GB2089136A - Electric motor - Google Patents

Abstract

A rotor 10 has projections 11 which confront similar projections 17 on poles 15 of a stator 14; and in any position of the rotor 10, the projections 17 on only one opposed pair of the poles 15 registered with the projections 11. Field windings 16 on the poles 15, and an armature winding 12 on the rotor 10, are energisable to provide either continuous or incremental operation. For incremental operation, winding 12 is not energised but direct current is passed through the windings 16 successively from one pole pair to the next, the speed of rotation of the rotor 10 being determined by the frequency with which the current is switched from one pair of windings to the next. For continuous operation, all of the windings 16 are energised together and a direct current is passed through the armature winding via slip rings (19), the current flow direction through each pair of windings 16 being reversed as a control axis (N) of the armature winding passes the centre line of the respective pole pair. <IMAGE>

Description

SPECIFICATION Electric motor This invention relates to electric motors, particularly those for moving a load to a well-defined position.

Conventional d.c. motors when used for the abovestated purpose work in a closed loop servo system, with resultant problems of stability, feedback, drift and gearing to provide accuracy. Such motors do not operate advantageously when required to rotate at very slow speeds or from stationary to move a very small angular increment either forwards or backwards. Moreover, because d.c. motors are analogue devices, linear amplifiers are used for their control which makes it difficult for them to be interfaced with a digital control system.

In addition, because of commutation the brushes of the motors are subject to wear and generate electrical noise.

These problems can be overcome by using a stepping motor, but this gives rise to fresh difficulties. For example, stepping motors do not operate advantageously at medium to high seed, and they must be carefully pulsed with an increasing frequency pulse rate to ensure that they accelerate to their top speed without losing synchronisation. Similar problems of course occur during deceleration. Since stepping motors are essentially digital devices, ie.

one digital electrical field change causes the rotor to increment one step, there is an upper limit to how fast the fields can be made to switch due to their inherent inductance. The need for careful acceleration and deceleration means that such motors are slow to reach their top speed. Furthermore, as inertial and frictional load is added to the motor as it drives the said load, the speed/torque curve drastically deteriorates in the medium to high speed range.

This is because the motor operates on magnetic attraction (minimum reluctance) which is basically not a very stiff drive. Stepping motors usually have to be over-rated for particular applications, or are fitted with rotary shaft encoders and complicated digital switching electronics in order to achieve maximum performance. In addition, with some motors resonance can cause rotor instability.

It is an object of the present invention to provide a motor which overcomes the above-described problems and disadvantages associated with conventional d.c. motors and stepping motors.

Accordingly to the present invention, there is provided an electric motor comprising a fixed component and a rotatable component, one component having thereon a plurality of projections and an armature winding, the other component having a plurality of poles and a field winding, each pole having projections which confront the projections on said component such that at any given position of the rotatable component, the projections on some of the poles are in registration with respective projections on said one component and the projections on the remainder of the poles are out of registration with respective projections on said one component first electrical supply means operable to supply electric current to said windings in a manner to rotate the rotatable component continuously, and second electrical supply means operable to supply electric current to said windings in a manner to rotate the rotatable component incrementally, the first and second electrical supply means being selectively operable.

With such an arrangement, the motor can be operated continuously, i.e. as a normal d.c. or univeral motor, to move a load rapidly for most of its travel between two points and can be operated incrementally, i.e. as a stepping motor, to control the precise positioning of the load at the end of its travel.

In one particular construction according to the invention, a field winding is provided for each pole of said other component, and the second electrical supply means is arranged to supply current successively to the field windings when the projections on their associated poles are out of registration with the projections on said one component. Preferably, the second electrical supply means supplies current simultaneously to the field windings associated with a pairofopposed poles in such a mannerthatthe poles become magnetic north and south poles respectively.

The turns of the armature winding can be disposed between the projections on said one component. Alternatively, the projections on said one component can be formed on a memberwhich overlies the armature winding.

Desirably, said one component and said other component are a rotor and a stator respectively.

