GB1566622A

GB1566622A - Self-starting miniature synchronous motors

Info

Publication number: GB1566622A
Application number: GB3304777A
Authority: GB
Original assignee: Jeco Corp
Current assignee: Jeco Corp
Priority date: 1977-08-06
Filing date: 1977-08-06
Publication date: 1980-05-08

Description

(54) SELF-STARTING MINIATURE SYNCHRONOUS MOTORS (71) We, JECO Go., LTD., a corporation organised and existing under the laws of Japan, of 790, Kuji, Takatsu-ku, Kawasaki City, Kanagawa Prefecture, Japan, 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: This invention relates to miniature syn; chronous electric motors.

Although various types of miniature synchronous motors have been developed, many have suffered from the difficulty of not being able always to self-start in a definite direction and produce relatively large torque. For example, according to a synchronous motor disclosed in U.S. Patent No.

3,256,453 N and S magnetic poles are arranged alternately about the periphery of a rotor, and a plurality of stator arms each having a pole face are disposed to surround the rotor. With this construction, however, since the number of pairs of poles is an even number, for example 8, when one of the stator main poles is magnetized to produce a repulsive force between it and an opposed first rotor pole (for example an N pole), the other stator main pole which is diametrically opposite to said one stator main pole does not produce any repulsive force between it and a second rotor pole (N pole) which is diametrically opposite to said first rotor pole but instead creates a repulsive force between it and a rotor pole (S pole) adjacent said second rotor pole.

In other words, the rotor torque is not produced by diametrically opposite rotor poles.

This means that the rotor shaft will be subjected to side pressure which causes one sided wear of the shaft as well as shortening the motor life. Furthermore, with this construction, one pair of rotor poles is always idle thus decreasing the efficiency of utilization of the poles.

According to another example disclosed in U.S. Patent No. 3,984,972, a motor comprises a rotor including diametrically opposite poles and a stator having two semicircular opposing portions encircling the rotor, the semicircular opposing portions being disposed in a point symmetrical relationship with respect to the rotor shaft.

With this construction, however, if the frequency of stepping pulses supplied to a coil common to the stator portions were too high, then before the rotor stops at a stable position due to rotor inertia the next driving pulse would be applied resulting in hunting. Furthermore, as this motor is of the stepping pulse drive type, if it is energized by an alternating current pulse having no zero current interval between adjacent driving pulses the rotor will often assume a stable stationary state in which the rotor does not rotate. For this reason, the motor of this construction could be operated only with a stepping pulse having a relatively low frequency.

According to this invention, there is provided a self-starting miniature synchronous electric motor comprising a rotor having a plurality of substantially equally spaced poles of alternate polarities, the number of pole pairs n being five or a larger odd integer, a stator comprising first and second substantially semi-circular stator portions of substantially the same diameter disposed to surround said rotor the centre points of said portions being displaced substantially symmetrically at opposite sides of the axis of rotation of said rotor such that said portions are point symmetrical with respect to said axis, each said stator portion having a main stator pole at each end thereof adjacent the other stator portion each pole comprising a pole piece projecting from the respective stator portion towards the rotor.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic end view showing the basic construction of a self-starting miniature synchronous motor embodying the invention; Fig. 2 is an enlarged view of a portion of the motor shown in Fig. 1; and Fig. 3 is an enlarged view corresponding to Fig. 2 and showing a modified embodiment of this invention.

A self-starting miniature synchronous motor 10 embodying the invention and shown in Fig. 1 comprises a rotor 12 having a shaft 11 and made of such magnetic material as isotropic barium ferrite.

Equally spaced N and S poles are alternately formed, by magnetization, around the periphery of the rotor 12 thus forming a total of n pole pairs. In this embodiment n is equal to 7. A pair of stators 14 and 15 are provided to surround the rotor, and these stators include stator poles 16 and 17 in the form of C or semicircular configurations facing the rotor. The ends of the stators opposite the stator poles are innerconnected by a magnetic yoke 13 on which a single exciting coil is wound.

Although not shown in the drawing, it should be understood that the rotor shaft 11 is supported by a frame.

