Method and apparatus for determining the rotor position of synchronous motors

Described is a method of determining the rotary position of the rotor of a synchronous motor, in relation to the rotating magnetic stator field. The instantaneous rotor position is ascertained by measurement of the emf (E.s) induced in at least one stator winding by the rotor in a current gap in the stator current (I.s) flowing through said stator winding, wherein in accordance with the invention when using a sinusoidal or quasi-sinusoidal stator current an artificial current gap in the stator current is produced. In that case the induced emf (E.s) is directly derived from the tapped-off terminal potential (U.s) of the respective stator winding.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention concerns a method of determining the rotor position
 of synchronous motors, in particular multi-phase synchronous motors, for
 regulating the synchronous motors to optimum efficiency, as set forth
 pursuant to the present disclosure, a method of detecting a zero passage
 of a stator current changing in sign of a synchronous motor, as set forth
 in the disclosure.
 2. Discussion of the Prior Art
 Electronically commutated synchronous machines or motors which are operated
 on a dc voltage network or which are converter-fed are known from the
 literature and various situations of use in a practical context.
 Synchronous motors are also increasingly coming into use in the sector of
 low rotary speed dynamics as for example in connection with pumps,
 condensers or washing machines. Besides the high level of starting torque,
 synchronous motors have the advantage over asynchronous motors inter alia
 that they can be operated with larger air gap tolerances, thereby
 affording structural advantages such as for example the direct drive for
 the washing drum in washing machines or pumps and condensers with a wet
 rotor.
 It is known that, in the case of synchronous motors, an optimum torque and
 thus an optimum level of efficiency are achieved if the vector of the
 magnetic flux .phi..sub.R produced by the rotor is perpendicular to the
 vector of the magnetic flux .phi..sub.S generated by the respective stator
 winding, that is to say if the magnetic field of the rotor is oriented in
 perpendicular relationship to the magnetic field of the respective stator
 winding. That arises out of the fact that the torque vector T is
 proportional to .phi..sub.R.times..phi..sub.S or the magnitude of the
 torque vector T is proportional to sin.alpha., wherein .alpha. is the
 spatial setting angle between the two magnetic fluxes .phi..sub.R and
 .phi..sub.S. In this case the rotor of the synchronous motor is
 so-to-speak pulled along by the rotating stator rotary field.
 As the magnetic flux .phi..sub.R generated by the rotor is determined
 directly by the position of the rotor, synchronous motors can be regulated
 for example by detection of the position of the rotor in relation to the
 rotating stator field. In that respect, it is known from the state of the
 art to provide at the rotor shaft of the synchronous motor sensors which
 establish the position of the rotor at any moment in time. A regulating
 apparatus of that kind is known for example from DE-A1 195 27 982 in which
 detection of the position, speed of rotation and/or direction of rotation
 of the rotor is effected by the use of stationarily mounted,
 magnetosensitive sensors, the measurement signals of which are fed to the
 electronic control system.
 It is also known to manage without sensors of that kind when regulating
 synchronous motors. If the stator winding is acted upon by a so-called
 gappy current, that is to say in particular a current of staircase-shaped
 or rectangular configuration with phases in which the current is
 constantly zero, it is possible, in those so-called current gaps, to
 detect the voltage which is induced by the rotation of the rotor in the
 stator winding and which is also referred to briefly as the emf as
 potential applied to the corresponding motor terminal, and to obtain
 therefrom information about the position of the rotor. Regulation of the
 synchronous motor is then effected in such a way that emf in the middle of
 the current gap should have a zero passage. In that case the control value
 for regulation is either the frequency with which the stator field is
 switched or the amplitude of the stator current. Such a method of
 regulating synchronous motors is described for example in detail in
 "Sensorless Speed Controlled Brushless DC Drive using the TMS320C242 DSP
 Controller" by P. Voultoury, Intelligent Motion, May 1998 Proceedings,
 pages 169-180.
 At certain rotary speeds as are required for example in the case of
 synchronous motors for washing machines or dryers, the use of a gappy
 stator current however involves undesirable clicking or chattering which
 is generally not acceptable to a customer. That noise is evidently caused
 by the fact that the stator windings are acted upon in a pulse-like manner
 by the pulses of the gappy current, in which case the frequencies which
 occur here are in the audible range.
