GB2062974A - Electric motor - Google Patents

Abstract

A motor comprises a stator 71 having a plurality of windings (not shown in Figure 7) arranged to provide a rotating magnetic field. A windingless rotor 73 having laminations 74 is provided with rolling wheel portions 76 of smaller diameter than fixed rolling rings 72 so that the rotor is attracted by the magnetic field and performs a rolling hypocycloidal motion within the stator whilst rotating in the opposite direction to the field rotation. A spindle (Figure 9 not shown) is rotatably mounted within the rotor and driven by driving pieces 97 which take up the eccentric motion of the rotor. The surfaces of parts 71,74, 72, 76 may be tapered at the same angle and the rotor may be movable axially by a fork engaging bush neck (83 Figure 10 not shown) so as to vary the gap between wheel portions 76 and rings 72 thereby varying the motor speed. The motor may be balanced by providing a plurality of rotors rotating out of phase (as in Figure 13, not shown) or counterbalance weights arranged to move in opposition to the rotor motion (as in Figure 14, not shown). The rotor may be cylindrical (as in Figure 15 not shown). The windings of the stator may be energised with rectified A.C. (Figures 2, 3, 5 not shown) or commutated D.C. (Figure 6 not shown). <IMAGE>

Description

SPECIFICATION Electric motor This invention relates to an electric motor and may be used as an a.c. or d.c. variable or fixed low speed motor.

According to the invention there is provided an electric motor comprising a stator having a plurality of windings, means for energising said windings to cause a magnetic field to rotate around the stator, and a windingless rotor mounted within the stator via a mounting arranged to permit the rotorto roll hypocyclicallywithin the stator under the influence of the rotating magnetic field while rotating in the opposite direction to the field rotation.

The motor according to the invention may be used on an a.c. supply, such as a 3-phase supply, in which case a plurality of diodes may be used for sequentially applying the half-cycles of the supply to respective windings of the stator. The motor may also be used on a d.c. supply in which case a brush and a commutator arrangement may sequentially apply the supply to various windings of the stator to provide a rotating magnetic field. In operation the rotor is attracted to the stator by the rotating magnetic field and is drawn round in a rolling motion within the stator. A rolling wheel portion formed on the rotor may be arranged to run in a rolling ring fixed with respect to the stator and of a larger diameter than the rolling wheel portion.This allows eccentric motion of the rotor in one direction within the statorwhile causing the rotor to rotate about its own axis in the opposite direction.

The rotor, stator, rolling wheel portion and rolling ring may be tapered with the same angle whereby axial movement of the rotor and rolling wheel portion relative to the stator and rolling ring varies the eccentricity of the rotor movement to vary the motor speed. Such speed variations may cover a continuous range of, for example, zero to 1,000 r.p.m. and can be achieved without a cumbersome gearbox or the like.

Certain embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 shows a stator of a motor according to the invention in a three-phase, one-wave, one-pole embodiment; Figure 2 shows the method of connecting the windings of Figure 1; Figure 3 is a curve of the three phase currents; Figure 4 shows a stator of a motor according to the invention in a three-phase, one wave, multi-pole embodiment; Figure Sshows the method of connecting the windings of Figure 4; Figure 6shows the method of connecting the windings in a d.c. motor according to the invention; Figure 7is a section through the outer shell and stator assembly of a motor according to the invention, showing the inclined inner surfaces of the stator but not the individual pole pieces;; Figure 8 is a section of the rotor of the motor of Figure 7; Figure 9 is a section of the spindle of the motor; Figure 10 is a section of a pedestal piece and axle bush of the motor; Figure 11 shows a driving piece of the motor; Figure 12 shows a spring and a further pedestal piece of the motor; Figure 13 is a section of a motor according to the invention having three rotors driving the same shaft; Figure 14 shows a counterbalance weight method of balancing a motor according to the invention; and Figure 15 shows the rotor assembly of a motor according to the invention in which the rotor is cylindrical.

