GB2156167A - Induction stepping motor - Google Patents

Abstract

An AC (or pulsed D.C.) induction stepping motor comprises a stator having multiple windings (11, 17, 18) associated with switching means for supplying different windings in sequence to alter the angular position of the field, and a rotor (12) provided with at least two coils or windings (13, 15) mutually displaced about the rotor axis so as to provide two or more stable rotor positions. By switching the stator windings the rotor can thus be stepped. The motor may be run at a continuous slow speed. <IMAGE>

Description

SPECIFICATION Induction stepping motor This invention re!ates to a rotary stepping induction motor designed more particularly for use with single phase alternating current.

Conventional stepping motors are designed to operate on a DC supplyand sufferfrom a numberof disadvantages in addition to the fact that they cannot be used where there is no DC supply. By contrast a conventional induction motor will operate on an AC supply, but is quite incapable offunctioning as a stepping motor and if it does not rotate continuously near its designed speed the rotor merely tends to stall.

One of the problems of applying the induction motor principle to a stepping motor is that the rotor has no natural steps or positions to assume and from which it can progress from one to the next. The present invention has overcome this problem and provides a simple effective induction stepping motor which will operate on alternating current.

The invention may be used with advantage in situations where accurate positional steps are needed combined with an appreciable power output, for example in pipeline valves and mechanisms for operating electrical rotary switch contacts. The invention may also be employed as an induction motor designed to produce low speed continuous rotation.

The actual speed depends upon the number of poles, number of phases, and switching frequency.

Thus the term "stepping motor" as used herein is intended also to include a motor which is designed to operate on a stepping principle, but may be caused to run art a continuous slow speed by appropriate control of the pulsing input. Thus, if pulses or cycles of current are supplied at an appropriate frequency in relation to the load the induction machine will, in fact, produce a continuous slow speed rotation.

Broadly stated the invention comprises a rotary stepping induction motor comprising a rotorposi- tioned within a stator having multiple windings and means to switch a supply current to different windings to alterthe angular position ofthefield,the rotor being provided with at least two electrically separate windings, angularly spaced about two axes which are magnetically perpendicular. There is no direct mutual coupling between the windings on one axis and those on the other axis, and as a result there are two or more stable rotor positions in relation to the stator.

According to a preferred feature the rotor is provided with pairs of windings lying in approximate ly parallel planes on opposite sides ofthe two rotor axes, and in many embodiments ofthe invention the rotor may be provided with four or more angularly spaced windings.

In any case, as mentioned above, the power supply is conveniently a single phase alternating current (the term alternating including also for present purposes a biased AC or pulsing DC) and the motorwill include a switching circuit for directing pusles of current in sequence to differentstatorwindings.

The invention may be performed in various ways and a number of embodiments will now be described by way of example with reference to the accompanying drawings, in which: Figures 1 and 2 are diagrams of a simple form of induction machine with a single rotor winding shown in two different positions, Figures 3 and 4 are similar end views of a machine with a modified rotor having two separate windings set at 90 , Figure 5 is an end view of a rotor somewhat similar to that of Figure 4, but with each winding doubled, Figure 6 is another end view of a further modification having four pairs ofwindings set at 45e intervals, Figures 7 and 8 illustrate further embodiments having respectively six and eight pairs of windings, Figure 9 is a simlified diagram illustrating an alternative form of rotor winding for a 4-pole motor using electrically connected coils in two electrically separate groups which are magnetically perpendicular, Figure 10 is a simplified diagram illustrating a three phase stator winding, and Figure 11 is a diagram illustrating one form of switching circuitforthe statorwindings.

The induction machine illustrated in Figure 1 comprises a stator 10 formed of laminated magnetic material having a single current carrying coil or loop 11, and a rotor 12 provided with a single current carrying loop or coil 13. The two arms ofthe loop 13 on the rotor are cross-connected or shorted by crossbars 14 while the arms ofthe loop 11 in the stator are connected to an external supply. When current is supplied to the stator coil a magnetic field is generated as indicated by the dotted lines andthe rotor tends to turn to the position illustrated in Figure 2 with the rotor loop or coil 13 aligned with the magnetic field.

This is not a positional stepping machine since it only has one stable position and the machine does not provide any impulse for moving the rotor from its stable position into another position.

