GB2159337A - Induction stepping motor - Google Patents

Abstract

A stepping induction motor comprises a rotor or linear moving element positioned adjacent a stator having multiple windings, and means to switch a supply current to different windings in sequence to alter the position of the field, the rotor or element being provided with at least two separate spaced windings to provide two or more stable equilibrium positions thereof in relation to the stator. Conveniently both the rotor and the stator have salient poles and the windings are preferably positioned in undercut slots arranged to increase the span of the poles. <IMAGE>

Description

SPECIFICATION Induction stepping motor This invention relates to a rotary or linear induction stepping motor comprising a stator element and a motor element which is capable of moving relative to the stator. From one aspect the invention is an improvement in or modification of the invention disclosed in copending Application No. 8404213, which describes a rotary induction stepping motor designed for operation with alternating current. In this design the laminated rotor carries two electrically separated short circuited windings angularly placed and magnetically separated by 90'. The whole rotor unit moves in steps as with other stepping motors when the stator current is switched. The model described in the aforesaid patent application develops a sinuisoidal torqueangle characteristic for "lumped" short circuited conductors on the rotor.Since the stepping performance is better for very sharp "cross-over" of the developed torque at the equilibrium positions of the rotor any improvements in this regard arising from changes in the basic structure of the device must be an advantage. One of the objects on the present invention is to reshape the characteristic by altering the rotor structure, and according to one preferred aspect of the invention this is realised by introducing "salient poles" on the rotor along angularly spaced axes, such as "direct" and "quadrature", as illustrated hereinafter.

The invention relates in general terms to a stepping induction motor of the kind comprising a rotor or "motor" positioned adjacent a stator having multiple windings, and means to switch a supply current to different windings in sequence to alter the position of the field, the rotor being provided with at least two separate spaced windings to provide two or more stable equilibrium positions thereof in relation to the stator.

From one aspect of the invention one or both elements are provided with salient poles.

Preferably the salient poles are each associated with one of the windings. The windings on opposite or spaced poles may be electrically connected, and according to a possible preferred feature the windings are positioned in undercut slots arranged to increase the span of the adjacent poles.

In a motor having, 'lumped" windings, the adjacent winding limbs of adjacent windings may be positioned in two layers in the same slot or closely adjacent side by side.

Some or all of the windings on the rotor or motor may be joined or completed by end rings.

The rotor may be associated with twin axially displaced stators, and the two stators may have poles or windings which are angularly shifted or offset. Alternatively the rotor may have two separate elements each coupled with a part of the stator and the two rotor elements have poles or windings which are angularly shifted or offset.

According to another aspect of the invention the rotor windings or poles and the stator windings or poles are angularly spaced at different angles.

The invention may also be applied to a linear motor having the same essential features. The motor may include means for switching the stator windings first in one direction and then the opposite direction to provide for reciprocating motion of the field and the "motor".

As seen in Fig. 1 the mutual coupling between a stator winding A and a short circuited rotor winding located on quadrature axis poles depends on the airgap length between the stator and rotor pole pieces. For uniform airgap all around the periphery of the rotor the mutual inductance distribution between stator and rotor in sinuisoidal. The configuration of the present invention is flexible in this regard because reshaping the poles or the inner stator surface alters this mutual inductance with respect to position. For the rotor position shown in Fig. 1 the mutual coupling between the A winding and q- axis rotor winding is maximum. If the airgap length is slightly altered towards the pole tips in the order of uniform change from the central axis of the pole, then the mutual inductance itself varies with position.In other words the total coupling is now dependent on two functions such as airgap length and rotor position, the former being adjustable. Identical arrangement on the direct axis poles gives the exact inverse replica of the mutual inductance appearing on the quadrature axis winding. The rotor thus occupies any of the zero torque equilibrium positions with an improved stability due to high torque angle slope.

In Fig. 2 the direct and quadrature windings are each placed on the rotor poles, and the coils located on the same axis poles are shorted to each other, ss (30 ) indicates the step angle which can be achieved by this rotor construction for the specified number of stator phases and poles indicated in the Figure. The stator phases are displaced by 120 with the conventional induction machine design.

The saliency of the rotor windings can also be introduced into the stator as shown in Fig. 3.