Referring to the drawings, the stator of the motor has a yoke 1 and a plurality of windings. The stator is made of silicon steel laminations and its inner surface defines projecting poles of alternating polarity, the groove between the poles being used for fitting the windings. Stators for a.c. motors are shown in Figures 1 to 5. In Figure 1 sixwindings 2,3 to 12, 13 are provided and these are connected to the three phases of the supply 14, 15, 16 via diodes as shown in Figure 2. The supplyto lead 14 is shown by curves 17 and 20 in Figure 3, lead 15 is provided with current 19, 22 and lead 16 with current 18, 21. During half-cycle 17 maximum current flows through winding 2,3 and smaller currents flow in windings 12, 13 and 4, 5.At the next current maximum 18 maximum current flows in winding 4, 5 with smaller currents in windings 2,3 and 6, 7. Thus it may be seen that the magnetic field produced by the stator extends over one half only of the stator and rotates. The pole arrangement shown in Figure 1 is referred to as a three-phase, one-wave and one-pole arrangement.

A one-wave multi-pole arrangement is shown in Figures 4 and 5. As compared to the Figure 1 arrangement each pole is divided into three and the magnetic flux of each pole is divided into two portions going to the two adjacent different poles through the yoke. The width of the yoke and the width of the poles are in due proportion and it is possible to reduce these widths without affecting the capacity of the motor: in Figure 4 the width of yoke 23 is two-fifths of the width of that of the six pole stator of Figure 1. The leads of the eighteen windings are numbered from 24 to 59 and the leads of a three-phase power supply are numbered 60, 61 and 62 (Figure 5). The windings of three adjacent poles are connected in series and will be energised together so that the rotating magnetic field is similar to that of Figure 1 embodiment.

In addition to the aforesaid two kinds of stators, the stator may be provided with twelve poles, twenty-four poles or more, but the more poles, the narrower the width of the yoke and the shorter the spacing between the poles and thus the possibility of a short circuit in the magnetic iines of force increases. Whichever type of stator is used the rotational direction of the magnetic field may be reversed by changing the phase sequence by reversing two of the power supply leads.

Figure 6 shows a method of connecting the windings of the stator when the motor is to be operated with a d.c. supply. In order to provide a strong magnetic field on one side of the statorwhich magnetic field rotates, a wiring distribution device is necessary. In Figure 4 one end of each winding is connected to a conductive member of a commutator 63 and the other end of each winding is connected to a common terminal of the d.c. supply. A slip ring 64 is connected to the other supply terminal and a brush 65 connects the slip ring and commutator.

Current flows from the slip ring through the brush commutator and some of the windings; spindle 66 drives the brush 65. As soon as the power applied to each winding is cut off by the brush, a high e.m.f. will be generated as a result of self-induction; a diode 67 is connected across each winding to short-circuit this eiectro-motive force so as to avoid burning of the cummutator.

Most of the three-phase and multi-pole stators are suitable for use with a d.c. power supply. If the stator is designed exclusively for use with a d.c. power supply, its poles need not be in a multiple of six; it may be designed using a multiple of two. Since all the windings of the poles of the stator are connected in parallel each winding only takes a small part of the total current and the spark generated on the interruption of current for each conducting member of the commutator is rather smaller than that found using series or shunt wound motors.

One of the laminations of the rotor is shown in the form of an annulus in Figure 1 and Figure 4. The lamination is of siiicon steel and there are no windings nor grooves defining poles. The lamination acts merely as a magnetic conductor. In order to reduce the weight of the rotor during its eccentric rotation the weight of the laminations of the rotor should be as low as possible. The width of the rotor and statoryokes are about one half of the width of the poles and so by using a multi-pole statorthe width of the yokes may be reduced.

Figure 7 shows a stator assembly schematically; the grooves defining the poles in the stator 71 are not shown.

Figure 8 shows the rotor. It comprises a base member 73 on to which are fitted the silicon steel laminations 74. Thereafter a fixing cap 75 is screwed on to retain the laminations. The rotor includes two rolling wheel portions 76 of equal size which are arranged to roll within respective rolling rings 72 of the stator. The diameter of the rolling wheel portions 76 is less than that of the rolling rings 72 to permit the rotorto roll hypocyclicallywithin the stator. The surface of the rolling rings 72 is preferably made of a flexible and friction proof material and a space is provided inside the rolling rings to serve as lubricant chamber to allow a lubricant to flow on to the ring surfaces.The rotor parts apart fro the laminations 74 shouid be light and durable and are preferably made of non-magnetically conductive materials so as to avoid the generation of eddy currents.

The maximum radially outward extent of the rotor motion is referred to as the rolling orbit The diameters of the stator 71, rotor 73, rolling wheel portions 76 and rolling rings 72 are selected so that there is a proper space between the rollin orbit and the statdr so as to avoid direct friction which would cause damage.