The detail of the poles of the stators 14 and 15 and the rotor will now be described with reference to Fig. 2 in which the opposed semicircular portions of the stator poles 16 and 17 have the same radius and disposed symmetrically with respect to the axis of the rotor shaft 11. However, the centers p and q of the semicircular portions do not coincide with the axis of the shaft 11 but are slightly eccentric with respect to the axis. More particularly, these centers are point symmetrical with respect to the axis of the shaft 11 and equally spaced therefrom. The semicircular portions of the stator poles 16 and 17 are formed with main pole pieces 16a, 16b, 17a and 17b and interpole pieces 16c, 16d, 17c and 17d respectively and notches 19a, 20a, 19b, 2gob; and 19c, 20e are formed between the main poles pieces 16a, 17a and interpole pieces 16c, 17c and between interpole pieces 16c, 17c and 16d, 17d, and between interpole pieces 16d, 17d and main pole pieces 16b, 17b respectively. The bottoms of these notches lie on circles having the same centers p and q as the semicircular portions respectively.

As above described since the centers of the semicircular portions of the stator poles are offset from the axis of rotor shaft, the main pole piece 16b of the stator pole 16 is closest to the rotor, and the distance to the rotor increases in the order of interpole pieces 16d and 16c and main pole piece 16a. On the other hand, the main pole piece 17a of the stator pole 17 is closest to the rotor and the distance to the rotor increases in the order of interpole pieces 17c, 17d and the main pole piece 1Th. Accordingly, when one considers the relationship between adjacent main pole pieces and the rotor, the main pole piece 17a is closest to the rotor whereas the main pole piece 1 6a is located most remotely from the rotor.

In, the same manner, the main pole piece 16b is located closest to the rotor while the main pole piece 17b is located most remotely from the rotor.

The number of poles of the rotor and the number of pole pieces of the stator are determined under the following conditions so as to always apply a repulsive force to the rotor. Thus, one half of the number of rotor poles is equal to m, where n is an integer, 7 in this example, and the number of the stator pole pieces is selected to be n+1 equal to , that is 4 in this example.

2 When the number of poles of the rotor and the number of stator pole pieces are selected in this manner, the pole pieces 17a, 17b, 17c, 16a, 16b and 16d of the stator poles 16 and 17 will come to exactly confront N and S poles of the rotor respectively with the result that when the stator pole pieces are magnetized by the exciting coil 18, the attractive forces or repulsive forces created between the stator pole pieces and the rotor poles are all in the same direction and add each other to create a torque.

Since the semicircular portions of the stator poles are made eccentric with each other, the pole pieces 17d and 16c do not exactly confront the corresponding poles of the rotor, but they are located at such displaced positions as to increase the torque. In this manner, according to this invention it is possible to eliminate idle poles which are inherent to the construction taught by U.S.

Patent No. 3256453.

Furthermore in accordance with this invention, the number of the rotor pole pairs is determined to be an odd number, (in this example 7) so that when the stator pole piece 1 7a comes to confront a S pole 21 a of the rotor as shown in Fig 2, a N pole 21c symmetrically opposite the pole 21a will confront the stator pole piece 16b.

This means that when the stator pole piece 1 7a is magnetized to become a S pole so as to create a repulsive force between it and the rotor pole 21a, the stator pole piece 16b would be magnetized to become a N pole so that a repulsive force will also created between it and rotor pole 21c. On the other hand, when the stator pole 17 is magnetized to act as a N pole and when the stator pole 16 is magnetized to act as a S pole attractive forces will be created be tween the rotor pole 21a and the stator pole piece 17a and between the rotor pole 21c and the stator pole piece 17b. Accordingly, it is possible to eliminate the problem of the one sided wear of the rotor shaft which has been inevitable in the construction shown in U.S. Patent No. 3256453, thereby prolonging the life of the motor.

In this invention, as can be understood from the description of operation to be described later n is an integer larger than 5 because interpole pieces are provided in addition to the main pole pieces.

The angular width of the main pole pieces 17a, 17b, 16a and 16b is selected to be about 65-75% of a value 360"/number of rotor poles whereas that of the interpole pieces to be 40 to 50%. When angular widths deviate from these values the torque of the motor would decrease greatly.

Especially where the angular width of the main pole pieces is set to be 3600/number of rotor poles, the operating range of the exciting voltage of coil 18 will become narrow thus causing hunting.

The motor of this invention operates as follows.

Suppose now that when the rotor 12 is standstill at a position shown in Fig. 2, a pulse voltage is impressed upon the exciting coil 18 to magnetize stator pole 17 to act as a S pole and stator pole 16 as a N pole.

