GB2028598A - D.C. motors - Google Patents

Abstract

A d.c. motor has stator poles (1, 2, 3, 4) with respective windings (18a, 18b, 18c, 18d) connected between adjacent segments (20a-20d, Fig. 3 not shown) of a commutator (20) so as to form a series loop around the commutator; and a rotor (14) which can be a permanent magnet or an electromagnet, carries brushes (28a, 28b) which contact the commutator (20) to establish a constant polarity sequence of N,N,S,S, for the stator poles which advances around the poles in synchronism with the rotation of the rotor. The rotor North pole is thus always repelled by a North pole and attracted by a South, and the rotor South pole is always repelled by a South pole and attracted by a North. Electronic commutation is also envisaged. <IMAGE>

Description

SPECIFICATION Improvements in and relating to D.C. machines The present invention related to d.c. machines and is particularly concerned with a d.c. machine in the form of a d.c. motor.

An object of the present invention is to provide a d.c. motor wherein a unidirectional magnetic flux summats with an alternating flux to produce a mechanical power output.

In accordance with the present invention there is provided a d.c. motor comprising at least two pairs of opposed stator poles coupled by a yoke, each pole carrying a respective stator winding, a rotor journalled on a shaft for rotation between the stator poles, the rotor having two magnetic poles, and means for connecting the stator poles to a d.c. source to establish a constant polarity sequence (N,N,S,S, for a four pole machine) for the stator poles which advances around the poles in synchronism with the rotation of the shaft so always to maintain the same relationship with the rotor poles whereby, throughout the majority of each revolution of the rotor, the North pole of the rotor is being repelled by a North pole and attracted by a South pole and the South pole of the rotor is being repelled by a South pole and attracted by a North.

Thus, the force exerted on the rotor North pole is proportional to the sum of the forces due to the rotor North pole and the adjacent stator North and South poles and the force exerted on the rotor South pole is proportional to the sum of the forces due to the adjacent stator South and North poles.

Said means for connecting the stator poles to the d.c. source can be electronic as mechanical. In the latter case, the motor can conveniently include a mechanical commutator which is fixed relative to the yoke and which has the same number of segments as there are stator poles, each pole winding being connected between two adjacent commutator segments whereby the pole windings are connected in said series loop around the commutator, and a pair of brushes carried by the rotor to rotate therewith, the brushes co-operating with the commutator to establish said constant polarity sequence.

In a four pole machine, and when using a commutator the polarity sequence is preferably established by said brushes sequentially connecting the commutator segments to a d.c. supply such that between each pair of segments whenever they are so connected there are two stator windings in series establishing poles of the same polarity and two stator windings in series establishing poles of opposite polarity.

The electrical power converted to mechanical power by the motor is less than the mechanical output power by the amount of power converted to mechanical power by the unidirectional flux.

It is a characteristic of the motor that the speed is proportional to the sum of the alternating and unidirectional fluxes whilst in known designs the speed is inversely proportional to the unidirectional flux.

The invention is described further hereinafter, by way of example, with reference to the accompanying drawings, in which: Fig. lisa diagrammatic longitudinal section through one embodiment of a d.c. motor in accordance with the present invention; Fig. 2 is a section on the line A-A in Fig. land Fig. 3 is a section on the line B-B in Fig. 1 showing the commutator plate of the motor and its orientation in relation to the stator poles.

The stator of the d.c. motor illustrated diagrammatically in the drawings comprises four stator poles 1, 2, 3, and 4 attached to a yoke 10, the yoke and poles being fabricated from a plurality of steel sheets riveted together. As shown in Fig. 2, adjacent poles are separated by equal distances xl.

Journalled on a shaft 1 2 for rotation within the generally cylindrical space defined by the poles 1 to 4 is a rotor 14 having a rotor winding 1 6. As shown in the sectional view of Fig. 2, the rotor has two arcuate ends 1 4a, 1 4b, of diameter slightly less than that of the imaginary cylinder on which the stator pole faces lie, whereby the radial distance X2 between the arcuate surfaces of the rotor and an adjacent stator pole is considerably less than the distance X1. The rotor pole faces are arcuately extended, for a reason explained further below. The rotor also has parallel flat side surfaces 1 4c, 1 4d around which the rotor winding is wound whereby the rotor can be magnetically polarised along its longer transverse axis, i.e. in the vertical direction when viewed as in Fig. 2.

Each stator pole 1 to 4 carries a respective winding 1 boa, 1 8b, 1 be, 1 Bd, connected in series, the junctions between the latter windings being connected to a commutator plate 20 rigidly mounted by means of an adjustable electrical insulator 23 on the yoke 10. The commutator plate 20, which preferably is fabricated from copper sheet, is divided into as many segments as there are poles i.e. four in this instance. The relationship between commutator segments and the poles is best illustrated by comparison of Figs. 2 and 3.