By comparison the induction machine illustrated in Figures 3 and 4 is capable of stepping motion. The essential difference is that here the rotor 12 is provided with an additional loop or winding 15 set at right angles to the winding 13. As a resultwhenthe stator coil 11 is energised the reaction on the coils of the rotortends to turn the rotorto a position where the magnetic field is effectively counterbalanced between the two coils 13, 15. In the present example this provides a first step of about 45" rotation.The stator in this example is provided with further angularlyspaced separately energised coil windings 17 and 18 displaced for example at 60" intervals. Itwiil be seen that after the rotor has shifted through 45" from its original position, if one of the otherstatorwindings 17 or 18 is energised, the rotor will shift or step through a further angle of about 30 and thereafter by switching the supply in sequence around the separate windings 11, 17,18 of the statorthe rotor can be made to step accurately through the same angular positions.

It has been found that there is an improvement in performance if each of the separate windings on the rotor is duplicated with one coil on either side ofthe mtary axis. As a practical matter it is also necessary for the crossbars of each loop orwinding to avoid the rotary axis itself since space must be allowed for the rotor shaft and bearings. Figure 5 shows an example in which the coil 15is replaced by two separate parallel coils 1 5a and 1 5b on opposite sides of the rotor longitudinal axis 20 and each completed buy a crossbar 1 sic, 1 sod which is bent as shown to avoid the shaft of the rotor. Likewise, the other rotor coil 13 is replaced by a pair of separate parallel coils 13a and 13b.It is found thatthe use of parallel separate coils allows the magnetic flux to pass between and improves the performance.

The further modification illustrated in Figure 6 shows a rotorwith four pairs of spaced parallel coils set at 45" intervals and in this case the two coils of each pair are moved further apart towards the periphery of the rotor. The pairs of coils are indicated at 21 a, 21 b, 22a, 22b, 23a, 23b, 24a, 24b. The stator for use with this rotorwill normallyhavewindingstoprovidefour poles, as indicated at 25.

Figure 7 illustrates a further developed rotor in which the number of separate windings is increased to six pairs providing in effect six poles and forthis rotor a statorwith a corresponding number of coils form six poles is required. In Figure 8the number of separate windings on the rotor has been further increased to provide eight poles.

In the examples illustrated each individual coil or winding is electrically separate from all the others, but in fact this need not be so in all cases. The essential feature is that the magnetically perpendicularwindings should be electrically separate so that the stepping feature is obtained: others may be linked.

Figure 9 illustrates an alternative rotor winding layoutfor4-polewinding in 24slots in which every third slot is omitted. The winding has eight pairs of concentric coils electrically connected to form two electricallyseparatewindings d and qterminating at S1-S2 and S3-S4 respectively. Terminals S1-S2 are shorted eitherdirectlyorthrough an impedance.

Terminals S3-S4 are similarly shorted.

Figure 10 illustrates a conventional three phase winding which, according to the invention, is suitable as the stator winding, having three spaced windings, as illustrated for example in Figure 4. Here a single phase alternating or pulsed supply system is connected to terminals Al -A2, B1-B2, and C1-C2, as required. It will be seen that by appropriate switching this alternating supply can be connected in sequence to a number of coils spaced around the periphery of the machine.

Figure 11 illustrates diagrammatically a possible form of elecron ic switching circuit for this purpose. A single phase alternating supply is connected as an input atthe terminals 30 and this is supplied to the three phase windings A, B and C by the triacs T1, T2 which are selectively operated in sequence bythe switching circuits illustrated. The speed and direction of movement can be controlled readily by appropriate design or control of the switching circuit to varythe switching rate and the sequence in which the pulses are suppliedtothe differentstatorwindings.

Claims (7)

1. Arotary"stepping" induction motorcomprising a rotor positioned within a stator having multiple windings and means to switch a supplycurrentto differnt windings to alter the angular position of the field, the rotor being provided with at least two electrically separate windings, in planes angularly spaced about the axis of the rotorto provide two or more stable rotor positions in relation to the stator.

2. An induction motor according to claim 1, in which the rotor is provided with pairs of windings, lying in approximately parallel planes on opposite sides of the rotor axis.

3. An induction motor according to claim 1 or claim 2, inwhich the rotor is provided with four or more angularly spaced windings.

4. An induction motor according to claim 3, in which the windings and the cross connecting winding limbs are located physically adjacent the periphery of the rotor.

5. An induction motoraccordingtoanyofthe preceding claims, in which the stator has three, four or more angularly spaced windings.

6. An induction motor according to any ofthe preceding claims, in which the power supply is a simple phase alternating current, and including a switching circu it for directing pulses of current in sequence to different stator windings.

7. A D.C. induction motor substantially in any of the forms described with reference to figures 3 - 11 of the accompanying drawings.