Abstract:
An electronically commutated motor has at least two winding phases ( 112, 114 ) which are wound together or otherwise inductively coupled. Current in each phase is controlled by a respective power transistor ( 124, 128 ). An integrated circuit controller ( 146 ) receives signals from a Hall sensor ( 118 ) and generates rotor position output signals (OUT 1 , OUT 2 ) which are oppositely phased and are applied to the bases of the respective power transistors ( 124, 128 ) so that the power transistors never both conduct at the same time. Further, a pair of latching transistors ( 162, 172 ) and a pair of base drain resistors ( 164, 174 ), connected to respective bases of the power transistors ( 124, 128 ) are provided, in order to assure “soft” switching of the power transistors at low RPM, yet prompter switching and higher efficiency at high RPM. These additional components also ensure a sufficiently long current gap between switch-off of one power transistor and switch-on of the other power transistor.

Description:
FIELD OF THE INVENTION 
     The invention relates to an electronically commutated motor having at least two stator winding strands or phases, each of which can be actuated by means of an associated power transistor. Such motors are often referred to as “two-phase” motors and are used in great quantities, above all in fans. FIG. 6 shows one of the circuits presently used by the Applicant for controlling such motors. 
     BACKGROUND 
     Often, such motors must be capable of being operated on very differing voltages, e.g. a fan with a nominal voltage of 24 V is, in practice, operated with voltages ranging between 12 V and 32 V, which represents a voltage deviation range from −50% to +30%. Depending upon voltage, such a fan runs at a desired speed (RPM), i.e. slowly at lower voltages, and fast at higher voltages. 
     Upon commutation of the motor current from one stator winding phase to another, the switchover can occur in a “hard” or in a “soft” manner. A hard switchover offers good efficiency, but high structure-borne noise levels caused by commutation noise and, additionally, EMC problems (EMC=Electro-Magnetic Compatibility). Further, protective measures must be taken for the end-stage transistors, so that the critical limit values of the components (permissible collector voltages etc.) are not exceeded. This can be accomplished by internal Z-diodes contained in the end-stage transistors or by external Z-diodes (for limiting these voltages) or by recovery diodes which feed back the shutoff energy of the windings to an operating voltage link circuit containing a capacitor capable of receiving this shutoff energy. 
     FIG. 6 shows a motor  10  with two stator winding strands or phases  12 ,  14  and a (schematically indicated) permanent magnet rotor  16 , in whose vicinity a Hall generator  18  is located. Hall generator  18  is also shown in the left portion of FIG.  6 . Ohmic resistors of phases  12  and  14  are designated  20  and  22 , respectively. Phase  12  is connected in series with an npn Darlington transistor  24  with built-in recovery diode  26 , and phase  14  is connected in series with an npn Darlington transistor  28  with built-in recovery diode  30 . The emitters of transistors  24 ,  28  are connected