USOO7355436 B2

(12) United States Patent (10) Patent No.: US 7,355.436 B2

Zehentner et al. (45) Date of Patent: Apr. 8, 2008

(54) METHOD FOR ERROR DETECTION IN A 5,363,039 A * 11/1994 Kumar et al. ............ 324, 158.1
DRIVE MECHANISM 5,796,231 A * 8/1998 Kyodo ....................... 318,608
6, 160,414 A 12/2000 Matsubara et al.
(75) Inventors: Georg Zehentner, Teisendorf (DE); gigs R. 92.8 E",- - - - - - - - "...
Norbert Huber, Teisendorf (DE); -W ealey et al. .... ---
Eugen Kellner, Berchtesgaden (DE) 2003/0001537 A1* 1/2003 Yang et al. ................. 318,727
FOREIGN PATENT DOCUMENTS
(73) Assignee: Dr. Johannes Heidenhain GmbH,
Traunreut (DE) EP
WO
O sis 492 A1
WO 97,14205 A1
gic
4f1997
(*) Notice: Subject to any disclaimer, the term of this
patent is extended or adjusted under 35 OTHER PUBLICATIONS
U.S.C. 154(b) by 638 days. Rene Spee et al., “Remedial Strategies for Brushless DC Failures.”
IEEE Transactions. On Industry Applications, vol. 26, No. 2, Mar?
(21) Appl. No.: 10/633,275 Apr. 1990, pp. 259-266.
(22) Filed: Aug. 1, 2003 * cited by examiner
O O Primary Examiner Ha Tran Nguyen
(65) Prior Publication Data Assistant Examiner—Roberto Velez
US 2004/OO85087 A1 May 6, 2004 (74) Attorney, Agent, or Firm—Brinks Hofer Gilson &
Lione
(30) Foreign Application Priority Data
(57) ABSTRACT
Aug. 2, 2002 (DE) ................................ 102 36 377
(51) Int. Cl. A method for error detection in a drive mechanism, having
GOIR 3L/34 (2006.01) a multiphase electric motor and a converter connected
GOIR 3L/02 (2006.01)
upstream
-
thereof, wherein the converter controls
-
voltages of
individual phases of the electric motor, and individual phase
(52) U.S. Cl. .........r irrir. 324/772; 318/.490 currents in the individual phases of the electric motor each
(58) Field of Classification Search ................ 324/772, extend periodically. The method includes measuring a phase
324/ 158.1; 318/.490 current of the electric motor at a predetermined point of a
See application file for complete search history. respective period, simultaneously varying a Voltage that is
(56) References Cited associated with the measured phase current and evaluating a
measured value of the measured phase current as a function
U.S. PATENT DOCUMENTS of the Voltage that is associated with the measured phase
4,757,242 A * 7/1988 Prather ....................... 1847 current.
5,008,608 A * 4, 1991 Unsworth et al. .......... 3.18/729
5,266,891 A 11/1993 Kumar et al. 35 Claims, 38 Drawing Sheets

21 21
PM1

31 21b 11

22 22d
P2

32 22b 2

23 230
PM3 l
23b
33
3
U.S. Patent Apr. 8, 2008 Sheet 1 of 38 US 7,355.436 B2
U.S. Patent Apr. 8, 2008 Sheet 2 of 38 US 7,355.436 B2

100 Total Sequence

Motor Test
Fig. 2 Test
PWM interface

103
Motor Test Error Message
With "PWM interface
Current faulty"
measurement

104
YeS Abort
because of GBT
error?
Error
106
Test for phase during motor
short circuit test ?

Short circuit
detected?

Test for line

to-ground
fault

Error Message: Error Report: Error Message:

Phase short circuit Line-to-ground fault or Phase open, or IGBT
GB Short Circuit does not switchfor
U.S. Patent Apr. 8, 2008 Sheet 3 of 38 US 7,355.436 B2

69

89

/9

99 61
£
MOT9J6OeJ? OEA
Vs
I 99

ZHS
U.S. Patent Apr. 8, 2008 Sheet 4 of 38 US 7,355.436 B2

PWM-Signals Fig. 4
PWM1: O 1 O O 1 0 1 1
PWM2: O O 1 0 1 1 0 1
PWM3: O O 0 1 1 1 1 0

Test. Out:
Low Voltages OK
1, O, O, O, O, 1, 1, 1 || all OK

1, 1, 0, 0, 1, 1, 0, O Il PWM.U1 Stuck on O
1, 0, 1, 0, 1, 0, 1 0 1 PWM.U2 Stuck on 0
1, 0, 0, 1, 1, 0, 0, 1 II PWM.U3 Stuck on 0
1, 1, 1, 0, 0, 0, 0, 1 Il PWM.U1 = PWM.U2
1, 0, 1, 1, 0, 1, 0, 0 (1 PWM.U2 = PWMU3
1, 1, 0, 1, 0, 0, 1, O (1 PWM.U1 = PWM.U3
1, O, O, O, O, O, O, O II PWM.U1 = PWM.U2 = PWM.U3

