Abstract:
An electronically commutated one-phase motor ( 20 ) has a stator having at least one winding strand ( 30, 32 ) and a permanent-magnet rotor ( 22 ). The rotor, as it rotates, induces a voltage (u ind ) in the at least one winding strand ( 30, 32 ). The motor has an electronic calculation device ( 26 ), preferably a microcontroller μC, which is configured to execute, during operation, the steps of a) sensing the value of the instantaneous operating voltage (Ub); (b) using the operating voltage value (Ub) and optionally further parameters, adjusting a time duration (T ON ) of a switch-on current pulse (i 30 ) for the motor, in order to apply a consistent amount of electrical energy to the windings during start-up attempts, thereby maximizing the probability of successful start-up, regardless of possible fluctuations in motor operating voltage and related operating parameters. The switch-on current pulse duration (T ON ) can be adjusted longer or shorter, as a function of operating experience.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of our U.S. application Ser. No. 12/830,492 filed 6 Jul. 2010, and also claims priority from our German application DE 10 2010 004 361.1 filed 12 Jan. 2010. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to an electronically commutated motor (ECM) and, more particularly, to an ECM with measures to make its start-up more reliable. 
       BACKGROUND 
       [0003]    One-phase electronically commutated motors are very inexpensive, and are often used for specific driving tasks, e.g. for fans or centrifugal pumps. They are usually controlled by means of a Hall sensor which magnetically detects the instantaneous rotational position of the rotor. Commutation without a sensor, referred to using the term “sensorless,” is, however, desirable, since better efficiency is obtained as a result. 
         [0004]    The terminology of such motors is somewhat confusing. For accurate definition of an ECM, firstly the number of stator current pulses per rotor rotation of 360° el. is indicated, e.g. single-pulse, two-pulse, three-pulse, etc.; also the number of winding strands in the stator is indicated, e.g. single-strand, two-strand, three-strand, etc. 
         [0005]    An ECM can therefore e.g. be described as single-strand and two-pulse, or two-strand and two-pulse. The expression “collectorless motor” is also used instead of “ECM”. Because there is no difference between the single-strand and two-strand motors in terms of physical operation, and because simplified terminology is always desirable for practical use, such motors are generally referred to as “one-phase” ECMs, even though they can have either only a single strand or, alternatively, two strands. 
         [0006]    Because the rotor in such motors has rotational positions at which the motor cannot generate any electromagnetic torque, an auxiliary torque is used that is effective at those zero positions. This can be a magnetically generated auxiliary torque, which is referred to as reluctance torque. Alternatively, this auxiliary torque could be generated mechanically, for example by means of a spring that is tensioned in certain rotational positions and delivers its stored energy at the zero positions. The result is that the rotor, at a standstill, is rotated sufficiently that at startup it is not in a rotational position in which the motor cannot generate an electromagnetic torque, since otherwise the motor would not be able to start. This starting position is also referred to as a “cogging” position. 
         [0007]    When such motors are currentless, normally the rotor is at a standstill and is in a so-called cogging position, into which it is pulled by the aforesaid auxiliary torque. When current is applied to the motor with the rotor in this position, the rotor will move; it is, however, only possible to guess how strongly it will move. 
       SUMMARY OF THE INVENTION 
       [0008]    It is therefore an object of the invention to make available a novel electronically commutated motor (ECM) with a reliable minimum rotor movement during start-up. 
         [0009]    According to the invention, this object is achieved by sensing the instantaneous motor operating voltage, and using this value to adjust the time duration of a switch-on current pulse applied to the motor. Sensing of the operating voltage creates the possibility of correctly metering energy delivery at startup. This is because energy is delivered at startup as a so-called current-flow block and, by means of the invention, this block can be metered so that, regardless of the instantaneous operating voltage, approximately the same quantity of energy is delivered at startup. Subsequent thereto, a check is made as to whether that quantity of energy is to small or too large, and corresponding corrective measures are taken, as appropriate, to apply a consistent quantity of energy. 