via a common emitter resistor  32  to a negative conductor  34 . Phases  12 ,  14  are connected to a positive conductor  36 , as shown. Conductors  34 ,  36  are, during operation, connected to a power supply device  38  which contains a storage capacitor  40 . This serves to take up the shutoff energy of phases  12 ,  14 , which is fed back via recovery diodes  26 ,  30  into this capacitor  40 . To the extent that the motor is supplied from an accumulator, the shutoff energy is fed back into the accumulator. 
     Hall generator  18  is connected via a resistor  42  with positive conductor  36  and via a resistor  44  with negative conductor  34 . Its output signal U H  (FIG. 7A) is applied to both inputs IN 1  and IN 2  of an IC (Integrated Circuit)  46  which generates signals OUT 1  and OUT 2  for controlling transistors  24 ,  26  and simultaneously serves as blocking or stall protection for motor  10 , i.e. when it is unable to turn, it is switched off by IC  46 . 
     This IC is preferably the ROHM BA6406. FIG. 7A shows the signal U H , FIG. 7B shows the signal OUT 1  of IC  46  and FIG. 7C shows the signal OUT 2 . Signals OUT 1  and OUT 2  run in phase opposition to each other. FIG. OUT 1  is fed via a resistor  50  (e.g. 8.2 kΩ) to the base of transistor  24 , which is connected via a capacitor  52  (e.g. 1 Nf) to the collector and via a base drain-off resistor  54  (e.g. 1.2 kΩ) to negative conductor  34 . In the same manner, signal OUT 2  is fed via a resistor  56  to the base of transistor  28 , which is connected via a capacitor  58  with its collector and via a base drain-off resistor  60  with negative conductor  34 . 
     During operation, transistors  24 ,  28  are alternately switched on by signals OUT 1 , OUT 2 . Through the combination of resistor  50  and capacitor  52  and the common resistor  32 , a soft switching of transistor  24  is achieved. Similarly, resistor  56 , in combination with capacitor  58  and common resistor  32 , effects a soft switching of transistor  28 . However, these measures cause an increased warming of transistors  24 ,  28  and therefore a reduction in efficiency. Furthermore, the circuit can be optimally configured only for a single operating point, i.e. for a specified RPM and a specified torque. This results in many compromises, especially with respect to voltage overruns and temperature overruns. One also obtains, in practice, a voltage range only of ±15%. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a new electronically commutated motor. 
     In accordance with the invention, this object is achieved by adding latching circuitry, including two transistors, to prevent simultaneous conduction of both power transistors. By the alternate biasing of the power transistors, one achieves that such a motor is usable over a large range of its operating voltage U B ; and further, that, during commutation, both power transistors are briefly blocked, which minimizes braking torques and improves efficiency, and that the kind of commutation can be adapted to the rotation speed (soft commutation at low RPM, hard commutation at higher RPM). This can be achieved by making the kind of ascent and descent of the commutation signal dependent upon the RPM. Such a solution is also very economical. 
    
