GB1578025A - Electric motors - Google Patents

Description

(54) IMPROVEMENTS IN AND RELATING TO ELECTRIC MOTORS (71) We, THE METTOY COM PANY LIMITED, a British Company of 14 Harlestone Road, Northampton NNS 7AF, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates to electric motors and more particularly to an improvement in or modifications of the electric motor described and claimed in our application No.

18389/72 (Serial No. 1,434,192).

We have found that in some constructions of motors difficulty is encountered in starting rotation in the correct direction. We have now discovered that this effect can be overcome by ensuring that there is a hysteresis which must be provided.

The present invention provides an electric motor according to any one of the claims of our Patent No. 1,434,192 wherein the means arranged to respond to the poles of the rotor during rotation is replaced by a commutating means including means responsive to rotation of the rotor which have inherent hysteresis, or a circuit exhibiting hysteresis is connected between the rotation responsive means and the means for controlling energisation of the coil.

Features and advantages of the present invention will become apparent from the following description of an embodiment thereof given by way of example when taken in conjunction with the accompanying drawings, in which: Figure 1 is a partly simplified plan view of a combined gramophone turntable and motor, with a portion of the turntable removed; Figure 2 is a section on the line 2-2 of Figure 1; Figure 3 is a block diagram of a motor control system; Figure 4 is a more detailed circuit diagram of a motor control system; Figure 5 is a magnetic energy waveform diagram pertaining to the operation of the complete gramophone; and Figure 6 is another magnetic energy waveform diagram pertaining to the operation of the complete gramophone.

A gramophone turntable and motor assembly is shown in Figures 1 and 2. The assembly includes a support panel or deck 10, on which the turntable 11 is rotatably mounted. The deck has a circular recessed portion 10a in the centre of which is a boss 13; the boss has a blind bore in which fits a shaft 14. The turntable is mounted for rotation by being attached to a further boss 15 which is supported by and rotates on the upper end of the shaft 14 extending from the boss 13. An extension 15a of the boss 15 projects through a hole in the centre of the turntable 11 and a bearing ball is received in the extension 15a between the upper end of the shaft 14 and the end of the bore in the extension.

The boss 15 can be made from a material such as a polytetrafluoroethylene coated acetal resin; such a bearing requires no lubrication and can be injection moulded.

The turntable comprises a generally flat upper surface having a circular recessed portion lla in the centre and a depending peripheral portion llb. A mat 16 is placed on the flat upper surface of the turntable, the extension 1Sa projecting through a hole in the mat mat 16 and forming the spindle of the turntable. The mat 16 is shown in Figure 2 to have a central annular projection 16a which is received in the recessed portion 1 lea in the turntable when records having the standard small central hole are to be played. However, if it is wished to play records having a larger central hole, the mat 16 is turned over and the annular projection 16a is used to centre the record on the turntable.

The turntable 11 is retained in poistion by a number of resilient latching arms 18 only one of which is shown in the drawings. The ends of the arms 18 are barbed and located such that if the turntable 11 and attached boss 15 are lifted vertically upwards the barbed ends of the arms 18 engage a shoulder 19 round the exterior of the boss 13. The turntable can he removed from the deck by inserting a suitable device through apertures 20 in the recessed portion 10a to move the arms 18 away from the latching position.

The turntable and deck can both be formed of moulded plastics material, but in the present case the turntable is made of metal to give it a higher moment of inertia and hence make it rotate more smoothly.

The depending peripheral portion 1 1h of the turntable 11 carries a ring 24 of magnetic material. The preferred material for ring 24 is a ferrite material bonded by a suitable plastic, applied as a strip to the inner surface of the periphery of the turntable. The ring can be accommodated in a recess, not shown, at this point. It is also possible, if the turntable is moulded for the ferrite to be moulded in or integrally with the turntable. In some circumstances when a non-magnetic material is used for the turntable it may be preferable or desirable to include an outer ring 21, shown in broken lines in Figure 2 od high permeability magnetic material, to provide a flux return path.

