Abstract:
The invention relates to an electronically commutated motor ( 10 ) and to a method of controlling an electronically commutated motor ( 10 ). In order to reduce commutation noise, it is proposed to influence the working range of the power-stage transistors ( 20, 22 ) with the aid of a component ( 48 ), in such a way that each transistors produces, during energization of each respective stator winding, a substantially constant current through the stator winding ( 12, 14 ). Preferably, each power-stage transistor operates within a pinch-off range.

Description:
CROSS-REFERENCE 
       [0001]    This application is a section 371 of PCT/EP2004/014759, filed 28 DEC. 2004, published 18 AUG. 2005 as WO 2005/076 456-A1. It also claims priority from German application DE 10 2004 006 449.0, filed 3 FEB. 2004, the entire content of which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to an electronically commutated motor and to a method for controlling an electronically commutated motor. 
       BACKGROUND 
       [0003]    With such motors, the occurrence of commutation noise is problematic for a number of applications. A number of suggested solutions to this are known from the existing art. 
         [0004]    A characteristic common to all the suggested solutions is that they are very complex and cost-intensive. 
       SUMMARY OF THE INVENTION 
       [0005]    It is the object of the present invention to reduce commutation noise. This object is achieved by a motor in which a respective field-effect transistor (FET) controls the current through each stator winding phase, and a further component influences the working range of the FET to make the current through the winding phase substantially current while that phase is energized. In a preferred method, the FET is operated as a pinch-off current source. 
         [0006]    The invention is based on the recognition that low-noise commutation can be achieved by means of a current through the stator winding that is substantially constant at least during the on-time of the stator winding. A basic idea of the invention is therefore to influence the working range of the field-effect transistor associated with the stator winding in such a way that the transistor generates, during the respective energization, a substantially constant current through the stator winding. A component configured for the purpose is provided therefor. 
         [0007]    According to a preferred embodiment of the invention, the component is configured in such a way that the field-effect transistor is operated as a pinch-off current source. 
         [0008]    In a further preferred embodiment of the invention, the component comprises a transistor. In other words, the field-effect transistor associated with the stator winding is shifted into the pinch-off region with the aid of a further transistor. This further transistor is preferably controlled by means of a variable resistor or by means of a microcontroller. This control action results in a change in the conductivity of the transistor, which results in a displacement of the working point into the desired region. The control action on the transistor modifies the current intensity through the stator winding, and thus the rotation speed of the motor. 
         [0009]    In a further embodiment of the invention, provision is made to continuously keep the current in the stator winding substantially constant during operation of the motor. 
         [0010]    As compared with known suggested solutions, the present invention enables low-noise commutation with a comparatively small outlay of material, and using a comparatively simple circuit. The invention is not limited to a specific type of motor. 
     
    
     