 It is therefor already known for troublesome noises of that kind to be
 avoided in the case of synchronous motors by a procedure whereby, in those
 rotary speed ranges, instead of the gappy current, a sinusoidal or
 quasi-sinusoidal stator current is used. A quasi-sinusoidal stator current
 of that kind is produced by the power switches of the three-phase bridge
 of synchronous motor being operated with pulses which are controlled in
 pulse width modulated (PWM) manner in such a way that a quasi-sinusoidal
 stator current is produced. Production of the quasi-sinusoidal stator
 current by PWM-actuation is described in greater detail for example in
 "Digitale Steuerung eines Dreiphasen-Induktionsmotors" ("Digital Control
 of a Three-Phase Induction Motor") by B. Maurice et al in Design &
 Electronik 8 of 07.04.1992, pages 40-46. In this case the control circuit
 has recourse to stored tables with values for the pulse duty factors of
 the bridge arms of the synchronous motor.
 Due to the use of a quasi-sinusoidal stator current however it is no longer
 possible to measure the emf induced in the stator windings and to use the
 measurement result for regulation of the synchronous motor, as was the
 case when using the gappy current.
 SUMMARY OF THE INVENTION
 Therefore the object of the present invention, in a synchronous motor, when
 using a quasi-sinusoidal or sinusoidal stator current, is to provide a
 possible way of detecting the position of the rotor without the use of
 sensors, and in particular measuring the emf induced in the stator
 windings in order to regulate the synchronous motor to optimum efficiency
 by means of those measurement values.
 By virtue of the actually sinusoidal or quasi-sinusoidal stator current
 being set to zero for a certain period of time, that is to say, an
 artificial current gap is produced, it is possible--similarly as in the
 case of the gappy stator current--to measure in that current gap the
 voltage induced in the stator winding by virtue of the rotation of the
 rotor--the emf--and in particular to measure the phase position between
 the induced emf and the stator current. Regulation of the synchronous
 motor is then effected in such a way that the emf induced in the stator
 winding and the stator current are in phase as, in that case, the greatest
 possible torque is achieved. The duration of the current gap is in that
 case kept very short in relation to the period duration of the
 quasi-sinusoidal stator current in order not to have an adverse influence
 on the drive of the synchronous motor.
 Preferably the current gap in the quasi-sinusoidal stator current is
 provided in the proximity of and in particular after recognition of a zero
 passage of the stator current as in that case the current can be switched
 off to the value zero more quickly than in other regions of the
 quasi-sinusoidal current configuration with higher absolute values in
 respect of the stator current.
 In this case detection of a zero passage of the stator current is
 advantageous effected by detecting the terminal potentials during
 so-called dead times in actuation of the corresponding stator winding. By
 comparison of the successively detected terminal potentials, upon a change
 in the terminal potential it is possible to detect a zero passage of the
 stator current between the respective dead times.
 Detection of the respective terminal potential is preferably effected by a
 procedure whereby, during the artificially produced current gap in the
 stator current in one stator winding, the other stator windings are
 short-circuited and the emf induced in the one stator winding, in the
 current gap, is detected by measurement of the terminal potential of the
 corresponding stator winding in relation to the common terminal potential
 of the other stator windings. In a preferred embodiment in this respect
 the short-circuiting stator windings are connected to the negative pole or
 the positive pole of the voltage intermediate circuit.
 A further object of the present invention is to provide a method of
 detecting a zero passage of stator current without the use of current
 sensors so that it is easily possible to provide the artificial current
 gap for detecting the position of the rotor without the use of sensors in
 accordance with the above-described method in the proximity of a current
 zero passage.
 In the method of detecting a zero passage of a stator current which changes
 in sign of a synchronous motor the terminal potential is detected during
 successive dead times of actuation of the corresponding stator winding;
 the comparison of the terminal potentials detected during two dead times
 is used to detect a zero passage of the stator current between the two
 dead times if the terminal potential has changed.
 Still another object of the present invention is to provide an apparatus
 for carrying out the above-indicated methods.
