fig1 schematically illustrates a motor system 10 comprising an electrical machine . the motor system 10 has a three - phase machine with hybrid excitation as electrical machine 11 with three phase windings 12 with the phase connections u , w , v . the phase windings 12 are connected to one another in the electrical machine 11 in star . the electrical machine 11 further has a field winding 13 which serves to generate , or contributes to generating , a magnetic field in the rotor of the electrical machine 11 with hybrid excitation and which is electrically connected to the star point 14 . the phase windings 12 are supplied with corresponding phase voltages by means of a driver circuit 15 , here an inverter with six switches s 1 to s 6 which are connected to one another in the form of a fully controlled 6 - pulse bridge ( b6 ) which is known per se . the switches s 1 to s 6 of the driver circuit 15 are driven by a drive unit 26 , wherein the switches s 1 to s 3 correspond to the pull - up switches , and the switches s 4 to s 6 correspond to the pull - down switches . the driver circuit 15 has three inverter branches comprising the switches s 1 and s 4 and , respectively , s 2 and s 5 and , respectively , s 3 and s 6 . the switches can be designed , for example , in the form of semiconductor switches , in particular as transistors , thyristors , mosfets , igbts , igcts and the like . at the input end , the driver circuit 15 is provided with a first supply voltage connection 16 and a second supply voltage connection 17 to which a supply voltage u 0 can be applied . the two supply voltage connections 16 , 17 can be , or can be electrically connected to , outputs of a dc voltage intermediate circuit or a dc voltage source , in particular a lithium - ion battery . the second supply voltage connection 17 can correspond , in particular , to a supply voltage ground . the field winding 13 is connected between the star point 14 and the second supply voltage connection 17 , that is to say the supply voltage ground . a shunt 24 can be used as current measurement apparatus 27 for current measurement purposes , said shunt being arranged between the second supply voltage connection 17 and an input of the driver circuit 15 , which is in the form of a b6 inverter , in order to implement the method described below . in this way , currents from the phase connections u , v , w or the phase windings 12 can be selectively measured depending on which of the lower switches ( pull - down switches ) s 4 to s 6 of the driver circuit 15 are closed . fig2 illustrates an exemplary pulse pattern 30 for driving the switches s 1 to s 6 of the driver circuit 15 . as shown in fig2 , pulse width modulated signals ( pwm ), which are preferably left - aligned , are used in this case . driving of the switches s 1 and s 4 in a switch section of the b6 inverter for supplying the first phase connection u is shown in the upper part of fig2 . in this case , the switches s 1 and s 4 are driven inversely in relation to one another in order to avoid a short circuit , this being indicated in the drawing by the negation bar over reference symbol s 4 . the central and the lower part of fig2 accordingly show driving of the switches s 2 and s 5 of the second switch section of the b6 inverter for the second phase connection v and , respectively , the switches s 3 and s 6 of the third switch section of the b6 inverter for the third phase connection w . as a result , sinusoidal ac voltages can be generated in particular . depending on the instantaneous pulse width , the electrical potential of the phase connections u , v , w is adjusted to an actual value which is in each case between the potentials of the connections 16 , 17 . in particular , the switches s 1 to s 6 are driven by the drive unit 26 such that an ac voltage is applied as a differential voltage between the two phase windings 12 . therefore , alternating currents are generated in the phase windings 12 , said alternating currents in each case adding to an adjustable constant current at the star point 14 . the star point 14 is drawn to a specific dc voltage potential ( star point potential ), which is varied depending on the operating point of the electrical machine 11 , by corresponding driving in order to generate a direct field current . the adjustable constant current then corresponds to the direct field current which flows through the field winding 13 . in order to measure the phase currents and the field current , a plurality of time measurement windows 31 , 32 , 33 are produced in a measurement period 37 of the measurement operation , wherein in each case one measurement current i measure is determined in each measurement window 31 , 32 , 33 , said measurement current flowing through one of the phase windings 12 or a parallel circuit of a plurality of the phase windings 12 overall with respect to a predetermined current direction . in this case , a measurement period 