    
     BRIEF FIGURE DESCRIPTION 
     Further details and advantageous features of the invention will be apparent from the embodiment described below and illustrated in the drawings, which is not to be understood as limiting the invention, as well as from the dependent claims. Shown are: 
     FIG. 1 illustrates a preferred embodiment of an electronically commutated motor according to the invention; 
     FIG. 2 is a graph showing the course of commutation during operation at a 50% undervoltage (12 V), the normal operating voltage being 24 V in this example; 
     FIG. 3 is a graph, analogous to FIG. 2, for the prior art motor shown in FIG. 6; 
     FIG. 4 is a graph showing the course of commutation, in a motor according to FIG. 1, during operation at a 16.5% overvoltage ,(28 V); 
     FIG. 5 is a graph, analogous to FIG. 4, for the prior art motor shown in FIG. 6; 
     FIG. 6 is a schematic diagram of a motor according to the prior art; 
     FIG. 7 is a signal diagram illustrating the operation of the motors shown in FIGS. 1 and 6; 
     FIG. 8 is a schematic diagram illustrating a first embodiment of the stator winding; and 
     FIG. 9 is a schematic diagram illustrating a second embodiment of the stator winding. 
    
    
     DETAILED DESCRIPTION 
     The electronically commutated motor  110  according to FIG. 1 has two stator winding phases  112 ,  114  and a (schematically illustrated) permanent magnet rotor  116 , in whose vicinity a Hall generator  118  is located, as also shown on the left side of FIG.  1 . Respective ohmic resistances of the windings  112  and  114  are designated  120  and  122 . 
     As shown in FIG. 8, by way of example, the two phases  112 ,  114  are preferably coupled with each other by making the winding with two parallel wires, i.e. as a so-called “bifilar” winding. The terminals of phase  112  are designated a and e, and the terminals of phase  114  are designated a′ and b′ as shown in FIGS. 8 and 9. In phase  112 , current flows from a to e, while in phase  114 , current flows from e′ to a′, so that these two phases create opposing magnetic fluxes. Motor  110  can, for example, be constructed as shown in DE 23 46 380 filed Sep. 14 1973, assigned Papst Motoren KG. 
     Alternatively, phases  112 ,  114  may be inductively coupled together via the metal lamination stack of the stator. FIG. 9 shows this, on a two-pole stator  210  of an external rotor motor. Phase  112  is wound on the upper stator pole  214 , and phase  114  is wound on the lower stator pole  216 . Part  218  of the lamination stack between poles  214 ,  216  effects a close magnetic coupling of the winding phases  112 ,  114 . The preferred rotation direction of the motor of FIG. 9 is designated  220 . In FIG. 9, the internal stator  210  remains still, and the two-pole rotor  116  rotates around it. 
     Phase  112  is, as shown in FIG. 1, in series with an npn Darlington transistor  124  with built-in recovery or free-running diode  126 , and phase  114  is in series with an npn Darlington transistor  128  with built-in recovery or free-running diode  130 . The emitters of transistors  124 ,  128  are, in the illustrated embodiment, connected in an advantageous manner via a common emitter resistor  132  to a negative conductor  134 . optionally, each of transistors  124 ,  128  can have its own emitter resistor. Phases  112 ,  114  are, as shown, connected to a positive conductor  136 . Conductors  134 ,  136  are, during operation, usually connected to a power supply  38  (FIG. 6) which includes a storage capacitor  40 . It serves the purpose of absorbing the shut-off energy of stator phases  112 ,  114 , which is fed back via recovery diodes  126 ,  130  into this capacitor  40 . 
     Depending upon how “softly” the end-stage transistors  124 ,  128  are switched during commutation, there is a reduction in the shut-off energy, which must be fed back into capacitor  40  during the commutation. In the most favorable scenario, such a capacitor becomes unnecessary, since a slow shut-off of transistors  124 ,  128  means that this energy can be completely converted into heat in the end stages. 
     Hall generator  118  is connected via a resistor  142  to the positive conductor  136  and via a resistor  144  to the negative conductor  134 . The generator output signal U H  (FIG. 7A) is applied to both inputs IN 1  and IN 2  of an IC  146 , which generates the signals OUT 1  and OUT 2  for control of transistors  124 ,  128  and which simultaneously provides protection against stalling of motor  110 ; that is, when rotor  116  cannot turn, the motor is turned off by IC  146 . FIG. 7A shows this signal U H , FIG. 7B shows signal OUT 1  of IC  146 , and FIG. 7C shows signal OUT 2 . The two last-mentioned signals are phased oppositely to each other. Signal OUT 1  is fed via a resistor  150  to the base of transistor  124 , which is connected via a capacitor  152  (Miller capacitor) to the collector of transistor  124 . In the same manner, signal OUT 2  is fed via a resistor  156  to the base of transistor  128 , which is connected via a capacitor  158  (Miller capacitor) to its collector. 
