Abstract:
A two-stranded electronically commutated DC motor has a permanent-magnet rotor ( 36 ), power supply terminals ( 28, 30 ) for connecting the motor to a current source ( 22 ) and a stator ( 102 ) having a winding arrangement which includes first and second winding strands ( 52, 54 ). The latter are controlled by respective first and second semiconductor switches ( 70, 80 ). The motor also has a third controllable semiconductor switch ( 50 ), arranged in a supply lead from one of the terminals ( 28, 30 ) to the winding strands ( 52, 54 ), which third switch is alternately switched on and off by applying to it a PWM (Pulse Width Modulated) signal  24 . During switch-off intervals, magnetic flux energy stored in the motor causes a decaying loop current (i 2 ) to run through the windings, continuing to drive the rotor. This facilitates conformal mapping of temperature information in the PWM signal onto a target motor rotation speed.

Description:
CROSS-REFERENCE 
       [0001]    This application is a section 371 of PCT/EP2006/00483, filed 20 Jan. 2006, and published 21 Aug. 2006 as WO 2006-089605-A. It further claims priority from German application DE 20 2005 003 414.2, filed 24 Feb. 2005, which is incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method of operating a two-stranded electronically commutated motor, and to a motor adapted for carrying out such a method. The invention refers preferably to motors of low and moderate output such as those used to drive fans that could be characterized, for example, using the term “compact fans” or “equipment fans,” e.g. in an output range from approximately 0.5 W to approximately 30 W, preferably approximately 3 W to approximately 20 W. 
       BACKGROUND 
       [0003]    In the context of such fans, a desire exists for them to run at full output, i.e. at a rotation speed of, for example, 4000 rpm, only when the temperature of the object to be cooled is high. It is possible for this purpose, by means of a sensor on or in said object, to generate a temperature signal, and with that a PWM (Pulse Width Modulated) signal whose duty factor depends on the temperature of said object, so that, for example, at a temperature of 20° C., the duty factor is low and the fan consequently runs slowly, since little heat needs to be removed. If, in contrast, the temperature of the object is 70° C., the duty factor is then increased to, for example, 80% and the fan runs correspondingly faster, so that the larger quantity of heat can reliably be dissipated. 
         [0004]    The result of this is that such fans have a longer service life, and at low temperatures a fan of this kind is almost or entirely inaudible, since it is running slowly. 
         [0005]    With fans of this kind, which must be very inexpensive, the problem arises of mapping the temperature information contained in a PWM signal of the kind described onto the rotation speed of the motor or fan as completely as possible and in the manner of a conformal mapping; in other words, as little as possible of this temperature information should be lost. 
         [0006]    For example, it may be that a rotation speed of 15% of the maximum rotation speed of the fan should correspond to a duty factor of 15%, a rotation speed of 20% of maximum speed to a duty factor of 20%, etc. 
         [0007]    If, as a result of peculiarities of the circuit and the design of the motor, the latter works in such a way that the rotation speed consistently has a value of 15% of the maximum rotation speed in a duty factor range from 15 to 50%, and rises only above a duty factor of 50%, the temperature information in the duty factor range from 15% to 50% is therefore lost. This is undesirable, because conformal mapping is not taking place, and because the risk exists that the object to be cooled will overheat. 
       SUMMARY OF THE INVENTION 
       [0008]    It is therefore an object of the invention to make available a novel method of operating a two-stranded electronically commutated motor, and a motor for carrying out such a method. 
         [0009]    According to the invention, this object is achieved by using a commutation controller to actuate first and second semiconductor switches, each arranged in series with a respective winding strand of the motor stator, and also periodically interrupting power supply to the windings by using a third semiconductor switch, located between the DC power source and the motor windings, and controlled by a PWM (Pulse Width Modulated) signal, with the result that, during switch-off intervals, a decaying loop current flows in the windings, and continues to drive the rotor of the motor. When the third semiconductor switch is blocked, energy delivery from the DC source to the one winding strand that is at that instant switched on is interrupted. But because the current in this one winding strand attempts to continue flowing without change, it continues to flow in the one and in the other winding strand. This current flows through the other winding strand, however, in a direction opposite to the “normal” direction, so that the current acts in driving fashion in the other winding strand as well. 