Low Voltages Faulty

0, 1, 1, 1, 1, 0, 0, 0 || PWM Signals OK
0, 0, 1, 1, 0, 0, 1, 1 II PWM.U1 Stuck on 0 + low voltage
0, 1, 0, 1, 0, 1, 0, 1 1/ PWM.U2 Stuck on 0 + low voltage
0, 1, 1, 0, 0, 1, 1, 0 // PWM.U3 Stuck on 0 + low voltage
0, 0, 0, 1, 1, 1, 1, O II PWM.U1 = PWM.U2+ low voltage
0, 1, 0, 0, 1, 0, 1, 1 1/ PWM.U2 = PWM.U3 + low voltage
O, O, 1, 0, 1, 1, 0, 1 II PWM.U1 = PWM.U3 + low voltage
0, 1, 1, 1, 1, 1, 1, 1 II PWM.U1 = PWM.U2 = PWM.U3 + low voltage

General errors
0, 0, 0, 0, 0, 0, 0 O // SH1 or SH2 not "O"
1, 1, 1, 1, 1, 1, 1, 1 11 Test - output open
U.S. Patent Apr. 8, 2008 Sheet 5 Of 38 US 7,355.436 B2

200 9
Current
Measurement
202
V- Apply voltage.
Determination of the Amplitude sinusoidal
measuring voltage Angle = 120
2O1 203

Yes <> No
204
Apply voltage.
Amplitude sinusoidal
Angle = 240

Apply voltage:
Amplitude sinusoidal
Angle = 360'

Determination of the 208

measuring current
mess

209
Rating the result

210
Evaluation by
the table
U.S. Patent Apr. 8, 2008 Sheet 6 of 38 US 7,355.436 B2

Fig. 6
U.S. Patent Apr. 8, 2008 Sheet 7 of 38 US 7,355.436 B2

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U.S. Patent Apr. 8, 2008 Sheet 38 of 38 US 7,355.436 B2

Test for line-to- 400

ground fault

PWM.U123="OOO"