The ends of the rotor winding 1 6 are connected to an electrical d.c. supply by way of respective slip rings 22 on the rotor shaft 1 2 and co-operating brushes 24. The rotor also carries brush gear 26 which is keyed to the shaft to rotate therewith and includes a pair of brushes 28a 28b which engage the commutator plate 20 at positions spaced 1 80' apart for supplying d.c. current to the stator windings.

For this purpose the rotor shaft carries further slip rings 30 connected respectively to the brushes 28a 28b and co-operating with further brushes 32 connected to the d.c. supply.

In other embodiments a single pair of slip rings may be used for both rotor and stator connections.

In other embodiments the rotor may be a permanent magnet in which case the necessity for a rotor winding is of course obviated.

In further embodiments the switching of the current to the stator windings may be performed electronically in which case the slip rings 30, commutator plate 20 and brushes 28a 28b are replaced by rotor position sensing devices such as reed relays or photodiodes.

The stator field windings 1 8a to 1 8d are connected in series around the commutator plate. For this purpose, each commutator segment 20a to 20d has a connection lug marked 21 a to 21 d. The winding 18a is connected between lugs 21a and 21 b; the winding 1 8b is connected between lugs 21b and 21 c; the winding 1 8c is connected between lugs 21 c and 21 d; and the winding 1 8d is connected between lugs 21 d and 21 a. The polarity of the winding connections is such that when a d.c. supply is connected to two opposite segments the two windings in one of the two series winding pairs thus formed have the same polarity but this polarity being opposite to that of the other series pair.This results in the stator poles 1 to 4 having a magnetic polarity sequence of North (N) North (N) South (S) South (S) which moves around with the rotation of the rotor.

Thus, for example, when the brushes 28a, 28b connect the supply to the commutator segments 20a and 20c the statorwindings 1 8a and 1 8b are connected in series such that poles 1 and 2 are Norths and the stator windings 1 8c and 1 8d are connected in series such that poles 3 and 4 are Souths.

When the rotor moves on to connect the supply to the commutator segments 20b and 20d the stator windings 1 8a and 1 8d are then connected in series such that poles 1 and 4 are Souths and the stator windings 1 8b and 1 8c are connected in series such that poles 2 and 3 are Norths, and so on. The poles and yoke are thus subjected to an alternating flux, the poles producing a first magnetic field which rotates synchronously with the rotation of the rotor.

In the four pole machine illustrated, the stator field is advanced in a series of steps of one quarter revolution per step. In the case of an eight pole machine, the stator magnetic field is advanced one eighth of a revolution per step and so on.

In simple terms, the basic operation of the illustrated motor is as follows; Consider the rotor 14 in the position illustrated in Fig. 2. The polarity of the rotor winding 16 is such that the upper end of the rotor is a South pole and the lower end a North pole. The stator windings 1 8c and 18d are then in series and are arranged so that poles 3 and 4 are South poles. Stator windings 1 8a and 1 8b are also in series and make poles 1 and 2 into North poles. In this condition, the South pole end of the rotor is repelled by pole 4 and attracted by pole 1 and the North pole end of the rotor is repelled by pole 2 and attracted by pole 3. All four poles thus act to apply a torque to the rotor which acts in a clockwise direction.The polarities remain the same until the rotor passes through the diagonal bisecting poles 1 and 3 whereupon commutator segments 20b and 20d are connected across the supply. Windings 1 8a and 1 8d are then in series making poles 1 and 4 Souths and windings 1 8b and 1 8c are in series making poles 2 and 3 Norths. The South pole end of the rotor is then repelled by pole 1 and attracted td pole 2 and the North pole end is repelled by pole 3 and attracted to pole 4. All four poles again act to apply torque in the clockwise direction. This condition applies each time the different pairs of commutator segments are connected to the supply so that the rotor is subjected to a pulsating but unidirectional torque which results in rotation of the shaft.

The torque on the shaft is proportional to the flux in the airgaps which in turn is proportional to the sum of the mmf s on the rotor and stator coils, thus Torque a stator mmf + rotor mmf 1.

