Abstract:
An electronically commutated one-phase motor ( 20; 20 ′) has a stator having at least one winding strand ( 30, 32; 30 ′), and it has a permanent-magnet rotor ( 22 ) that induces, as it rotates, a voltage (u ind ) in the winding strand. The motor further has an electronic calculation device or microcontroller ( 26 ) which is configured to execute, during operation, the following steps repetitively: sampling the induced voltage (u ind ) in a currentless winding strand, for example, during a half-wave of the induced voltage, in order to obtain a plurality of analog voltage values; digitizing the analog voltage values in order to obtain a plurality of digitized voltage values; and processing the plurality of digitized voltage values to ascertain the instantaneous rotation direction of the motor rotor. 
     The control circuit then can use these data to assure reliable motor start-up, regardless of any external driving forces which occur.

Description:
CROSS-REFERENCE 
       [0001]    This application claims priority from our German Application DE 10 2009 033 526.9, filed 11 JUL. 2009, the entire content of which is hereby incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to one-phase electronically commutated motors (ECMs) and, more particularly, to an improved motor configuration which facilitates reliable motor start-up. 
       BACKGROUND 
       [0003]    One-phase electronically commutated motors (ECMs) are inexpensive and are therefore often used for specific driving tasks, e.g. for fans or centrifugal pumps. They are usually controlled by means of a Hall (magnetic position detecting) sensor. Commutation without a sensor, referred to using the term “sensorless,” is, however, desirable. 
         [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. Because there is no difference between these two types of motor in terms of physical operation, and because simplified terminology is always desirable for practical use, such motors are generally referred to, in the trade, as “one-phase” ECMs, even though they can have either only a single strand or 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 said zero positions. The function of this auxiliary torque is chiefly to rotate the rotor so 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. 
         [0007]    Such motors have a preferred rotation direction in which they start easily, and a rotation direction opposite to the preferred rotation direction, in which startup is more difficult but not impossible. 
         [0008]    In the case of fans or pumps, an additional difficulty arises from the fact that they can be driven by the medium being transported, for example by a high wind or a storm; in such a case it is not known whether the rotor is being rotated in, or (alternatively) against, the preferred direction by that external driving force. 
         [0009]    In a storm, the rotation speed of the rotor can become fairly high; and in such a case, with a motor that has no Hall sensor, firstly the rotation direction must be identified and, if it is the wrong one, the motor must be reversed and (as an example) switched over from a rotation speed of −3800 rpm to a speed of +4100 rpm. A prerequisite for this is an identification of the rotation direction. 
         [0010]    It is known from DORNHOF EP 1 596 495 A2 and corresponding US 2005-253546-A1 (US attorney docket 8703-190) that in the context of a “one-phase” ECM of this kind with auxiliary reluctance torque, the rotation direction can be identified from the shape of the induced voltage, i.e. from the voltage induced in a currentless winding strand by the permanent-magnet rotor as it rotates. 
         [0011]    It is therefore an object of the invention to make available a novel one-phase ECM, i.e. an ECM that can be either single-strand or two-strand. This object is achieved by sampling induced voltage in a currentless winding, digitizing them, and using the sampled values to ascertain the instantaneous direction of rotation. The invention makes it possible to identify the rotation direction in an ECM of the kind cited initially, thereby making it possible to use such an ECM even in a location where it can be caused to rotate by an external driving force since, once the rotation direction is identified, certain countermeasures can be taken if a determination has previously been made that the motor is (usually as a result of external driving force) rotating in a wrong rotation direction. 
     