     IC  146  is, as shown, connected via its terminal VCC to the positive conductor  136  and via its terminal GND to the ground or negative conductor  134 . Optionally, a resistor  147  can be placed in the connection to positive conductor  136 , and terminal VCC can be connected via a Z-diode  149  to negative conductor  134 . The Zener voltage of diode  149  can be selected to be above the nominal operating voltage U B , e.g. 28 V if the motor is designed for 24 V. Thereby, the amplitudes of signals OUT 1 , OUT 2  rise to a voltage of 28 V and then remain constant, even if U B  rises still higher. In FIG. 7B, the variable amplitude of signal OUT 1  is indicated by the double-ended arrow  151 . The same applies for the signal OUT 2 , but this is not shown. 
     Thus, below the Zener voltage, one has a dependence of signal amplitudes upon U B , but not above the Zener voltage. Without the Z-diode  149 , there is this dependence over the entire range of U B . 
     The base of transistor  124  is connected via a base drain resistor  160  and the collector-emitter path of an npn transistor  162  to the negative conductor  134 . The base of transistor  162  is connected via a resistor  164  to the base of transistor  128  and via a resistor  166  to the output OUT 2  of IC  146 . The value ratio of resistor  164  to resistor  166  is about, for example, 3:100, i.e. the influence of the signal on the base of transistor  124  is stronger than that of signal OUT 2 . 
     In a fully symmetrical manner, the base of transistor  128  is connected via a base drain resistor  170  and via the collector-emitter path of an npn transistor  172  to the negative conductor  134 . The base of transistor  172  is connected via a resistor  174  to the base of transistor  124 , and via a resistor  176  to the output OUT 1  of IC  146 . The values of resistors  174 ,  176  correspond to those of resistors  164 ,  166 . 
     The transistors  162 ,  172  can, on the one hand, be called “safety or latching” transistors, since they latch the two power transistors  124 ,  128  with respect to each other, and prevent them from both being conductive (ON) at the same time. On the other hand, these transistors do not simply switch ON or OFF; rather, they activate, during the commutation interval, the base drain resistors  160  and  170 , respectively, while in the time ranges outside the commutation, these resistors  160 ,  170  are not active. 
     According to the prior art circuit shown in FIG. 6, the base drain resistors  54 ,  60  are continuously active. This is particularly disadvantageous during an undervoltage condition, since, for example, in FIG. 6 the resistor  60  and resistor  56  together form a voltage divider which, if there is a low operating voltage U B , may prevent transistor  28  from receiving an adequate base current. 
     However, since in the circuit according to FIG. 1, the base drain resistors  160 ,  170  are only active in the interval of the commutation process, and not active in the time interval between two commutations, problems in the latter time interval arising from resistors  160 ,  170  are avoided, even in case of low operating voltage U B , since they are not then active, as already described. 
     In the circuit, various monitoring points P 1  to P 6  are as shown. If, for example, power transistor  124  is conductive, monitoring points P 1  and P 3  have differing potentials, since a base current flows via resistor  150  from the positive (high) output OUT 1  to the base of transistor  124 , and upon shut-off of transistor  124 , the signal OUT 1  is indeed low, but the shut-off voltage of winding  112  is transmitted via capacitor  152  to the base of transistor  124 , and has the effect that this base receives base current for a little while longer. Due to the symmetry of the circuit, the relationships at transistor  128  are identical. 
     PREFERRED VALUES OF COMPONENTS 
     Motor  110  is, in this example, designed for an operating voltage of 24 V direct current, and has a power draw of 2.4 W at this nominal voltage. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Hall generator 118 
                 HWA 101 
               