     
    
     
       BRIEF FIGURE DESCRIPTION 
         [0010]    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. 
           [0011]      FIG. 1  is an overview diagram that schematically shows various situations that can occur during operation of a sensorless one-phase motor and must therefore be taken into account in its software, in order to ensure reliable starting; 
           [0012]      FIG. 2  is a schematic depiction to explain an ECM that operates with a reluctance torque; 
           [0013]      FIG. 3  is a circuit diagram of an embodiment of a one-phase motor that is configured to take into account situations of  FIG. 1 , the motor being illustrated as a two-strand motor; 
           [0014]      FIG. 4  is a depiction to explain  FIG. 3 ; and 
           [0015]      FIG. 5  shows a routine to optimize a time period Tv whose length is important for optimum commutation. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The diagram of  FIG. 1  schematically shows problems that must be taken into account, when developing a “sensorless one-phase ECM.” 
         [0017]    After being switched on, the motor is in situation  10  of  FIG. 1 , i.e. it has either a rotation speed n=0 or (if externally driven) a rotation speed n≠0, and its rotational position “pos” is unknown (to the motor&#39;s electronic control circuit), since a rotational position sensor is not present. 
         [0018]    According to situation  12 , this can mean that the rotation speed is zero, and the rotor is in one of its cogging positions that is defined by the cogging torque. 
         [0019]    The motor can, however, also be in situation  14 , meaning that an external driving force is acting on it. In the case of a fan, for example, a wind gust or a storm can be driving the motor, so that, although it is receiving little or no motor current, its rotation speed n is nevertheless not equal to zero, since the rotor (in the case of a fan) can be driven like a windmill by a storm. 
         [0020]    Under these conditions, however, the motor can also run in both rotation directions (see positions  16  and  18  of  FIG. 1 ) while the normal motor current is flowing. If the motor is rotating in the wrong rotation direction, corresponding countermeasures are then necessary. The “wrong” rotation direction thus means that reversing must occur after startup. 
         [0021]    Step  14  measures whether an induced voltage u ind  is present, i.e. whether the magnitude of u ind  is greater than zero. This can also be the case, for example, when a fan is being passively driven by wind. In addition, a measurement is made as to whether the magnitude of rotation speed n is greater than zero. 
         [0022]    If the response to both queries is NO, the program goes to step  12 , which indicates that the rotation speed has a value of zero, and also that the rotational position of the rotor is defined by the so-called cogging torque, i.e. the rotor has “snapped” into one of its cogging positions. 
         [0023]    If the responses in step  14  are YES, the rotor is either rotating in its preferred direction PRDIR (step  16 ) or rotating oppositely to its preferred direction PRDIR (step  18 ). The rotation direction cannot, however, be immediately deduced from the available data. 
         [0024]    The motor can, however, also rotate in either of the two rotation directions as a result of external influences; the normal motor current is flowing, but the motor can rotate in the wrong direction. The “wrong” rotation direction means that it must be reversed after starting. 
         [0025]      FIG. 2  shows the circuitry of an Electronically Commutated Motor (ECM)  20  that operates in sensorless fashion. Motor  20  has a permanent-magnet rotor  22  (indicated merely schematically) that is depicted with four poles, but can also have two, six, eight, etc. poles. Rotor  22  can be an internal rotor, external rotor, the rotor of a motor having a flat or conical air gap, etc. 
         [0026]    Motor  20  has a microcontroller (μC)  26 , preferably a PIC12F629 from Microchip Technology, Inc., Chandler, Ariz., 85224, USA. The power supply system of μC  26  is, as usual, not depicted. Motor  20  has two stator winding strands  30 ,  32  that are usually magnetically coupled via the magnetic circuit of the motor, as indicated by symbol  34 ′. Placed in series with first winding strand  30  is a first semiconductor switch, here e.g. an n-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor)  34 , which has a recovery diode  38  connected antiparallel to it and which is controlled by μC  26  via a control line  36 . Together with semiconductor switch  34  and diode  38 , strand  30  forms a first series circuit  40  that can optionally contain further elements. 