    
     
       BRIEF FIGURE DESCRIPTION 
         [0012]    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 shown in the drawings. 
           [0013]      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 of the motor; 
           [0014]      FIG. 2  is a schematic depiction to explain an Electronically Commutated Motor (ECM) that operates with a reluctance torque; 
           [0015]      FIG. 3  is a circuit diagram of an embodiment of a one-phase motor that is equipped and configured to handle the situations shown in  FIG. 1 , the motor being illustrated as a two-strand motor; 
           [0016]      FIG. 4  is a depiction to explain  FIG. 3 ; 
           [0017]      FIG. 5  is a depiction to explain  FIGS. 6 and 7 ; 
           [0018]      FIG. 6  is the flowchart of a first part of a program sequence to ascertain the rotation direction; 
           [0019]      FIG. 7  is the flowchart of the second part of the program sequence of  FIG. 6 ; 
           [0020]      FIG. 8  is a circuit diagram, analogous to  FIG. 3 , of a one-phase ECM that in this case is implemented in a single-strand manner, i.e. has only a single strand that is controlled via an H-bridge; and 
           [0021]      FIG. 9  is a set of current graphs to explain  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  schematically shows problems that must be taken into account when developing a “sensorless one-phase ECM.” 
         [0023]    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 electronics), since a rotational position sensor is not present. 
         [0024]    Step  14  measures whether an induced voltage u ind  is present, i.e. whether u ind  is greater than zero. This can also be the case, for example, when a fan is being driven by wind. In addition, a measurement is made as to whether the magnitude of n is greater than zero. 
         [0025]    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 so-called “cogging torque” has a value of zero, i.e. the rotor has “snapped” into one of its detent or cogging positions. 
         [0026]    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 deduced from the existing data. 
         [0027]    In this case, the motor can rotate in either of the two rotation directions; the normal motor current is flowing, but the motor might rotate in the wrong direction. The “wrong” rotation direction means that it must be reversed after starting. 
         [0028]    If external driving, e.g. as a result of wind, is occurring, the first task, after switching on, is therefore to identify the rotation direction of the rotating rotor. 
         [0029]      FIGS. 2   a ) and  2   b ) show, highly schematically and only as an example, the structure of a motor  20  that uses an auxiliary reluctance torque. It has a permanent-magnet rotor  22 , in this case an external rotor having two poles, and it has a stator having a stator winding  30  and a stator winding  32 . The stator lamination stack is labeled  35 , and, in this example, has a shape characteristic of such motors, approximately comparable to two sawteeth. 
         [0030]    The pole gaps of rotor  22 , one of them labeled  0  and the other located directly opposite, position themselves approximately in the rotational position depicted when the stator is currentless, provided no external driving force is present; in other words, the pole gaps seek out the location with the largest air gap. When rotor  22  is being driven from the outside, it generates in stator windings  30 ,  32  an induced voltage u ind  that is measured and analyzed when current flow is occurring (in rotation direction DIR). 
         [0031]    The present invention relates to instances  16  and  18 , i.e. firstly, when external driving is occurring, the rotation direction of the motor being driven by the wind (or other forces) must be identified, so that the motor can then be operated in the correct rotation direction. 
         [0032]      FIG. 3 . shows the circuitry of an ECM  20  that operates in a sensorless manner. 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. 
         [0033]    Motor  20  has a microcontroller μC  26 , for example a PIC12F629 of MICROCHIP TECHNOLOGY INC. of Chandler, Arizona, USA. Relevant datasheets are, at the time of this writing, available from the website www.microchip.com/TechDoc.aspx?type=datasheet. 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  35 . Placed in series with first winding strand  30  is a first semiconductor switch, here e.g. an n-channel (Metal Oxide Semiconductor Field Effect Transistor (MOSFET)  34 , which has a recovery diode  38  connected in antiparallel with 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. 
         [0034]    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 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. 
         [0035]    As  FIG. 3  shows, the two series circuits  40 ,  50  are connected in parallel, to form a parallel circuit  52  whose bottom node  54  is connected to ground  56 , optionally via a diode  55 . 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 DC 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. 
         [0036]    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 operating voltage Ub, e.g. 12, 24, 48, 60 V, etc. is applied toward ground  56  during operation. A DC current source  63  of any suitable kind is depicted symbolically. A diode  61  is located antiparallel to third semiconductor switch  60 . Third semiconductor switch  60  is controlled by μC  26 , via a control line  64 . 
         [0037]    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  65  from overvoltage. 
         [0038]    A potential from the drain 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. 
         [0039]    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 (A/D) converter in μC  26 , is connected between drain D of first semiconductor switch  34  and ground  56 . 
       Measuring Ub 
       [0040]    This measurement is made via voltage divider  75 ,  76 . The latter is dimensioned so that the internal reference voltage (in this case 5 V) of the A/D converter in μC  26  cannot be exceeded. This prevents measurement errors. Alternatively, this voltage divider can also be placed between source S of third semiconductor switch  60  and ground  56 . 
         [0041]    Voltage divider  75 ,  76  also 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 true shape of the induced voltages at inputs  65  and  71 , respectively, which sensing would be impeded by voltage limiting. In this instance, the induced voltage is therefore sensed by way of voltage divider  75 ,  76  and input A/D of μC  26 , with the result that the true shape of the induced voltage can also be detected. 
         [0042]    The signals, at drains D of first semiconductor switch  34  and of second semiconductor switch  44 , are sensed at comparators  65 ,  71  in μC  26 . 
       Manner of Operation of FIG. 3 
       [0043]    Reference is made, for this purpose, to the graphs of  FIG. 4 . 
         [0044]    Shortly before time t 0  in  FIG. 4 , all three semiconductor switches (transistors)  34 ,  44 ,  60  in  FIG. 3  are blocked, and motor  20  consequently receives no energy from terminal  62 , i.e. energy delivery from DC source  63  is blocked or interrupted. 
         [0045]    At time to, 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 , transistor  34 , and diode  55  (if present) to ground  56 . Graph  4   a ) shows the shape of current i 30 , which of course depends on the value of the motor rotation speed and on other factors. 
         [0046]    Commutation time t 0  is followed by a commutation time 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. 
         [0047]    Located in a time interval Tv before t 4  is a time t 2  at which transistor  60  is 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. 
         [0048]    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*î 2  (1)
 