 In accordance with the invention, when using a sinusoidal or
 quasi-sinusoidal stator current at least one motor terminal and preferably
 all motor terminals, for detection of the terminal potential of the
 associated stator winding, are connected to the motor control so that the
 terminal potential can be used as a regulator input parameter for
 regulation of the synchronous motor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 As a basis for the following considerations, the equivalent-circuit diagram
 of a single-phase synchronous motor will first be described with reference
 to FIG. 1. The stator winding of a synchronous motor respectively has a
 line resistance R.s and a line inductance L.s. The voltage U.s in the form
 of an ac voltage is applied to the stator winding by a converter. In
 addition, a voltage E.s which is also briefly identified as emf is induced
 in the stator winding by the rotor which for example has a permanent
 magnet.
 FIG. 2 shows the electrical parameters illustrated in FIG. 1 and the
 relationships thereof in the form of a vector diagram. The vector diagram
 in FIG. 2 is rotor flux-oriented, that is to say the d-axis always points
 in the direction of the magnetic flux .phi..sub.R of the rotor. The q-axis
 is in leading relationship through 90.degree. relative to the d-axis. By
 virtue of the induction law (induced voltage E.s is proportional to the
 differential quotient of the magnetic flux in relation to time
 d.phi..sub.R /dt) the emf induced in the stator winding is always on the
 q-axis, that is to say it leads the magnetic flux .phi..sub.R of the rotor
 by 90.degree..
 The voltage U.s applied to the stator winding is shown by way of example
 for a given point of operation. The alternating current I.s lags by the
 angle .phi. behind the voltage (U.s-E.s) effectively applied to the stator
 winding. The angle .phi. results from the motor characteristic parameters
 and the considered rotary speed .omega. of the rotor in accordance with
 .phi.=arctan (.omega..multidot.L.s/R.s).
 The invention is now based on the realisation that the point of operation
 of the synchronous motor with the optimum level of efficiency, that is to
 say with the greatest torque, is that at which the current I.s flowing in
 the stator winding and the emf E.s induced in the stator winding are in
 phase.
 That arises on the one hand out of the fact that, as already set forth in
 the preamble to the description, the greatest torque exists when the
 magnetic flux of the rotor .phi..sub.R and the magnetic flux of the stator
 winding .phi..sub.S are perpendicular to each other. In addition, with a
 sinusoidal configuration in respect of the rotor flux .phi..sub.R the
 induced emf E.s leads the magnetic flux of the rotor .phi..sub.R by
 90.degree., as described above with reference to FIG. 2. In addition. in
 the case of a sinusoidal stator current I.s on the one hand the applied
 voltage U.s leads the current I.s flowing in the stator winding by
 90.degree. (U.s proportional to dI.s/dt) and on the other hand the applied
 voltage U.s leads the magnetic flux of the stator .phi..sub.S by
 90.degree. (U.s proportional to d.phi..sub.S /dt). The above-indicated
 relationship between optimum efficiency and phase position as between I.s
 and E.s follows from those three prerequisites set forth above.
 An embodiment of a circuit arrangement will now be described with reference
 to FIG. 3 while reference will be made to FIGS. 4 to 6 to describe the
 method of determining the instantaneous rotational position of the rotor
 of a synchronous motor in relation to the rotating magnetic stator field
 in accordance with the present invention.
 FIG. 3 shows the equivalent-circuit diagram of an electronically switched
 synchronous motor. The embodiment of FIG. 3 involves a three-line,
 six-pulse synchronous motor with permanent excitation. It should be
 expressly pointed out at this stage however that the present invention can
 basically be applied to all kinds of synchronous motor and the circuit
 arrangement described hereinafter is only a preferred embodiment given by
 way of example.
 The synchronous motor has in its stator S three star-connected stator
 windings which are connected on the one hand at a star point and on the
 other hand respectively to a connecting terminal u, v, w of the motor. The
 synchronous motor further has a rotor which carries a permanent magnetic.
 The connecting terminals u, v, w are each further connected to the central
 tapping between an upper switching means 3a and a lower switching means 3b
 of the power bridge 2, which for example are in the form of power
 transistors.
