Synchronous electric motor

The motor comprises a rotor with permanent magnets with at least one pair of poles, a stator having a winding wound around a magnetic circuit, position-detectors for providing driving signals in dependence on the angular position of the rotor, and a driver circuit for controlling the flow of current in the stator winding in dependence on the signals supplied by the position-detectors. The winding is divided into a first half-winding and a second half-winding wound around the magnetic circuit in opposite directions. The driver circuit comprises first and second controlled switches in series with the first stator half-winding and with the second stator half-winding, respectively, in respective circuit branches arranged in parallel between the supply terminals. These switches have respective control inputs connected to the position-detectors in an arrangement such that, during steady-state operation, respective unidirectional currents having a frequency equal to half of the frequency of the alternating supply voltage flow through the half-windings alternately.

BACKGROUND OF THE INVENTION
 The present invention relates to a synchronous electric motor.
 More specifically, the subject of the invention is a single-phase
 synchronous electric motor comprising:
 a rotor or inductor with permanent magnets, with at least one pair of
 poles,
 a stator or armature having a winding wound around a magnetic circuit,
 position-detecting means associated with the rotor for providing driving
 signals in dependence on the angular position of the rotor, and
 a driver circuit connected to the detecting means and to the stator winding
 and having a pair of terminals for connection to an alternating voltage
 supply, the driver circuit being arranged to control the flow of current
 in the stator winding in dependence on the signals supplied by the
 position-detecting means.
 Synchronous motors of this type are characterized in that, during
 steady-state operation, the angular velocity .omega. (in
 revolutions/minute) is related to the frequency f of the alternating
 supply voltage (in Hz) and to the number p of pairs of poles by the known
 equation:
EQU .omega.=f.multidot.60/p
 Thus, for example, a single-phase synchronous motor with two poles (p=1)
 supplied with a 50 Hz alternating voltage rotates, during steady-state
 operation, at an angular velocity .omega.=3000 revolutions/minute.
 A synchronous electric motor with 4 poles (p=2), also supplied with a 50 Hz
 alternating voltage, rotates, during steady-state operation, at an angular
 velocity of 1500 revolutions/minute.
 SUMMARY OF THE INVENTION
 An object of the present invention is to provide a synchronous electric
 motor of the type specified above having, in general p (p=1, 2 . . . )
 pairs of poles but having, during steady-state operation, a synchronism
 rate of revolution equal to that of a motor with 2p poles.
 This and other objects are achieved, according to the invention, by a
 synchronous electric motor, the main characteristics of which are defined
 in appended claim 1.

