Abstract:
A method for determining the position of the rotor of an electric machine having multiple phases in relation to the stator. The change Δİ in the time derivative İ of the current I flowing through at least one of the power inputs is determined. The change Δİ is caused by a change ΔU in the potential U on at least one of the power inputs of the electric machine. A measurement signal that is representative of a position of the rotor is determined from multiple simultaneously or successively determined changes Δİ.

Description:
The present application is a 371 of International application PCT/DE2012/100009, filed Jan. 16, 2012, which claims priority of DE 10 2011 008 756.7, filed Jan. 17, 2011, the priority of these applications is hereby claimed and these applications are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for determining the position of the rotor of an electrical machine having several phase strands, in relation to the stator, and an apparatus for carrying out the method. 
     EP 1 005 716 B1 discloses a method for determining the position of the rotor of an electrical machine, wherein the changes of the potential at the star point caused by changes of the potential of the rotor at the current supply inputs are utilized for determining a signal which is representative for the rotor position. 
     The invention is based on the object of creating a novel method for determining the position of the rotor of an electrical machine which can be carried without picking up an electrical value within the electrical machine. 
     SUMMARY OF THE INVENTION 
     The method according to the invention which meets this object is characterized in that, for at least one of the current supply inputs of the electrical machine, a change Δİ of the time derivation İ of the current flowing through the respective current supply input is determined as a result of a change ΔU of the potential U on at least one of the current supply inputs of the electrical machine, and a signal representative for the position of the rotor is determined from several changes Δİ occurring simultaneously or successively. 
     In accordance with the invention, the determination of the position of the rotor takes place exclusively with the aid of currents which can be measured outside of the electrical machine and with changeable voltages. 
     Preferably, inductivity of one or more phase strands or/and a quotient of these inductivities at a given time are determined as representative measurement signals. 
     The change ΔU of the potential U takes place within such a short period of time that voltages induced in the phase strands in the meantime and currents flowing through the current supply inputs do not noticeably change. 
     The phase strands can be switched in a star configuration or/and in a triangular configuration. 
     Preferably, the change ΔU of the potential U from a potential zero point to a direct voltage takes place, particularly a battery voltage, or vice versa. 
     In accordance with a particularly preferred embodiment, the change ΔU of the potential U takes place within the scope of a current supply of the electrical machine through pulse width modulation (PWM). The determination of the position now does not require an intervention in the electrical machine itself, or in the manner of operation thereof. 
     The change Δİ can be determined, for example, by means of a measuring transformer which, for example, produces a voltage signal which may have to be reinforced. For example, in the case of a voltage switch occurring within the scope of the pulse width modulation, a voltage leap results at the secondary winding of the measuring transformer which corresponds to the voltage leap during switching. 
     Several changes Δİ caused by several potential changes ΔU can be determined for only one of the current supply inputs. The expenses for measurement circuits are correspondingly low. Alternatively, in the extreme case, changes Δİ affected by a single potential change ΔU can be determined for several current supply inputs, so that a measuring circuit is required for each current supply input. 
     In accordance with a preferred embodiment of the invention, the representative measuring signal is determined from equations which contain the sum of the voltage components over the individual phase strands prior to and after the voltage change. 
     In the following the invention will be explained in more detail with the aid of embodiments and the enclosed drawing which refers to these embodiments. In the drawing: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a schematic illustration of an electrical machine with phase strands switched in a star configuration and devices for determining the position of the rotor in accordance with the method of the invention, 
         FIG. 2  shows a measuring device for determining changes Δİ of the time derivation İ of the current I in a phase strand, and 
         FIG. 3  shows a schematic illustration of an electrical machine with phase strands switched in a triangular configuration, and devices for determining the position of the rotor in accordance with the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Three phase strands  1 ,  2 ,  3  of an electrical machine  4  switched in a star configuration each form an inductive resistance  5  and an ohmic resistance  6 . The ends of the phase strands  1 ,  2 ,  3  facing away from the star point  7  are connected to connections  1 ′,  2 ′,  3 ′ for current supply lines  1 ″,  2 ″,  3 ″. 
     A current supply device  8  connects the current supply lines  1 ″,  2 ″,  3 ″ through switching devices  9  to  11  with the direct voltage U B  of a battery  12  or the voltage zero point corresponding to the pulse width modulation method (PWM-Method). 
     Measuring devices  13  to  15  in the current supply lines  1 ″,  2 ″,  3 ″ serve for determining changes Δİ, i.e. changes of the first derivation of the current I flowing through the respective current supply line over time. This refers to short changes which result from switching by the switching devices  9  to  11 . 
     The measurement devices  13  to  15  are in connection with an evaluating device  16  which, in turn, is connected to the current supply device  8  through a control line  17 . 
     As illustrated in  FIG. 2 , the measuring devices  13 ,  14 ,  15  may have, for example, a measuring transformer  18  whose secondary winding  19  delivers a measurement for İ, i.e. the first derivation of the current I over time. An amplifier  21  ensures that the inductivity of the primary winding  20  can be small in comparison to the inductivity of the phase strands  1 ,  2 ,  3 . 
     With respect to the voltages U 1  applied to the individual phase strands İ (İ=1, 2, 3), the following is true at any point in time:
 