         [0000]    where
 
e=energy stored in the magnetic field of the relevant strand
 
L=inductance of that strand
 
i=current at time t 2 .
 
         [0049]    This stored energy now causes a loop current i* to flow through strand  30  because transistor  34  is once again conductive. This loop current i* flows from the lower terminal  54  of strand  30  through transistor  34 , node  54 , recovery diode  48 , and the two strands  32  and  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 t 3  of  FIG. 4   a ), reaches a value of zero. Transistor  34  can therefore be blocked in wattless manner as of time t 3 , since loop current i* has become zero. 
         [0050]      FIG. 4  shows the sequence over time. 
         [0051]    Transistor  60  is blocked at time t 2 , so that from that time onward a loop current i* flows. This current becomes zero at time t 3 , so that transistor  44  can be blocked without switching losses. 
         [0052]    While loop current i* is flowing, drains D of transistors  34  and  44  are substantially grounded. After loop current i* ends, a signal  68  that corresponds to the induced voltage in the currentless strand  32  is produced at drain D of the nonconductive transistor  44 . At time t 4  this signal  68  causes commutation, i.e. causes the (hitherto blocked) transistors  44  and  60  to switch on, and causes transistor  34  to be blocked, so that a current i 32  now flows through strand  32 . 
         [0053]    The above-described processes then iterate or repeat continuously, as is evident in view of the symmetry of the circuit, i.e. transistors  34  and  44  become alternatingly conductive or blocked, and corresponding loop currents flow. When transistor  34  is conductive, loop current i* flows counter-clockwise; and when transistor  44  is conductive, loop current −i* (not depicted) flows clockwise. 
       Sensing the Rotation Direction 
       [0054]    To sense the rotation direction of motor  20 , motor current i mot , is briefly switched off in order to make motor  20  currentless, for example, during one electrical rotation, i.e. during approx. 360° el. The rotating rotor magnet  22  then induces a voltage in both strands  30  and  32 . 
         [0055]    This results in the situation illustrated in  FIG. 5 . 
         [0056]    At time t 2 , transistor  60  becomes blocked, so that motor current i mot  converted into a loop current i* that becomes zero at time t 3 , so that as of t 3  the induced voltage u ind , which causes transistor  34  to be blocked at time t 4 , can be measured at drain D of transistor  44 . 
         [0057]    Beginning at t 4  two half-waves  70 ,  72  of the induced voltage are obtained, and are delivered to μC  26 . At the end of second half-wave  72 , current i mot  is switched back on. 
         [0058]    A timer  80  in μC  26  is switched on at time t 4  in order to measure the combined duration T 1  of the two half-waves  70 ,  72 . This timer  80  also measures duration T 2  of first half-wave  70 , so that duration T 3  of second half-wave  72  can be calculated as 
         [0000]        T 3= T 1− T 2  (2).
 