402
Store IGBT
error as Erro

PWM.U123="111"
4O3 Fig. 18

Store GBT
error as Err1

Error message
indicate line-to
ground fault

IGBT of +UZ has GBT of -UZ has Unknown Error

a short circuit a Short circuit
 US 7,355,436 B2
1. 2
METHOD FOR ERROR DETECTION IN A electric motor, without having to use separate measuring
DRIVE MECHANISM devices for the converter on the one hand and the electric
motor on the other. Moreover, this allows a remote diagnosis
Applicants claim, under 35 U.S.C. S 119, the benefit of of errors be a modem or via the internet.
priority of the filing date of Aug. 2, 2002 of a German patent 5 The possibility for distinguishing between errors in the
application, copy attached, Serial Number 102 36 377.3, converter on the one hand and the electric motor on the other
filed on the aforementioned date, the entire contents of results from the fact that not merely the phase currents in the
which is incorporated herein by reference. individual phases of the electric motor at defined angles of
current flow are measured for error detection, but that
BACKGROUND OF THE INVENTION 10 simultaneously a variation of the electric Voltages generating
the currents takes place. The behavior of the currents as a
1. Field of the Invention function of the underlying electric Voltage allows an evalu
The present invention relates to a method for error detec ation leading on the one hand to a distinction between errors
tion in a drive mechanism, having a multiphase electric in the converter, and in the electric motor on the other hand.
motor and a converter connected upstream thereof, wherein 15 This will be explained in detail further down by means of
the converter controls the voltage of the individual phases of examples. It is important here that the Voltages are varied in
the electric motor, and the phase currents in the phases of the the course of the measurements in Such a way that the angle
electric motor each extend periodically and wherein, for of current flow does not change, i.e. that the phase currents
error detection, several phase currents of the electric motor are each checked at a single constant point of the period.
are measured at a predetermined point of the respective The errors to be detected in connection with the electric
period, while the associated Voltages at the respective phases motor relate in particular to a break in the lines of a phase
of the electric motor are simultaneously varied, and mea of the electric motor, for example as the result of a line break
Sured values of the phase currents are evaluated as a function or a contact which is not closed. The errors to be detected
of the voltage at the respective phase of the electric motor. inside the converter can mainly relate to a faulty, i.e. not
2. Discussion of Related Art 25 correctly Switching, converter valve in the form of a tran
Such a method is intended for detecting errors in a drive sistor, in particular a so-called IGBT (insulated gate bipolar
mechanism having a multiphase motor, in particular a three transistor).
phase motor, and a converter, wherein the converter controls A disruption in a phase of the electric motor is detected in
the Voltage at the phases of the electric motor and the phase that the respective phase current is always approximately
currents (which are a function of said Voltages) each extend 30 equal to Zero, regardless of the value of the underlying
periodically in the individual phase of the electric motor. voltage (as a rule, the exact value of “0” does not occur
The phase currents are measured here for detecting errors. because of measurement effects). In contrast to this, in case
Methods for error detection in a drive mechanism by of a faulty, not correctly Switching converter valve, a drop to
measuring phase currents are known, for example, from EP the current value Zero (not occurring with a correctly Switch
0 848 492 A1 and U.S. Pat. No. 5,266,891, the entire 35 ing converter valve) can only be observed in a partial area
contents of which are incorporated herein by reference. of the underlying electrical Voltage curve.
An angle of current flow is selected for evaluating the
OBJECT AND SUMMARY OF THE INVENTION phase currents in Such a way that at least one of the phase
currents always equals Zero during the measurement. For
An object of the present invention is based on making 40 example, in case of sinusoidally extending phase currents,
available an improved method for error detection in a drive the measurement of the phase currents can take place at an
mechanism. angle of current flow at which one of the phase currents
This object is attained in accordance with the invention by shows a Zero-crossing.
a method for error detection in a drive mechanism, having In the course of measuring the phase currents, the Voltages
a multiphase electric motor and a converter connected 45 are preferably varied in such a way that the ratio of the
upstream thereof, wherein the converter controls voltages of Voltages during the measurement remains constant. In par
individual phases of the electric motor, and individual phase ticular, two Voltages can be varied in Such a way that the Sum
currents in the individual phases of the electric motor each of the Voltages during the measurement always equals Zero.
extend periodically. The method includes measuring a phase In a preferred embodiment of the present invention, the
current of the electric motor at a predetermined point of a 50 voltages at the individual phases of the electric motor are
respective period, simultaneously varying a Voltage that is periodically varied, wherein each of the Voltages passes
associated with the measured phase current and evaluating a through a full period in the course of the measurement.
measured value of the measured phase current as a function The variation of the Voltages can take place in a simple
of the Voltage that is associated with the measured phase way on the basis of a sine curve, so that the Voltages show
Current. 55 Zero-crossings which are important for a Solid evaluation,
In accordance therewith, for error detection the phase which will be explained further down below.
currents are measured at a predetermined position of the The measurement of the phase currents can take place at
respective period, i.e. at a predetermined angle of current several different angles of current flow, wherein the number
flow, while simultaneously varying the Voltages at the of the measurements at different angles of current flow
phases of the electric motor, and the measured values are 60 preferably corresponds to the number of the phases of the
evaluated as a function of the Voltage at the respective phase electric motor, and wherein a different one of the phase
of the electric motor. currents has the value of Zero in each one of the measure
The Solution in accordance with the present invention is mentS.
suitable for error detection in connection with drive mecha For evaluating the measurement results in regard to error
nisms for machine tools, and makes possible the locating of 65 detection, the measured phase currents can each be evalu