The ferrite material chosen has a high retentivity and is permanently magnetised in a direction to present a series of magnetic pole faces exposed on the inner surface of the ring of material and alternatively of opposite polarity, and cooperating with a stator magnetic assembly indicated generally at 22 in Figure 1. This magnetic assembly includes two similar pole members 23, each as described in detail in our main case Serial No. 1,434,192.

One pole member 23 is attached to each end of a coil 25 such that the fingers of one pole piece interdigitate with the fingers of the other pole member 23. The pole members are attached to the coil by means of a screw which also fixes the thus formed stator to the recess 10a in the deck 10.

The series of outwardly facing pole pieces will be of alternate polarity when an energising current is passed through the coil 25. The peripheral extent of each such pole piece corresponds to the peripheral extent of the magnetic pole faces formed on the ferrite mag net ring 24. Other ways of producing the stator pole faces can be employed.

If desired, the pole members 23 can extend completely around the periphery of the turn table or they can be reduced to a number of discrete sections, each less than 900; the section shown in Figure 1 occupies about 60".

There is some advantage if an even number of such sections is used, to give a more balanced drive to the turntable, but the use of a single element as shown has the advantage of economy and has been found to work satisfactorily.

Means are provided, responding to the rota tion of the turntable, to derive an electrical signal. A variety of devices can be used for this purpose, such as a reed switch as in our main application, a Hall effect probe, a magnetoresistive element or contacts can be used. However, a convenient and effective method of deriving the desired electrical signal is by means of an optical system including a light source and sensor with apertures or reflective portion on the turntable rim supported on a sub-plate 29 attached to the bottom of recess 10a. The optical system shown comprises a light emitting device 28 and a light sensitive device 30 for sensing light transmitted through one or more apertures in the turntable rim. The device 30 is housed in a tubular member 30a which projects through an aperture 10b in the recess 10a and the open end of which faces the apertures in the turntable rim. The light emitting device 28 is housed in a corresponding tubular housing 28a mounted to emit its light through an aperture 10c in the recess 10a toward the turntable rim.

Means are provides, for example by means of a screw moving in an aperture 29a in the bottom of the recess 10a, to allow the position od the optical system on installation to be adjusted relative to the apertures on the turntable rim. The optical system senses the rotation of the rotor and is associated with a circuit controlling a supply d.c. in order to produce driving current for the motor stator. A suitable control system for producing driving pulses from a direct current supply e.g. dry cells is indicated in Figure 3. In this type of control, means are provided for operating the motor in a manner which provides a uniform speed of rotation the speed being controllable.

The arrangement includes a timing circuit 40, which feeds a timed signal to a comparator 41 which also receives a signal from the rotor position sensor 42, such as the optical system 28,30. The comparator output, with suitable amplification at 43, energises the operating coil of the motor with direct current pulses.

A feed-back signal from the motor is applied, over conductor 44, to the timing circuit to reset it. There will be a functional relationship, indicated by the broken line 45, from the motor drive to the rotor position sensor 42.

The timing circuit defines a reference time period which can be identified as Tr. The timing circuit is reset each time the current to the stator is reversed. The rotor position sensor will develop, a signal of period Ts which is indicative of rotor position. In the comparator circuit 41, if the period Ts is greater than the period Tr, the next reversal of motor current will occur on the switching of the commutator.

This will continue to be the case until the time interval Ts is equal to Tr, whereupon the timing circuit conrols the current reversals.

By this operation, the motor will run up from rest to a constant speed condition. If there is a change of speed from this uniform speed, for example due to change of load on the turntable, there will be a change in the relative timing of the rotor sensor pulses, and the circuit will operate in a corrective sense to bring the rotor speed back to the desired controlled value.

With the operation described, the rotor shows no preferred direction of rotation, and could run in either direction. In order to impose a preferential direction, the optical system or the reflective portions or slots can be positioned slightly to one side of a pole position, and there is then a strong preferential direction, to the extent of inhibiting reverse rotation.

It can also be arranged that the rotor will always come to rest in a position, with respect to the stator, such that a starting torque, in the desired direction, will be produced when the motor is energised. This can take the form of a positioning detent device.