               
                   
                 IC 146 
                 ROHM BA6406 
               
               
                   
                 Resistor 132 
                 1 Ω 
               
               
                   
                 Resistors 150, 156 
                 8.2 kΩ 
               
               
                   
                 Resistors 160, 170 
                 560 Ω 
               
               
                   
                 Resistors 164, 174 
                 1.5 kΩ 
               
               
                   
                 Resistors 166, 176 
                 47 kΩ 
               
               
                   
                 Capacitors 152, 158 
                 3.3 nF 
               
               
                   
                 Transistors 124, 128 
                 ZTX600 
               
               
                   
                 Transistors 162, 172 
                 BC847A 
               
               
                   
                   
               
             
          
         
       
     
     MODE OF OPERATION 
     Since the arrangement is symmetrical, it suffices to describe the processes involved in the switching of current from phase  114  to phase  112 , since the processes in the reverse direction, ie. from phase  112  to phase  114 , take the same course. 
     Whenever phase  114  is carrying current, the signal OUT 1  is low and the signal OUT 2  is high. This corresponds to instant t, in FIG.  7 . Thus, transistor  124  is blocked, and transistor  128  is conducting, as is transistor  162 , since its base is subjected to the high signal OUT 2 . However, since OUT 1  is low, no current flows over its collector-emitter path, i.e. in this state, transistor  162  has no influence on the function of the motor and also does not influence its efficiency. 
     Upon commutation at instant t 2  (FIG.  7 ), signal OUT 1  goes high and signal OUT 2  goes low. Transistor  162  continues to conduct, since it receives, as before, a base current from the base of transistor  128 , and thereby transistor  124  remains initially blocked, although signal OUT 1  is high, since its base current is drained away via resistor  160 . Transistor  128 , in this state, is still conductive, since it is still receiving a base current, via capacitor  158 . 
     At monitoring point P 3 , one obtains a potential which is determined by the resistors  150 ,  160  and the capacitor  152 . Point P 1  is, as previously described (arrow  151  in FIG.  7 B), at a potential which is dependent upon the potential of positive conductor  136 . The potential at point P 3 , due to capacitor  152 , is delayed in reaching its end value, which is determined by resistors  150 ,  160 . At this time, transistor  172  is still not conducting. The potential on the base of transistor  172  is determined by the signal OUT 1 , which in this case is high, and by the voltage divider  176 ,  174 , connected to the base of transistor  174 , which at this instant still has a low potential. Thus, transistor  172  remains temporarily blocked. 
     Due to the increase in charge on capacitor  152 , the potential at P 3  rises, so that transistor  172  begins to conduct, which assures blocking of transistor  128 . Upon blocking of transistor  128 , the time constant of elements  158  (capacitor) and  170  (resistor) becomes effective. The current amplification of transistors  162 ,  172  has little influence on the time delay. As soon as transistor  172  becomes conductive, this removes the base current from transistor  162 , which hitherto had been supplied via resistor  164 . Thereby the current over the base drain resistor  160  stops, and transistor  124  is released and can turn on according to a time constant which is determined by the resistor  150  and the capacitor  152 . Transistor  128  is, at this instant, currentless, i.e. between the shut-off of transistor  128  and the turn-on of transistor  124   124 , a current gap is created, in which neither transistor  124  nor transistor  128  is conductive. 
     These processes are supported by the inductive coupling between phases  112  and  114 , examples of which are shown in FIGS. 8 and 9. 
     FIG. 2 illustrates the voltage courses at a low voltage U B  of 12 V. The curves U C128 , U C124  show the collector voltages of transistors  128 ,  124 , respectively. 
     At instant t 2 , signals OUT 1  and OUT 2  change. During a transition phase until instant t 3  voltage U C124  decreases and voltage U C128  increases, because transistor  128  starts to block. Due to the inductive coupling (FIG. 8) of windings  112 ,  114 , the increase in U C128  between t 2  and t 3  corresponds essentially to the decrease in U C124 , without transistor  124  being conductive yet. In the interval from t 3  to t 4  the shut-off voltage of phase  114  causes a rise  180  in U C128  and thus a recovery current through diode  130 . This keeps transistor  162  conductive, so that a portion of the base current of transistor  124  flows via resistor  160 , and transistor  124  can first switch on at instant t 4 , at which time its base voltage U B124  has become sufficiently positive because transistor  162  has blocked. 