 The control connections of the power transistors 3a, 3b are electrically
 connected to the outputs uh, ul, vh, vl, wh, wl of a motor control 1, for
 example a microcontroller, wherein the control connections of the
 switching means 3a, 3b are preferably actuated by way of MOS-gate drivers
 (not shown).
 Connected in parallel with the connections (Gnd and +HV) of a constant
 current source or an intermediate circuit inverter is an intermediate
 circuit capacitor 4 which serves as a smoothing capacitor for the
 intermediate circuit voltage +HV. In addition, connected in parallel with
 the switching means 3a, 3b is a respective free-running diode 5 with
 opposite forward direction.
 In addition the terminal potentials U.u, U.v and U.w of the motor terminals
 u, v, w are respectively taken off at the centre tapping of the bridge arm
 3a, 3b and passed by way of a suitable resistor to the motor control 1. As
 described hereinafter, the terminal potentials U.u, U.v, U.w detected in
 that way serve as input parameters for the detection of a current zero
 passage of the stator current and the phase position as between the stator
 current I.s and the induced emf E.s.
 FIG. 4 now shows by way of example the sinusoidal or quasi-sinusoidal
 configuration of the stator current I.s as is obtained by pulse width
 modulated (PWM) control of the power transistors 3a, 3b in phase-shifted
 relationship in each stator winding of the synchronous motor.
 As already mentioned in the opening part of this specification the
 production of a quasi-sinusoidal stator current I.s by a PWM-actuation
 arrangement is described in greater detail for example in "Digitale
 Steuerung eines Dreiphasen-Induktionsmotors" by B. Maurice et al in Design
 & Electronik 8 of 07.04.1992, pages 40-46. In this case the control
 circuit has recourse to stored tables with values for the pulse duty
 factors of the bridge arms of the synchronous motor.
 The use of a sinusoidal or quasi-sinusoidal stator current is desirable in
 particular in relation to rotary speeds as are required for example for
 washing machines or dryers in order to avoid the undesirable noises which
 occur when using a gappy stator current. When using a quasi-sinusoidal
 stator current however it is no longer possible, as in the case of the
 gappy stator current, to measure the emf induced in the stator winding in
 the current gap of the stator current and to use the measurement result to
 regulate the synchronous motor.
 Therefore, as diagrammatically shown in FIG. 4, in the quasi-sinusoidal
 current configuration (broken line) the stator current I.s is set to zero
 (solid line) for a given period of time, that is to say an artificial
 current gap .mu. is produced, in which then the emf E.s induced in the
 respective stator winding can be measured. The duration of the sampling
 time .mu. is for example constant at 50 .mu.s while the period duration
 .tau. of the quasi-sinusoidal stator current I.s, for example in the case
 of washing machines, is about 60 ms for the wash phase and about 3 ms for
 the spin phase. As the sampling time .mu. is very short in relation to the
 period duration .tau. of the current I.s, the current configuration and
 thus control of the synchronous motor is only immaterially influenced. The
 actual conditions are shown in greatly exaggerated form in FIG. 4 for
 enhanced clarity of the drawing.
 So that the stator current I.s switches off or falls to the value zero as
 quickly as possible in order then to be able to measure the emf E.s
 induced in the stator winding, it is advantageous for the current gap .mu.
 to be provided as closely as possible to a current zero passage .sigma. a
 of the stator current I.s. Advantageously, that occurs as directly as
 possible after a current zero passage .sigma. as in that case, by
 monitoring of the current configuration, it is possible to detect such a
 current zero passage .sigma. and directly afterwards produce an artificial
 current gap .mu..
 Now, in the artificial current gap .mu. produced in that way, the emf E.s
 induced in the stator winding by virtue of rotation of the rotor is
 measured and, by means of the sign and optionally the magnitude of the emf
 E.s and the current zero passage, it is possible to determine the phase
 position between the emf E.s and the stator current I.s. Ascertainment of
 the phase position is advantageously effected in all stator windings of
 the multi-phase synchronous motor, whereby it is possible to achieve a
 higher degree of accuracy in terms of control.