37 corresponds to a pulse width modulation period 38 of the left - aligned pulse width modulation , wherein different drive vectors 34 , 35 followed by a zero vector 36 are positioned one after the other . in order to produce the suitable measurement windows 31 , 32 , 33 , the relevant phase windings 12 are simultaneously coupled to the connection 28 of the specific reference potential in accordance with a predefined pulse pattern 30 . provision can be made , in particular , for in each case the first measurement window 31 to last from the beginning of the measurement period 37 up to a first flank of a first switching signal for a relevant switch ( a relevant pair of switches ) of a first inverter branch of the b6 inverter , for the second measurement window 32 to last from the first flank of the first switching signal of the relevant switch of the b6 inverter to a first flank of a second switching signal for a switch of a second inverter branch of the b6 inverter , and for the third measurement window 33 to last from a last flank , as seen over all of the switching signals in the relevant measurement period , up to the end of the measurement period 37 . in the example in fig2 , the two drive vectors 34 , 35 are positioned at the beginning of the pulse width modulation period 38 . in contrast to the prior art , it is therefore possible for not only two phase currents , but also a phase current and the sum of another phase current and the field current to be measured . therefore , the phase current of the first phase winding 12 at the phase connection u is measured in the first measurement window 31 . a sum of a second phase current at the phase connection v and the field current through the field winding 13 is measured in the second measurement window 32 . in order to measure the field current in the shunt 24 by measuring the voltage across the shunt 24 , all pull - down switches s 4 , s 5 , s 6 of the inverter or of the driver circuit 15 have to be closed ( and accordingly all of the pull - up switches s 1 , s 2 , s 3 have to be opened ), so that the measurement current during a zero vector 36 is measured . this state typically occurs at the end of the pulse width modulation period 38 . if the field current is now known by measurement , the second phase current and therefore , by means of the node law at the star point 14 , the third phase current too can also be calculated . this takes place by means of an evaluation unit 25 which determines the phase currents and the field current from the respective measurement currents i measure which were determined during the measurement period 37 . therefore , all of the actual values of the phase currents are known and can be supplied to a current control system 23 . depending on the rotor position , other variants also occur for the measurement currents measured during the drive vectors 34 , 35 . to this end , the pulse pattern 30 can be adjusted in such a way that driving of the switches s 1 to s 6 is optimized in such a way that measurement can be performed with as little noise as possible . if the maximum amplitude of the three - phase voltage system , which connects the inverter or the driver circuit 15 to the machine terminals or phase connections u , v , w , is set and the electrical machine 11 therefore runs at high rotation speeds and under high load , the zero vector 36 at the end of the pulse width modulation period 38 is too short to carry out a measurement of the measurement current i measure . the field current cannot be measured in this case . provision can therefore be made for the operating state in which excessively short zero vectors 36 occur in the pulse width modulation period 38 to be identified and , in this case , for measurement of the field voltage across the shunt 24 at the end of the pulse width modulation period 38 to be prevented . however , this is not disadvantageous for adjusting the field current in comparison to a variant with a current sensor in the field coil current path since the star point voltage and therefore the voltage across the field winding 13 can no longer be changed in any case . the zero potential of the three - phase voltage system which connects the driver circuit 15 to the phase connections u , v , w can no longer be varied upward or downward in this case . therefore , a current control system 23 for the field current has no way of changing the field current by means of the actuating variable “ duration of the zero vector 36 of the three - phase voltage system ”. furthermore , the field current is however required for calculating the phase currents from the current values which are measured with the drive vectors 34 , 35 . therefore , in this case , instead of the measured value for the field current , a model - based value , in particular a value calculated by software , is used , said value being corrected in an appropriate manner as soon as the field current can be measured again .