Abstract:
A low noise synchronous motor including a permanent-magnet rotor and a stator with at least two pairs of stator poles and corresponding winding further includes an electronic power supply comprising a capacitor that is serially connected to one winding of only one pair of poles to act as a 90° phase shifter. A static switch that is driven by an output of the electronic power device is connected to drive another winding of said two pairs. The static switch is controlled by a sensor detecting the position at the rotor.

Description:
This is a Continuation of application Ser. No. 09/630,732 filed Aug. 2, 2000 now abandoned. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to an electronic power supply for a synchronous motor with permanent-magnet rotor with two pairs of poles, supplied directly from the mains. 
   A synchronous motor with permanent-magnet rotor has many applications, especially in the civil sector, where the power levels involved are low or, as an approximate indication, lower than 100 W. 
   In their most basic structure, these motors have a fixed stator part and a part which rotates about its own axis of symmetry and is termed rotor. In these motors, the stator is constituted by an iron core which is shaped like a tuning fork and on which spools are fitted which support the windings, powered by the mains with an electronic device interposed. 
   The rotor is constituted, in its simplest form, by a cylinder of magnetic material which is rigidly coupled to the rotation shaft. The stator windings, supplied by the mains, produce a magnetic field which interacts with the magnetic poles of the rotor, causing the rotation of the rotor and therefore of the device connected thereto, which can be for example the impeller of a pump. 
   The advantages of the synchronous motor with permanent-magnet rotor with respect to an induction motor are both technical and economical. Technically, this type of motor is more compact, for an equal power level, and is always far more efficient than an asynchronous motor. 
   For instance, the U.S. Pat. No. 5,276,392 to Beckerman or the U.S. Pat. No. 5,325,034 to Reynolds both relate to a single phase AC motor including at least a winding connected in series with a phase shift capacitor and a second winding connected to a switch. 
   This kind of induction motor is asynchronous and there is no need to know the rotor position. Moreover, the rotor doesn&#39;t have permanent magnets and doesn&#39;t require a specific electronic control for driving the starting phase. However, the motor structure is complex, expensive and not suitable for low power applications. 
   Another reason to prefer synchronous motors is their low cost and the simple structure of the rotor and of the stator. This type of motor is single-phase, since the only winding is supplied with power by the mains voltage. 
   The U.S. Pat. No. 5,434,491 to this applicant relates to a synchronous motor with a permanent magnet rotor and including an electronic device for driving the starting phase of the motor. 
   However, this motor has application limits, particularly when low noise in operation is required. Because of its operating characteristic, the generated torque is in fact not constant at each instant during rotation. 
   In particular, the torque oscillates about a medium value and the oscillation frequency depends on the frequency of the supply voltage. 
   The torque that oscillates about the medium value can be considered as the sum of a constant term, which is responsible for moving the load, and of a pulsed term, which produces vibrations in the motor. 
   Additionally, the asymmetry of the stator pack means that there is a preferential direction for the attraction force which is in any case applied between the stator, constituted by iron laminations, and the rotor, which is made of magnetic material. This axial interaction and the pulsed nature of the torque, lead to pulsed stresses and therefore to vibrations which are generated in the stator of the motor. 
   The stator is always rigidly coupled to a supporting structure and therefore the structure is affected by these vibrations unless damping is provided, assuming of course this is possible; such damping is in any case expensive. In some applications, for example in a circulation pump for heating systems, these vibrations are in the audible frequency range and therefore produce an undesirable and unacceptable noise. These vibrations can be reduced, at least theoretically, by means of different electronic or mechanical refinements, but such refinements are expensive and scarcely reliable and in any case are only palliatives, since they tend to reduce the effect, but do not contrast the cause, of the noise. 
   SUMMARY OF THE INVENTION 
   The aim of the present invention is to provide a power supply for a synchronous motor with a permanent-magnet rotor which allows to eliminate the vibrations and the hence noise of the motor. 
   An object of the present invention is to provide an electronic starting device which allows complete structural symmetry, which is a further assurance of elimination of vibrations and of noise. 
   Another object of the present invention is to provide an electronic power supply for a synchronous motor with permanent-magnet rotor which is simple and safe. 
   Another object of the present invention is to provide an electronic power supply for a synchronous motor with permanent-magnet rotor and a synchronous motor with permanent-magnet rotor and two pairs of stator poles which is very efficient, inexpensive and safe. 
   These and other objects which will become better apparent hereinafter are achieved by an electronic power supply for a synchronous motor including a permanent-magnet rotor and two pairs of stator poles, according to the present invention, this motor comprises:
         a winding for each stator pole;   an electronic power device for driving the windings of each pair of poles, said electronic power device receiving directly as power input a main voltage supply;   a capacitor serially connected to one winding of each pair to act as a 90° phase shifter;   a static switch driven by said electronic power device and connected to drive the other winding of said each pair;   at least a position sensor connected to an input of said electronic power device for detecting the position and polarity of the rotor.       

   In order to optimize the performance of the motor, particularly during starting, it can be convenient to provide at least one of the electronic circuits with a booster coil which is engaged during starting and disconnected when steady-state operation is achieved. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further characteristics and advantages of the present invention will become better apparent from the following detailed description of a preferred embodiment thereof, given by way of non-limiting example and shown in the accompanying drawings, wherein: 
       FIG. 1  is a sectional view of the diagram of a motor according to the present invention; 
       FIG. 2  is a basic diagram of the electronic power supply circuit according to the present invention; 
       FIG. 3  is a basic diagram of the power supply circuit with the rotor position sensor; 
       FIG. 4  is a diagram of a phase shifter for the signal that arrives from the rotor position sensor; 
       FIGS. 5 and 6  are views of logic devices for driving the static switch; 
       FIG. 7  is a diagram for driving an auxiliary starting coil; 
       FIG. 8  is a diagram of the control of both coils of static switches; 
       FIGS. 9 and 10  are views of the control of one of the two coils and of the starting coil by means of static switches. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference to the above-cited figures, the motor for which the electronic power supply device is designed is a synchronous motor with at least two pairs of poles with a permanent-magnet rotor, designated by the reference numeral  10  in FIG.  1 . The motor is composed of a stator  11  which has four pole shoes, designated by the reference numerals  12   a  and  12   b  for the first pair and  13   a  and  13   b  for the second pair. 