         [0010]    Because the current, flowing through the two winding strands in this state separated from the DC source, jumps from a larger to a smaller value in the manner of a jump function, energy stored in the magnetic circuit of the motor continues to be used to drive the rotor, and little or no reactive power is generated. 
         [0011]    When the third semiconductor switch is then switched back on, energy is once again delivered from the DC source. The current in the other winding strand jumps immediately back to zero, and the current in the one winding strand jumps back to its full value, since here as well a jump function takes effect because at the instant of the jump, nothing changes in terms of the energy content of the magnetic circuit of the motor. As a consequence of this jump, the full drive energy is then once again generated by the current flowing through the one winding strand, while the contribution of the other winding strand returns to zero. 
         [0012]    Another manner of achieving the stated object is provided by a motor structure in which first and second semiconductor switches are respectively arranged in series with first and second stator winding strands, and the winding-remote terminals of the switches (e.g. their source terminals) are connected via a diode to a ground bus. When a PWM signal having a variable duty factor is used to control the third semiconductor switch, the motor can convert this duty factor into a rotation speed in such a way that the information contained in the duty factor is appropriately converted into a corresponding rotation speed; or, in other words, that little or none of the information contained in the duty factor is lost. This is particularly important when the motor drives a fan and the rotation speed of the fan is controlled by a temperature, since in this context the motor rotation speed should increase approximately monotonically when the monitored temperature rises monotonically. 
     
    
     
       BRIEF FIGURE DESCRIPTION 
         [0013]    Further details and advantageous refinements of the invention are evident from the exemplifying embodiments, in no way to be understood as a limitation of the invention, that are described below and depicted in the drawings. In the drawings: 
           [0014]      FIG. 1  is a circuit diagram of a preferred embodiment of a two-stranded electronically commutated motor whose rotation speed is controllable by a PWM (Pulse Width Modulated) signal; 
           [0015]      FIGS. 2 and 3  are two circuit diagrams which explain the invention; 
           [0016]      FIG. 4  is a schematic depiction to explain the manner of operation; and 
           [0017]      FIG. 5  is a set of oscillograms to explain the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  shows a preferred embodiment of an electronically commutated motor  20  according to the invention. The motor obtains its energy from any DC source  22 , which is depicted symbolically as a battery but is usually configured as a power supply powered from an alternating-current or three-phase power grid, as is known to one skilled in the art. The left half shows a “DC link” circuit. 
         [0019]    The rotation speed of motor  20  is controlled by means of a PWM signal  24  that is generated by any PWM generator  26  and that has, for example, a frequency in the range from 16 to 30 kHz, preferably approximately 25 kHz. The period length of signal  24  is labeled T in  FIG. 1 , and its pulse duration is labeled t. The ratio 
         [0000]        pwm=t/T* 100%  (1) 
         [0000]    is referred to as the “duty factor” (or “PWM duty cycle”). In other words, when t=T, the duty factor pwm=100%. 
         [0020]    Any item of information can be encoded into this duty factor, e.g. a datum regarding temperature, relative humidity, radioactivity, etc. It is usually desirable for the rotation speed to rise with a rising duty factor, but it is also possible for the rotation speed to decrease with a rising duty factor, or for it to remain constant in certain ranges as the duty factor rises. 
         [0021]    It is moreover often desirable for the information contained in the duty factor to be converted into a rotation speed of motor  20  according to certain rules, for example with a so-called “switch-on hysteresis.” 
         [0022]    Motor  20  has a positive terminal  28  (+UB) and a negative terminal  30  (GND). Located between these terminals is the series circuit made up of an RC element having a capacitor  32  (e.g. 470 nF) and a resistor  34  (e.g. 10 ohm). Also located between terminals  28  and  30  is a commutation controller  34 , e.g. a commutation module of known design, or a correspondingly programmed microcontroller. 
