air gap on the clutch magnetic circuit, due to the periodic passage of these notches.

These air gap variations can be detected by an auxiliary coil or by the coil which generates the magnetic field, and the corresponding signal can then be used to activate disengagement of the clutch, via a control circuit. As can be seen from Figs. 1 7 and 18, the clutch is equipped with an anchor 105 of ferromagnetic material, which is braze-welded 111 with similar braze-welding 11 2 on the rotor 106, also of ferromagnetic material, to interrupt the magnetic flux and stiffen the structure. Similar results can also be obtained with slots and spokes created by punching. Inside the magnetic core 110 there is an ancillary coil 113, linked with the same magnetic flux produced by coil 109.On the corresponding facing surfaces of ring 105 and pulley rotor 106, as shown in Figs. 1 8 and 19, there are radial recesses, 114 and 11 5 respectively, to create surface discontinuity. In the event of relative slippage between anchor 105 and rotor 106, these recesses cause variations in the magnetic flux produced by coil 109. In fact, as shown by Figs. 20 and 21, when recesses 114 are opposite recesses 11 5, the pole surface is reduced by the air in the said recesses, whereas when recesses 114 and 11 5 are not facing, the pole surface is reduced by the sum of the air in recesses 114 plus the air in recesses 11 5.

Hence, in the event of slippage between ring 105 and pulley 106, there are periodic variations in the reluctance of the magnetic circuit and hence in the magnetic flux.

These variations thus cause a current in auxiliary coil 113, which can control an automatic electronic protection circuit, to activate disengagement of the clutch in the event of the slippage being greater than a predetermined value. This circuit is illustrated in Fig.

22, which shows the activation circuit for the connected parts, comprising a start-up switch 11 6, which cuts the supply to coil 109 via an external command.

On the supply to coil 109, there is a protection circuit 117, which, on receipt of a signal from auxiliary coil 11 3, cuts-off the electrical supply to coil 109, so that pulley 106 idles.

The protection circuit 11 7 comprises a power transistor 118, which controls the clutch supply coil 109.

This transistor 118 is controlled by a storage circuit 119, which receives a signal from comparator circuit 1 20.

This comparator, fed by the negative voltage generator 121, compares the signal supplied by auxiliary coil 113, which is amplified by amplifier 122, with a predetermined threshold value.

On start-up of the connected parts, switch contact 11 6 cuts-off; transistor 11 8 is then in conduction mode, coil 109 is fed and draws ring or anchor 105 against pulley rotor 106, deflecting elastic ring 104, thus initiating rotation of component 101.The storage circuit 11 9 maintains transistor 11 8 in conduction, in accordance with a signal emitted by comparator 120; in the event of slippage of anchor 105 on pulley 106, the electromotive force generated by auxiliary coil 113, which is proportional to the relative speed between anchor 105 and pulley 106, is compared with the predetermined threshold signal and, if it is higher than this value, comparator 1 20 sends a signal to memory 11 9 which determines opening of transistor 1 18, cutting-off current to coil 109.

Since, in the start-up phase, there is a brief period of relative slippage between anchor 105 and rotor 106, due to start-up inertia in component 101, a cut-off signal must be prevented from being applied to transistor 11 8 in this period. To this effect there is a time-delay circuit 123, which, after cut-off of thermostat circuit 116, prevents the signal from comparator 1 20 from being sent to the memory 11 9 to activate opening of transistor 11 8, for a given period.

It is, however, possible to have a protective device which prevents or reduces rupture or wear on the engine transmission belt; to this effect, the clutch is so dimensioned that there is slippage with torque values lower than the maximum torque that can be transmitted by the transmission belt, hence there is slippage in the clutch, which is able to withstand this for a brief period and activates consequent intervention of the protective device, but never between the belt and pulley. By taking a signal from coil 109, as shown in Fig. 22 by a chain dotted line 124, and with a suitably arranged amplifier 122, it is also possible to avoid the use of auxiliary coil 113, using instead the voltage oscillations in coil 109 produced by slippage of anchor 105 to control disengagement.If the sensitivity of the electronic circuit so permits, recesses 11 4 and 11 5 can also be avoided, generating an electromotive force in a coil linked to the magnetic flux in the event of slippage, following an increase in air gap due to the roughness of the facing surfaces sliding against each other, forming powders of abrasive material and such like. Circuit 11 7 can also be equipped with a device which activates a fault signal or similar on the vehicle's dashboard in the event of intervention of the clutch disengagement device.

Numerous applications can be found in addition to those illustrated for ancillary parts on car engines.

As regards requisite applications and con structional problems, and usage connected with this, numerous options can be introduced without going outside the scope of this invention in terms of its general characteristics.

Claims

1. An electromagnetic clutch with a rotational speed detector device, suitable for coupling rotatable parts to a drive component by means of a belt transmission or the like in motor vehicles or the like, comprising a fixed annular electromagnet, a rotor coupled to the said transmission and connectable to a said rotatable part by means of a magnetizable anchor means and detector means for detecting the rotational drag of the said rotor, which detector means comprise at least one sensor means on said fixed electromagnet and generator means on said rotor to generate an electromagnetic signal which can be detected by the sensor means to provide an output signal dependent upon the rotational speed of said rotor.