errors, so that it is possible to distinguish between an error ated at defined points of the respective current flow which
in the converter and an error in the downstream connected are based on the electrical Voltage, for example at the
 US 7,355,436 B2
3 4
maximum of the respective phase current between a maxi BRIEF DESCRIPTION OF THE DRAWINGS
mum and a Zero-crossing of the associated Voltage. In this
case the evaluation of the measurement results can be FIG. 1 represents a schematic electrical circuit diagram of
performed in a simple manner by a table. an embodiment of an electric motor with an upstream
It is not necessary to measure all phase currents directly, connected frequency converter,
FIG. 2 is a flow chart of an embodiment of a method for
since one of the phase currents can be indirectly determined error detection in a drive mechanism in accordance with
from the remaining phase currents. FIG. 1 in accordance with the present invention:
The input signals of the converter are determined by FIG. 3 is a schematic representation of an embodiment of
output signals of a controller, in particular by pulse-width 10 an evaluation circuit for detecting errors at the interface
modulated signals (PWM signals). It is therefore useful to between the frequency converter from FIG. 1 and an
check the output signals of the controller for errors at the embodiment of an upstream-connected controller in accor
interface between the controller and the converter. In the dance with the present invention;
course of this a check is made whether defined, predeter FIG. 4 is a possible table for evaluating the circuit in
mined patterns of the output signal of the controller generate 15 accordance with FIG. 3 in accordance with the present
the expected input signals (voltage signals) at the converter. invention;
An error has occurred if defined, predetermined output FIG. 5 is a flow diagram for the more detailed explanation
signals of the controller do no result in the expected Voltage of a possible method step from FIG. 2 regarding the dis
signals. The latter are linked with each other via a logical crimination of errors in the electric motor, on the one hand,
gate, in particular an EX.OR gate, for evaluating the Voltage and the upstream-connected frequency converter on the
signals. other in accordance with the present invention;
FIG. 6 is a representation of the ideal path of the phase
Within the scope of the present invention it is further also currents in the electric motor from FIG. 1;
possible to check whether a short circuit between two phases FIGS. 7a to 16c represent different relevant measurement
of the electric motor exists. The check for a short circuit is 25 results when executing the method in accordance with FIG.
always performed when the measurement of the phase 5,
currents has to be stopped because of an overcurrent in one FIG. 17 is a representation in the form of a table of the
of the converter valves of the converter. The detection of results of an embodiment of a method for detecting a short
Such an overcurrent can take place in a simple manner at the circuit between two phases of the electric motor in FIG. 1 in
respective converter valve itself which had shut off because 30 accordance with the present invention; and
of an overcurrent. FIG. 18 is a flow chart for an embodiment of a method of
Respectively two phases of the electric motors are placed detecting a line-to-ground fault in one of the phases of the
on the same electrical potential for checking for a short electric motor in FIG. 1.
circuit, and a check is then made whether the strength of one DETAILED DESCRIPTION OF THE
35
of the phase currents indicates a short circuit. The determi PREFERRED EMBODIMENTS
nation of the electrical potential of the individual phases can
here take place by the suitable selection of an angle of
current flow. By sequentially placing all possible combina A drive mechanism for machine tools is represented in
tions of two phases of the electric motor on the same FIG. 1, which has a multiphase electric motor in the form of
potential, it is possible in case of the appearance of a short 40 a three-phase motor 1, a converter in the form of a frequency
circuit to determine which of the two phases is short converter 2 connected upstream of the motor 1, as well as a
circuited, since the short circuit can not be caused in the controller 3 for generating PWM signals for the frequency
respective phases which are on the same potential. converter 2. The three phases 11, 12, 13 of the drive motor
If short-circuit currents should appear with all combina 1 are schematically represented in FIG. 1 as coils with
45 associated feed lines. A numerical control NC controls the
tions, it is possible to perform a check for a line-to-ground motor 1 as a forward feed motor or a spindle motor.
fault of one of the phases. To this end the individual The frequency converter 2 (power element) connected
electrical circuits of the converter which are respectively upstream of the drive motor 1 is constituted by three pairs of
assigned to one of the phases of the electric motor are IGBTs 26a, 26b, 2.7a, 27b, 28a, 28b with associated recov
charged with identical input signals. Each of these circuits 50 ery diodes. Moreover, an electrical feed line 21a, 21b, 22a,
can be constituted by transistors or thyristors arranged in 22b and 23a, 23b is assigned to each IGBT 26a, 26b, 27a,
pairs, in particular in the form of IGBTs arranged in pairs 27b, 28a, 28b.
with associated recovery diodes.
Respectively one of two possible Voltages is applied as Each one of these pairs of IGBT 26a, 26b, 27a, 27b, 28a,
the electrical potential to the respective phase of the electric 28b, together with the associated recovery diodes and the
55 feed lines 21a, 21b, 22a, 22b, 23a, 23b, constitutes an
motor by the electrical circuits assigned to the individual electrical circuit 21, 22 or 23 which, as a function of the
phases of the electric motor as a function of the respective input signals from the frequency converter 2, or the output
input signal of the converter. In this case a line-to-ground signals from the controller 3, Switches an upper intermediate
fault exists if a current appears, although all phases of the circuit Voltage UZ/2, or a lower intermediate circuit Voltage
electric motor have been placed on an identical electrical 60 -UZ/2 to the respectively associated phase 11, 12 or 13 of the
potential. drive motor 1. Two of the phase currents l1, 12 in the first
The Solution in accordance with the present invention, phase 11 or the second phase 12 of the drive motor 1
and its further developments described above, are particu resulting from this are measured by the current sensors 41,
larly suitable for use in a three-phase motor. 42. The phase current 13 in the third phase 13 of the drive
Further characteristics and advantages of the present 65 motor 1 can be determined from the measured values of the
invention will become apparent in the course of the subse two other phase currents 11, 12. In this case a current can only