         [0023]    Motor  20  has a permanent-magnet rotor  36  that is depicted symbolically as a two-pole rotor but of course can have more than two poles, e.g. four, six, etc. poles. Rotor  36  controls a Hall IC  38  that is depicted twice in  FIG. 1  and only in symbolic form, i.e. the power supply to Hall IC  38  is not depicted because it is known. Controlled by the signal of Hall IC  38 , module  34  supplies to two outputs  40 ,  42  commutation signals  44 ,  46  that serve to control motor  20  (cf.  FIG. 1 ). 
         [0024]    Also connected to terminal  28  is source S of a p-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor)  50  whose drain D is connected via a connector  51  to upper terminals a 52 , a 54  of two winding strands  52 ,  54 . These strands  52 ,  54  preferably have a close magnetic coupling that is indicated at  56 . This coupling is produced on the one hand by the magnetic circuit of motor  20 , and on the other hand by the fact that the winding wires of the two strands  52 ,  54  are wound in parallel-wire fashion; this is referred to in practice as “bifilar” winding. As indicated by points  58 ,  60 , strands  52 ,  54  generate different magnetic fields; i.e. when, for example, a current i 1  flows in strand  52  from upper terminal a 52  to terminal e 52 , the North pole of rotor  36  is attracted, and when a current i 3  flows in strand  54  from upper terminal a 54  to e 54 , the South pole S of rotor  36  is attracted by the same stator pole. Conversely, when a current i 3  ( FIG. 1 ) flows in strand  54  from e 54  to a 54 , it then has the same effect as a current i 1  flowing in strand  52  from a 52  to e 52 , i.e. it intensifies its effect. This is explained further with reference to  FIG. 4 . 
         [0025]    Provided between negative lead  30  and positive branch  51  of the “DC link” circuit is a diode  55 , whose purpose will be explained below. 
         [0026]    A Z-diode  64  is connected antiparallel to p-channel MOSFET  50 . PWM pulses  24  are delivered from PWM generator  26  to gate G of this MOSFET  50  via a control lead  66 . (The positive pulses block MOSFET  50 .) 
         [0027]    The current through first winding strand  52  is controlled by an n-channel MOSFET  70  whose drain D is connected to terminal e 52  of strand  52 , whose source S is connected to a connector  72 , and to whose gate G pulses  44  are delivered from output  40  of commutation controller module  34 . 
         [0028]    Connector  72  is connected to terminal  30  via a blocking element in the form of a base diode  74 . The latter prevents a current from flowing from ground terminal  30  to connector  72  when connector  72  becomes more negative than ground terminal  30 . 
         [0029]    A recovery diode  76  is arranged antiparallel to MOSFET  70 . A diode of this kind is usually already integrated into MOSFET  70 . 
         [0030]    The current through second winding strand  54  is controlled by an n-channel MOSFET  80  that has a recovery diode  81  connected antiparallel to it. Drain D of MOSFET  80  is connected to terminal e 54  of strand  54 , and its source S is connected to connector  72 . Control pulses  46  are delivered to its gate G from terminal  42  of commutation controller module  34 . 
         [0031]    Located between the gate and drain of MOSFET  70  is the series circuit made up of a capacitor  82  (e.g. 220 pF to 3.3 nF) and a resistor  84  (e.g. 510 ohm to 10 kohm). The function of this RC element is to slow down the switching operations in MOSFET  70 . 
         [0032]    The same RC element is provided analogously for MOSFET  80 , namely the series circuit made up of a capacitor  86  and a resistor  88 . 
         [0033]    Also located between terminal e 52  and terminal  30  is the series circuit made up of a capacitor  90  and a resistor  92  (e.g. 10 kohm), and analogously located between terminal e 54  and terminal  30  is the series circuit made up of a capacitor  94  and a resistor  96 . Their function is to suppress oscillations of the drain voltages of MOSFETs  70  and  80  that might otherwise occur when MOSFET  50  is shut off and switched on. 
         [0034]    Operation 
         [0035]    When the duty factor of PWM signal  24  is 100%, i.e. when FET  50  is continuously conductive, motor  20  receives a continuous current and operates in the usual way as a two-phase, two-pulse motor whose manner of operation is assumed to be known. (“Two-pulse” refers to a motor that has two current pulses delivered to its stator winding for each rotor revolution of 360° el.; cf.  FIG. 5 .) 