Then the lefthand portion 17al of the pole piece 17a is located closer to the S pole 21a of the rotor than the righthand portion 17a2, a repulsive force is created between the pole piece 17a and the S pole 21a so that the rotor 12 rotates in the counterclockwise direction. In the same manner, as the lefthand portion 17cl of the interpole piece 17c is located closer to the S pole 21b of the rotor than the righthand portion 17c2 a repulsive force is also created between the interpole piece 17c and the S pole 21b of the rotor, with the result that the rotor is subjected to a torque also in the counterclockwise direction. In the same manner, a repulsive force is created between the stator pole pieces 16b and 16d and the S poles 21c and 21d of the rotor to rotate. the rotor in the counterclockwise direction. When the rotor rotates a predetermined angle the repulsive force between the stator pole piece 17a the S pole 21a of the rotor 12 disappears and now an attractive force appears between a N pole 21e of the rotor adjacent the S pole 21a and the stator pole piece 17a. Similar attractive forces are created between other stator pole pieces and the other rotor poles, thus rotating the rotor until the N pole 21e of the rotor comes to confront the stator pole piece 17a. The angle of one step rotation caused by one pulse is equal to 360"/14=25.7". When the excitation of the pole piece 17a disappears, the rotor will stop at a position in which the rotor has been slightly attracted toward the lefthand portion 17al of the stator pole piece 17a.

Upon application of a pulse voltage of the opposite polarity upon the exciting coil 18, similar operation would be repeated.

Thus, by alternately applying positive and negative pulse voltages the rotor 12 is rotated stepwisely. Since notches 20a, 20b, 19a and 19b are formed between the main pole pieces and the interpole pieces, the magentic forces between these notches and the rotor are considerably smaller than those created between the stator pole pieces and the rotor. Consequently, the magnetic forces created between the stator pole pieces 17a, 17c, 16b and 16d and the rotor poles are larger that those created in the prior art motor, thus increasing the torque. In addition, since the main pole pieces 16a and 1 7b are more remotely located from the rotor than the other main pole pieces the influence of these pole pieces 16a and 17b upon starting is not large but these pole pieces act to provide paths for the magnetic flux flowing between them and adjacent stator main pole pieces 17a and 16b.

As above described, since notches are provided for the stator poles, a suitable number of interpoles are provided between the main stator pole pieces and since the semicircular portions of the stator poles are eccentric and main pole pieces 16a and 17b are located more remotely from the rotor, a larger torque is applied to the rotor irrespective of the angular position thereof thus positively self-starting the rotor without the trouble of dead centers as in the prior art construction.

When a pulse voltage is impressed upon the exciting coil 18, the stator poles 16 and 17 are magnetized alternately in the opposite directions for creating attractive and repulsive forces between the stator pole pieces and the rotor poles whereby the rotor rotates at a speed corresponding to the frequency of the pulse voltage. When a pulse voltage having such low frequencies as one or two pulses per second is impressed upon the exciting coil the rotor rotates stepwisely at a speed corresponding to the pulse frequency in the counterclockwise direction, whereas when a pulse voltage having higher frequency as 25 or 35 Hz per second is applied to the exciting coil 18, the rotor will rotate continuously instead of a stepwise rotation. The direction of rotation is counterclockwise or clockwise.

With these high frequencies, either a source of a pulse voltage having a unity duty ratio or a source of sine wave may be used.

In the above described embodiment, the semi-circular portions of the stator poles 16 and 17 are provided with main pole pieces 16a, 16b, 17a and 17b and interpole pieces 16c, 16d, 17c and 17d which are separated by notches 19a, 19b, 19c, 20a, 20b and 20c respectively, so that at an instant when the notches come to confront the rotor poles the attractive and repulsive forces created therebetween are decreased. For this reason, no restraining force is created between the rotor poles and the stator poles as in the prior art construction. This advantage is independent of the frequency of the pulse voltage applied to the exciting coil 18. Thus, the motor of this invention operates stably without the troubles of hunting and dead centers over a wide frequency range from low to high frequencies.

Where the motor is driven with a low frequency voltage, although the motor operates satisfactory with only the main stator pole pieces 17a, 17b, 16a and 16b for the purpose of increasing the starting torque and for assuring stable operation it is advantageous to add interpole pieces 17c, 17d, 16c and 16d.

Fig. 3 shows a modified embodiment of this invention which is different from that shown in Fig. 2 in that the interpole piece 17'c is brought closer to main pole piece 17a by an angle of about (360"/number of rotor poles)/4 than the interpole piece 17c shown in Fig. 2. In the same manner, the interpole piece 17'd is brought closer to the main pole piece 17b by an angle of about (3600/number of rotor poles)/4 than the interpole piece 17d shown in Fig, 2.