         [0059]    When rotor  22  is rotating continuously and uniformly, T 2  and T 3  are of approximately equal magnitude. If this is not the case, the rotation of rotor  22  has been disturbed, for example by a wind gust; the measurement therefore cannot be used and must be repeated. 
         [0060]      FIG. 6  and  FIG. 7  show how the rotation direction is identified. The program can be stored in a ROM  74  or RAM  79 , for example in μC  26 . 
         [0061]    After startup at S 100 , in a step S 102  three memories X, Y, and Z are set to zero, likewise timer  80  at S 103 . 
         [0062]    Timer  80  is then switched on in S 104  at time t 4 . 
         [0063]    S 106  checks whether induced voltage U ind , has a value differing from zero. If NO, a measurement of time spans T 1  and T 2  is initiated at S 108  by timer  80 . 
         [0064]    At S 110 , the A/D converter in μC  26  is then switched on, in order to digitize a measured value. 
         [0065]    At S 112 , A/D conversion is ended, and a new digital value is obtained that is usually different from the old, i.e. previous, value. 
         [0066]    In S 114 , the new value is compared with the previous “old” value. If the new value is greater than the old value, at S 116  a constant a is added to memory X. If, at S 118 , the new value is less than the old value, constant a is then added to memory Y in S 120 . 
         [0067]    If the values are equal in S 122 , then in S 124  the value a is added to memory Z. 
         [0068]    The program then goes to step S 126 , which checks whether induced voltage u ind  is different from zero. If YES, the program goes to step S 128 , where the previous value is replaced by the new value from S 112 , and then a new analog value of the induced voltage is digitized in S 110 . 
         [0069]      FIG. 7  shows further processing of the results from  FIG. 6 . If it is found, in S 126 , that induced voltage u ind  has dropped to zero, timer  80  is switched off in S 130 , and S 132  checks whether the value of duration T 2  corresponds approximately to one-half of T 1 . This situation exists when rotor  22  is rotating. at approximately constant speed. If that is not the case, an “error flag” or error signal is set in S 134 . 
         [0070]    If the response at 5132 is YES;  5136  then first checks whether the value in memory Z is greater than the sum of the values in memories X and Y. This means that in most of the instances tested, induced voltage u ind  has not changed. This indicates an error, and in this case the program therefore goes to S 134 , where the error signal is set. 
         [0071]    If the response at S 136  is NO, the program goes to S 138  to see whether the values in memory X exceed the values in memory Y. If the response there is YES, rotor  22  is therefore running forward, i.e. in preferred direction PRDIR, and in S 140  the forward bit is set. 
         [0072]    If the response in S 138  is NO, then in S 142  the reversing bit is set; this means that motor  20  must be reversed. The program then goes to step S 144 , i.e. the routine is complete. 
         [0073]    It is possible in this manner, for example, to identify the rotation direction during a single revolution of rotor  22 , motor  20  being briefly currentless during this process. 
         [0074]      FIG. 8  shows the implementation for an ECM that is implemented as a single-strand, two-pulse ECM  20 ′. 