         [0036]      FIG. 2  shows this state, in which FET  50  is continuously conductive and left FET  70  is made conductive by a positive signal at output  40  of module  34 , while output  42  is at ground potential with the result that right FET  80  is blocked. In this case, a current i 1  flows from terminal  28  via FET  50 , strand  52 , FET  70 , and base diode  74  to terminal  30 . 
         [0037]    This occurs, under the control of Hall IC  38  ( FIG. 1 ), over a rotation angle of approximately 180° el. of rotor  36 . In the case of a two-pole rotor  36  such as the one depicted in  FIGS. 1 to 4 , 180° el. corresponds to an angle of 180° mech., i.e. the state according to  FIG. 2  persists for approximately half a mechanical revolution; during the next half-revolution, FET  70  is blocked and FET  80  is instead made conductive, with the result that a current i 3  flows (cf.  FIG. 1 ). This is referred to as electronic commutation. 
         [0038]      FIG. 4  shows a portion of a bifilar winding with strands  52  and  54 .  FIG. 4  is, of course, only an example; a large number of designs for two-phase, two-pulse motors is known, and the depiction according to  FIG. 4  serves only to explain the manner of operation with reference to a simple example, without thereby limiting the invention to this specific design. The invention does not require a bifilar winding, but the latter is advantageous in terms of efficiency. 
         [0039]    In the switching state according to  FIG. 2 , a current flows from terminal a 52  to terminal e 52 , i.e. from top to bottom. This current produces, for example, a North pole at lower side  100  of a stator pole  102 , so that the South pole of rotor  36  is attracted. 
         [0040]    When current i 1  is switched off by FET  70  and FET  80  is instead switched on, current i 3  ( FIG. 1 ) flows through strand  54 , specifically from a 54  to e 54 , i.e. from bottom to top in  FIG. 4 . This current therefore produces a South pole at lower side  100  of stator pole  102 , so that the North pole of rotor  36  is attracted. 
       Effect of PWM Signal  24   
       [0041]    When signal  24  has a duty factor of less than 100%, FET  50  is briefly interrupted, for example, 25,000 times per second. 
         [0042]      FIG. 3  shows what happens during such an interruption, at the moment when left FET  70  is conductive and right FET  80  is blocked. 
         [0043]    No further energy can now be delivered from terminal  28  to motor  20  from DC source  22 , i.e. current i 1  is interrupted. 
         [0044]    In the context of an inductance, however, magnetic flux density B cannot change abruptly, so that as a result of this flux density, a current i 2  continues to flow through strand  52 ; lower terminal e 52  of strand  52  becomes positive, and upper terminal a 52  becomes negative. The consequence is that current i 2  flows through FET  70 , then on through connector  72  to diode  81  and through the latter to terminal e 54  of strand  54 , then through the latter to terminal a 54  and back to terminal a 52 . Current i 2  thus flows in a loop or circuit, and it is supplied from the decaying energy that is stored in the magnetic circuit of motor  20 , and that energy is consequently converted into drive energy for rotor  36  and thereby “consumed.” 
         [0045]      FIG. 4  shows an example of the path of this current. It flows from terminal a 52  from top to bottom through strand  52  to terminal e 52 , so that strand  52  generates a North pole at side  100  of stator pole  102 . 
         [0046]    From terminal e 52 , current i 2  flows through FET  70  and diode  81  to terminal e 54  and from there, again from top to bottom, through strand  54  to terminal a 54 , so that strand  54  once again generates a North pole at pole side  100 . 
         [0047]    Because the magnetic flux in pole  102  does not change abruptly but merely decreases continuously because of the conversion into rotational energy of rotor  36 , this means that when in  FIG. 2  the magnitude of current i 1  is, for example, 1 A, then in  FIGS. 3 and 4  current i 2  will be equal to only half that magnitude, i.e. 0.5 A, because of course 0.5 A is flowing through strand  52  and likewise 0.5 A through strand  54 , resulting in  FIG. 4  in a total current of 
         [0000]      2* i   2 =2*0.5 A=1 A. 