quent description of the drawing figures. flow in the drive motor if at least two of the three voltages
 US 7,355,436 B2
5 6
U1, U2, U3 at the phases 11, 12, 13 of the electric motor 1 103 takes place and, if required, an error message 105 is
are different, i.e. at least one of the voltages U1, U2, U3 has issued, which indicates an open phase in the drive motor 1
been set to the value UZ/2, and another to the value -UZ/2. or an error in the frequency converter 2 (disconnected
Of the controller 3, whose output signals PWM1, PWM2 IGBT).
and PWM3 determine the input signals of the frequency If, however, an overcurrent is detected in one of the
converter 2, only the electric lines 31, 32.33 are represented IGBTs, a check is made in the next step 106 whether it is a
in FIG. 3, which constitute the interface with the frequency phase short circuit, i.e. whether two phases of the drive
converter 2 and are assigned to respectively one of the motor 1 are short-circuited. Again, measurements at the
electrical circuits 21, 22, 23 of the frequency converter 2. different angles of current flow are performed, wherein the
The output signals generated by the controller 3 are respec 10 angles of current flow in the individual measurements are
tively rectangular signals, whose pulse-duty factor is a selected in Such away that respectively two of the phases are
function of the desired reference values for the current in the on the same electrical potential, i.e. that the same interme
drive motor 1. diate circuit voltage has been switched in. In the course of
Drive mechanisms for machine tools of the type repre evaluating the measurements, consideration is given to the
sented in FIG. 1 are known. A description of how an error 15 fact that in the presence of a short circuit of two phases
detection can be performed for such a drive mechanism will during the current measurement Switch-off because of an
be provided by the further drawing figures, which not only overcurrent does not take place only if there is no potential
makes possible a dependable detection of occurring errors, difference between the two affected phases at that time, i.e.
but also their localization. It is intended in particular to make the two phases have been switched to the same intermediate
a distinction between the occurrence of an error in the drive circuit voltage. Following the detection of a short circuit, the
motor 1 and in the frequency converter 2 without the test is terminated with the error message 107 “phase short
provision of additional measuring devices, in particular circuit'.
separate measuring devices for the detection of errors in the If a switch-off because of an overcurrent takes place at all
drive motor 1, on the one hand, and the frequency converter three angles of current flow, a test 108 for a line-to-ground
2, on the other. 25 fault is performed. In the course of this the output signals
FIG. 2 shows an overview of the total sequence of an error (PWM signals) of the controller 3 are initially set fixed to
test 100 for the drive mechanism from FIG. 1 by a flow “1”, and then fixed to “0”. If there is a line-to-ground fault,
chart. a switch-off because of an occurring overcurrent will take
The interface between the controller 3 and the frequency place with both measurements. But in case of a short circuit
converter 2 is checked in a first test step 101. i.e. a test is 30 in an IGBT, an overcurrent will only occur when the further
performed whether the PWM signals generated by the IGBT, which is assigned as a partner to the defective IGBT,
controller arrive correctly at the frequency converter 2 and has been switched on. By this it is therefore possible to
whether all required auxiliary voltages for controlling the distinguish between whether an overcurrent is caused by a
IGBTs are present. If an error is detected during this test, the line-to-ground fault of one of the phases of the drive motor
error message 102 “Error in PWM interface” and the test is 35 1, or by a short circuit in an IGBT of the frequency converter
terminated, since further test functions cannot be performed 2.
in a meaningful way as long as it has not been assured that In what follows, the error tests which are represented
correct input signals have been provided to the frequency together in FIG. 2 will be individually explained in greater
converter 2. detail.
If no errors appeared during the test of the interface 40 FIG.3 represents an evaluation circuit for performing and
between the frequency converter 2 and the controller 3, a evaluating the test 101 (FIG. 2) for errors at the interface
“motor test with current measurement' is performed in the between the controller 3 and the frequency converter 2. The
next step 103. In the course of this the electrical voltages, on evaluation circuit contains three connections, which are
which three different angles of current flow of the three respectively assigned to one of the feed lines 31, 32, 33
phase current driving the drive motor 3 are based, are 45 (FIG. 1) at the interface between the controller 3 and the
sinusoidally varied. In this case the angle of current flow is frequency converter 2. The first two connections are made
selected in such a way that the current is zero in one of the via respective electrical lines 51, 52 with the inputs of a
three motor phases, therefore the associated Voltage also has logical gate 56 in the form of an EXOR gate. The output
the constant value of Zero. signal from this first EXOR gate 56 is supplied to a second
With three different angles of current flow, a total of six 50 EX.OR gate 57, to whose input the signal occurring at the
measurements of the phase current which can be evaluated third connection of the evaluation circuit is Supplied via a
result from a separate evaluation of the two half-waves of line 53. In turn, the output of the second EX.OR gate is
the sinusoidally extending Voltages. Evaluation is performed connected with an input of a third EX.OR gate 58, which is
by a table, from whose entries it can be determined whether in addition supplied with the result of a self-test circuit 54,
an error in the drive mechanism is caused by an IGBT, which 55 which checks the presence of all low voltages (of 15 V or 24
does not switch on, or a disruption in a phase of the drive V) required for controlling the IGBTs. The output signal of
motor. In the course of this, use is made of an effect wherein the third EX.OR gate 58 is supplied to a logical gate 59 in
the respectively induced phase current, depending on the the form of a NOR gate, whose input is furthermore con
sign, flows off via the associated recovery diode either to the nected with a circuit 55 for activating the test of the
positive or the negative intermediate circuit Voltage bar in 60 interface.
the respective electrical circuit 21, 22, 23 of the frequency For performing the error test at the interface between the
converter 2. controller 3 and the frequency converter 2, the controller is
In the course of this test a determination is simultaneously operated in such a way that defined patterns of PWM signals