2. An electromagnetic clutch according to claim 1, wherein said detector means further comprises control means having an input coupled to receive said sensor output signal and an output coupled to an automatic electric protection means arranged to disengage said clutch in the presence of said sensor output signal.

3. An electromagnetic clutch according to claim 2, wherein the output of said control means is further coupled to an indicator means for indicating a drive fault in the presence of said sensor output signal.

4. An electromagnetic clutch according to claim 1, further comprising control means having an input coupled to receive said sensor output signal and an output coupled to an indicator means for indicating a drive fault in the presence of said sensor output signal.

5. A clutch according to claim 2 or 3, wherein the control means controls actuation of the said automatic protection means when the sensor output signal representative of the detected speed of the rotor deviates from a normal operating value.

6. A clutch according to any one of claims 1 to 5, wherein said sensor means comprises a coil located on the outer surface of the electromagnet, and the generator means comprises a corresponding recess on the inner surface of the said rotor, said coil being arranged to detect variations in magnetic flux produced by the electromagnet when the clutch is in operation each time the recess passes said coil as a result of variations in the air gap due to said recess, to provide said electrical output signal having a frequency equivalent to the rotational speed of the rotor.

7. A clutch according to any one of claims 1 to 5, wherein said sensor means comprises a coil integral with the electromagnet, and said generator means comprises a corresponding magnet on the rotor, said coil being arranged to detect a magnetic signal, independently of the operation of the clutch, each time said magnet passes it during rotation of said rotor, producing said electrical signal having a frequency equivalent to the rotational speed of the rotor.

8. A clutch according to claim 7, wherein said magnet is a permanent magnet.

9. A clutch according to claim 7, wherein said magnet is an electromagnet, energised with the clutch coil or independently of it.

10. A clutch according to any one of claims 1 to 5, wherein said sensor means comprises the same electromagnet coil and there is a permanent magnet in this electromagnet, with a recess, aperture or the like constructed in the rotor, corresponding to the pole piece of the electromagnet, this coil detecting a variation in magnetic flux each time the said recess, aperture or similar passes in front of the permanent magnet.

11. A clutch according to any one of claims 1 to 5, wherein the sensor means comprises a permanent magnet integral with the fixed electromagnet of the clutch, with an annular or localized coil on the fixed electromagnet, and the generator means comprises a recess, aperture or the like on the rotor opposite the permanent magnet.

1 2. A clutch according to any one of claims 1 to 5, wherein there is at least one recess on the outer surface of the fixed electromagnet, with a similar number of corresponding recesses on the rotor, producing an electrical signal that can be detected by the electromagnet coil or by an auxiliary coil adjacent to it, each time the recesses face each other.

13. A clutch according to claim 6, 10, 11 or 12, wherein a permanent magnet is inserted in the rotor recess.

14. A clutch according to any one of the preceding claims, wherein an optical reflection sensor device is mounted in the fixed electromagnet on the surface opposite the rotor, with a reflective area on the rotor corresponding to the sensor device.

1 5. A clutch according to any one of the preceding claims, wherein said sensor means is linked to an electronic circuit to compare the signal produced by the sensor means with one or more predetermined comparative values and to cause actuation of alarm signals and/or protective devices if this signal is incompatible with the said comparative value or values.

1 6. A clutch according to claim 15, wherein said reference value of the electronic circuit is a fixed threshold value, corresponding to the minimum admissible rotational speed, or one or more pairs of values defining the normal operating range of the parts to which the clutch is connected.

1 7. A clutch according to claim 15, wherein said reference value of the electronic circuit is a signal produced by a revolution detector device on the drive component, generating an alarm signal and/or an actuation signal for automatic protection devices if the rotational speed of the rotor differs from the appropriate rotational speed for the drive component, taking into account the transmission ratio, or a speed proportional to this, or if they are incompatable.

1 8. A clutch according to any one of claims 1 to 5, wherein the sensor means solid with the fixed electromagnet is the actuating coil of the electromagnet, or an annular auxiliary coil adjacent to it, and the generator means, which generates a signal that can be detected by the sensor, consists of a plurality of corresponding notches or discontinuities on the facing surface of the magnetizable anchor and rotor, producing periodic variations in the magnetic field, due to variations in the air gap, in the event of relative slippage, the said sensor means activating protective devices when such slippage is registered.

1 9. A clutch according to claim 18, fed by an electronic circuit comprising a protective device consisting of a power transistor supplying the electromagnet, powered by a circuit generating an opening signal when the auxiliary coil supplies a voltage higher than a predetermined value to a comparator circuit linked to it.

20. A clutch according to claim 19, wherein said electronic circuit comprises an activation time-delay component.

21. A clutch according to any one of the preceding claims, arranged to link auxiliary motor vehicle parts to the engine by means of a belt transmission or the like, said parts comprising at least one of the cooling fan, power steering pump and air conditioning compressor unit.

22. A clutch according to claim 24, linked to the air conditioning compressor and the said automatic protection devices cause disengagement of the clutch in the event of shutdown of the compressor, protecting the transmission belt from slippage and allowing operation of the other parts linked up to it and activation of a fault signal as appropriate.

23. An electromagnetic clutch substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.