One form of the circuit shown in block schematic form in Figure 3 is given in Figure 4. The operation of this circuit is as follows.

The timing period Tr is set by the charging of a 0.22,uF capacitor via one of two potentiometers. The capacitor and potentiometers are fed from two antiphase voltage sources. The value of the selected potentiometer is varied to set the running speed. The instantaneous value of the voltage at the junction of the capacitor and selected potentiometer is amplified by a differential input amplifier (1).

Amplifier 1 in this circuit is assumed to be one quarter of a quad IC package of low input impedance operational amplifiers. e.g. type MC 3401 P or LM 3900 N, chosen because they are relatively inexpensive. The current flowing through the commutator, here taken to be a light sensitive resistor (photocell), is compared with that from a reference current source, here shown compensated for variations in battery voltage by connection to the light emitting diode 28, on the inputs of amplifier 3.

This is given positive feedback, i.e. is electronic hysteresis, via a 100 K Q resistor. The square wave output of amplifier 3 is compared with the voltage from amplifier 1 at comparator 2.

The square wave output of the comparator is current amplified by transistors to drive the coil. Amplifier 4 inverts the output of amplifier 2 to provide one voltage source for the timing circuit. The other antiphase voltage source comes from the transistor output stage.

Suffice to say, the optical system does not have any inherent hysteresis and therefore this is provided by the amplifier 3 and the feedback resistor from its output to its non-inverting input. The amplifier 1 and associated input elements forms the timing circuit and the amplifier 2 forms the comparator.

In order to better understand why hysteresis is necessary attention is directed to Fig. 5 which discloses the case of a position sensor not having hysteresis. Diagram 5(a) shows the two magnetic elements of the motor, the upper reperesenting the rotor and the lower represent ing the stator coil energised by DC current.

With the rotor and stator poles as shown in Fig. 5(a), small displacements of the rotor will result in the rotor returning to its initial position due to the attractive force between opposite poles. Such a position is termed hereinafter minimum magnetic potential energy. Conversely, when the rotor is moved such that a north pole on the stator is aligned with a north pole on the rotor, small displacements of the rotor from this initial position will result in the rotor moving to a position with unlike poles of the rotor and stator aligned. Such an initial position is termed hereinafter maximum magnetic potential energy.

Diagram 5(b) is a plot of the magnetic potential energy as the moving rotor is passed over the stator. This waveform may be more nearly sinusoidal in shape in a practical motor, but in order to demonstrate the principals and quantify the starting performance, it is more conveniently shown as a serrasoid. Tests on the above gramophone motor have shown that many of the effects observed on starting may be explained by this simplified model. The minima correspond to the stable condition with opposite poles aligned. The maxima are unstable positions with like poles aligned. Thus if the rotor is placed at position a and released it will move from left to right, while if at "b" it will move from right to left toward the position of minimum potential energy at "c". In diagram 5(b) it is assumed that no commutation of the stator takes place and so the rotor will come to rest in position "c".

Diagram 5(d) shows the waveform of the magnetic energy if commutation takes place at positions ';x" on diagram 5(a). It is assumed that the commutator has been set to give equal length "on" and "off" states in response to the moving rotor, i.e. 1:41 mark/space ratio. Referring now to the optically commutated motor, a convention has been chosen in which the effect of a commutation is to invert the waveform of the magnetic energy by switching the stator current, when the photocell senses a light intensity less than the threshold value.

The phase relationship of the light intensity 5\(c) may also be varied with respect to the magnetic strip i.e. similar to the sector segment in the main application.

Diagram 5(d) shows the commutated waveform of the magnetic potential energy. The overall effect of this is to drive to the right, in this convention called forward, but only if the turntable happens to start off with the relationship between magnetic strip and drive coil corresponding to the heavily lined region since only from here will the turntable gain sufficient momentum to overcome the shorter region of increasing magnetic potential energy which it will experience after commutation. In all other initial positions it may be seen that the turntable will merely oscillate backwards and forwards about the same commutation point and will not drive forward.