made whether one of the IGBTs of the frequency converter can be expected in the lines 31, 32, 33, which constitute the
2 Switches off because of an overcurrent. If no IGBT error 65 interface. Each one of these patterns corresponds to a
104 in the form of an overcurrent is detected, the evaluation defined distribution of the values “1” and “O'” of the PWM
103a of the measured values from the current measurements signals PWM1, PWM2, PWM3, which are to be made
 US 7,355,436 B2
7 8
available by the controller 3 in the corresponding lines 31, respective control lines SH1, SH2. In the second case the
32, 33 of the interface. The voltage signals PWM.U1, output of the test circuit (“test.out') is open.
PWM.U2, PWM.U3 generated because of these signals in At the termination of the checks of the signals present at
the lines 51, 52, 53 on the input side of the evaluation circuit the interface between the controller 3 and the frequency
5, which respectively represent the value “1” or “0” of the converter 2 described by means of FIGS. 3 and 4, the actual
associated PWM signal, are linked together in the first two core element of the error diagnosis takes place, namely the
EXOR gates 56, 57. Added to this is the linkage with the detection of errors in the frequency converter 2 or the drive
self-test circuit 54, which checks for the existence of all motor 1 by means of the measurement of the phase currents
required low Voltages and which, in the case of an error, 11, 12, 13 in the individual phases 11, 12, 13 of the drive
provides the value “1”, otherwise the value “0”, to the 10 motor 1. An overview regarding the sequence of the respec
corresponding input of the third EX.OR gate 58. The result tive measurements is represented in the flow chart in accor
of these three linkages in the EXOR gates 56, 57, 58 is dance with FIG. 5. The sequence of measurements repre
supplied to the NOR gate 59 (inverter) whose input, at the sented in FIG. 5 is performed by a microprocessor within the
activation of the test circuit, is additionally charged with the numerical control NC of FIG. 1.
value “0” by the control lines for test activation, since both 15 The aim of this test is not only to detect the presence of
the control lines SH1, SH2 are each set to the value “0”. an error, but also to localize the source of this error, in
For evaluating the values obtained by the evaluation particular to distinguish between a disruption of a phase of
circuit 5, the patterns of the three PWM signals PWM1, the drive motor 1, for example because of a break in a line
PWM2, PWM3, as well as the output signal of the NOR gate (also a break in the winding of the motor), or an open
59 measured as a function of the respective pattern, are contact, and an error in the frequency converter 2, for
stored in a table. example as the result of a disconnected IGBT.
In the form of a table, FIG. 4 shows for all eight possible Following the activation of the current measuring device
patterns of the three PWM signals, from “000 to “110, 200, the underlying voltages are initially determined. For
what values (“test.out”) can appear at the output of the NOR this purpose, the Voltage, which induces the nominal current
gate 59, and what conclusions can be drawn from them. 25 desired for the measurement in the motor, is determined
The first three lines of the table in FIG. 4 provide the from the electrical data of the motor which can be polled
possible combinations (patterns) of the three PWM signals, from the motor control. The actually induced current will be
as a function of which the output signal (test out) is checked again later during the measurement. If the differ
generated at the NOR gate 59. In this case the eight columns ence from the desired nominal current exceeds a predeter
of the table relate to the eight different possible combina 30 mined value, the measuring Voltage is matched, so that the
tions of the three PWM signals. maximally permissible current of the motor is induced for a
The next eight lines of the table under the heading “low new measurement in the drive motor 1. This matching of the
voltages ok” provide the possible output signals (“test.out') Voltage is based on a linear connection between the current
for all eight combinations of PWM signals in the case where and the voltage. If the electrical data of the motor are not
no low voltage error has occurred. The first of these eight 35 available, there is the alternative option of slowly approach
lines represents the case where no error has been observed ing the nominal current from a Voltage which initially is
at the interface between the controller 3 and the frequency selected to be low. The measuring Voltage is selected in Such
converter 2. In this case, with the signal combination "000 a way that the phase currents induced in the drive motor 1
at the inputs of the evaluation circuit, the value “1” will are clearly distinguished over the background noise, on the
appear at the output of the NOR gate 59, with a signal 40 one hand, and that on the other hand damage to the drive
combination “100 at the output, the value “0” appears, etc. motor 1 by too strong currents is prevented.
Each of the next seven lines represents results wherein the After determining the measuring Voltage, in a next step
signal appearing at the interface does not correspond to the 202 with a first angle of current flow wherein one of the
signal expected based on the setting of the controller. The phase currents l1, 12, 13 is Zero, a measurement of the
corresponding errors resulting from the respective evalua 45 remaining two phase currents as a function of a sinusoidal
tion of the eight possible combinations of the desired PWM variation of the measuring Voltage takes place in Such a way
signals, as well as the associate output signal (“test.out'), at that the Voltage passes through exactly one period.
the inverter 59, are respectively indicated at the end of the Subsequently thereto a check is made whether in the
table. course of the measurement an IGBT error 203 has occurred,
The next eight lines relate to the case wherein an error has 50 i.e. whether one of the IGBTs of the frequency converter 2
occurred during the self test of the low voltages, so that was switched off because of an overcurrent. In this case the
accordingly the value “1” is set at the input of the third measurement is terminated with an appropriate error mes
EX.OR gate 58 by the appropriate circuit 54. Sage.
The error message “PWM.U1” is stuck on 0, for Otherwise, the current measurement is continued, namely
example, indicates that in the corresponding feed line 51 the 55 at two further angles of current flow of the three-phase
signal PWM.U1 represented by a voltage always assumes current used for Supplying the drive motor 1, so that the
the value “0” although, as a function of the respective measurement is performed at a total of three angles of
pattern of the three PWM signals, this value should swing current flow, and during each measurement a different one of