         [0027]    Located in series with second strand  32  is a second controllable semiconductor switch  44  that is controlled by μC  26  via a control line  46 . This switch can likewise be an n-channel MOSFET  44  that has a recovery diode  48  connected antiparallel to it. Together with second semiconductor switch  44 , second strand  32  forms a second series circuit  50  that may contain further elements. 
         [0028]    As  FIG. 2  shows, the two series circuits  40 ,  50  are connected in parallel, to form a parallel circuit  53  whose bottom node  54  is connected to ground  56 . The upper ends of strands  30 ,  32  are also connected to a DC link circuit  58 . This means that when semiconductor switch  34  is conductive, a current i 30  flows from link circuit  58  through first strand  30 , and when semiconductor switch  44  is conductive, a current i 32  flows through strand  32 . This statement must, however, be modified for the time intervals just before a commutation, as will be explained below. 
         [0029]    Link circuit  58  is connected via a third semiconductor switch  60  (here a p-channel MOSFET) to a motor terminal  62  to which a positive voltage Ub, e.g. 12, 24, 48, 60 V, etc. is applied toward ground  56  during operation. A DC source  63  of any kind is depicted symbolically. A power supply, for example, often serves as a DC source. 
         [0030]    A diode  61  can be located antiparallel to third semiconductor switch  60 . Third semiconductor switch  60  is controlled by μC  26  via a control line  64 . 
         [0031]    A potential from drain D of semiconductor switch  34  is delivered to a comparator input  65  of μC  26  through a sensor line  66  and a resistor  67 . Input  65  is connected via a Zener diode  69  to ground  56 , in order to protect said input from overvoltage. 
         [0032]    A potential from drain D of second semiconductor switch  44  is likewise delivered to a comparator input  71  of μC  26  through a sensor line  68  and a resistor  73 . Input  71  is connected via a Zener diode  69 ′ to ground  56 , in order to protect input  71  from overvoltage. 
         [0033]    In addition, a voltage divider, made up of two resistors  75 ,  76 , whose connecting node  77  is connected to input A/D of an analog-to-digital converter in μC  26 , is connected between drain D of first semiconductor switch  34  and ground  56 . 
       Measuring Ub 
       [0034]    This measurement is made via voltage divider  75 ,  76 . The latter of the two resistors is dimensioned so that the internal reference voltage (in this case 5 Volts) of the A/D converter in μC  26  cannot be exceeded. This precludes measurement errors. Alternatively, this voltage divider can also be placed between source S of third semiconductor switch  60  and ground  56 . 
         [0035]    Voltage divider  75 ,  76  also, additionally, has another function: depending on the amplitude of the voltages that are induced in strands  30 ,  32 , said voltages are limited by protective diodes  69 ,  69 ′. It is important for rotation direction detection, however, to sense the shape of the induced voltages at inputs  65  and  71 , respectively, which would be prevented by voltage limiting. In this instance, the induced voltage is therefore sensed by voltage divider  75 ,  76  and input A/D of μC  27 , with the result that the shape of the induced voltage can also be detected. 
         [0036]    The signals at drains D of first semiconductor switch  34  and of second semiconductor switch  44  are sensed at comparators  65 ,  71  in μC 
       Manner of Operation of FIG. 2 
       [0037]    Reference is made for this purpose to  FIG. 3 . 
         [0038]    Shortly before time instant t 0  in  FIG. 3 , all three semiconductor switches  34 ,  44 ,  60  in  FIG. 2  are blocked, and motor  20  consequently receives no energy from terminal  62 , i.e. energy delivery from DC source  63  is blocked. 
         [0039]    At time t 0 , transistors  34 ,  60  are switched on by μC  26  so that a current i 30  flows from terminal  62  through transistor  60 , link circuit  58 , strand  30 , and transistor  34  to ground  65 .  FIG. 3   a ) shows the shape of current i 30 , which of course depends on the value of the motor rotation speed and on other factors. 