         [0048]    This means that, in this case, a jump function exists, i.e. current i 1 , of 1.0 A in  FIG. 2  is divided into two currents of 0.5 A without thereby producing any change in the effect on rotor  36 . 
         [0049]    When FET  50  is once again made conductive by signal  24 , the state according to  FIG. 2  is immediately restored, i.e. the current through strand  54  immediately becomes zero, and current i 1 , immediately jumps back to a value that is now less than 1.0 A, energy now being once again delivered from DC source  22 . 
         [0050]    When FET  50  is blocked by PWM signal  24 , the current i 1 , flowing at that instant is therefore halved but the current 
         [0000]        i   3 =0.5* i   1   (2) 
         [0000]    flows through twice the number of windings, namely both strands  52  and  54 , as depicted in  FIG. 4 , so that nothing changes in terms of the magnetic effect, as is evident to one skilled in the art without further explanation. 
         [0051]    The abrupt change in current in the context of the switching operations of FET  50  is made possible by the fact there is no change in the magnetic flux through rotor pole  102  in the context of these switching operations, i.e. the magnetic energy remains unchanged at the instant of the abrupt change. 
         [0052]    The effect of base diode  74  is that the current induced, upon shutoff of FET  50 , can flow only through connector  72 . Instead of diode  74 , an active semiconductor switch (with no recovery diode) that is controlled by signal  24  could also be used, but this solution is more complex. 
         [0053]    In order to improve efficiency, it is advantageously possible to make both FETs  70  and  80  conductive, via a connector  67  that leads from PWM generator  26  to commutation module  34 , simultaneously with the blocking of FET  50 , since the voltage drop at a conductive FET is less than the voltage drop at a current-carrying diode. 
         [0054]    Upon blockage of FET  50 , the potential at its drain D becomes negative, i.e. there occurs there, because of the inductance of winding strand  52  or  54 , a negative voltage peak whose height depends on the coupling factor of the bifilar winding (cf.  FIG. 4 ) and on the switching speed of FET  50 . 
         [0055]    There are two possibilities for limiting this negative voltage peak (without having to switch FET  50  more slowly): 
         [0056]    a) Z-diode  64  that is arranged between the drain and source of FET  50 . This diode cuts off the negative voltage peak as soon as it reaches a specific amplitude. 
         [0057]    or: 
         [0058]    b) Diode  55 , whose cathode is connected to drain D of FET  50  and whose anode is connected to terminal  30  (GND). 
         [0059]      FIG. 1  shows both variants. Both variants harmonize with the procedure for loop current i 2  depicted in  FIG. 3  and do not negatively affect it. 
         [0060]    Both variants are advantageous because with them, the switching speed of FET  50  does not need to be slowed down. If that were done, what might result would be a correlation, between duty factor pwm and the rotation speed of motor  20 , that greatly deviates from linearity. 
         [0061]    The invention provides the advantage that by modifying the duty factor of signal  24 , motor current i 4  ( FIG. 1 ), and therefore also the rotation speed of motor  20 , can be modified over a wide range in low-noise, almost linear, and EMC-compliant fashion so that, for example, a duty factor of 20-100% also corresponds to a rotation speed change of approximately 20-100% of maximum rotation speed. This would not be possible if FETs  70  and  80  were directly controlled by PWM signal  22 . Experiments by the Applicant have shown that, in this case, motor  20  simply remains at a standstill at a duty factor below 50%. 
         [0062]    Using PWM signal  24  in the context of the invention, it is therefore possible to control motor  20  directly without needing to modify said signal with electronic manipulations. Such manipulations are of course also not precluded within the scope of the invention, for example in order to generate a hysteresis already described. 
         [0063]    It is also very important that the FETs  70  and  80  that produce electronic commutation do not experience greater stress as a result of the above-described switching operations in FET  50 , since upon blockage of FET  50 , the current in FET  70  or  80  that is conductive at that instant temporarily drops by half, so that the power dissipation also correspondingly drops. 