In this construction the rotor is designed to house "lumped" conductors, but is not provided with salient poles. The general appearance of the poles on the rotor or stator are like any synchronous machine but the physical shape of the pole face or the rotor surface or both pole face and the rotor surface can be carefully designed for improved torque-angle characteristic. A, B and C are the stator windings and are wound on the stator poles.

Fig. 4 represents a 4 pole rotor with salient poles. The windings on each pole piece are connected as shown diagrammatically in this Figure. Fig. 5 illustrates how the stator can be constructed with solid or salient poles and the rotor with double winding "layers", i.e. two windings in each slot. Fig. 6 shows a similar arrangement on the stator, while the rotor has two rotor windings slots closely positioned side-by-side under each pole.

When the stator winding currents are switched in sequence, the rotor steps angularly through a predeterminded angle called the "step angle", which for a particular machine depends on the number of machine poles, the number of stator phases and the number of short circuited windings on the rotor. This step angle can be adjusted by varying the machine poles and the stator phases. These two constraints alter the size of the machine, but the objective may be otherwise solved by arranging the rotor windings as described below.

The product of the step angle and the stepping rate at which the stator currents are fed controls the speed of the machine in r.p.m. Lower step angles are desirable for slow speed machines. Fig. 7 illustrates a motor having a three winding 2 pole stator, and four mutually non-coupled short circuited rotor windings placed perpendicular to each other and individually separated electrically from one another. The windings have conductors located in the slots diametrically opposite to each other so that at least one such short circuited rotor coil is at its minimum linkage position, when the other one perpendicular to it is at its maximum linkage point. Each rotor coil axis is separated by 45n e This arrangement provides a step angle of 15' which would correspond to a 4 pole machine neglecting the size and the magnitude of torque.

The step angle for an induction stepping motor can be obtained from the relationship 180" p 2pqr where ss = step angle p = number of stator pole pairs q = number of stator phases r = the number of short circuited rotor winding pairs per pole and the pairs are magnetically arranged perpendicular to one another.

For Example, with 's' the total number of rotor windings per pole = 2r o S Fig 1 1 3 2 300 2 2 2 3 2 150 4,5,6, 3 1 6 2 150 4 2 6 2 750 - 5 1 3 4 150 6 2 3 4 750 10 7 1 6 4 12 8 2 6 4 3,750 - 9 2 6 8 1.8750 In Fig. 8 the lumped rotor windings are replaced by a salient pole construction. Only the coils wound on diametrically opposite poles are shorted to each other. The stator in this example is of conventional induction motor type. In Fig. 9 the rotor windings are of the cage type and the stator windings are wound on solid salient poles. The arrangement of rotor conductors is similar to Fig. 7 but with two parallel windings on each rotor axis.

Fig. 10 shows a possible arrangement of rotor windings for a 4 pole stator with a cage type rotor having intermediate rotor windings between direct and quadrature axes. The step angle for this rotor configuration is 7.5 . The cage type rotor and wound stator can be replaced by a salient pole design similar to Figs. 8 and 9.

Fig. 11 shows how the rotor coil end connections can be accommodated when a single winding is placed on each rotor axis instead of two parallel ones as in Fig. 9. Each limb of the coil is connected to the other limb by the semi-circular conductors in parallel. This type of connection is introduced to avoid the balancing difficulties when the rotor is not wound with two parallel windings. At the opposite end of the rotor all the terminals can be shorted by one common end ring or plate.

Fig. 12 is a diagramatic end view of a "dual stack" arrangement with a single long rotor. The stator of each "stack" has three phase windings say Al, B1 and C1 which are at conventional 120 spacings in stack 1. Stack 2 also has a similar arrangement, but is mechanically displaced by 30 from the reference axes of stack 1. The rotor is shown with 4 short circuited windings, and Fig. 1 3 is a block diagram side view.

Instead of angularly displacing each stack on the stator the rotor can be constructed separately with two independent rotor elements each having an arrangement of rotor windings as illustrated in any of Figs. 1 to 11. Thus for each stack the corresponding rotor element will be displaced independently, so as to produce sequential torque in one direction with respect to the excited phases. Two or more stator phases can be excited to get larger torque. Furthermore with half-stepping operation very accurate and small step angles can be achieved. The construction of Fig. 12 gives a step angle of 7.5 and for half stepping operation it is reduced to 3.75 . The provision of a larger number of phases on the stator greatly improves the pullout torque.The stator and rotor can individually or both have salient pole construction, as described in the earlier examples.