 Detection of a current zero passage .sigma., production of an artificial
 current gap .mu. and measurement of the induced emf E.s during the current
 gap .mu. will now be described with reference to FIGS. 5A to D by means of
 different operating conditions of the synchronous motor in detail. The
 description hereinafter relates in this respect to the three-line
 six-pulse synchronous motor shown in FIG. 3. It should be expressly
 pointed out once again at this stage however that the method can basically
 be applied to all kinds of synchronous motors.
 FIGS. 5A to D respectively show in relation to a common time axis: a) the
 emf E.u induced in the stator winding u; b) the stator current I.u flowing
 in the stator winding u; c) the actuation signals uh and ul of the two
 switching means of the stator winding u; d) the terminal potential, U.u at
 the motor terminal u which is fed to the motor control as a control
 signal; e) the actuation signals vh and vl of the two switching means of
 the stator winding v; f) the terminal potential U.v at the motor terminal
 v which is fed to the motor control as a control signal; g) the actuation
 signals wh and wl of the two switching means of the stator winding w; and
 h) the terminal potential U.w at the motor terminal w which is fed to the
 motor control as a control signal. Shown in each case is only a narrow
 time window in the proximity of a zero passage of the stator current I.u
 of about some 100 .mu.s. During that short period of time the
 quasi-sinusoidal configuration of the stator current I.u as such cannot be
 recognised and the emf E.u induced in the stator winding u can be assumed
 to be constant. In a similar manner the zero passages of the stator
 currents I.v and Iw in the other two stator windings v and w are
 preferably also monitored and the respective phase positions as between
 the induced emfs E.v and E.w and the stator current I.v and I.w
 respectively ascertained.
 FIG. 5A firstly shows the case of a negative current zero passage (change
 in sign of the stator current I.u from + to -), wherein the induced emf
 E.u is still positive. This means that the emf E.u and thus the rotor R
 lag the stator current I.u in the stator winding u.
 To detect a zero passage of the stator current it is possible to provide in
 the current path of each stator winding a current measuring device which
 transmits the detected current measurement values to the motor control 1.
 In order however and in particular for reasons of cost to be able to
 forego additional electronic components of that nature detection of a zero
 passage in the stator current I.u is detected as follows:
 Upon actuation of the power transistors 3a, 3b of a bridge arm, so-called
 dead times are interposed, during which neither the upper nor the lower
 power transistor is switched in a conducting condition. That is intended
 reliably to prevent overlapping switching of two switching means in a
 bridge arm, which would cause a short-circuit. During the dead times the
 voltage potential U.s at the corresponding phase or motor terminal u is
 determined by the current direction of the stator current I.s in the same
 phase.
 When the stator current I.s flows into the stator winding the current
 switches during the dead time to the free-running diode 5 of the lower
 switching means 3b of the corresponding bridge arm and the potential U.s
 at the motor terminal of the corresponding phase corresponds to the
 negative pole of the voltage intermediate circuit, in this case Gnd. If in
 contrast the stator current I.s flows out of the stator winding, then the
 current switches during the dead time by way of the free-running diode 5
 of the upper switching means 3a of the corresponding bridge arm to the
 positive pole of the voltage intermediate circuit (+HV). Upon a current
 zero passage the stator current changes its polarity and thus the voltage
 potential U.s also changes during the dead time at the corresponding motor
 terminal.
 This is shown in FIG. 5A in the time intervals T-1 and T. During the first
 dead time (time interval T-1), in which both switching means 3a, 3b of the
 bridge arm associated with the motor terminal u are switched in a
 non-conducting condition (uh and ul OFF), the stator current I.u is
 positive and therefore flows into the stator winding u: It thus switches
 by way of the free-running diode 5 of the lower switching means 3b of the
 corresponding bridge arm to the negative pole (Gnd) of the voltage
 intermediate circuit. In the time interval T-1 a terminal potential
 U.u=Gnd is accordingly detected. After that dead time, in the time window
 shown in FIG. 5A, there is a negative zero passage of the stator current
 I.u, that is to say the stator current I.s changes its polarity from + to
 -. During the next dead time (time interval T) of that bridge arm the
 stator current I.u is thus negative and therefore flows out of the stator
 winding u. It now switches by way of the free-running diode 5 of the upper
 switching means 3a of the corresponding bridge arm to the positive pole
 (+HV) of the voltage intermediate circuit. Accordingly a terminal
 potential U.u=+HV is detected in the time interval T. In the two time
 intervals T-1 and T of the two directly successive dead times the terminal
 potential U.u therefore has opposing voltage potential, whereby the motor
 control 1 recognises that a zero passage of the stator current I.u has
 occurred between the two time intervals T-1 and T. As the terminal
 potential U.u has changed from Gnd to +HV in the time intervals T-1 and T,
 this case involves a negative current zero passage of the stator current
 I.u.