         [0075]    The only winding strand  30 ′ is arranged between drains D of the two lower n-channel MOSFETs  34 ,  44  of an H-bridge  150  whose upper bridge transistors  152 ,  154  are implemented as p-channel MOSFETs that each have a respective recovery diode  156 ,  158  connected in antiparallel with them. Drain D of transistor  152  is connected to drain D of transistor  34 , and drain D of transistor  154  is connected to drain D of transistor  44 . Transistor  152  is controlled by μC  26  via a control line  160 . Transistor  154  is likewise controlled by μC  26  via a control line  162 . (Control is usually applied to upper transistors  152 ,  154  via interposed amplifiers, which are not shown in this schematic diagram.) 
         [0076]    During operation, a current flows from terminal  62  through transistor  152 , winding strand  30 ′, and transistor  44  to ground  56 ; and after a rotation of rotor  22  through less than 180° el., commutation occurs to a current from terminal  62  through transistor  154 , winding strand  30 ′, transistor  34 , and to ground  56 . 
         [0077]    In order to measure the induced voltage for an identification of the rotation direction, both upper transistors  152 ,  154  are blocked, so that then, initially, a loop current i* flows through winding strand  30 ′ and the two lower transistors  34 ,  44  and recovery diodes  38 ,  48 . 
         [0078]    Once loop current i* has reached a value of zero, the induced voltage and its profile are measured in the same manner as described in detail with reference to  FIGS. 3 ,  6 , and  7 , thereby yielding the rotation direction. 
         [0079]    Reference is made to  FIG. 9  regarding the manner of operation of  FIG. 8 . 
         [0080]    At time t 10 , both transistors  152  and  44  are switched on, so that a current flows from terminal  62  through transistor  152 , strand  30 ′ (from left to right), and transistor  44  to ground. 
         [0081]    At time t 12  transistor  152  is blocked, so that energy delivery from outside is interrupted. Transistor  44  remains conductive. 
         [0082]    Since current can no longer flow from terminal  62  to the motor, the current in strand  30 ′ is maintained by the energy stored in that strand, and a loop current i* now flows (clockwise), during time period T 20  of  FIG. 9 , through strand  30 ′, through (still conductive) transistor  44 , and back through recovery diode  38  to strand  30 ′. 
         [0083]    This loop current i* continues to drive rotor  22  and drops quickly (within time period T 20 ) to zero. 
         [0084]    When loop current i* has dropped to zero, the voltage u ind  (analogous to signal  68  of  FIG. 4   b ) that is induced by rotor  22  in strand  30 ′ can be measured at drain D of transistor  44 ; this voltage indicates that commutation can now occur. The curve is in this regard identical to  FIG. 4  between times t 2  and t 3 . 
         [0085]    In the course of commutation, transistor  44  becomes blocked at time t 14  and, after a brief pause of, for example, 30 μs, the two transistors  154  and  34  are switched on, so that a current flows from terminal  62  through transistor  154 , strand  30 ′ (from right to left), and transistor  34  to ground  56 . 
         [0086]    At time t 16  transistor  154  is blocked, and a loop current −i* then flows (counter-clockwise) through (still conductive) transistor  34 , recovery diode  48 , and strand  30 ′. This current −i* quickly drops to zero, after which it is possible to measure at drain D of transistor  34  (analogously to signal  68  of  FIG. 4 ) the induced voltage that is induced by rotor  22  in strand  30 ′ and that brings about a new commutation at time t 18 , as depicted in  FIG. 9 . 
         [0087]    Sensorless commutation in the desired rotation direction, which ensures that motor  20  rotates in the required rotation direction even under difficult conditions, is thereby achieved. 
         [0088]    Many variations and modifications are of course possible, within the scope of the present invention.