         [0040]    Commutation time instant t 0  is followed by a commutation time instant t 4 , at which transistor  34  is switched off and transistor  44  is switched on, so that current i 30  is shut off and current i 32  (through strand  32 ) is switched on. 
         [0041]    Located in a time interval Tv before t 4  is a time instant t 2  at which transistor  60  becomes blocked, so that energy delivery from terminal  62  is interrupted, i.e. no energy is delivered from DC source  63  to motor  20  during this time period Tv. 
         [0042]    A specific current i flows in strand  30  shortly before time t 2 , so that a specific energy E is stored in strand  30  in accordance with the formula 
         [0000]        E= 0.5 *L*i   2   (1)
 
         [0000]    where
       E=energy stored in the magnetic field of the relevant strand   L=inductance of that strand   i=current at time instant t 2 .       
 
         [0046]    This stored energy now causes a loop current i* to flow through strand  30  because transistor  34  is still conductive. This loop current i* flows from the lower terminal of strand  30  through transistor  34 , node  54 , recovery diode  48 , and the two strands  32  &amp;  30  so that, as before, it generates a driving torque on rotor  22 , with the result that loop current i* rapidly drops and, at time instant t 3  of  FIG. 3  reaches a value of zero. Transistor  34  can therefore be blocked in a wattless manner as of time instant t 3 , since loop current i* has become zero. 
       Measuring Operating Voltage Ub 
       [0047]    It is important for startup purposes to know the operating voltage Ub of the motor. While most ECMs have a fixed operating voltage, they can also be operated in an extended range of operating voltages. ECMs for a voltage of 48 V should therefore be able to be operated within a voltage range extending approximately from 36 V to approximately 72 V. The result of these voltage differences is to produce, for the same current-flow duration, very different rotor accelerations for the first current-flow block: the rotor is more strongly accelerated at higher operating voltages, and it may happen that the change in induced voltage is therefore not detected, so that commutation cannot take place. In order to ensure detection of the induced voltage after the first current-flow block, the first current-flow blocks must be adapted with respect to the operating voltage. When the motor is at a standstill, the operating voltage can be identified very easily by way of voltage divider  75 ,  76  and the A/D converter in microcontroller  26 . This can be done by switching on semiconductor switch  60  and switching off the two power stage transistors  34  and  44 . In this case, the operating voltage is measured directly at drain terminal D of power stage transistor  34 . 
         [0048]    If the motor is additionally being driven from outside (referred to as an “external driving force”), an induced voltage additionally occurs at the measuring points of the winding. This voltage is overlaid on the operating voltage, and the latter therefore cannot reliably be detected. in this case, the operating voltage can be measured at a point upstream from transistor  60 . This variant, however, requires additional circuit complexity. One solution is to proceed in such a way that when an external driving force is identified, the induced voltage and its zero crossings are observed. This is possible if all three semiconductor switches are left nonconductive, with the result that no operating voltage is present at winding strands  30 ,  32 , and the change in induced voltage can be observed. When a zero crossing of the induced voltage then occurs, transistor  60  is switched on and the operating voltage is then present at the winding strands. At that moment, the induced voltage has no influence, and only the operating voltage is measured. Depending upon the rotation speed, this method must function very quickly, since before and after the zero crossing the induced voltage has a steep edge slope that might otherwise cause measurement errors. 
         [0049]    In addition to the lack of information about rotor position, there are other important factors that require attention in the context of a sensorless startup. Different winding resistance values, and hence different winding currents and different startup torques, occur, depending upon the operating voltage, winding design, and winding temperature. The startup torque is opposed by frictional torques that change with temperature and with the age of the motor. Attention must also be paid to the differences in axial moment of inertia among different rotors. Different angular accelerations are also produced depending on the rotation direction. 
         [0050]    If the angular acceleration achieved by means of the first current-flow block is sufficient for evaluation of the voltage induced by the rotor, this makes it possible to ascertain the rotation direction in which the rotor was accelerated. This means that the first current-flow block that is selected must not be too long, so that after current flow and after the subsequent current loops, the induced voltage can also be measured. 