 It is not absolutely necessary for the terminal potential U.u to be
 detected and evaluated during each dead time of the corresponding bridge
 arm. The more frequently however that the terminal potential U.u is
 detected and evaluated in the dead times, the more quickly is it possible
 to recognise a zero passage of the stator current I.s and the closer is it
 possible for an artificial current gap to be produced to such a current
 zero passage, whereby the length of the current gap or the change in
 current configuration can be shorter, as will be further described
 hereinafter.
 In accordance with the invention the current zero passage of a stator
 current can therefore be detected both by means of suitable current
 measuring devices and also by detection of the change in the terminal
 potential between two dead times.
 Immediately after detection of a current zero passage of the stator current
 I.u in the time interval T, in the time interval T+1 both switching means
 3a, 3b of the bridge arm of the motor terminal u become non-conducting (uh
 and ul OFF) and the other two motor terminals v and w, independently of
 their preceding switching states, are switched to a common voltage
 potential, that is to say short-circuited. Desirably, for that purpose the
 two motor terminals v and w are connected by way of the switching means
 3a, 3b of the associated bridge arms to the negative pole (Gnd) or the
 positive pole (+HV) of the voltage intermediate circuit.
 In order to provide for rapid decay of the stator current I.u to the value
 zero, in the case of a negative current zero passage of the stator current
 I.u as shown in FIGS. 5A and B the two short-circuiting motor terminals v
 and w are switched in the time interval T+1 to the negative pole (Gnd) of
 the voltage intermediate circuit by closure of the respective lower
 switching means 3b (vl and wl ON). At a positive current zero passage of
 the stator current I.u in contrast, as shown in FIGS. 5C and D, the two
 short-circuiting motor terminals v and w, in the time interval T+1, are
 switched to the positive pole (+HV) of the voltage intermediate circuit by
 closure of the respective upper switching means 3a (vh and wh ON). As a
 result the stator current I.u must respectively start against the highest
 possible potential so that it switches down to the value zero in a
 correspondingly short time.
 After the stator current I.u has decayed to the value zero (time interval
 T+2) the two switching means 3a, 3b of the motor terminal u remain
 switched in the non-conducting condition during measurement of the emf E.u
 induced in the stator winding u. Likewise the switching means 3a, 3b of
 the other two motor terminals v and w remain in their unchanged switching
 state during the time interval T+2 relative to the time interval T+1. The
 voltage potential U.u at the bridge point of the motor terminal u in
 relation to common potential (Gnd) of the other two motor terminals v and
 w is in this case precisely 3/2.multidot.E.u, as the considerations
 hereinafter show.
 FIG. 6 illustrates the conditions in the time interval T+2 for the
 situation shown in FIG. 5A, that is to say the two switching means 3a, 3b
 of the motor terminal u are switched in a non-conducting condition and no
 current flows in the stator winding u (I.u=0). In addition the two motor
 terminals v and w are connected together and switched to the negative
 potential (Gnd) of the voltage intermediate circuit.