between “0” and “1”. This error can be caused by a break in the phase currents l1, 12, 13 has the value “0”. Furthermore,
the line, for example. The indication “PWM.U1=PWM.U2' 60 following each one of the further current measurements 204,
indicates that the same signal is always measured in the two 203, an inquiry as to the presence of an IGBT error 205 or
respective lines 51, 52 of the evaluation circuit 5, although 207 is performed and, if such should be the case, the
this should only be the case in four of the total of eight measurement is again terminated with an error message.
possible combinations of the PWM signals. If no IGBT error has occurred in any of the current
The two last lines of the table indicate “general errors' in 65 measurements, in a next step 208 the maximum of the
the evaluation circuit. In the first case the test circuit is not current amount which had occurred during the measurement
activated at all, therefore the error already lies in the is determined and is used in the form of lmess for standard
 US 7,355,436 B2
9 10
izing the results. Subsequently a rating 209 of the measure the two measured phase currents l1, 12 each run through a
ment results takes place for preparing the error detection, as complete period in accordance with the underlying Voltages
well as an evaluation 210 of the results of the rating by U1, U2.
means of a table. In the measurement represented in FIG. 7b at an angle of
In what follows, the performance of the individual current current flow of 360°, the first phase current 11 and the
measurements, the rating of the measurement results, as well associated Voltage U1 are each constantly equal to Zero,
as the evaluation of the results for error detection will be since this phase current 11 has a Zero-crossing at an angle of
described more accurately. The evaluation is performed only current flow of 360°, see FIG. 6. This is correspondingly
if none of the IGBTs was switched off because of an applicable for the measurement represented in FIG. 7c at an
overcurrent during the current measurement, and the mea 10 angle of current flow of 240° in regard to the second phase
Surement as a whole was not interrupted. Because in that current 12 and the associated Voltage U2.
case a check for short-circuited phases, or line-to-ground FIGS. 8a to 8c show three measurements under the same
faults, as explained above by means of FIG. 2, would be conditions as described by FIGS. 7a to 7c wherein, however,
performed, instead of an evaluation of the current measure the upper IGBT 26a of the first electrical circuit 21 of the
ment. 15 frequency converter 2 is malfunctioning, i.e. does not cor
FIG. 6 shows the ideal path of the three phase currents l1, rectly switch on. This becomes apparent from the behavior
12, 13 of the drive motor 1 within an angular range between of the first phase current 11 at a positive associated Voltage
0° and 500°. Each of the three phase currents 11, 12, 13 U1.
extends sinusoidally, and they are phase-shifted by 120° As explained above by FIG. 1, with a positive voltage
from each other. The path of the phase currents is determined (corresponding to the value “1” of the PWM signal) gener
by the PWM signals which are supplied as output signals of ated by the appropriate PWM signal PMW1, PMW2 or
the controller 3 to the frequency converter 2 and therefore PMW3, the appropriate upper IGBT 26a, 27a, 28a of the
determine the potential which is supplied to the individual corresponding electrical circuit 21, 22, 23 is switched on.
phases 11, 12, 13 of the drive motor 1 at specific times. But if the associated PWM signal PMW 1, PMW2 or PMW3
For performing the current measurements, by which 25 assumes the value “0”, and a negative Voltage is accordingly
errors in the frequency converter 2, on the one hand, and in created, the corresponding lower IGBT 26b, 27b, 28b of the
the drive motor 3, on the other, are intended to be detected, corresponding pair 26a, 27a, 28a, or 26b, 27b, 28b is
switched on.
the PWM signals PWM1, PWM2 and PWM3 are here set in Therefore the fact, that in accordance with FIGS. 8a to 8c
Such a way that respectively one of the three phase currents
11, 12, 13 is Zero, and the other two phase currents have the 30 at an angle of current flow of 120°, as well as 240°, the first
same value, but opposite signs. This is the case at angles of phase current 11 is always Zero when the associated Voltage
current flow of 120°, 240°, as well as 360°, as can be seen U1 assumes a positive value, indicates that the correspond
in FIG. 6. ing IGBT 26a is not correctly switched on. Otherwise, at a
Thereafter, at the respectively preset angle of current flow, positive value of the said Voltage U1, a corresponding value
the underlying Voltages (potentials) at the phases 11, 12, 13 35 of the first phase current 11 greater than Zero would have to
of the drive motor 1 are chronologically varied in such a way have been measured, as represented in FIGS. 7a to 7c for an
that the phases of the two currents which are not equal to error-free system.
Zero at the actual angle of current flow pass through a full In a corresponding way it can be seen by FIGS. 9a to 9c,
sinus period and have the same value, but opposite signs, at that the lower IGBT26b of the first pair 26a, 26b (of the first
any time. The third of the three voltages U1, U2, U3 is in this
40 electrical circuit 21 of the frequency converter 2) is defec
case Zero and belongs to the phase current which has the tive, i.e. does not switch on. Because here the first phase
current 11 assumes the value Zero in case the associate
value Zero at the corresponding angle of current flow. In this voltage U1 or U2 is negative, i.e. the lower IGBT26b should
case the amplitude of the chronologically varied Voltages is have switched on.
fixed by the measuring voltage which had been determined 45 In a corresponding way it is possible to conclude in
at the start, as described above. connection with the current and voltage paths in FIGS. 10a
Only the first two phase currents 11, 12 (provided they to 10c, 11a to 11c, 12a to 12c and 13a to 13c that the upper
equal Zero) and the associated Voltages U1, U2 (provided IGBT 27a of the second pair 27a, 27b (FIGS. 10a to 10c) is
they do not equal Zero) are represented in the Subsequent faulty, that the lower IGBT 27b of the second pair 27a, 27b
drawing figures, which show the results of the above 50 (FIGS. 11a to 11c) is faulty, that the upper IGBT 28a of the
described current measurements, since in accordance with third pair 28a, 28b (FIGS. 12a to 12c) is faulty, or that the
FIG. 1, a current sensor 41 or 42 is associated only with lower IGBT 28b of the third pair 28a, 28b (FIGS. 13a to
these two phase currents l1, 12 for current measurement. The 13c) is faulty.
conditions in the third phase 13 of the drive motor 1 can be In contrast to this, FIGS. 14a to 14C each show a current
calculated from the results regarding the first two phases 11, 55 measurement at the three angles of current flow of 120°,
12. 240°, as well as 360°, corresponding to the explanations in
FIGS. 7a to 7c show the results of the measurements at the FIGS. 7a to 7c, in connection with a drive mechanism in
