Abstract:
An electric motor ( 10 ) with a low number of revolutions comprises a rotor ( 17 ) powered by three-phase alternating voltage, a series of magnets ( 21, 22 ) and a coil forming a stator ( 24 ), in which the rotor ( 17 ) and the stator ( 24 ) have the same number of magnetic poles. The stator ( 24 ) is powered with direct voltage, and the frequency of the rotor voltage ( 17 ) is varied to obtain a predetermined number of rpm of the motor and to vary the acceleration and deceleration conditions of the motor. A thrust bearing opposes the effort of the falling brake disc.

Description:
TECHNICAL FIELD 
       [0001]    This invention refers to an electric motor with a low number of revolutions. 
         [0002]    In particular, this invention refers to an electric motor with a low number of revolutions which can be advantageously used for applications requiring torque. 
         [0003]    By way of example, the electric motor according to the invention can be used to drive lifting devices, such as lifts. 
       BACKGROUND ART 
       [0004]    Electric motors used in applications with a heavy torque requirement, for example electric motors for lifts, are generally positioned inside relatively small spaces, which is a source of problems as the motors are usually fairly large due to the presence, in addition to the motor itself, of a reduction unit and obviously the pulley around which the cables that raise and lower the lift cabin are wound. 
         [0005]    The reduction units used in these applications often have electromechnical characteristics that are fairly inconstant over time, leading to functioning irregularities that cause a considerable reduction in the performance of the motor. 
       DESCRIPTION OF THE INVENTION 
       [0006]    This invention proposes to provide a solution to the background art problems, thanks to a new design of motor which is small and is not equipped with a reduction unit. 
         [0007]    In particular, in the lifts sector, this invention proposes to provide an electric motor with a low number of revolutions (for example a motor with approximately 50 rpm corresponding to a lifting speed of around 1 m/sec) thanks to which the components necessary for the movement and the braking of the machine can be directly contained inside the drive pulley. 
         [0008]    This is achieved by means of an electric motor with the features described in the main claim. The dependent claims describe particularly advantageous embodiments of the electric motor according to the invention. 
         [0009]    The most typical configuration of the electric motor according to the invention consists of a rotor (with a high number of magnetic poles) powered by three-phase alternating voltage, and of a stator with the same number of magnetic poles as the rotor, created by a coil powered by direct voltage. 
         [0010]    According to a particularly advantageous embodiment of the invention, the direct voltage consists of a single-phase rectified and thus pulsating alternating voltage. This direct voltage makes it possible to obtain excellent results in terms of motor performance. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0011]    Other features and advantages of the invention will become clear on reading the description of some embodiments of the invention, given as non-binding examples, with the help of the accompanying drawing which shows a cross-section of an electric motor according to the invention. 
       
    
    
     DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION 
       [0012]    In the figure, the reference number  10  indicates overall an electric motor with a low number of revolutions used to drive lifting devices according to the invention. 
         [0013]    The motor  10  is supported by a fixed metal shaft  11  by means of a pair of bearings  12 ,  12 ′ (either roller or ball bearings) and is enclosed in the casing  13  of a pulley, the outer surface of which is equipped with a series of slots  14 ,  14 ′ . . . designed to accommodate cables (not shown in the figure) for driving a lifting device, for example a lift. The casing  13  is rigidly fixed by means of screws  15  to a pair of diamagnetic side plates  16 ,  16 ′ which work in conjunction with the said roller bearings  12 ,  12 ′. 
         [0014]    The actual electric motor is housed inside the space formed by the diamagnetic plates  16 ,  16 ′ and the pulley casing  13 . 
         [0015]    In particular, integral with the inner surface of the pulley  13  is a rotor pack  17  fitted with a series of slots (not shown in the figure) designed to accommodate a three-phase winding powered by alternating voltage. This three-phase winding is connected to three respective conductor rings  18 ,  18 ′,  18 ″ inserted in appropriate slots cut in the side plate  16 ′. In addition, a protective guard  19  consisting of two portions joined together and fixed to the shaft  11  supports three respective brushes  20 ,  20 ′,  20 ″ in contact with the said conductor rings  18 ,  18 ′,  18 ″. 
         [0016]    Respective magnets  21 ,  22  are positioned next to each other and fixed to the shaft  11 . These magnets  21 ,  22  define a space which houses a stator-powered coil  24  powered by direct voltage in order to create the magnetic polarities of the stator. 
         [0017]    Finally, the motor  10  comprises an electromagnetic brake  25  consisting of a pair of metal discs, one of which is integral with the side plate  16 ′ and the other with the magnet  22 , and of disc-pushing springs contained in appropriate slots cut in the magnet  22 . 
         [0018]    Such a motor is particularly suited for application as a machine without a reduction unit in the lifts sector, since the drive pulley can contain all the equipment necessary both to move and to brake the machine. 
         [0019]    According to the invention, the respective power supplies of the stator coil  24  (advantageously direct voltage obtained by rectifying single-phase alternating voltage) and of the rotor windings  17  (three-phase alternating voltage) are governed by means of an appropriate control device, until the required number of revolutions is reached, which depends on the number of poles of the motor and on the frequency of the three-phase power supply. 
         [0020]    The control device that can be used for these applications is the variable frequency type, for example the ac-dc-ac or dc-ac type. 
         [0021]    According to the invention, the stator coil  24  is powered by direct voltage. In this embodiment, the motor comprises a high number of magnetic poles, both stator and rotor having the same number, and in the stator the poles are the clawed type as in automobile alternators. Some features of the synchronous motor in terms of performance are lost in this case, achieving however an acceleration torque that makes it possible to obtain the required number of revolutions from the rotor by merely varying the frequency of the three-phase power supply of the rotor itself. 
         [0022]    In order to avoid flow leakage between the driving shaft and the coil, the side plates  16 ,  16 ′ are made from diamagnetic material. 
         [0023]    As far as the steady state speed and the braking and acceleration phases are concerned, the stator  24  is constantly powered by direct voltage, while the rotor  17  has a variable frequency achieved by means of a frequency converter external to the rotor. In this case, during the acceleration and deceleration phases the frequency of the three-phase voltage of the rotor  17  is adjusted by frequency converters (ac-dc-ac or dc-ac) which provide the frequencies, currents and thus the torques required. 
         [0024]    The invention is described above with reference to a preferred embodiment, in which the main shaft is fixed, the stator is integral with the shaft and the rotor turns around the shaft and the stator. 
         [0025]    Embodiments of this invention are nevertheless foreseen in which the conformation of the motor is inverted, and the main shaft rotates, while the motor casing integral with the three-phase windings is fixed. 
         [0026]    In this case, the main shaft is equipped with a pair of conductor rings connected to appropriate cables inserted in slots in the shaft and connected to windings of the coil and of the rotating magnets. 
         [0027]    A pair of brushes is then connected to the rings for transmission of the current, while the mechanical drive organs, in particular the lifting organs, are connected to the main shaft.