Diameter of stator - Gap x 2 = diameter of rolling orbit ........................................ (1) The difference between the diameters of the rolling orbit and the rotor is defined by the difference between the diameters of the rolling rings and rolling wheel portions: Diameter of rolling orbit - diameter of rotor = Diameter of rolling ring - diameter of rolling wheel ........................................ portion (2) Figure 9 shows a spindle which fits inside the rotor but the inner surface of the rotor does not contact the spindle. Drive is transferred from the rotor 73 to the spindle via two driving pieces 97 shown in Figure 11. The spindle is provided with two sets of teeth 79 which engage corresponding driving grooves 86.

The driving pieces 97 are provided with shock absorbing recesses 84 on the front and 85 on the rear for taking up the eccentric motion of the rotor.

Rolling balls 78 and rubber sleeved shock absorbing studs 77 of the rotor fit into the recesses of the driving pieces. In operation the shock absorbing studs 77 swing round within the recesses 84 so that the danger of impact and friction have been changed into a free rolling effect; the diameter of the recesses 84 and studs 77 are selected such that their difference equals the maximum difference between the diameters of the rolling wheel portions 76 and the rolling rings 72. A lubricant chamber is designed inside the driving pieces so as to allow-lubricant to overflow to the recesses 84 and grooves 86.

A spring 87 (Figure 12) urges the driving pieces and rotor assembly to the left in the drawing against the reaction of a pedestal piece 88 fixed to a holding flange 80 of the spindle. At the other end a pedestal piece 81 is mounted on the shaft and the bush 82 engages the driving piece 97. This prevents the rotor assembly from moving axially when the motor iis used with the spindle vertical, in which case the pedestal piece 81 is positioned at the bottom so that the effect of gravity on the rotor does not compress spring 87. Spring 87 allows some flexibility around the rolling balls 78. Axially outside of the pedestal pieces 81 and 88 are provided conventional bearings and end caps. The bush 82 is provided with a neck 83 which may be engaged by a fork for moving the rotor assembly axially to vary the speed of the motor as will be described.

In operation the magnetic field rotates around the stator as described above and the rotor is attracted towards the pole having the greatest field. Thus the rotor rolls hyjpocyclically around inside the stator and after 360 electrical degrees the magnetic field will have completed one cycle and the rotor surface will roll a little more than one revolution because the circumference of the rolling wheel portions 76 is less than the circumference of the rolling rings 72. Thus after completely rolling one circle around the rolling orbit the rotor passes over its starting point and generates a setback effect; if the rotor continues to roll a continuous setback effect exists. Between the rolling and setback of the rotor a shock effect appears because the centre of gravity ofthe rotor moves eccentrically; however since in the driving mechanism between the rotor and the spindle the gap between the shock absorbing studs 77 and the recesses 84 is greater than the shock amplitude, the shock of the rotor is not transmitted to the spindle.

However the setback of the rotor can still drive a driving piece through the shock absorbing studs 77 to transmit a stable torque to the spindle.

As shown in Figures 7 and 8 the stator 71, rolling rings 72, rotor 74 and rolling wheel portions 76 are tapered with the same angle. Also the laminated portion 74 of the rotor is longerthan the stator 71 and the rolling wheel portions 76 are longer than the rolling rings 72. Thus by moving the rotor assembly axially relative to the stator and rolling rings the gap between the rolling wheel portions 76 and rolling rings 72 may be varied thereby varying the eccentric ityofthe motion of the rotor and the size of the setback effect. The setback produced in each cycle is equal to the difference between the circumference of the rolling orbit and the circumference of the rotor.

(Circumference of rolling orbit - circumference of rotor)/circumference of rotor = revolutions of rotor after each cycle of the magnetic field (3) This may also be expressed as (Diameter of rolling orbit - diameter of rotor)/ diameter of rotor ........................................ (4) The speed of rotation of the magnetic field is in synchronism with the frequency is 50 Hz the speed of the magnetic field circulation of an a.c. stator will be 3,000 r.p.m. The speed of a d.c. stator magnetic field circulation is dependent upon the design; the maximum r.p.m. of a d.c. type may reach 10,000 r.p.m.The relation between the field circulation rate and the motor speed is: (Diameter of rolling orbit - diameter of rotor) x magnetic field circulation rate/diameter of rotor = r.p.m ........................................ (5) Thus the bigger the difference between the rolling orbit diameter and the rotor diameter the higherthe speed of the motor but the efficiency and stability would be relatively reduced; consequently it is preferred that the rotor diameter should not be less than 90% of the diameter of the rolling orbit. Based on this ratio, the maximum r.p.m. of an a.c. type motorwould be 360 r.p.m. and of a d.c. type motor 1,000 r.p.m. In practice a lower r.p.m. is usually selected for higher efficiency. The motor may be designed for very low speeds down to virtualiy zero.