Furthermore, the interpole pieces 16'c and 16'd of the stator pole 16 are also displaced by an angle of about (360"/rotor pole number)/4 toward the main pole pieces 16a and 16b when compared with the interpole pieces 16c and 16d shown in Fig. 2. The construction of other elements is the same as that of Fig. 2.

As above described in the embodiment shown in Fig. 3, although the main pole pieces 16a, 16b, 17a and 17b substantially exactly confront corresponding poles of the rotor as in the embodiment shown in Fig.

2, the interpole pieces 16'c, 16'd, 17'c and 17'd do not confront the rotor poles.

Suppose now that the rotor is at a standstill at a position shown in Fig. 3. When a voltage is impressed upon the exciting coil 18 to magnetize the stator poles 16 and 17 to become N and S poles respectively. Then, an repulsive force will ;be created between the lefthand portion 17'cl of the interpole piece 17'c and the S pole 21b of the rotor thus applying a counterclockwise torque to the rotor. As above described, since the interpole piece 17'c is located closer to the main pole piece 17a than the interpole piece 17c shown in Fig.

2, the angular rotation at this moment is larger than that of Fig. 2. Accordingly, the S pole 21a of the rotor is brought closer to the righthand portion 1 7a 1 of the main pole piece 17a. In the same manner, similar repulsive force is created between the righthand portion 16!dl of the interpole piece 16'd and the rotor N pole 21d thus repelling the N pole 21c of the rotor toward the righthand portion 16bl of the main pole piece 16b. For this reason, the attractive and repulsive forces between the rotor poles and the stator pole pieces are larger than those of Fig. 2, with the result that the rotor rapidly self-starts without the trouble of any dead center.

The interpole pieces contribute little to the rotor torque so that they may be omitted.

Although in the foregoing embodiments a single exciting coil 18 was wound about a common yoke each stator portion may be made up of a stator pole and a yoke and the yokes of respective stator portions may be interconnected by a yoke wound with an exciting coil.

In other embodiments of the invention the or at least one interpole of each stator portion may be spaced from that main stator pole which is nearer the rotor so that when the main stator pole registers with a rotor pole of one polarity the or at least one interpole is displaced by a predetermined angle away from registration with another rotor pole of said one polarity in a sense away from that main stator pole.

WHAT WE CLAIM IS: 1. A self-starting miniature synchronous electric motor comprising a rotor having a plurality of substantially equally spaced poles of alternate polarities, the number of pole pairs n being five or a larger odd integer, a stator comprising first and second substantially semi-circular stator portions of substantially the same diameter disposed to surround said rotor the centre points of said portions being displaced substantially symmetrically at opposite sides of the axis of rotation of said rotor such that said portions are point symmetrical with respect to said axis, each said stator portion having a main stator pole at each end thereof adjacent the other stator portion each pole comprising a pole piece projecting from the respective stator portion towards the rotor.

2. A motor as claimed in claim 1 wherein the stator portions have a common exciting coil.

3. A motor as claimed in claim 1 or claim 2 wherein each stator portion has at least one stator interpole spaced by a recess from that main stator pole which is nearer the rotor.