The energy expended in attracting the rotor is equal to the magnetic energy in the airgaps at the end of the attraction phase and the energy expended in repelling the rotor is equal to the energy in the airgaps at the start of the repelling phase. Since each pair of stator poles attract and repel two rotor poles during each revolution and the flux density in the airgaps is the sum of the flux densities due to each source of mmf, the energy expended in moving the rotor through a complete revolution is: and

rotor mmf x yo. ,urr Br = Teslas 3 2. Lg. Lr and stator mmf x ,uo. yrs Bs = Teslas 4 2. Lg.Ls when Br = flux density in the airgaps due to rotor mmf Bs = flux density in the airgaps due to stator mmf Lg = length of airgaps - M Lr = length of rotor iron circuit - M Ls = length of stator iron circuit - M ps = number of stator poles pr = number of rotor poles Vg = volume of airgaps - M3 ,urr= relative permeability of rotor iron yrs = relative permeability of stator iron = = permeability of air and the power expended in rotating the shaft is the product of the energy expended per revolution and the number of revolutions per second thus::- Wo = E. n Watts 5 when E = magnetic energy dissipated per revolution - Joules n = revolutions of shaft per second Wo = power dissipated in airgaps -- watts The total energy E (eqn 2) is the sum of the energies supplied to the airgaps by the rotor and stator mmf s. Since the rotor flux is unidirectional and can be supplied by a permanent magnet, the energy required from the supply is that required to produce the alternating mmf in the stator coils and this energy is therefore the effective electrical input to the motor. Since the motor operates on a d.c. supply the total power supplied to the stator circuit is Vs x Is watts and the power dissipated in each coil is; Vs. Is.

Pc = Watts 6 ps when Vs = stator terminal voltage Is = stator current ps = number of stator poles Some of this power, usually less than 10%, is dissipated as heat losses.

Stator copper losses = ps (lc2. Rc.) - Watts 7 when Ic = current per coil Rc = resistance of stator coil and the remainder is the power required to provide the alternating mmf according to the equation; Ws = ps [Ic (Vc - lc.Rc)] Watts 8 When the rotor is a permanent magnet, the minimum volume of the rotor for a given magnet material is; Volume = Length x area Br.Lg Br. a volume of rotor= x M 9 H1.o B1 when a = crossectional area of airgap - M Br = flux density produced in airgap -- teslas B, = flux density in the magnet for (BH)max H, = magnetic field strength for (BH)max Lg = length of airgap - M = = permeability of air The energy supplied to the airgaps by the rotor is used to attract and repel both poles of the rotor to and from each pair of stator poles over a complete revolution and the power expended in moving the rotor, due to the rotor flux is the product of the energy dissipated per revolution and the number of revolution per second;;

Since, in this instance, the rotor is a permanent magnet no external power is required to maintain the magnetic power dissipated in the airgaps. The explanation for the source of power from a permanent magnet belongs to the field of advanced physics and is beyond the scope of this simple description of a practical application of a well know phenomenon.

When the rotor consists of a wound rotor, the energy supplied to the airgaps is equal to the energy in the rotor coil less the energy in the rotor iron; energy in rotor coil = energy in airgaps + energy in iron Br2. Br. Vr Joules = .Lr. Ir2 = Vg+ = ----- Vg 1 ------ 11 2.,uo 2.,uo ,"r and since the energy stored in the iron is small compared with the energy in the airgaps, the power dissipated in the airgaps due to a wound rotor becomes:- Wr # ps. pr. (+.Lr.lr2.).n Watts 12 Since no energy is required to maintain the unidirectional flux, the power supplied to a wound rotor is that required to maintain the rotor copper losses:: rotor copper losses = Vr.lr = Ir2Rr Watts 13 when Ir = rotor current Rr = resistance of rotor coil Vr = rotor terminal voltags Neglecting iron magnetic and mechanical losses the output of the motor becomes:- Wo = Ws + Wr Watts 14 and the electrical to mechanical efficiency becomes; mechanical output Ws + Wr efficiency = 15 x 100 = x 100 electrical input Ws + losses The instantaneous value of reverse emf induced in the stator coils is proportional to the reluctance of the stator magnetic circuit, which, in turn, is proportional to the mutual inductance between rotor and stator. Immediately after commutation, i.e. when the stator flux opposes the rotor flux, the mutual inductance has a maximum negative value and the stator current has a corresponding maximum value.