Since the difference between the rolling orbit diameter and the rotor diameter determines the motor speed the variation of this difference during the running of the motor provides a means for achieving stepless speed variation. If the iarger end of the stator 71 is aligned with the larger end of the rotor 74, the gap between the rotor and the rolling orbit (or the gap between the rolling rng 72 and the rolling wheel portion 76) would be equal to zero; then the motor would be bound in the middle of the stator without any rolling and the speed of the motor would naturally be equal to zero.If the small end of rotor 74 is aligned with the small end of stator 71, the diameter difference between the rolling orbit and the rotor and the diameter difference between the rolling rings and rolling wheel portions are equal and at a maximum and the motor speed would be highest.-Stepless speed variation may be achieved by moving the rotor assembly axially relative to the stator assembly by means of a fork inserted into bush neck 83 as mentioned above. If, however the stator, rotor, rolling rings and rolling wheel portions were made cylindrical as indicted in Figure 15 the motor would be a single speed motor. The cost of manufacturing a stepless variable speed motor is just a littie higher than that of such a fixed speed motor; however, a variable speed motor might be considered much cheaper in view of the cost of conventional gearboxes and the like.

The setback effect referred to above generates a torque. The relation between the rolling of the rotor and the torque is: Attractive force on rotor x rolling speed = torque x rotation speed .................................... (6) The rolling speed referred to in equation (6) is the r.p.m. of the rotor on condition that the rolling orbit diameter is greater than the rotor diameter. Wheneverthe magnetic field rotates one cycle, the rotor will roll over one cycle along the rolling orbit. Consequently: Rolling speed = (1 + (rolling orbit diameter-rotor diameter)/rotor diameter) x magnetic field circulation speed ........................................ (7) The difference between the rolling orbit diameter and the rotor diameter is the main factor determining the efficiency of the motor.The bigger the difference the motor the magnetic leakage and the less the attractive force on the rotor; as long as the rotor diameter is not less than 90% of the diameter of the rolling orbit the efficiency of this motor will be higher than that of an induction motor.

As mentioned above some shock is caused by the eccentric motion of the rotor. This may be reduced by reducing the weight of the rotor, or limiting the difference of the diameter of the stator and the rotor so as to reduce the degree of eccentricity. Further, the shock may be reduced by using shock aborbing base frame. If these techniques were all adapted the shock could be made low enough for practical use for a motor of several horse-power with a speed of 300 r.p.m. (a.c.) or 800 r.p.m. (d.c.). For completely eliminating the shock a symmetrical device may be provided as shown in Figure 13. It includes three stators, 91,92 and 93 installed in the same casing and three rotors 94,95 and 96 driving the same spindle 98. The size and weight of rotors 94 and 96 are half those of rotor 95. 97 refers to the driving pieces. The magnetic fields of stators 91 and 93 are generated in the same phases so as to attract the corresponding rotors in the same direction. The magnetic field of stator 92 is 180" out of phase so as to attract its rotor in the opposite direction. The magnetic field of each stator is rotated in the same direction and consequently the total weight moving in opposite directions within the motor is the same.

Thus a stable smooth running state of the motor may be achieved.

Referring to Figure 14, an alternative method of balancing the motor is shown which does not involve effectively three motors in one shell. The figure shows a counterbalance weight mechanism near one end cap 101 and a similar mechanism may be provided at the other end of the motor. The outer body shell 102, spindle 103, driving piece 104, rolling wheel portion 105 and rolling ring 106 are similar to the corresponding parts mentioned above. The parts 107 to 121 form the counterbalance weight mechanism which is installed in the centre of each pole of the stator; for instance if there are twelve poles, twelve counterbalance weight sets are installed. A swing arm 107 is supported by a fulcrum 108. On the left end of the swing arm is a counterbalance weight 109 which is urged buy a spring 120 towards the inner surface of the motor shell.All of the counterbalance weights 709 are installed at the same distance from the axis of the motor. If the rolling wheel portion 105 moves a little to the bottom side, the supporting arm 121 will push the swing arm 107 so that the lower counterbalance weight 109 moves against the force of its spring 120. The upper supporting arm 121 reduces its resistance force on its swing arm and the return spring 120 pulls the upper counterbaiance weight away from the centre of the motor. Two opposite counterbalance weights will move in the same direction but opposite to the direction of movement of the rotor end. Thus there is no overall movement of weight within the motor. If the rotor is moved axially its eccentric motion is changed and the degree of movement of the counterbalance weights is changed accordingly. Thus the motor remains balanced at all speeds.