The basic principles of the invention as explained for rotary induction stepping motor may also be extended to a linear stepping motor. For this the "stator" and "motor" windings are placed on their equivalent stationary and moving parts so that when one of the short circuited "motor" windings is at maximum linkage position the other one is at the minimum linkage point. Thus Fig. 1 5 is the linear version of Fig. 14, in which the position of the rotor d-axis winding is at maximum linkage point. Assuming the motion to be from left to right as viewed in Fig. 15, when A winding is switched on the "motor" moves one quarter of its stator winding spread. To move it further in the same direction, the B winding is switched on, then C, and A and so on.

Thus the "motor" moves each time a step length equivalent to 30 step angle for a 2 pole rotary machine windings. Figs. 1 6 and 1 7 illustrate the position of the "motor" one and two step lengths to the right from the position shown in Fig. 1 5. If the switching sequence is reversed after a certain length of "motor" movement linear reciprocation can be achieved. This corresponds to a double acting solenoid.

Figs. 18 and 1 9 illustrate one type of stator and rotor winding connection for a 2 pole design.

Windings carrying currents in one particular direction can be connected to a common base thus forming a parallel connection of all the A-phase windings to increase the effective torque and reduce the switching circuits. Alternatively one spread length of one phase can be independently switched one at a time in turn. For example in Fig. 18, two A-phase windings are illustrated having two independent connections. The + point indicates an upward direction of current flow and the - point indicates the current flowing to the source terminal, thus completing one loop.

Separate loops of this type can even be connected to one switching circuit, designating the A phase. Each rotor winding may be independently shorted to form a loop as in Fig. 19, whenever the A winding has independent loops. In some instances when multiple loops of stator phases are connected in parallel or series to one switching circuit, then the rotor windings can be arranged in series and finally shorted as shown in Fig. 20.

Smaller step lengths can be obtained by arranging the rotor windings as shown in Fig. 21 with intermediate short circuited rotor windings placed at intervals equivalent to the 45 step angle of Fig. 7. In Fig. 22 a solid conductor such as an aluminium bar is placed on each axis, such as 'd' and 'q'. For such a rotor a 2 pole rotor winding connection may be made as shown in Fig. 23. One end of all the rotor conductors can be shorted by a single end bar as in Fig. 24.

Fig. 25 illustrates one example of using salient poles on the stator to improve the torque distribution. A similar construction can be adopted for the rotor as in Fig. 26. However if the rotor is held rigid, the stator steps over the rotor the same step length as described in earlier examples.

Claims (14)

1. A stepping induction motor comprising a rotor or "motor" positioned adjacent a stator having multiple windings, and means to switch a supply current to different windings in sequence to alter the position of the field, the rotor being provided with at least two separate spaced windings to provide two or more stable equilibrium positions thereof in relation to the stator.

2. A stepping induction motor according to Claim 1, in which one or both elements are provided with salient poles.

3. A motor according to Claim 2, in which the salient poles are each associated with one of the windings.

4. A motor according to Claim 2 or Claim 3, in which the windings on opposite or spaced poles are electricaly connected.

5. A motor according to any of Claims 2 to 4, in which the windings are positioned in undercut slots arranged to increase the span of the adjacent poles.

6. A stepping induction motor according to Claim 1, having "lumped" windings, the adjacent winding limbs of adjacent windings being positioned in two layers in the same slot, or closely adjacent side by side.

7. A stepping induction motor according to any of the preceding claims, in which some or all of the windings on the rotor or motor are joined or completed by end rings.

8. A motor according to any of the preceding claims, in which the rotor is associated with twin axially displaced stators.

9. A motor according to Claim 8, in which the two stators have poles or windings which are angularly shifted or offset.

10. A motor according to Claim 8, in which the rotor has two separate elements each coupled with a part of the stator and the two rotor elements have poles or windings which are angularly shifted or offset.

11. A motor according to any of the preceding claims, in which the rotor windings or poles and the stator windings or poles are angularly spaced at different angles.

12. A stepping induction motor according to any of the preceding claims, in which the motor and stator are linear.

1 3. A motor as claimed in Claim 12, including means for switching the stator windings first in one direction and then the opposite direction to provide for reciprocating motion of the field and the "motor".

14. A stepping induction motor substantially as in any of the forms described with reference to the accompanying drawing.