GB1598796A

GB1598796A - Shaded-pole induction motors

Info

Publication number: GB1598796A
Application number: GB4183677A
Authority: GB
Original assignee: National Research Development Corp UK
Current assignee: National Research Development Corp UK
Priority date: 1978-05-05
Filing date: 1978-05-05
Publication date: 1981-09-23

Description

(54) SHADED-POLE INDUCTION MOTORS (71) We, NATIONAL RESEARCH DEVELOPMENT CORPORATION, a British Corporation established by Statute, of Kingsgate House, 66 - 74 Victoria Street, London, S.W.1, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to shaded-pole single-phase induction motors arranged for reversal of the direction of rotation.

As is well known the direction of rotation of a single-phase induction motor may be predetermined by providing an inductive loop (or shading ring) over a corresponding extremity of each main pole. In general the magnetic circuit of a shaded-pole motor must include an auxiliary short-circuited winding magnetically displaced from the main winding. The resultant phase delay in the growth and decay of flux in the region of the short-circuited winding relative to that on the main pole axis causes the rotor to move in the direction from the axis to the phase delay region. A ring may be placed at the appropriate pole extremity to produce rotation in either direction and a motor can in principle be made reversible by using a ring in each position with provision for open-circuiting the one which is not required at any time. The practical construction then becomes more complex since a substantial copper ring must be preplaced by a multi-turn coil suitable for connection to a low-current external switching circuit.

By such means the direction of rotation from rest can be selected but it may be impossible to achieve the ability to reverse by switching at full speed.

An object of the invention is to provide a motor having improved reversing performance.

According to the invention there is provided a single phase reversible induction motor having a stator comprising equal numbers of main salient poles and of auxiliary salient poles interspaced between the main poles, each auxiliary pole having a shading ring, and the winding of the main poles and the winding of the auxiliary poles being so arranged for connection to the power supply as to enable one of the windings to be reversed in connection.

On such reversal of connection each auxiliary pole is caused to become of like polarity with that one of the adjacent main poles which was previously of opposite polarity, whereby reversal of rotation is induced.

The main pole windings and the auxiliary pole windings may be arranged in series or in parallel with each other.

Preferably the auxiliary poles are symmetrically interspaced between the main poles.

Preferably each shading ring is magnetically symmetrical with respect to the adjacent main poles.

Preferably each shading ring has a value of resistance such that in operation the forward torque/speed characteristic over the speed range +coo lies above the speed axis.

The arc of each auxiliary pole may be in the region of one-third of the main pole pitch.

An embodiment of the invention will be described with reference to the accompanying drawings in which: Figure 1 represents diagrammatically the pole structure viewed in the axial direction of a motor employing conventional shading rings; and Figure 2 represents the pole structure of a motor arranged for reversing according to the invention.

Referring to Figure 1 a conventional design of stator lamination 10 has four poles 12, 14, 16, 18 shown before winding. As viewed in the drawing the left-hand tip of each pole is shaded by a ring 20 which is mounted in a slot 22. The field distribution resulting from the presence of the rings 20 has the effect of inducing motion in a rotor 24 in the anti-clockwise direction and no means is provided for reversing this direction of motion.

In Figure 2 a modified stator lamination 26 has main poles 12, 14, 16, 18 as before and includes an auxiliary pole 28 positioned centrally in the gap between the pair of main poles 12 and 14, and similar poles 30, 32, 34 in the gaps 14-16, 16-18, 18-12 respectively.

Each pole 28 to 34 has a shading ring 36 symmetrically about the radial axis of the pole. The main pole windings are not shown but are conventionally arranged to produce alternate polarities. Each pole 28 to 34 also carries an auxiliary winding 38 which is conveniently wound over the ring 36, the direction of winding again being arranged so that alternate polarities are produced when all the windings 38 are connected in series with the power supply. The series connection is made via a polarity change-over switch 40 to enable the sense of the connection to windings 38 to be reversed, while the sense of the connection to the main winding remains unchanged. The main windings may be connected to the power supply in parallel with the auxiliary windings or in series with these windings. The connecting block 42 between the switch 40 and the power supply input provides for the series connection to be made, the direction of current in the main windings remaining unchanged when switch 40 is operated.

