DC ELECTRIC MOTOR WITH TWO COMMUTATORS AND PERMANENT MAGNET ROTOR The invention relates to electric motors, and in par- ticular to miniature DC motors. Such motors are useful eg in mobile telephones where such a motor can have a mass mounted eccentrically on the motor shaft. In addi- tion to being acoustically signalled, or as an alterna- tive thereto, incoming calls can thereby be signalled by vibrations generated by the motor rotating. Signal- ling by means of vibrations is preferably not audible but can be sensed by a user carrying the mobile tele- phone eg in a pocket.

In mobile phones power consumption of the various com- ponents is of great interest. It is therefore important that all components have low power consumption for the desired function. It is thus desirable to a motor, which can be used as vibrator motor in mobile phones, and which has low power consumption and yields a high torque both at start-up and when running. Furthermore the motor should generate minimum possible electric noise and have a long expected lifetime. The motor ac- cording to the invention achieves these goals.

WO 00/41289 discloses a miniature DC-motor with a single set of coils and a single commutator brush commutating the total current from one coil to another. With the ar- rangement of the invention the current path includes two coils, one from each set of coils, in series. The current is maintained in one of the coils in series and only switched in the other. Hereby electric noise and electro- magnetic interference (EMI) is reduced, and the motor has a higher torque for the same consumed electric power-or

a lower power consumption for the same torque, and the motor can be made very compact.

In the following the invention will be described with reference to the drawings, in which Figure 1 is a longitudinal section through a motor in a preferred embodiment of the invention, Figure 2 is a schematic drawing illustrating the con- tacting of the motor in figure 1, Figure 3 shows schematically the structure of the coils of the motor in figure 1, and Figure 4 shows schematically a cross section through the motor in figure 1 and illustrates the contacting of the motor.

Figure 1 shows a motor with a stator 1. In this embodi- ment the stator is tubular and is made of a soft mag- netic material. Inside the tubular stator 1 is a flexi- ble sheet 2 with coils formed on one side. The coils are indicated at 2a. The coils can also be formed on both sides of the sheet 2. Alternatively, conventional wound coils may also be used. At the ends of the tubular sta- tor 2 there are a first bearing 4 and a second bearing 4a respectively, which receive the opposite ends of a shaft 6 of an electrically conductive material. The shaft 6 carries a magnet 3, which is magnetised in the direction of its diameter, ie transversally to the motor axis. The two bearings 4 and 4a are preferably identical and have a first set of contactors Cl and a second set of contactors C2 respectively, which are formed as flat

surfaces perpendicular to the motor axis and facing in- wards in the stator. At both sides of the magnet 3 the shaft 6 has contact springs 5 and 5a respectively, which are made of a resilient, electrically conductive metal wire. The contact springs 5 and 5a have a narrow end and a wider portion with a free end. The contact springs are secured to the shaft and electrically connected to the shaft at their arrow ends. Each of the free ends of the contact springs is in electrical contact with a contact surface of respective ones of the bearings. At the bear- ing 4a the shaft protrudes beyond the bearing, and the protruding end of the shaft carries an eccentrically mounted mass 7.

Figure 2 shows a first set of coils S1, S3 and S5. Each of the coils S1, S3 and S5 in the first set has one coil wire end connected to a first terminal T1 of the motor, and the other coil wire ends are connected to respective ones of the contactors Cla, Clb and C1c of the first set of contactors Cl situated at the first bearing 4. Figure 2 also shows a second set of coils S2, S4 and S6. Each of the coils S2, S4 and S6 in the second set has one coil wire end connected to a second terminal T2 of the motor, and the other coil wire ends are connected to re- spective ones of the contactors C2a, C2b and C2c of the second set of contactors C2 situated at the second bear- ing 4a. The rotor with the motor shaft 6 and contact springs 5 and 5a is for illustrative purposes indicated by a circle. The structure in figure 2 is shown"folded out". In the motor the shown structure is cylindrical, and in each set of coils the coils are spaced 120 de- grees apart. In each set of contactors the three contac- tors are also spaced 120 degrees apart. Each contactor extends for slightly less than 120 degrees so that a

small gap exists between neighbouring contactors.. The first set of coils and the first set of contactors are angularly offset 60 degrees relative to the second set of coils and the second set of contactors.

In operation the terminals T1 and T2 are connected to a DC power source In the shown position of the motor shaft an electric current path is established from the first terminal T1 through the coil S5 of the first set of coils, the contactor Clc of the first set of contactors, the first contact spring 5, the shaft 6, the second con- tact spring 5a, the contactor C2c of the second set of contactors, the coil S4 of the second set of coils and the second terminal T2. The electric current in the coils S5 and S4, generates a magnetic field, which inter- acts with the rotor magnet 3 and causes the rotor to ro- tate. Upon rotation of the rotor the shaft 6 with con- tact springs 5,5a shown schematically in figure 2 will move from the position shown to a position where the current path is maintained in the coil in one set of coils, whereas the current in the coil in the other set of coils is shifted to another coil in the same set of coils. The current path thus always includes one coil in one set of coils and one coil in the other set of coils connected in series, and upon rotation of the rotor switching of the current path is done alternatingly be- tween the first set of coils and the second set of coils. Each switching operation thus only switches half of the current path and half of the coils, while the other half of the current path and the other half of the coils is switched subsequently.

Figure 3 shows schematically the layout of the coils on the flexible sheet. The sheet will be rolled and placed

inside the tubular stator near its cylindrical inner surface, where it will be held in place by means of the two bearings. It is seen that each coil has a lateral extension corresponding to 180 degrees when placed in the stator, and that the coils are spaced apart corre- sponding to 60 degrees.

Figure 4 shows a schematical cross section through the motor. The outermost structure is the rolled sheet with the coils. Also shown are the two sets of contactors, where, for illustrative purposes, the first set of con- tactors Cla, Clb and Clc are shown having a larger di- ameter than the second set of contactors C2a, C2b and C2c. Also shown is a contact brush Cbr illustrating the shaft with the two contact springs. The contact brush Cbr is shown bridging contactors in the two sets, in this case contactor Clc in the first set and contactor C2b in the second set.