DETAILED DESCRIPTION OF THE INVENTION
 With reference to FIG. 1, a single-phase, synchronous electric motor 1
 according to the invention comprises a rotor or inductor 2 with permanent
 magnets with (at least) one pair of poles. The rotor 2 is mounted for
 rotating relative to a stator comprising an assembly of plates 3 around
 which is wound a winding formed by two half-windings L1 and L2,
 interconnected at 4. In the embodiment shown, the assembly 3 is
 substantially C-shaped. It could, however, be of any other known shape.
 The half-windings L1 and L2 are wound on the stator magnetic circuit 3 in
 opposite directions. They may be constituted by a single, uninterrupted,
 insulated conductor wire, and in this case, the point 4 represents a
 central tap of the winding as a whole formed by the half-windings L1 and
 L2. Alternatively, the half-windings L1 and L2 may be formed separately
 and then connected at the point 4.
 In the embodiment shown by way of example, a position-detecting device,
 generally indicated 5 in FIG. 1, is associated with the rotor 2 of the
 motor 1. This device can provide electrical driving signals in dependence
 on the instantaneous angular position of the rotor 2.
 In the embodiment shown, the position-detecting device 5 comprises a disk 6
 fixed for rotation with the rotor 2. An arc sector 7 of opaque material
 extending through an angle, for example, of 120.degree., extends on one
 face of the disk 6. The device 5 also comprises two
 photoemitter/photodetector pairs, indicated 8 and 9, disposed in
 respective fixed positions along the path of the sector 7, for example,
 spaced 180.degree. apart. The pair 8 comprises a photodiode (an emitter)
 PD1 and an associated phototransistor (or receiver photodiode) PT1,
 disposed facing one another on opposite sides of the path of the opaque
 sector 7.
 Similarly, the pair 9 comprises a photodiode (an emitter) PD2 and an
 associated phototransistor (or receiver photodiode) PT2.
 The half-windings L1, L2, as well as the photodiodes PD1, PD2 and the
 phototransistors PT1 and PT2 are connected to a driver circuit, generally
 indicated 10 in FIG. 1. This circuit has a pair of terminals A and B for
 connection to an alternating supply voltage, for example, to the
 electrical supply mains.
 FIG. 2 shows the behaviour of the signals P1 and P2 supplied by the
 phototransistors PT1 and PT2 in the course of one complete electrical
 revolution (360.degree.) of the rotor 2, and hence of the disk 6,
 performed clockwise from the starting position shown in FIG. 1,
 assuming--as indicated above--that the opaque sector 7 extends through an
 angle of 120.degree..
 FIG. 3 shows a detailed diagram of an embodiment of the driver circuit 10.
 In the embodiment illustrated, this circuit comprises a first
 silicon-controlled rectifier (SCR) and a second SCR, indicated Q1 and Q2,
 connected substantially in series with the first half-winding L1 and with
 the second half-winding L2, respectively, in respective circuit branches
 B1 and B2 connected in parallel between the supply terminals A and B.
 The photodiodes PD1 and PD2 are connected in series with one another and
 with a resistor R1 in a further circuit branch B3 connected in parallel
 between the terminals A and B.
 The phototransistor PT1 has its emitter connected to the terminal B and its
 collector connected to the terminal A via a resistor R2, and to the gate
 of Q1 via a resistor R3. A resistor R4 is connected between the gate of Q1
 and the terminal B.
 Similarly, the phototransistor PT2 has its emitter connected to the
 terminal B and its collector connected to the terminal A via a resistor R5
 and to the gate of Q2 via a resistor R6. A resistor R7 is connected
 between the gate of Q2 and the terminal B.
 Two filtering capacitors C1 and C2 are also connected between the terminals
 A and B.
 The operation of the synchronous electric motor according to FIGS. 1 and 3
 will now be described with reference also to the graphs shown in FIG. 4.
 The graphs of FIG. 4 show the curves of a plurality of quantities during
 steady-state operation (that is, when the rotor 2 of the motor is rotating
 at synchronism speed).
 In FIG. 4, the curve of the alternating supply voltage applied between the
 terminals A and B is indicated V.sub.AB. The enabling signals of the
 phototransistors PT1 and PT2 are indicated P1 and P2.
 With the curves shown by way of example in FIG. 4, when the opaque sector 7
 intercepts the radiation between the photodiode PD1 and the
 phototransistor PT1, the collector of the latter is brought to "high"
 level. Q1 is correspondingly made conductive as soon as a positive
 half-wave of the voltage V.sub.AB starts. When Q1 becomes conductive, a
 voltage V.sub.L1 substantially corresponding to the voltage V.sub.AB is
 applied to the half-winding L1. A current, indicated I.sub.L1 in FIG. 4,
 which is phase-shifted with a delay relative to the voltage V.sub.L1,
 correspondingly flows in the half-winding L1. The controlled diode Q1 is
 cut off as soon as the current I.sub.L1, becomes zero. When this occurs,
 the voltage V.sub.L1, which in the meantime has become negative, ceases.
 As soon as the opaque sector 7 intercepts the radiation between the
 photodiode PD2 and the phototransistor PT2, the collector of PT2 changes
 to "high" level. The controlled diode Q2 then becomes conductive as soon
 as a positive half-wave of the supply voltage V.sub.AB arrives. A voltage
 V.sub.L2, the curve of which corresponds substantially to that of the
 voltage V.sub.AB is thus localized in the half-winding L2 and a current
 I.sub.L2 which is phase-shifted with a delay relative to the voltage
 V.sub.L2 flows in this half-winding, as indicated in the fifth graph of
 FIG. 4. The controlled diode Q2 remains conductive until the current
 I.sub.L2 becomes zero. When this occurs, the controlled diode Q2 is cut
 off and the voltage in the half-winding L2 ceases.
 During steady-state operation, respective unidirectional currents I.sub.L1
 and I.sub.L2 having a frequency equal to half of the frequency of the
 alternating supply voltage V.sub.AB thus flow through the half-windings L1
 and L2.
 The sixth graph of FIG. 4 shows qualitatively the curve of the
 magnetomotive force MMF relating to the entire winding formed by the
 half-windings L1 and L2. As is known, the magnetomotive force is
 proportional to the current intensity. Since the half-windings L1 and L2
 are wound on the same magnetic circuit in opposite directions, the
 magnetomotive force MMF has a curve with half-waves alternately of
 opposite sign, with a frequency of half of the frequency of the supply
 voltage V.sub.AB, as shown in the sixth graph of FIG. 4.
 The seventh graph of FIG. 4, on the other hand, shows the curve of the
 electromotive force EMF developed as a result of the rotation of the rotor
 2. It also has a frequency which is half the frequency of the supply
 voltage V.sub.AB.
 The last graph of FIG. 4 shows qualitatively the curve of the torque T
 developed by the motor which--as is known--is substantially proportional
 to the product of the magnetomotive force MMF and the electromotive force
 EMF.
 Although the motor described above has only one pair of poles, it rotates
 with a synchronism speed equal to that of a corresponding synchronous
 motor having two pairs of poles, and hence at a speed reduced by half.
 The motor described above thus behaves as a synchronous electric motor with
 4 poles although it has a rotor with two poles and a stator which differs
 from that of a normal two-pole motor purely in that the two half-windings
 are wound in opposite directions.
 The invention thus enables a motor with the performance of a normal motor
 with four poles to be produced simply by a modification of the stator
 portion of a normal two-pole motor, with clear advantages from an
 industrial point of view.
 Naturally, the principle of the invention remaining the same, the forms of
 embodiment and the details of construction may be varied widely with
 respect to those described and illustrated purely by way of non-limiting
 example, without thereby departing from the scope of the invention as
 defined in the appended claims.
 In particular, the invention is not intended to be limited to electric
 motors with a single pair of poles. In fact, the invention enables motors
 with performance corresponding to that of normal motors with 2 p pairs of
 poles to be produced with the use of a rotor with p pairs of poles.
 Moreover, the invention is not limited to motors in which the angular
 position of the rotor is detected by optical sensors. For example, amongst
 the various possible alternative solutions are devices using Hall-effect
 sensors.