     Transistor  128  is thus switched off at instant t 3 , and transistor  124  is first switched on at t 4 , thereby producing, between t 3  and t 4 , a current gap. The shut-off and turn-on processes run gently, so that the motor produces little structure-borne noise during the commutation. This is important in a fan, because at low RPM the fan noises should be minimized. 
     FIG. 3 shows the analogous processes in the motor according to FIG. 6, also at a low voltage of 12 V. There, transistor  28  shuts off about at instant t 5 , and at instant t 6 , transistor  24  is turned on, i.e. the current gap is shorter here. Furthermore, as previously mentioned, the resistors  54  and  60  are continuously active, which is unfavorable for the operation of the motor at low voltages. 
     FIG. 4 shows the processes in the motor of FIG. 1, when it is operated at an overvoltage of 28 V. Rotor  16  here rotates significantly faster, i.e. the processes of commutation must be executed faster, so that the power of the motor will be sufficiently large and that it can achieve a high rotation speed. 
     At instant t 7 , the signals OUT 1 , OUT 2  change, which means, due to the inductive coupling of phases  112 ,  114 , that the voltage U C124  on the collector of transistor  124  decreases and the voltage U C128  of transistor  128  increases until instant t 8 , in such a way that the decrease and the increase correspond. This inductive (transformer) coupling of the windings (cf. FIG. 8) is indicated by an arrow  184 . 
     Subsequently, the shut-off current spike on winding  114  causes (at  186 ) an increase in voltage U C128 , which lengthens the current gap further up to an instant t 9 , at which, due to the blocking of transistor  162 , the base voltage U B124  rises so far that this transistor becomes conductive. The increase of U B124  is designated  188 . Thus, at high voltages and high RPM, the current gap lasts from t 8  to t 9 . 
     Upon comparison of FIGS. 2 and 4, it is significant that the steepness  192  of the rise of collector voltage U C128  in FIG. 2 is low, and this steepness  194  in FIG. 4 is high. This is a result of the fact that the amplitudes of signals OUT 1 , OUT 2 , in the high state, are approximately proportional to the operating voltage U B , so that the charging of capacitor  152  (or  158 ) and the switching of transistor  172  (or  162 ) occurs faster at high U B . This is a result of the voltage divider  166 ,  164  between points P 2  and P 4  and the voltage divider  176 ,  174  between points P 1  and P 3 . Due to the higher input voltage at P 1  or P 2 , at points P 5  or P 6  the necessary turn-on voltage is achieved sooner, if the voltage U B  is higher. 
     FIG. 5 shows that, in the motor of FIG. 6, the steepness  196  of the increase in U C28  is not much changed from that of FIG.  3  and that, already at instant t 10 , at which the signals OUT 2 , OUT 1  change, the signal U B24  goes so high that transistor  24  turns on, so that for a while both transistors  24  and  28  conduct, which leads (at  198 ) to an increase in total current i total . 
     At instant t 11 , transistor  24  blocks again and at t 12 , it becomes conductive again, which leads (at  200 ) to a corresponding reduction in the total current i total  The shut-off between t 11  and t 12 , leads to corresponding heating up of transistor  24  (due to the unnecessary switching processes) and to a reduced efficiency of the motor. 
     By practicing the invention, one thus achieves, at all rotation speeds, a sufficiently large switching pause during the commutation. At low voltage U B , during the commutation, the current in a winding rises and falls only relatively slowly, i.e. one obtains a switching signal edge with a lower slope, which o results in a quiet running of the motor. At higher operating voltage U B , during the commutation, the current rises and falls in a winding quickly. This improves the efficiency and increases the maximum attainable RPM, at which, however, the structure-borne noise of the motor rises. At high RPM, that is not disturbing, since all the noises are rising anyway. The base drain resistors  160 ,  170 , which are turned on and off by the transistors  162 ,  172 , respectively, are only effective during the commutation, as already described, so that they do not degrade the efficiency of the motor. This permits dimensioning for a large voltage range. The signal at the base of power transistor  124  controls, via the resistor  174 , the transistor  172 . The signal at the base of power transistor  128  controls, via the resistor  164 , the transistor  162 . The transistors  162 ,  172  are so latched with respect to each other that only one of the two can conduct at a given time, and this excludes the possibility that both transistors  124 ,  128  would conduct at the same time, and a defined current gap is created at commutation, which is a precondition for obtaining a “soft” switching of transistors  124 ,  128 . Comparison of FIGS. 4 and 5 shows this. According to FIG. 5, for a time both transistors  24 ,  28  conduct simultaneously, whereas by contrast in FIG. 4, a sufficiently long “switching pause” is created. 
     Naturally, within the scope of the present invention, numerous variations and modifications are possible. In particular, the various functions of latching transistors  162 ,  172  could be performed by a larger number of transistors, to the extent that the cost thereof is not a factor.