4. A motor as claimed in claim 3 wherein

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. 16a, 16b, 17a and 17b and interpole pieces 16c, 16d, 17c and 17d which are separated by notches 19a, 19b, 19c, 20a, 20b and 20c respectively, so that at an instant when the notches come to confront the rotor poles the attractive and repulsive forces created therebetween are decreased. For this reason, no restraining force is created between the rotor poles and the stator poles as in the prior art construction. This advantage is independent of the frequency of the pulse voltage applied to the exciting coil 18. Thus, the motor of this invention operates stably without the troubles of hunting and dead centers over a wide frequency range from low to high frequencies. Where the motor is driven with a low frequency voltage, although the motor operates satisfactory with only the main stator pole pieces 17a, 17b, 16a and 16b for the purpose of increasing the starting torque and for assuring stable operation it is advantageous to add interpole pieces 17c, 17d, 16c and 16d. Fig. 3 shows a modified embodiment of this invention which is different from that shown in Fig. 2 in that the interpole piece 17'c is brought closer to main pole piece 17a by an angle of about (360"/number of rotor poles)/4 than the interpole piece 17c shown in Fig. 2. In the same manner, the interpole piece 17'd is brought closer to the main pole piece 17b by an angle of about (3600/number of rotor poles)/4 than the interpole piece 17d shown in Fig, 2. Furthermore, the interpole pieces 16'c and 16'd of the stator pole 16 are also displaced by an angle of about (360"/rotor pole number)/4 toward the main pole pieces 16a and 16b when compared with the interpole pieces 16c and 16d shown in Fig. 2. The construction of other elements is the same as that of Fig. 2. As above described in the embodiment shown in Fig. 3, although the main pole pieces 16a, 16b, 17a and 17b substantially exactly confront corresponding poles of the rotor as in the embodiment shown in Fig. 2, the interpole pieces 16'c, 16'd, 17'c and 17'd do not confront the rotor poles. Suppose now that the rotor is at a standstill at a position shown in Fig. 3. When a voltage is impressed upon the exciting coil 18 to magnetize the stator poles 16 and 17 to become N and S poles respectively. Then, an repulsive force will ;be created between the lefthand portion 17'cl of the interpole piece 17'c and the S pole 21b of the rotor thus applying a counterclockwise torque to the rotor. As above described, since the interpole piece 17'c is located closer to the main pole piece 17a than the interpole piece 17c shown in Fig. 2, the angular rotation at this moment is larger than that of Fig. 2. Accordingly, the S pole 21a of the rotor is brought closer to the righthand portion 1 7a 1 of the main pole piece 17a. In the same manner, similar repulsive force is created between the righthand portion 16!dl of the interpole piece 16'd and the rotor N pole 21d thus repelling the N pole 21c of the rotor toward the righthand portion 16bl of the main pole piece 16b. For this reason, the attractive and repulsive forces between the rotor poles and the stator pole pieces are larger than those of Fig. 2, with the result that the rotor rapidly self-starts without the trouble of any dead center. The interpole pieces contribute little to the rotor torque so that they may be omitted. Although in the foregoing embodiments a single exciting coil 18 was wound about a common yoke each stator portion may be made up of a stator pole and a yoke and the yokes of respective stator portions may be interconnected by a yoke wound with an exciting coil. In other embodiments of the invention the or at least one interpole of each stator portion may be spaced from that main stator pole which is nearer the rotor so that when the main stator pole registers with a rotor pole of one polarity the or at least one interpole is displaced by a predetermined angle away from registration with another rotor pole of said one polarity in a sense away from that main stator pole. WHAT WE CLAIM IS:

1. A self-starting miniature synchronous electric motor comprising a rotor having a plurality of substantially equally spaced poles of alternate polarities, the number of pole pairs n being five or a larger odd integer, a stator comprising first and second substantially semi-circular stator portions of substantially the same diameter disposed to surround said rotor the centre points of said portions being displaced substantially symmetrically at opposite sides of the axis of rotation of said rotor such that said portions are point symmetrical with respect to said axis, each said stator portion having a main stator pole at each end thereof adjacent the other stator portion each pole comprising a pole piece projecting from the respective stator portion towards the rotor.

4. A motor as claimed in claim 3 wherein

the or at least one said interpole of each said stator portion is spaced from that main stator pole which is nearer the rotor so that when the main stator pole registers with a rotor pole of one polarity the or at least one said interpole registers with another rotor pole of said one polarity.

5. A motor as claimed in claim 3, wherein the or at least one said interpole of each said stator portion is spaced from that main stator pole which is nearer the rotor so that when the main stator pole registers with a rotor pole of one polarity the or at least one said interpole is displaced by a predetermined angle away from registration with another rotor pole of said one polarity in a sense away from that main stator pole.

6. A motor as claimed in claim 5, wherein said predetermined angle is (360 /2n)/4.

7. A motor as claimed in any one claims 3 to 6, wherein the number of main poles and interpoles provided for each said stator portion is (n+1)/2, each said main pole and interpole and the interposed said recesses being arranged so as simultaneously to substantially confront different ones of said rotor poles.

8. A motor as claimed in claim 1 including at least first and second stator interpoles between the main stator poles of each said stator portion, said stator poles and interpoles being spaced so that when the main stator poles register with rotor poles of one polarity said interpoles are each displaced by a predetermined angle away from registration with other rotor poles of said one polarity in senses towards the respective nearer main stator pole.

9. A self-starting miniature synchronous electric motor substantially as described herein with reference to Figures 1 and 2, or Figure 3, of the accompanying drawings.