       BRIEF FIGURE DESCRIPTION 
         [0011]    Further details and advantageous refinements of the invention are evident from the exemplifying embodiments described below and depicted in the drawings, in which: 
           [0012]      FIG. 1  is a greatly simplified circuit diagram of an electronically commutated motor according to a first embodiment of the invention; 
           [0013]      FIG. 2  is a greatly simplified circuit diagram of an electronically commutated motor according to a second embodiment of the invention; 
           [0014]      FIG. 3  schematically depicts the commutation current through a stator winding according to the existing art (curve A) and according to embodiments of the invention (curves B and C); 
           [0015]      FIG. 4  is a greatly simplified circuit diagram of an electronically commutated motor according to a third embodiment of the invention; and 
           [0016]      FIG. 5  shows a family of characteristic curves of an n-channel field-effect transistor. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  is an exemplary depiction of a two-phase electric motor that can be used with the present invention. The electronically commutated DC motor  10  comprises two stator winding phases  12 ,  14  and a permanent-magnet rotor  16  (depicted merely symbolically). Arranged in the vicinity of rotor  16  is a Hall sensor  18 . For reasons of clarity, the latter is illustrated at a different location in the present circuit diagram. Phase  12  is in series with a first power-stage transistor  20  (MOSFET), and phase  14  is in series with a second power-stage transistor  22  (MOSFET). Phases  12 ,  14  are connected to a positive lead  24 . Positive lead  24  and negative lead  26  are connected, during operation, to a power supply (not depicted) or to a battery. Phases  12 ,  14  are usually coupled to one another in transformer fashion via the iron of the stator lamination stack. 
         [0018]    Hall sensor  18  is connected on the one hand via a resistor  28  to positive lead  24 , and on the other hand to negative lead  26 . The output signal of Hall sensor  18  is delivered, through resistors  30 ,  32  and a capacitor  34 , to the two inputs IN 1  and IN 2  of a microcontroller (pC)  36 . pC  36  is connected at its terminal VCC to positive lead  24 , and at its terminal GND to negative lead  26 . A storage capacitor  38  is arranged between positive lead  24  and negative lead  26 . pC  36  generates signals OUT 1  and OUT 2  to control power-stage transistors  20 ,  22 , and at the same time provides stalling protection for motor  10 . The control signals are generated by means of a program routine or control routine executing in pC  36 . Signal OUT 1  is delivered through a resistor  40  to the gate of power-stage transistor  20 . In the same fashion, signal OUT 2  is delivered through a resistor  42  to the gate of power-stage transistor  22 . The gate of power-stage transistor  20  is connected via a resistor  44  to negative lead  26 . 
         [0019]    In the same fashion, the gate of power-stage transistor  22  is connected via a resistor  46  to negative lead  26 . 
         [0020]    Source terminals S of power-stage transistors  20 ,  22  are connected via a control transistor  48  (MOSFET) to negative lead  26 . Gate G of control transistor  48  is connected to a variable resistor  50  that is arranged between positive lead  24  and negative lead  26 . According to the present invention, power-stage transistors  20 ,  22  are respectively controlled in the source region by control transistor  48  in such a way that the current through stator windings  12 ,  14  is substantially constant at least during commutation. For that purpose, power-stage transistors  20 ,  22  are operated as a pinch-off current source (cf. Tietze/Schenk, Halbleiter-Schaltungstechnik [Semiconductor Circuit Engineering], 12th ed., pp. 411 ff.). For example, when power-stage transistor  20  is controlled by control signal OUT 1 , control transistor  48  acts as a resistance with respect to ground. In this embodiment of the invention, the current intensity through stator windings  12 ,  14 , and thus the rotation speed of motor  10 , can be adjusted by means of variable resistor  50  at the gate of control transistor  48 . This embodiment is especially suitable for those applications in which a change in motor rotation speed during operation is not necessary. 
         [0021]      FIG. 5  shows a family of output characteristic curves of an n-channel field-effect transistor, which family has four characteristic curves  311 ,  312 ,  313 , and  314 . The drain current I D  is plotted for four different gate-source voltages U GS =2.5 V, 3.0 V, 3.5 V, and 4.0 V as a function of drain-source voltage U DS . The family of characteristic curves displays an ohmic region (triode region) OB  300  in which characteristic curves  311  to  314  extend, at the origin U DS =0 V, almost linearly through the origin, yielding a behavior similar to that of an ohmic resistor. 
         [0022]    In addition to ohmic region OB  300 , there is a so-called pinch-off region AB  302  in which characteristic curves  311  to  314  exhibit an almost constant drain current I D . A line  301  marks the boundary between ohmic region OB  300  and pinch-off region AB  302 . 
         [0023]    What is achieved by way of control transistor  48  of  FIG. 1  is that drain-source voltage U DS  is modified, and the magnitude of the current (i) through stator winding  12  is thus also influenced. Because boundary  301  between ohmic region OB  300  and pinch-off region AB  302  is likewise dependent on drain-source voltage U DS , what is also achieved, if applicable, is that transistors  20 ,  22  operate in the pinch-off region. 
         [0024]    All types of field-effect transistors exhibit a pinch-off region of this kind. 
         [0025]      FIG. 2  is the circuit diagram of a motor  10  according to the present invention in a second embodiment, in which the motor rotation speed can be controlled without difficulty. 
         [0026]    For this variable control function, gate G of control transistor  48  is connected to output OUT  3  of a pC  36  via a low-pass filter constituted by a resistor  52  and a capacitor  54 . The low-pass filter converts the digital control signals  51  of pC  36  into an analog voltage signal whose magnitude is dependent on the pulse duty factor of control signals  51 . The remainder of the circuit arrangement corresponds to that of  FIG. 1 . Appropriate control applied to control transistor  48  ensures that the current through stator windings  12 ,  14  is substantially constant. Modification of the conductivity of control transistor  48 , and thus a modification of the motor rotation speed, are accomplished in accordance with the program routines or control routines executing in pC  36 . 
         [0027]    When control transistor  48  is controlled in such a way that it exhibits a high resistance and thus poor conductivity, the potential at the source of the respective power-stage transistor  20 ,  22  rises. Less current flows through power-stage transistor  20 ,  22 , and it transitions into the pinch-off region. 
         [0028]    When power transistor  48  is controlled in such a way that it exhibits a low resistance and thus high conductivity, the potential present at the source of the respective power-stage transistor  20 ,  22  is low. The high gate-source voltage associated therewith results in a correspondingly high current intensity in stator winding  12 ,  14 . 
         [0029]    As depicted in  FIG. 3 , a smoothing of the current curve is achieved with the present invention. In contrast to conventional commutation methods (curve A), according to the present invention the current through a stator winding is kept substantially constant either during energization (curve B) or throughout the entire operating period of the motor (curve C). 
         [0030]      FIG. 4  shows an exemplifying embodiment according to the present invention of a full-bridge circuit for a three-phase electronically commutated motor  10 ′. 
         [0031]    Identical or identically functioning components are labeled with the same reference characters and will not be explained again. 
         [0032]    Stator  220  comprises three winding phases  221 ,  222 ,  223  in a wye configuration, which are connected between a neutral point  224  and winding terminals L 1 , L 2 , and L 3 . 
         [0033]    Power stage  200  is implemented as a full bridge, and comprises three upper transistors  201 ,  202 ,  203  that are connected between positive lead  24  and respective winding terminals L 1 , L 2 , and L 3 ; as well as three lower transistors  204 ,  205 ,  206  that are connected between respective winding terminals L 1 , L 2 , and L 3  and control transistor  48 . 
         [0034]    Gate terminals G of power-stage transistors  201  to  206  are controlled by a power-stage control system  210  via terminals  211  to  216 . 
         [0035]    Control transistor  48  is adjusted in such a way that lower transistors  204 ,  205 ,  206  each operate in the pinch-off region. 
         [0036]    Commutation is accomplished by means of control input to power-stage transistors  211  to  216 , and therefore to winding terminals L 1 , L 2 , L 3 , as a function of the position of rotor  16 ′. 
         [0037]    In a preferred embodiment, the respective upper transistors  201 ,  202 ,  203  or lower transistors  204 ,  205 ,  206  are made conductive for the entire commutation period. In a further preferred embodiment, commutation off-times are provided upon a change in energization in order to prevent short-circuiting. It is additionally possible to control upper transistors  201 ,  202 ,  203  using a clock-timed control signal  201 ,  202 ,  203 . 
         [0038]    Numerous variants and modifications are of course possible within the scope of the present invention.