Thus during repulsion: Vs. Br isr a ----- amps 16 n The mutual inductance has a minimum value when the rotor poles are equidistant from the stator poles and increases to a maximum positive value at the end of the attraction phase, i.e. immediately before commutation when the instantaneous stator current: Vs isa a -- amps 17 Br. n Thus, immediately after commutation the stator current reaches a maximum value but for the majority of the cycle the change in mutual inductance is in a direction which increases the reverse emf and decreases the stator current thus:: Vs Is a ----- amps 18 Br. n Commutation is effected by making the width of the commutator brushes 28a 28b greater than the divisions between segments 20a to 20d, so that when the brushes move from one segment to the next both segments are momentarily short circuited by the brushes. The stator coil under commutation is therefore short circuited and the direction of current due to the reverse emf through the short circuit is in a direction which opposes the rotor flux. If the short circuit is applied before the flux in the airgap reaches its maximum value power is lost during the attraction phase. Similarly if the short circuit is applied after the flux in the airgap reaches its maximum value power will be lost during the repulsion phase.Ideally therefore commutation must be completed within the period of maximum flux, for this reason the arcuate ends of the rotor are extended so that the volume of the airgaps is maintained over a longer period than would otherwise be the case. To allow maximum commutation time, the commutator plate is adjusted so that the short circuit occurs when the leading edges of the stator poles coincide with the leading edges of the rotor poles. The maximum value of stator current then depends on the speed of the rotor, thus: n aVs. Br. r.p.s. 19 The above adjustment of the commutator plate gives maximum speed in one direction only.For omnidirectional applications the short circuit is applied when the rotor poles and stator poles are in alignment so that the time interval for commutation at a given speed, and consequently the maximum speed in either direction, is reduced. Since the coils to be switched are stationary, electronic switching may be preferable to the commutator plate system, in which case the commutator plate 20 brushes 28a 28b and 32 and sliprings 30 are replaced with rotor position sensing devices such as reed relays or - photo-diodes.

The characteristics of the motor are largely defined by equations 1 and 18, i.e. both torque and speed are proportional to the sum of the stator and rotor mmf s and therefore for a given input; torque x speed = constant 20 Ain unusual characteristic is obtained by varying. the rotor current, in that if the rotor current is decreased the stator current increases according to equation 1 8 but the power output falls due to the reduced maximum value of stator mmf according to eqn 1 6 and the reduction in Br. Conversely, if the rotor current is increased the power output increases but the effective input power decreases

Claims (10)

1. A d.c. motor comprising at least two pairs of opposed stator poles coupled by a yoke, each pole carrying a respective stator winding, a rotor journalled on a shaft for rotation between the stator poles, the rotor having two magnetic poles, and means for connecting the stator poles to a d.c. source to establish a constant polarity sequence (N,N,S,S, for a four pole machine) for the stator poles which advances around the poles in synchronism with the rotation of the shaft so as always to maintain the same relationship with the rotor poles whereby, throughout the majority of each revolution of the rotor, the North pole of the rotor is being repelled by a North pole and attracted by a South pole and the South pole of the rotor is being reprelled by a South pole and attracted by a North.

2. A d.c. motor as claimed in claim 1, including a commutator which is fixed relative to the yoke and which has the same number of segments as there are stator poles, each pole winding being connected between two adjacent commutator segments whereby the pole windings are connected in a series loop around the commutator, and a pair of brushes carried by the rotor to rotate therewith, the brushes co-operating with the commutator to establish said constant polarity sequence.

3. A d.c. motor as claimed in claim 2 including a pair of slip rings carried by the rotor and cooperating with a pair of fixed brushes for coupling the d.c. supply to the pole windings via the commutator.

4. A d.c. motor as claimed in claim 2 or 3 in which the switching points for the commutator, at which the brushes leave one commutator segment and contact another, lie at positions corresponding to the magnetic poles of the rotor being radially aligned with the stator poles.

5. A d.c. motor as claimed in claim 1, 2, 3 or 4 in which the rotor is magnetised by a winding carried thereby which is connected to a d.c. supply.

6. A d.c. motor as claimed in claim 5 in which the rotor winding is connected to its d.c. supply by way of slip rings carried by the rotor and co-operating with fixed brushes.

7. A d.c. motor as claimed in claim 6, when appendant to claim 3, in which said slip rings for connecting the stator pole windings to the d.c. source are also used to connect the latter d.c. source to the rotor winding.

8. A d.c. motor as claimed in claim 1 , 2, 3 or 4 in which the rotor is a permanent magnet.

9. A d.c. motor comprising at least two pairs of opposed stator poles coupled by a yoke, each pole carrying a respective stator winding; a commutator which is fixed relative to the yoke and which has the same number of segments as there are stator poles, each pole winding being connected between two adjacent commutator segments whereby the pole windings are connected in a series loop around the commutator; a rotorjournalled on a shaft for rotation between the stator poles, the rotor having two magnetic poles; a pair of brushes carried by the rotor to rotate therewith; the brushes cooperating with the commutator to establish a constant polarity sequence (N,N,S,S, for a four pole machine) for the stator poles which advances around the poles in synchronism with the rotation of the shaft so as always to maintain the same relationship with the rotor poles wereby, throughout the majority of each revolution of the rotor, the North pole of the rotor is being repelled by a North pole and attracted by å South pole and the South pole of the rotor is being repelled by a South pole and attracted by a North.

10. A d.c. motor substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.