 U   1   =U   i ind   +L   i   İ   i   +R   i   I   i   (1),
 
wherein U i ind  denotes the voltage induced in the phase strand i, L i  denotes the inductivity of the phase strand i, and R i  denotes the ohmic resistance thereof.
 
     Depending on whether the battery voltage U B  or the voltage zero point contact the phase strand i, U i =0, U i =U S , U i =U B −U S , or U i =U S −U B , wherein U S  refers to the potential at the star point  7 . 
     For example, if starting from a switching state in which all three phase strands  1 ,  2 ,  3  are connected to the voltage zero point, the following applies:
 
0= U   i ind   +L   1   İ   1   +R   1   I   1   (2)
 
0= U   2 ind   +L   2   İ   2   +R   2   I   2   (3)
 
0= U   3 ind   +L   3   İ   3   +R   3   I   3   (4).
 
     The ohmic resistances R i  can be considered as being equal and they can be considered to be constant during motor operation of the electrical machine. On the other hand, the inductivities L i  depend on the respective influence of the exciter field and, thus, on the position of the rotor relative to the magnetizations of the pole winding cores. Within half a magnetic period there is always an unequivocal relationship between the inductivity of the phase strands and the position of the rotor which can be utilized for determining the position, as will be explained in the following. 
     After switching, for example, the phase strand  3  to the battery voltage U B  by means of the switching device  11 , the following result is obtained:
 
 U   S   =U   1 ind′   +L   1   İ   1′   +R   1   I   1′   (5)
 
 U   S   =U   2 ind′   +L   2   İ   2′   +R   2   I   2′   (6)
 
 U   B   −U   S   =U   3 ind′   +L   3   İ   3′   +R   3   I   3′   (7).
 
     The change of the switching state characterized by the equations (2) to (4) into the switching state according to the equations (5) to (7) takes place so quickly that neither the voltages U 1 ind  induced in the phase strands nor the currents I change significantly so that U i ind =U i ind′  and R i  İ 1 +R 1  I 1 . The switch essentially only has an effect on i i  ie., the first derivation of the currents over time. By subtracting (2)−(5), (3)−(6) and (4)−(7), the following is obtained.
 
 L   1 ( İ   1   −İ   1′ )= L   1   ●Δİ   1   =U   S   (8)
 
 L   2 ( İ   2   −İ   2′ )= L   2   ●Δİ   2   =U   S   (9)
 
 L   3 ( İ   3   −İ   3′ )= L   3   ●Δİ   3   =U   B   −U   S   (10).
 
     The values Δİ 1 , Δİ 2 , Δİ 3  can be determined by means of the measuring devices  13  to  15 . In the three equations (8) to (10), the inductivities L 1 , L 2  and L 3  as well as the potential U S  at the star point are then unknown. 
     From the three equations (8) to (10), the ratios L 1 /L 2 , L 1 /L 3 , L 2 /L 3  can be determined while eliminating U S  which each represent a dimension for the position of the rotor within half a magnetic period. 
     In addition, the following applies to the above described switch:
 
 U   B =( L   3 +1/(1/ L   1 +1/ L   2 ))●Δ İ   3   (11).
 