Diagram 3(e) shows the effect on the magnetic potential energy waveform of advancing the commutation points to the maxima and minima i.e. adjusting the phase of the sector segment in the main application. In principle, with a commutating device without hysteresis the motor will start every time, but this phase relationship using such a commutator will not give good running performance since in a real motor there will be inaccuracies e.g. run-out of the tumtable, errors in position of the strip of permanent magnetic material etc. which will cause errors in the commutator switching point.

If the commutator is set to operate at the 0 and 180 phase positions on the magnetic potential energy waveform for one region of the strip, then at another region these inaccuracies may cause the commutation to be after 1800. The overall effect is that during running d the motor the electronics will trans èr control from the timing circuit to the commutator. Thus the motor will drop out of synchronism and wow and flutter will be greatly increased. Consequently in a real motor, setting the commutator at or very close to the 0 and 180 points is not desirable.

Normally the phase angle of the commutator will be set lower so that such non-idealities can be accommodated without degrading the speed regulation of the motor as in Figure 5(d). Thus in a motor with a zero hysteresis commutator the requirement for good speed regulation implies imperfect starting performance.

Turning now to Figure 6, Figures 6(a), 6(c) and 6(d) depict the various magnetic potential energy waveforms for the optical commutator with electronic hysteresis, biased to switch at F800 intervals of the waveform.

The light intensity experienced by the photocell is shown in 6(b) in which the switching points with arrows to the right are for forward motion, and with the arrows to the left are for backward motion. Thus as we proceed through waveform 6(a) and respond to the appropriate commutation points, forward movement gives magnetic energy diagram 6(c) and backward movement 6(d).

Inspecting 6(c) we see that starting anywhere between a and b on the wave results in smooth forward drive since the rotor will gain sufficient energy to overcome the short interval of increasing potential energy after commutation.

Starting from region g is more complicated.

Following the arrowed line, the motion is initially forward, the motor commutates and the waveform moves into region g2. However, the turntable will not have gained sufficient energy to overcome the energy barrier and will fall back. As it does so we must now inspect the waveform for backward motion 6(d). The turntable moves down the energy slope from right to left, through a second commatation point (baclrwards). The turntable gains insufficient energy to surmount the energy barrier for backward motion so it comes to rest, then moves forwards. We now inspect the waveform for forward motion, 6(c), and find that the turntable is now in the region between a and b, so it starts and continues to run forwards. Region g1 has the figure "2" above it to show it starts forward and passes the original starting point after 2 commutations (1 forwards, 1 backwards). Similarly region g2 has 2 commutations (1 backwards, 1 for wards) before passing the original starting point moving forwards. The only other possibility is that the turntable starts off in the region g,, with the commutator set for backward motion. We see that the turntable will start forward after 1 backward commutation.

Thus, in the absence of friction, the turntable will always eventually run forward. This should be compared with the zero hysteresis situatiols, Figure 5(d) which will only start over part of its cycle.

Figure 6 has assumed a 1:1 mark/space ratio, but the bias on the commutator can be adjusted to give unequal mark/space ratio, i.e.

switching at intervals that are not 180 apart on the magnetic potential energy waveform.

It can be shown that the turntable will always eventually run forward though up to five commutations can occur before the turntable passes the starting point running smoothly forward. For comparison, a commutator with no hysteresis and an unequal mark/space ratio has only small regions which will result in forward motion. Thus the presence of hysteresis is again providing eventual smooth forward running from any starting position.

If the commutation point for backward motion occurs before 90 , the turntable will run either forwards or backwards.

The effect of friction on the starting characteristics is as follows. For a zero hysteresis commutator, the extent of the starting regions is reduced since not only is there the succeed ing energy barrier to surmount, but additional work must be done to overcome friction.

For the commutator with hysteresis, the situation is rather different. It can be shown that, in the starting condition, as the turntable passes through first one backward and then one forward commutation, its gain in momentum is proportional to the magnitude of the hysteresis in the commutator. This gain in momentum is available to do work and if this work is greater than that necessary to overcome frictional effects, then the motor will always start.