The manner of operation of the reversing motor from a stationary state can now readily be appreciated. It will be understood that idealised conditions are discussed. In the presence of saturation, the magnetic state of a practical machine cannot be simply or precisely defined but will cause the machine to perform, in general, in the manner described. In the conventional construction of Figure 1 the presence of shading ring 20 on the left hand tip of pole 12 is well known to cause anti-clockwise rotation of the rotor 24 without regard to the instantaneous polarity of pole 12. Considering first the embodiment of Figure 2 with the auxiliary coils 38 inoperative it will be seen that pole 28 occupies a symmetrical and magnetically neutral position between main poles 12 and 14. The shading ring 36 of pole 28 therefore has no effect on the direction of rotation of rotor 24. If pole 28 were displaced towards pole 12 there would bye a tendency to clockwise rotation and if towards pole 14, a tendency to anti-clockwise rotation. When current is passed through auxiliary coils 38 the auxiliary poles, such as the pole 28, can no longer be magnetically neutral. Suppose the coils 38 to be connected so that pole 28 is N during half-cycles when pole 12 is also N and pole 14 is S. Due to the presence of shading ring 36 on pole 28 the growth of flux in that pole region is delayed with respect to the flux in the region of pole 12. Clockwise rotation will therefore be induced in rotor 24 from pole 12 towards pole 28. Alternatively, switch 40 may be operated to change the direction of current flow in coils 38 and consequently to reverse the polarities of poles 28, 30, 32 and 34, the rnain pole polarities remaining unchanged.

Pole 28 is now of like polarity to pole 14 but the growth of flux at pole 28 is again delayed in phase so that rotor 24 experiences a force in the anti-clockwise direction.

The direction in which rotation is initiated is determined by the response of the rotor to a stator field which appears to rotate. The manner of connection to the power supply described with reference to Figure 2 provides for the direction of rotation of the field to be reversed by reversal of the connections to the auxiliary windings. Equally effectively, the connections to the main winding may be arranged for reversal by switching, while those to the auxiliary windings are unchanged.

The interspacing of auxiliary poles with main poles in the embodiment described provides for all gaps between poles to be of equal size and consequently there is no predisposition towards rotation in either direction. If, however, each auxiliary pole is displaced in the same direction from the centre of the respective main pole gap, the presence of the shading ring on the auxiliary pole will tend to cause rotation in a favoured direction in response to energisation of the main poles alone. Less energisation of the auxiliary poles will then be required to ensure rotation in that direction and correspondingly greater energisation for rotation in the opposite direction. Such an assymetric arrangement can be employed in applications in which there may be a requirement for preferential performance in one direction of rotation.

The main windings and auxiliary windings, although it is convenient from a production standpoint that they should be similar, need not have identical numbers of turns. In practice the space available for the auxiliary winding is likely to be restricted by the presence of the shading ring so that the auxiliary winding may have substantially fewer turns than the main winding.

In the preceding discussion a motor designed in accordance with the invention has been shown to be selectively bi-directional from rest. It is a further advantage if the motor can be made fully reversing i.e.

reversible at speed. The ability to reverse rapidly when the motor is switched to the reverse connection under light or heavy load requires active braking by the motor so that the rate of braking is determined by the torque/speed characteristics of the motor and not by the inertia of the load. For exampl, on switching to reverse the torque/ speed characteristic for a forward running machine may lie partly above the speed axis.

On light load such a machine will continue to run forwards but for a sufficiently heavy load a relatively slow reversal is achieved.

Since power dissipation is greatest at low speeds, overheating may result from slow reversal particularly if the operation is repetitive. In order to avoid this risk and to provide satisfactory reversing capability under all load conditions, the reverse torque/speed characteristics must lie completely below the speed axis for all speeds over the range -coo to + oo where coo, in the conventional manner, denotes the running light speed. The forward and reverse characteristics of the auxiliary pole machine are symmetrical so that the criterion may equally be expressed as requiring the forward characteristic for the range -coo to +ouzo to lie above the speed axis. Such a characteristic has been achieved and it is thought that the most important factor is the provision of a low value of resistance in the shading rings. The copper rings found to be suitable were of double the cross-sectional area which had been used in a motor of the conventional type shown in Figure 1 of generally similar size.