   Two pairs of windings, respectively designated by the reference numerals  14   a  and  14   b  and  15   a  and  15   b , are further present on each one of the two pairs. In the following pages of the description or in the electrical diagrams, the pair  14   a  and  14   b  is illustrated as a single coil  14  and the pair  15   a  and  15   b  is illustrated as a single coil  15 . 
   A rotor  16  of the permanent-magnet type is arranged, and can rotate, between the pole shoes  12   a ,  13   a ,  12   b  and  13   b . The motor according to the present invention is supplied with power directly by the mains by means of an electronic circuit. 
   Such circuit is shown schematically in FIG.  2  and substantially comprises the two coils  14  and  15 , also shown graphically as being offset by 90 geometric degrees; the coil designated by the reference numeral  14  is supplied by means of a TRIAC  18  which is driven by an electronic device  19 , while the coil designated by the reference numeral  15  is supplied with power by means of a capacitor  17  which phase-shifts through 90° the 5 current that circulates in the coil  15  with respect to the current that circulates in the coil  14 . 
   As shown more clearly in  FIG. 3 , the electronic circuit  19  has a first power input receiving a signal which arrives from the mains voltage and a second signal input receiving a signal which arrives from a position sensor  20  which detects the position and polarity of the rotor. 
   Driving occurs when the polarity of the mains can produce a torque which is favorable for starting, according to the polarity of the rotor that faces the pole shoe. This association is achieved by means of the XOR logic function shown in  FIGS. 4 and 5 . The current on the two coils mutually offset by 90° occurs by means of the capacitor  17 . 
   In order to improve the performance of the motor, depending on the position of the rotor position sensor or of the current-voltage phase shift, it can be necessary to apply, by means of the device  21  of  FIG. 4 , a phase shift to the signal that arrives from the position sensor. In order to further improve the efficiency of the electronic systems, the information related to the current zero-crossing of the motor is acquired so as to drive the TRIAC  18  only when necessary. 
   The current zero-crossing information can be acquired either by direct measurement of the current through the current sensor  28  or by other methods, for instance measuring the voltage drop across the TRIAC, as shown in  FIGS. 5 and 6 . 
   The current zero-crossing information is passed through an AND logic function together with the output of the XOR function, and the output of the AND function is used to drive the TRIAC  18 , as shown in FIG.  5  and in FIG.  6 . 
   In order to further improve the efficiency of the motor, it is possible to use an additional coil, designated by the reference numeral  22  in  FIG. 7 , which constitutes a booster coil which is designed to boost the stator field only during starting. 
   After starting, operation occurs only by means of the steady-state coil, while transition from the booster coil  22  to the steady-state coil  14  can occur by means of a timer or by means of a block, designated by the reference numeral  23  in  FIG. 7 , which is capable of detecting when the rotor  16  reaches the synchronous speed. 
   Moreover the device can be implemented with several configurations on the power section, shown by way of example in  FIGS. 8 ,  9  and  10 . In particular, in  FIG. 8  both coils  14  and  15  are controlled by means of TRIACs, designated by the reference numerals  24  and  25  respectively. In  FIG. 9 , control occurs by means of TRIACs  26  and  27  both on the booster coil and on the steady-state coil, as shown in  FIG. 9 , while  FIG. 10  is a view of a similar embodiment in which the booster coil is a fraction of the steady-state coil and both are TRIAC-controlled. With a configuration of this type, the resulting field is a rotating field which is equivalent to a pair of poles which also rotate about the same rotation axis as the rotor. 
   The interaction of the rotating pair of stator poles with the pair of poles of the rotor produces a torque at the axis which is constant in each instant and therefore free from vibrations. 
   The resulting motor has all the advantages of the high-efficiency synchronous motor with permanent-magnet rotor, while vibration is fully eliminated since the torque is no longer pulsed but is now constant. 
   Another advantage is that this synchronous motor with permanent-magnet rotor with two pairs of stator poles has a constant torque which tends to make it turn in a single direction. 
   During the transient starting step, the motor tends to accelerate monotonically in a direction which is determined by the phase of the power supply voltages. The control system obtains the intended phase shift between the power supply currents of the various phases by using an appropriate capacitor, and also allows direct power supply from the mains without AC/DC conversion. 
   The consequences of this technique are low cost, due to the reduction in power components, simplification of the control circuit, and great reduction in filtering requirements to avoid the harmonics that would be introduced as noise in the network. 
   The system is based on the recognition of the position and polarity of the rotor and of the polarity of the power supply voltage. The power supply of the coils is enabled only when the transient torque generated at that instant is suitable to start the motor. 
   In this way it is possible to achieve good pick-up and high steady-state efficiency. 
   Starting from the same inventive concept, it is possible to produce motors with multi-pole rotors and stators in which the number of stator pairs is twice the number of rotor pole pairs. 
   The dimensions, the materials and the components may of course be any according to requirements. 
   The disclosures in Italian Patent Application No. PD99A000190 from which this application claims priority are incorporated herein by reference.