 As this arrangement involves a symmetrical, sinusoidal emf-system, the
 following applies:
EQU E.u+E.v+E.w=0 (1)
 In addition the following relationships apply in the meshes M.sub.1,
 M.sub.2 and M.sub.3 :
EQU M.sub.1 : .phi..sub.Y =-L.v.multidot.dI/dt-R.v.multidot.I-E.v (2)
EQU M.sub.2 : .phi..sub.Y =L.w.multidot.dI/dt+R.w.I-E.w (3)
EQU M.sub.3 : .phi..sub.u =E.u+.phi..sub.Y (4)
 wherein .phi..sub.Y is the potential at the star point in relation to Gnd
 and .phi..sub.u is the potential of the motor terminal u in relation to
 the common terminal potential (Gnd) of the motor terminals v and w, that
 is to say corresponds to the terminal potential U.u. On the assumption
 that the stator windings are of the same structure, that is to say L.v=L.w
 and R.v=R.w, it can be deduced that:
 (2)+(3): 2.multidot..phi..sub.Y =-E.v-E.w (5)
EQU (1) in (5): 2.multidot..phi..sub.Y =E.u{character pullout}.phi..sub.Y
 =1/2.multidot.E.u (6)
EQU (6) in (4): .phi..sub.u =E.u+1/2.multidot.E.u{character pullout}.phi..sub.u
 =3/2E.u (7)
 This means that in the time interval T+2, that is to say during the
 artificially generated current gap in the stator current I.u, by detection
 of the terminal potential U.u=.phi..sub.u at the motor terminal u it is
 possible directly to ascertain the emf E.u induced in the stator winding
 u.
 If, as in the case shown in FIG. 5A, what is involved is a negative zero
 passage of the stator current I.u and a positive value of .phi..sub.u and
 E.u respectively, it directly follows therefrom that the emf E.u induced
 in the stator winding u lags behind the stator current I.u. If, besides
 the sign of the induced emf E.u, the absolute value thereof is also
 detected, it is additionally possible to deduce therefrom the degree of
 deviation of the phases as between E.u and I.u.
 Regulation of the synchronous motor is now effected in such a way that the
 emf E.u induced in the stator winding u is taken if possible to the value
 zero, during the current gap in the stator current I.u. For example either
 the frequency with which the stator field is switched or the amplitude of
 the stator current I.u can be used as the control value for that
 regulation effect.
 In contrast to FIG. 5A, FIG. 5B shows a case in which, at a negative
 current zero passage of the stator current I.u, the induced emf E.u is
 already negative, that is to say the emf E.u and thus the rotor lead the
 stator current I.u.
 As in FIG. 5A, in this case also, during the dead time in the time interval
 T a negative current zero passage of the stator current I.u is detected
 between the two time intervals T-1 and T. The stator current I.u is then
 also switched down to the value zero insofar as in the time interval T+1
 the two switching means 3a, 3b of the bridge arm of the motor terminal u
 are made non-conducting (uh and ul OFF) and the other two motor terminals
 v and w, independently of their preceding switching states, are jointly
 switched to the negative pole (Gnd) of the voltage intermediate circuit
 (vl and wl ON).
 After decay of the stator current I.u, in the time interval T+2 the
 terminal potential U.u is measured at the motor terminal u in relation to
 the common terminal potential U.v=U.w=Gnd. In the configuration shown in
 FIG. 5B, there is a potential .phi..sub.u =-3/2.multidot.E.u. That
 potential .phi..sub.u however cannot be measured as it is held by the
 free-running diode 5 of the lower switching means 3b of the corresponding
 bridge arm of the stator winding u at Gnd. For that reason, it is
 necessary in this case for the bridge arms of the other two motor
 terminals v and w to be changed over to the supply potential +HV of the
 voltage intermediate circuit (vh and wh ON). Now, in the time interval
 T+3, at the bridge arm of the motor terminal u, it is possible to measure
 the potential .phi..sub.u =+HV-3/2.multidot.E.u from which the induced emf
 E.u can be directly ascertained.
 FIG. 5C, in contrast to FIG. 5A, shows a situation in which, at a positive
 current zero passage of the stator current I.u, the induced emf E.u is
 still negative, that is to say the emf E.u and thus the rotor trail the
 stator current I.u.
 As in FIG. 5A, this case also involves ascertaining during the dead time in
 the time interval T a current zero passage (in this case positive) of the
 stator current I.u between the two time intervals T-1 and T. The stator
 current I.u is then switched down to the value zero insofar as in the time
 interval T+1 the two switching means 3a, 3b of the bridge arm of the motor
 terminal u are made non-conducting (uh and ul OFF) and the other two motor
 terminals v and w, independently of their preceding switching states, are
 jointly switched to the positive pole (+HV) of the voltage intermediate
 circuit (vh and wh ON).