A motor of the kind described above needs no special provision for starting current and the generation of reverse torque may easily be attained. Apart from variations in the supply voltage the torque will remain constant because the current and attractive force of the magnetic field on the rotor remain constant. Thus the motor runs with substantially constant current, torque and speed however, the efficiency of the motor changes during variation of the speed. Since the torque of the motor is constant it will stop if overloaded. Also the motor can be stopped by interrupting one of the three power supply lines.Under such circumstances the various poles of the stator will have a different spacing from the rotor; the windings which are close to the rotor will have less current because of higher reactance; the windings which have a larger spacing will have more current because of lower reactance. The relative increase in current depends on the structure of the stator and the space between the rotor and stator. The maximum current may reach about one and a half times the normal current. In the symmetrical motor of Figure 13 the current will not increase because windings which are 180" apart are connected in series. Therefore the windings of this motor are unlikely to be damaged by an accidental overload current.

Since a motor according to the invention can use d.c. power and have extensive stepless speed variation down to zero it may be used as the motor for a car. Such a car would not only have a high efficiency butwould require no clutch orgearboxwhich makes the mechanical structure very simple. During starting, running, load variation and braking the current is almost constant so starting and protecting devices need not be used. Since the motor can run directly at a low speed and needs no gearbox, its structure may be simple and light Compared with conventional variable speed motor arrangements the motor according to the invention not only is light in weight and small in size but may also save about 30% in manufacturing costs.

Claims (13)

1. An electric motor comprising a stator having a plurality of windings, means for energising said windingsto cause a magnetic field to rotate around the stator, and a winding less rotor mounted within the stator via a mounting arranged to permitthe rotor to roll hypocyclically within the stator under the infuence of the rotating magnetic field while rotating in the opposite direction to the field rotation.

2. A motor as claimed in claim 1 wherein the means for energising the windings comprise a plurality of diodes for sequentially applying the half-cycles of a three-phase supply to respective windings.

3. A motor as claimed in claim 1 wherein the means for energisingthe windings comprise a commutator and a brush for sequentially applying a d.c. supply to the windings.

4. A motor as claimed in any preceding claim wherein the mounting comprises a rolling wheel portion formed on the rotor and running in a rolling ring fixed with respect to the stator and of larger diameter than the rolling wheel portion so as to allow eccentric motion of the rotor without its magnetic portion contacting the stator.

5. A motor as claimed in any preceding claim wherein the rotor, stator, rolling wheel portion and rolling ring are tapered with the same angle whereby axial movement of the rotor and rolling wheel portion relative to the stator and rolling ring varies the eccentricity of the rotor movement to vary the motor speed.

6. A motor as claimed in any of claims 1 to 4 wherein the rotor and stator are cylindrical and the motor is a substantially fixed speed motor.

7. A motor as claimed in any preceding claim including a shaft mounted for rotation within the rotor and a driving piece mounted coaxially of the shaft for driving the shaft and arranged to be driven by the rotor.

8. A motor as claimed in any preceding claim including a plurality of sets of windings, the rotational phases of the rotors and the electricai phases of the windings being such as to balance the motor.

9. A motor as claimed in any of claims 1 to 7 including a plurality of counter balance weights positioned around the axis of the motor and arranged to move in opposition to the movement of the rotor to balance the motor.

10. An electric motor substantially as hereinbefore described with reference to Figures 1 to 12 of the accompanying drawings.

11. An electric motor substantially as hereinbefore described with reference to Figure 13 of the accompanying drawings.

12. An electric motor substantially as hereinbefore described with reference to Figure 14 of the accompanying drawings.

13. An electric motor substantially as herein be fore described with reference to Figure 15 of the accompanying drawings.