         [0051]    If an induced voltage cannot be measured after the first current-flow block (during time period T on  of  FIG. 3 ), there may be various reasons for this. The motor may have been blocked by an external influence; or the first current-flow block was set too short, and as a consequence of increased bearing friction, aging, or a high winding resistance, the electrical torque generated would then not be enough to accelerate the rotor sufficiently. In this case, an induced voltage cannot be identified. Provisions must be made for all these different instances. 
         [0052]      FIG. 4  shows a starting routine for the normal case, in which rotor  22  is in a predetermined rotational position from which it is to be started. 
         [0053]    Starting occurs at S 250 , S 252  checks whether the induced voltage Uind differs from zero, i.e. checks whether rotor  22  is rotating. If YES, the program goes to a special routine  254  for startup, and then (at S 256 ) transitions into a standard commutation in the desired rotation direction. One such commutation is described below with reference to  FIG. 5 . 
         [0054]    If the response at S 252  is No, the program goes to step S 258 , where transistor  60  is switched on and transistors  34  and  44  are switched off in order to measure the operating voltage Ub at input A/D of μC  26 . 
         [0055]    In the next step S 260 , a factor x is derived (for example from stored tables) from Ub and optionally from other factors, e.g. the instantaneous temperature, and in S 262  the operating voltage Ub is multiplied by this factor in order to calculate the duration T on  of the switch-on current pulse that is calculated on a predictive basis for startup of motor  20 . 
         [0056]    The two transistors  60  and  34  are then switched on at S 264 , with the result that current i 30  through strand  30  is switched on and rotor  22  is accelerated. 
         [0057]    After time T ON  calculated in step S 262  has elapsed, in step S 266  transistor  60  is switched off, thereby interrupting energy delivery from current source  63 . But because transistor  34  is still conductive, the magnetic energy stored in winding strand  30  causes a loop current I* to flow from node  54  through transistor  34 , recovery diode  48 , and the two winding strands  32  and  30  back to node  54 , and this loop current I* drives rotor  22  and thereby rapidly drops to zero. 
         [0058]    As long as loop current I* is flowing, drains D of the two transistors  34  and  44  are at ground potential; but when I* has become equal to zero, an induced voltage u ind  indicating the rotation of rotor  22  is obtained at drain D of transistor  34 . This voltage is sensed in step S 268 . 
         [0059]    If it is not possible to sense any such induced voltage, the program goes to step S 270 , where time span T on  is extended by an amount equal to an “Offset” value; the program then goes to step S 264  in order to repeat the startup attempt at an increased energy. 
         [0060]    If the response in S 268  is YES, S 272  checks whether the induced voltage at the drain of transistor  34  can be measured. If NO, time span T on  is too long, and in S 274  it is therefore shortened by an Offset correction time, in order to weaken the startup current pulse. 
         [0061]    The program then goes to step S 264  in order to repeat the startup operation at reduced energy. If, however, the response in S 272  is YES, i.e. if the induced voltage does occur at the drain of transistor  34 , this means that the loop current has dropped to zero at the correct time, and the program goes to step S 276  where ECM  20  is commutated normally. In this case motor  20  is running normally, and motor  20  usually starts without difficulty. 
         [0000]    Optimizing Commutation Time t 4   
         [0062]    Optimized commutation is important for optimum and low-loss operation of motor  20 , since the motor then runs quietly with good efficiency. 
         [0063]    Commutation optimization is of course particularly difficult with a sensorless motor because a rotor position sensor is not present, so that optimization requires working with other variables that can be measured. 
         [0064]      FIG. 3  shows at a) the currents i 30 , i 32  in the two strands  30  and  32  of motor  20 . The potential p 52  at node  52  of  2 , i.e. at drain D of FET  44 , is depicted at b). Because the arrangement is symmetrical, the potential p 54  at node  54  has the same profile but offset 180°, and is therefore not depicted in  FIG. 3 . 