In the motor described above which is designed as a gramophone motor, the work that is made available may be used to overcome the friction of the playing arm and stylus, friction in the bearings etc, and is not available from a motor commutated by a zero-hysteresis switch. Hysteresis thus helps to overcome the effects of friction and, that up to a limit, the wave hysteresis the better is the starting performance.

Thus, using a model based on a serrasoid magnetic potential energy waveform, a motor commutated by a sensor with hysteresis, whether inherent or added electronically, will always start even where there is some friction.

Without hysteresis the motor will not always start when there is friction. In addition, the motor will tolerate some inequality in the mark/space ratio; without hysteresis it will not always start in these conditions.

Further the motor will tolerate some inaccuracy in the phase setting; without hysteresis it will not always start in these conditions. In a real motor, as stated before, the magnetic potential energy waveform is likely to tube more sinusoidal in shape. Whilst this will give rise to regions where the driving force on the turntable is low and the starting not always assured (near the maxima and minima of the magnetic energy waveform), the argument presented above will still pertain in that the chances of starting and running will he higher if the motor is commutated by a commutator with hysteresis.

In practical terms, to set up a motor incorporating a commutator with hysteresis, the tolerance to commutator phase angle is an important factor. If this is high the phase can be set quickly by hand during manufacture. If it is low, then the motor is difficult to set up.

For a motor with hysteresis, the phase angle must be such that a backward commutation point does not lie in the quadrants 0 90 1180. 270 (otherwise the motor could run backwards) and that a forward commutation does not lie in the quadrants 0 90 and 180"-270" (to prevent degradation of speed regulation under maximum torque conditions).

In this way, both forward and backward commutation points must lie in the same quadrant.

To allow for the errors which will be present in the motor, e.g. deviation from equal mark/ space ratio and commutation phase angle variation arising from mechanical inaccuracies it will be necessary to place the first backward commutation point at a phase angle slightly greater than 90 , say 100 , and to place the first forward commutation slightly before 180 , say 1700. Thus if we provided 50 of hysteresis in the commutator, we would have a tolerance of + 100 on the setting of phase of the commutator. Consequently if the motor can provide enough torque to overcome friction, it will always run, even starting from regions very close to commutation points.

In order to achieve starting every time for a motor with no hysteresis (only possible in the idealised friction-free condition), we have seen that no variation in phase setting from 0" and 1800 is allowed during starting conditions and there is no tolerance to mechanical inaccuracies when at speed, resulting in a loss oic synchronism.

Hysteresis can be incorporated by using a commutator which has inherent hysteresis e.g.

a reed switch or by using a commutator without hysteresis and adding a Schmitt trigger circuit to interface the commutator to the stator coil drive circuit. Indeed, in principle, several alternative sensors are possible, e.g.

wound coil, or air blast and pressure sensor, provided that electronic hysteresis is added.

In the main case, Patent No. 1,434,192, various means arranged to respond to the poles of the rotor during rotation are disclosed. We disclaim from the scope of the present claims those means disclosed in the main case which have inherent hysteresis.

Subject to the foregoing disclaimer WHAT

Claims (7)

WE CLAIM IS:

1. An electric motor according to any one of the claims of our Patent No. 1,434,192 wherein the means arranged to respond to the poles of the rotor during rotation is replaced by a commutating means including means responsive to rotation of the rotor which have inherent hysteresis, or a circuit exhibiting hysteresis is connected between the rotation responsive means and the means for controlling energisation of the coil.

2. An electric motor according to claim 1, wherein the circuit exhibiting hysteresis is a trigger circuit.

3. An electric motor according to claim 2, wherein the trigger circuit is a Schmitt trigger.

4. An electric motor according to claim 1, wherein the circuit exhibiting hysteresis comprises an amplifier with positive feedback.

5. An electric motor according to claim 4, wherein the rotation responsive means comprises an optical system including a light source and a sensor.

6. An electric motor according to claim 5, wherein said optical system is disposed adjacent a cylindrical surface of the rotor and that at last one aperture is provided in the cylindrical surface of the rotor for passage of light from said source to said sensor.

7. An electric motor according to claim 1 substantially as hereinbefore described with reference to the accompanying drawings.