The present invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic axial end view of an electric motor according to the present invention; Figure 2 is a perspective view of a rotor which forms part of the motor shown in Figure I; Figure 3 is a schematic axial section through the rotor; and Figure 4 is a schematic end view of the rotor illustrating the direction of current flow through and armature winding thereof.

Referring to the drawings, the illustrated electric motor comprises a rotor 10 which has a series of axially extending projections 11 on its outerperipheral surface and an armature winding 12, the turns of which are disposed in slots 13 defined between adjacent projections 11 (see Figures 3 and 4). For the sake of clarity, the armature winding 12 is not shown in Figures 1 and 2. A stator 14 surrounds the rotor 10 and has a plurality of poles 15, each pole 15 having afield winding 16 there-around and a plurality of axial projections 17 which confront the projections 11 on the rotor 10. In the specific embodiment illustrated, eight poles 15 are provided and are arranged in four opposed pairs referenced A, B, C and D not shown on Figure 1 respectively.The field windings 16 on the poles in each opposed pair are connected as illustrated for the pole pair A, (the A phase) between terminals A' and A" such that when an electrical current is passed therethrough, the poles become magnetic north and south poles respectively. In any given angular position of the rotor 10, the projections 17 on one pair of poles are in registration with respective ones of the projections 11 on the rotor, while the projections 17 on the remaining poles are out of registration with other respective ones of the projections 11 by one half of the width of the projections.

Referring now particularly to Figures 3 and 4, the armature winding 12 is composed of a number of individual turns 18 which, as mentioned previously, are disposed in slots 13 defined between adjacent ones of the projections 11. Alternatively, the turns 18 can be laid in large slots on the rotor and can be overlain by a thin drum on which the projections 11 are formed. Electrical current is supplied to the armature winding 12 in use in a conventional manner by means of slip rings 19 and brushes 20. An encoder disc 21 is mouted for rotation with the rotor 10, and a reader 22 senses the angular position of the rotor 10 by means of the disc 21.

As will now be described, appropriate energisation of the field windings 16 and the armature winding 12 enables the motorto be operated either continuously or incrementally. For incremental operation, current is passed through the pairs of field windings 16 successively. When for example, current is passed from terminal A' to terminal A" in Figure 1, north and south magnetic poles are created as indicated causing the projections 11 on the rotor 10 to align exactly with the projections 17 on the pole pair A. This alignment is caused by the magnetic flux wishing to create a minimum path resistance or reluctance and this occurs when the projections are aligned. A digital control (not shown) switches the current in the A phase off and switches current in the B phase on.As a result, the rotor 10 rotates anticlockwise as viewed in Figure 1 by one half of the width of one projection to form a minimum reluctance path with the projections 17 on the pole pair B.

The current switching operation is continued to the C and D phases so that the rotor 10 rotates by one half of the projection width each time. Thus, switching of the phases in the order ABCD produces anticlockwise rotation of the rotor at a speed which is determined by the frequency at which the phases are switched. It will be manifest that rotation of the rotor in a clockwise direction will be obtained by switching the phases in the A,D,C,B,A. In a practical example, 50 projections are provided on the rotor so that the motor is capable of 200 steps per revolution. The above-described operation does not assume any contribution from the armature winding 12.

For continuous operation of the motor, current is fed into and out of the slip rings 19to energisethe armature winding 12, and the A, B, C and D phases of the field windings 16 are all switched on together. In the specific arrangement illustrated, the winding 12 is in the conventional form for a two-pole electric motor, the direction of current flow through the winding 12 being indicated in Figure 4. In orderto maintain the rotor in rotation electronic computation must be employed to reverse the current flow direction through each phase of the field windings as the central axis N of the armature winding passes the centre line of the respective pole pair, thereby reversing the magnetic polarity of that pole pair.

Such computation is easily achieved by means of the encoder disc 21 and the reader 22. The direction of rotation of the rotor can be reversed by reversing the direction of current flow through the armature winding 12 or by reversing the polarity of the stator field. A conventional d.c. motor connection such as a series or shunt can be used with this arrangement.