         [0065]    As long as FET  44  is conductive, its drain D is connected to ground  56 , so that a voltage induced by the permanent-magnet rotor  22  in strand  32  cannot be measured at node  52 . 
         [0066]    As soon as current I* has dropped to zero, however, this induced voltage (labeled in  3   b ) as  68 ) can be measured at node  52 , so that the occurrence of voltage  68  means that loop current I* has dropped to zero; this is the case at time t 3 , and means that as of that time wattless commutation can take place. 
         [0067]    Time span Tv, between time t 2  at which FET  60  becomes blocked and current i 30  is thereby switched off, and time t 4  at which FETs  44  and  60  are switched on so that a current i 32  flows through strand  32 , therefore has an optimum value when time span Tp between times t 3  and t 4  becomes as short as possible, since Tv then also has a minimal value. 
         [0068]    On the other hand, of course, Tv must not become too short, since then the switching on of current i 32  (time t 4 ) would fall in a time period Ti (between t 2  and t 3 ) in which loop current I* is still flowing, so that wattless commutation would not be possible. In this case, time span Tv must therefore be extended. 
         [0069]    The operations depicted in the flow charts of  FIGS. 4 and 5  serve this purpose. 
         [0070]    Time Tv ( FIG. 3 ), which is set at the startup of motor  20  to a default value, and at the beginning of which (at time t 2 ) the “prelude” to each commutation begins, can be optimized by means of μC  26 . The corresponding routine is depicted in  FIG. 5 . 
         [0071]    This routine begins at step S 88  and is preferably called at each commutation. In S 88 , Tv is set to a default value after switching on. The optimization of Tv begins in S 90 . S 92  checks whether the end (t 3 ) of current looping was detected before commutation time t 4 . If so, Tv is reduced in S 94  by an decrement ΔTv 1 . If not, then in S 96  Tv is increased by an increment ΔTv 2  that is greater than decrement ΔTv 1  in step S 94 . Optimization ends at step S 98 . 
         [0072]    The result is that an optimum value for Tv is automatically established within a few revolutions, even if the motor rotation speed has changed as a result of external influences, e.g. an air current. 
       Problems at Higher-Order Transistor  60   
       [0073]    At startup or in the event of a change in the load on motor  20 , it may happen that the higher-order transistor  60  becomes blocked too late, and a loop current is therefore still flowing through strands  30 ,  32  at the commutation time. A currentless commutation is not possible in such a case, and protective measures must be taken to prevent this. 
         [0074]    One possibility here is to use a link circuit capacitor, which is arranged between link circuit  58  and ground  56  and which absorbs the residual magnetic energy of the winding strand that is to be switched off and thereby limits the voltage at link circuit  58 . 
         [0075]    It is also possible to insert a Zener diode between link circuit  58  and ground  56  in order to limit the voltage at link circuit  58 . 
         [0076]    The drain voltages of FETs  34  and  44  can also be limited, using respective Zener diodes that are arranged between the pertinent drain D and ground  56 . 
         [0077]    Another, and possibly additional, action is to limit the drain voltages of FETs  34 ,  44  by slow switching. This can be achieved using a series circuit of a capacitor and a resistor that is connected between drain D and gate G of the relevant FET. 
         [0078]    The drain voltages of FETs  34 ,  44  can also be limited by slow switching of the relevant FET. This can be done using a series circuit of a Zener diode  124  and a resistor  126 . In this case, a series circuit of this kind is inserted between D and G of the relevant transistor. 
         [0079]    Current limiting can additionally be provided for motor  20 . This is not depicted in  FIG. 2 , so as not to make the depiction difficult to understand as a result of a proliferation of elements. Current limiting is preferably achieved by blocking higher-order transistor  60  in the event of an overcurrent, in order to interrupt energy delivery from DC source  63  to motor  20 . This results in a respective loop current I* as already described, and this loop current generates a torque, thus yielding current limiting with highly efficient motor operation. 
         [0080]    Many variations and modifications are, of course, possible within the scope of the inventive concept.