 After decay of the stator current I.u in the time interval T+2 the terminal
 potential U.u at the motor terminal is measured in relation to the common
 terminal potential U.v and U.w respectively. The situation in FIG. 5C
 involves a potential .phi..sub.u =+HV-3/2.multidot.E.u which is more
 negative than the terminal potential U.v=U.w=+HV and from which the
 induced emf E.u can be directly ascertained.
 Lastly FIG. 5D shows a case in which at a positive current zero passage of
 the stator current I.u the induced emf E.u is already positive, that is to
 say the emf E.u and thus the rotor lead the stator current I.u.
 As in FIG. 5C, this case also involves detecting during the dead time in
 the time interval T a positive current zero passage of the stator current
 I.u between the two time intervals T-1 and T. The stator current I.u is
 then also switched down to the value zero insofar as in the time interval
 T+1 the two switching means 3a, 3b of the bridge arm of the motor terminal
 u are made non-conducting (uh and ul OFF) and the other two motor
 terminals v and w, independently of their preceding switching states, are
 jointly switched to the positive pole (+HV) of the voltage intermediate
 circuit (vh and wh ON).
 After decay of the stator current I.u, in the time interval T+2, the
 terminal potential U.u is measured at the motor terminal u in relation to
 the common terminal potential U.v=U.w=+HV. In the situation shown in FIG.
 5D there is a potential .phi..sub.u =+HV+3/2.multidot.E.u. This potential
 .phi..sub.u however cannot be measured as it is held by the free-running
 diode 5 of the upper switching means 3a of the corresponding bridge arm of
 the motor terminal u at the supply potential +HV. For that reason it is
 necessary in this case for the bridge arms of the upper two motor
 terminals v and w to be changed over to the base potential Gnd of the
 voltage intermediate circuit (vl and wl ON). Now, in the time interval
 T+3, at the bridge arm of the motor terminal u, it is possible to measure
 the potential .phi..sub.u =+3/2.multidot.E.u from which the induced emf
 E.u can be directly ascertained.
 Upon detection of the terminal potential U.u during the current gap in the
 stator current I.u (time interval T+2 and T+3 respectively), as described
 with reference to FIGS. 5A to D, it is not only the sign/polarity of the
 induced emf E.u that is evaluated, but also the magnitude/amplitude of E.u
 for example by way of an A/D-converter provided in the motor control 1. In
 that way the current phase shift between stator current I.u and induced
 emf E.u can be deduced and powerful and efficient regulation of the
 synchronous motor can be achieved.
 In contrast thereto, it is also possible, as a configuration of the
 regulator which is simpler from the circuit engineering point of view, to
 operate without amplitude detection in respect of the induced emf E.u and
 to evaluate only the signs thereof. In that case, to measure the induced
 emf E.u, the short-circuiting stator windings v, w are in each case
 clamped to the negative pole (Gnd) of the voltage intermediate circuit (vl
 and wl ON).
 If the induced emf E.u is positive, then the terminal potential U.u
 measured at the open motor terminal u is also positive
 (U.u=+3/2.multidot.E.u), as is the case in the time interval T+2 in FIG.
 5A and in the time interval T+3 in FIG. 5D. If in contrast the induced emf
 E.u is negative, then the negative terminal potential U.u
 (U.u=+3/2.multidot.E.u) is clamped by way of the associated free-running
 diode 5 of the lower switching means 3b of the corresponding bridge arm to
 the negative pole (Gnd) of the voltage intermediate circuit, as is the
 case in the time interval T+2 in FIG. 5B and in the time interval T+3 in
 FIG. 5C. For this reason the magnitude of the negative terminal potential
 U.u in relation to the negative pole (Gnd) of the voltage intermediate
 circuit corresponds at a maximum to the diode forward voltage of about 1
 V.
 With this simplified regulation of the synchronous motor the motor control
 1 interrogates the detected voltage potentials U.u, U.v, U.w only for
 greater or smaller than the ground potential (Gnd). If the induced emf E.u
 is substantially greater than the supply voltage (Vcc) of the motor
 control 1, which is the case with most uses on a low-voltage network, the
 motor control 1 can also more easily interrogate the detected voltage
 potentials U.u, U.v, U.w for greater or smaller than Vcc/2.