     Consequently, four equations (8) to (14) are available for determining the unknown values L 1 , L 2 , L 3  and U S . Each of the inductivities L 1 , L 2 , L 3  may serve as a dimension for the position of the rotor within half a magnetic period. 
     It is understood that for obtaining several equations from which U S  can be eliminated and L 1 , L 2  or/and L 3  can be determined, several successive switching state changes can be considered as long as the condition is adhered to that over the total duration of the measurements the position of the rotor does not perceptibly change. Under these conditions, Δi does not have to be determined in all three phase strands. When considering several switching state changes, the measurement of Δİ in a single phase strand may be sufficient. Accordingly, only a single measuring device is required. 
     Equations for determining L 1 , L 2 , or/and L 3  can also be set up for the situation in which the phase strands  1 ,  2 ,  3  are switched in a triangular configuration, as illustrated in  FIG. 3 . 
     A first switching state change concerns, for example, switching of the connection  1   a′  from the voltage zero point to battery voltage U B . A second switching state change concerns the switching of the connection  2   a ′ from the voltage zero point to battery voltage U B . This results in the three equations
 
Δ İ   2   ●L   1   =U   B   (12)
 
Δ İ   3   ●L   3   =U   B   (13)
 
Δ İ   3   ●L   2   =U   B   (14),
 
from which the unknown values L 1 , L 2 , L 3  can be determined.
 
     The measuring devices ( 13 ) to ( 15 ) in the embodiment according to  FIG. 2  determine, or at least one such measuring device determines, a voltage signal S representative for the time derivation İ of the current I in the respective current supply line, wherein the voltage signal is supplied to the evaluating device  16 . Control signals received through the control line  17  and indicated by the switching devices  9  to  11  permit the determination of S prior to and after switching, and thus the determination of signal ΔS which is representative for Δİ. The evaluating device  16  determines from the signals ΔS, for example, with the aid of the above mentioned equations, the signals representative for the position of the rotor. 
     The above described method for determining the position could be combined with the known method which is based on the evaluation of the potential U S  at the star point. 
     The primary winding  20  of the measuring device shown in  FIG. 2  may result in an advantageous damping effect during the switching procedures. 
     From the determined inductivities L 1 , L 2 , L 3 , a flow vector can be determined whose direction coincides with the direction of the rotor flow vector Φ R  produced by the magnetic field of the rotor and which is proportional to the rotor flow vector Φ R  as long as the currents I 1 , I 2 , I 3  flowing in the phase strands do not significantly influence the total flow vector Φ. The latter may be the case, especially when the rotor field is weak and the air gap is large. In addition to the rotor flow vector Φ R  which is a function of L 1 , L 2 , L 3 , the stator flow vector Φ S , which depends on the currents I 1 , I 2 , I 3 , are determinative for the total flow vector Φ:
 
Φ( L   1   ,L   2   ,L   3 )=Φ R +Φ S ( I   1   ,I   2   ,I   3 )  (15).
 
     Consequently, in the case of known inductivities L 1 , L 2 , L 3  and known currents I 1 , I 2 , I 3 , the rotor flow vector Φ R  can be computed:
 
Φ R =Φ( L   1   ,L   2   ,L   3 )−Φ S ( I   1   ,I   2   ,I   3 )  (16).
 
     When the rotor flow vector Φ R  is known then the rotary position of the rotor is also known. 
     The currents I 1 , I 2 , I 3  can be measured. However, they can also be computed if the inductivities L 1 , L 2 , L 3  are known, if the values İ 1 , İ 2 , İ 3  are known (by measuring Δİ 1 ), if the resistances R 1 , R 2 , R 3  are known, and if the induced voltages U 1 ind , U 2 ind , U 3 ind  are known. They can also be computed by means of the above indicated equations. 
     The voltages U i ind  result from the components Φ i  of the total flow vector Φ multiplied with the determined rotary speed of the rotor.