         [0064]    Intelligent utilization of the flux energy stored in the magnetic circuit of motor  20 , which energy is used in the invention to drive rotor  26  during the periods in which FET  50  is not conducting a current, produces only a little energy in the form of reactive power during the switching operations, so that a small size is sufficient for capacitor  32 , e.g. 100 to 470 nF. If FETs  70  and  80  were controlled directly using signal  24 , a far larger buffer capacitor would be needed, for which no room would be available specifically in compact fans. Such a capacitor would also, because of its limited service life, implicitly shorten the service life of motor  20  and would not be capable of being processed as an SMD (Surface Mounted Device) component. 
         [0065]    The result of Z-diode  64  or diode  55  is that the negative voltage peaks occurring at FET  50  during operation are substantially damped. 
         [0066]    The result of RC element  32 ,  34  in coaction with diode  74  is to reduce the voltage increases in DC link circuit  51  that occur during normal commutation of motor  20 . 
         [0067]    Because the motor current flows through both strands  52 ,  54  during PWM operation, the invention produces a kind of hybrid between a two-pulse motor having two strands and a two-pulse motor having only one strand, so that overall efficiency is improved because the consequence of digital switching is that large losses do not occur in FET  50 . This allows a unit having somewhat lower performance to be used for FET  50 , thus decreasing costs. 
         [0068]      FIG. 5  is a set of oscillograms of the currents in motor  20 . Current i 4  that flows out of current source  22  into motor  20  is depicted at the bottom. For a pwm&lt;100%, this current continuously varies between a value of zero and an instantaneous maximum value, since FET  50  is switched off and on, for example, 25,000 times per second. 
         [0069]    Also depicted are currents i 1 , i 2 , and i 3  whose significance is evident from  FIGS. 1 to 4 . 
         [0070]    For a pwm&lt;100%, current i 1 , (and likewise current i 3 ) in the switched-on strand continuously jumps between 50 and 100%, as described in detail with reference to  FIG. 4 . 
         [0071]    For a pwm&lt;100%, current i 2  in the switched-off strand jumps between 0 and 50% with reference to the instantaneous value of current i 4 . 
         [0072]    Assuming that the motor is running at 6000 rpm, this corresponds to 100 revolutions per second. One revolution therefore takes 0.01 second. 
         [0073]    If the PWM signal has a frequency of 25,000 Hz, this then yields, for each complete rotor revolution, a number Z: 
         [0000]        Z= 25,000*0.01=250  (3) 
         [0000]    of current interruptions produced by FET  50 .
 
The duration of these interruptions is a function of duty factor pwm, and the latter consequently determines the actual value of currents i 1 , i 2 , i 3 , and i 4  and thereby the rotation speed of motor  20 .
 
         [0074]    Base diode  74  is particularly important for the commutation operation: When FET  50  is conductive while commutation is occurring, then for example, the previously conductive FET  70  is shut off and the previously blocked FET  80  is switched on. 
         [0075]    The shutoff of current i 1 , causes a positive potential at point e 52 , and that potential is transferred in transformer fashion from strand  52  to strand  54 , so that point e 54  therein becomes more negative and its potential can drop below the potential of point  30  (GND), so that, without diode  74 , a current would flow from point  30  to connector  72 . 
         [0076]    This could cause the source of the FET to become so negative that FET  70  would begin to conduct again and end up in a high-resistance state. 
         [0077]    Diode  74  prevents this, since, in such a case, it blocks, so that no current can flow from point  30  to connector  72 , and the shutting-off FET (in this case FET  70 ) remains blocked. 
         [0078]    The shape of currents i 1  to i 4  is a function of the shape of the voltage induced by rotor  36  as it rotates in strands  52  and  54 . This shape is characteristic of electronically commutated motors, whose rotors have an approximately trapezoidal magnetization of the rotor poles with narrow pole gaps. This type of preferred magnetization of the rotor poles has proven very valuable within the scope of the present invention. 
         [0079]    Many variants and modifications are of course possible, within the scope of the present invention. For example, transistors  50 ,  70 , and  80  could also be implemented as bipolar transistors, although FETs are preferred.