Theoretical analysis suggests that reversing torque is increased by making the arc of the shaded pole larger than that normally used in a shaded pole motor and the experimental motor of Figure 2 has been successfully operated with an arc of 70C (electrical).

An auxiliary pole machine has been found to be superior in reversing performance and simpler to construct than one in which the selected one of a pair of pole-tip windings is short-circuited to determine direction.

There is some additional stator power consumption in the new machine but it is considered that the value of bi-directional operation with outweigh this factor in applications such as fan drives in which the shaded pole-motor is commonly used.

WHAT WE CLAIM IS: 1. A single-phase induction motor having a stator comprising equal numbers of main salient poles and of auxiliary salient poles interspaced between the main poles, each auxiliary pole having a shading ring, the winding of the main poles and the winding of the auxiliary poles being so arranged for connection to the power supply as to enable one of the windings to be reversed in connection, whereby the direction of rotation of the motor is caused to be reversed.

2. A motor according to Claim 1 in which the auxiliary poles are symmetrically interspaced between the main poles.

3. A motor according to Claim 1 in which the auxiliary poles are asymmetically interspaced between the main poles such that preferential rotation may be caused in a predetermined direction.

4. A motor according to Claim 1 or Claim 2 in which each shading ring is magnetically symmetrically disposed with respect to the adjacent main poles.

5. A motor according to Claim 1 or Claim 2 in which each shading ring is magnetically asymmetrically disposed with respect to the adjacent main poles such that preferential rotation may be caused in a predetermined direction.

6. A motor according to any preceding claim in which each shading ring is substantially greater in cross-section than is used in a unidirectional motor of similar rating.

7. A motor according to any preceding claim in which each shading ring has a low value of resistance such that in operation the forward torque/speed characteristic over the speed range +ouzo lies above the speed axis.

8. A motor according to any preceding claim in which the arc of each auxiliary pole extends to substantially one-third of the main pole pitch.

9. A motor substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. reverse connection under light or heavy load requires active braking by the motor so that the rate of braking is determined by the torque/speed characteristics of the motor and not by the inertia of the load. For exampl, on switching to reverse the torque/ speed characteristic for a forward running machine may lie partly above the speed axis. On light load such a machine will continue to run forwards but for a sufficiently heavy load a relatively slow reversal is achieved. Since power dissipation is greatest at low speeds, overheating may result from slow reversal particularly if the operation is repetitive. In order to avoid this risk and to provide satisfactory reversing capability under all load conditions, the reverse torque/speed characteristics must lie completely below the speed axis for all speeds over the range -coo to + oo where coo, in the conventional manner, denotes the running light speed. The forward and reverse characteristics of the auxiliary pole machine are symmetrical so that the criterion may equally be expressed as requiring the forward characteristic for the range -coo to +ouzo to lie above the speed axis. Such a characteristic has been achieved and it is thought that the most important factor is the provision of a low value of resistance in the shading rings. The copper rings found to be suitable were of double the cross-sectional area which had been used in a motor of the conventional type shown in Figure 1 of generally similar size. Theoretical analysis suggests that reversing torque is increased by making the arc of the shaded pole larger than that normally used in a shaded pole motor and the experimental motor of Figure 2 has been successfully operated with an arc of 70C (electrical). An auxiliary pole machine has been found to be superior in reversing performance and simpler to construct than one in which the selected one of a pair of pole-tip windings is short-circuited to determine direction. There is some additional stator power consumption in the new machine but it is considered that the value of bi-directional operation with outweigh this factor in applications such as fan drives in which the shaded pole-motor is commonly used. WHAT WE CLAIM IS:

1. A single-phase induction motor having a stator comprising equal numbers of main salient poles and of auxiliary salient poles interspaced between the main poles, each auxiliary pole having a shading ring, the winding of the main poles and the winding of the auxiliary poles being so arranged for connection to the power supply as to enable one of the windings to be reversed in connection, whereby the direction of rotation of the motor is caused to be reversed.