From the above, it will be manifest that the electric motor combines in one unit the best characteristics of a d.c. oruniveral motor over most of its speed range with the best characteristics of a stepping motor over a portion of its speed range. Advantages of the motor include high torque for rapid acceleration and deceleration; high maximum r.p.m.; long life from two bearings and no commutation; simple digital control in both modes of operation; compatabilitywith digital output devices; non-cumulative rotary error; and the ability for accurate positioning and speed control.

It will be apparent that various modifications can be made to the above-described arrangement. For example, although the stator 14 has been described as having eight poles 15 arranged in four opposed pairs, other arrangements are equally feasible. In addition, the armature winding 12 has been described as for a conventional two-pole electric motor, but again other arrangements are equally possible.

The motor of the present invention has a variety of different applications. For example, it could be connected to a feedling lead screw of a machine tool to give rapid traverse of a tool towards a workpiece followed by highly accurate machining of the latter.

The motor could be connected to driving gears of a gun or guided missile launcher to give fast slew towards the target followed by accurate tracking when near the final position. The motor could drive a capstan for a tape cassette at high speed and then work in incremental mode for reading a block of characters. Similarly, the motor could be used to traverse a printing head across a page and stop the head accurately for printing. The motor could drive the paper feed on a printer where its fast mode would execute skips and its accurate indexing would be used for the lines of print. Guided weapons could be steered by such a motor operating the control surfaces. The motor could feed steel strip into a press requiring rapid traverse then accurate stopping for punching. Robots are an application where high traverse rates coupled with accurate positioning would be required. The motor could drive a warehouse crane that moves along and upwards to load and unload goods.

Claims (7)

CLAIM 1. An electical motor comprising a fixed component and a rotatable component, one component having thereon a plurality of projections and an armature winding, the other component having a plurality of poles and a field winding, each pole having projections which confrontthe projections on said one component such that at any given position of the rotatable component the projections on some of the poles are in registration with respective projections on said one component and the projec tions on the remainder of the poles are out of registration with respective projections on said one component, first electrical supply means operable to supply electric current to said windings in a manner to rotate the rotatable component continuously, and second electrical supply means operable to supply electric current to said windings in a manner to rotate the rotatable component incrementally, the first and second electrical means being selectively operable. New claims or amendments to claims filed on 26 November 1981 Superseded claims claim 1 New or amended claims:

1. A d.c. electric motor comprising a fixed component and a rotatable component, one component having thereon a plurality of projections and an armature winding, the other component having a plurality of poles and a field winding, each pole having projections which confront the projections on said one component such that at any given position of the rotatable component the projections on some of the poles are in registration with respective projections on said one component and the projections on the remainder of the poles are out of registration with respective projections on said one component, first electrical supply means operable to supply direct current to said windings in a manner to rotate the rotatable component continuously, and second electrical supply means operable to supply direct current to said windings in a manner to rotate the rotatable component incrementally, the first and second electrical means being selectively operable.

2. An electric motor as claimed in claim 1, wherein a respective field winding is provided for each pole of said other component, and the second electrical supply means is arranged to supply direct current successively to the field windings when the projections on their associated poles are out of registration with the projections on said one component.

3. An electric motor as claimed in Claim 2, wherein the second electrical supply means supplies direct current simultaneously to the field windings associated with a pair of opposed poles in such a manner that the poles become magnetic North and South poles respectively.

4. An electric motor as claimed in Claim 1,2 or 3, wherein the turns of the armature winding are disposed between the projections on said one component.

5. An electric motor as claimed in Claim 1,2 or 3, wherein the projections on said one component are formed on a memberwhich overlies the armature winding.

6. An electric motor as claimed in any preceding Claim, wherein said one component and said other component are a rotor and a stator, respectively.

7. A d.c. motor substantially as hereinbefore described with reference to the accompanying drawings.