angles of current flow of 120°, 360°, as well as 240°, namely accordance with FIG. 1, wherein a disruption (“phase open’)
in a drive mechanisms in which no error to be detected is exists in the first phase 11 of the drive motor 1. This can be
present in the frequency converter 2 or the drive motor 1. It 60 the result of a line break, or of an incorrectly closed contact.
can be clearly seen that the measured phase currents 11, 12 As a result thereof the phase current 11 in the first phase 11
each follow the associated voltage U1 or U2. This means of the drive motor 1 is always equal to Zero, regardless of the
that the Zero-crossings and the maxima of the phase currents value of the sinusoidally varying voltage U1.
11, 12 chronologically slightly trail the corresponding Zero In a corresponding way, in accordance with FIGS. 15a to
crossings and maxima of the underlying Voltages U1, U2. 65 15c, or FIGS. 16a to 16C, an evaluation of the measurement
The result of the measurement at an angle of current flow shows that a disruption exists in the second phase 12, or the
of 120°, corresponding to FIG. 7a, furthermore shows that third phase 13 of the drive motor 1, so that the corresponding
 US 7,355,436 B2
11 12
phase current 12 is always equal to zero (FIGS. 15a to 15c), The latter can have the value k=0.2, for example. The
or the first and second phase currents 11 and 12 add up to a maxima and minima of the phase currents 11, 12 determined
value of Zero, even if only one of the two associated Voltages in the manner explained above are then each evaluated as to
U1 or U2 is not equal to zero, see FIGS. 16b and 16c. whether their value is greater or less than k-times the
For the concrete, automated (computer-controlled) evalu 5 greatest measured current limess. The presence of a real
ation of the measurement results, such as has been repre current maximum or minimum is assumed only if the former
sented by way of example for defined error occurrences by is the case. Otherwise it is assumed that the respective
means of FIGS. 7a to 16c, use is made of the fact that the current is Substantially equal to Zero and that only the noise
phase currents l1, 12 trail the assigned voltages U1 or U2. was measured.
Thus, the minima, maxima and Zero-crossings of the Volt 10 In this case the following equations apply for forming a
ages U1 and U2 underlying the measured phase currents 11, table which can be systematically evaluated:
12 are first determined for the evaluation. Thereafter, the If Illdk*Ilmessl& 120, the value “1” is assigned to the
maximum or the minimum of the associated phase current 11 corresponding current 1 (in this case the current 1 selectively
or 12 is determined for each Voltage maximum and the represents the measured values of the phase current 11 or 12).
Subsequent Zero-crossing, as well as for each Voltage mini 15 Iflisk*Ilmess, the corresponding current is assigned the
mum and the Subsequent Zero-crossing. Thus, the respective value “O’.
maximum of the phase current 11 or 12, which lies between If Illdklmess& 1-0, the corresponding current is assigned
a maximum of the associated voltage U1 or U2 and the the value “-1.
Subsequent Zero-crossing, is determined, as well as the Thus, for detecting an error, the actually measured current
minimum of the respective phase current 11 or 12, which lies values are not directly evaluated, instead the current path
between a minimum and a Subsequent Zero-crossing of the between a maximum of the underlying Voltage and a Zero
associated voltage U1 or U2. crossing of the underlying Voltage is assigned the value “1”
This will be explained by way of example by FIG. 7a, or “0”, wherein the value “1” means that in the said range
which shows the two phase currents 11 and 12, as well as the a current maximum has occurred, and the value “0” means
associated voltages U1, U2 for an angle of current flow of 25 that the current in the respective range is Substantially equal
120° in a drive mechanism which does not show any errors to Zero. In a corresponding manner the respectively mea
to be detected. It can be seen that a maximum of the sured current is assigned the value “-1” between a voltage
associated phase current 11 is located between the maximum minimum and the Subsequent Zero-crossing, if an actual
and the Subsequent Zero-crossing of the one Voltage U1. minimum of the respective current has occurred here, oth
Correspondingly, a minimum of the associated phase current 30 erwise the value '0'. Thus, the results of the current mea
11 is located between the minimum of the one voltage U1 Surements can be represented in a table, wherein each one of
and its subsequent Zero-crossing. This applies correspond the observed angles of current flow of 120°, 240°, as well as
ingly to the other Voltage U2 and the associated phase 360°, is assigned four current values, namely two values of
current 12. the first phase current 11 and two values of the second phase
Therefore respectively two pairs of current values of the 35 current 12. In turn, these values are sorted to determine
phase current 11 and 12 result from each of the three series whether they represent a range between a maximum and a
of measurements which had been recorded at the angles of Subsequent Zero-crossing, or between a minimum and a
current flow of 120°, 240° as well as 360°, namely two Subsequent Zero-crossing of the associated Voltage.
current values for the first phase current 11 and two current This table can then be evaluated in the same way as the
values for the second phase current 12, since an associated 40 actual current values were evaluated above by FIGS. 7a to
current value of each of the corresponding phase currents 11 15c. This has already been explained by FIGS. 7a and 8a
or 12 is determined from each half period of the associated and also applies to the further drawing figures. The above
voltage U1 or U2. evaluation has shown that the entire measured current path
If now the representation in accordance with FIG. 8a is is not necessary for detecting an error, it is merely important
compared with the representation in accordance with FIG. 45 that the respectively measured phase current in a defined
7a, it can be seen that, because of the faulty upper IGBT 26a range of the associated electrical voltage is greater than Zero,
of the first pair 26a, 26b, the first phase current 11 does not equal to Zero or less than Zero. This can be determined by the
assume its own maximum between the maximum and the criteria applied for forming the table while detecting the
Subsequent Zero-crossing of the associated Voltage U1. The current maxima or minima.
phase current 11 is substantially equal to Zero over the entire 50 As demonstrated by FIGS. 7a to 15c, it is possible in
range (except for noise). Therefore the absence of the particular to distinguish whether there is an error in an IGBT,
current maximum indicates that here the phase current 11 or one of the phases 11, 12, 13 of the drive motor 1 is open.
does not act corresponding to the underlying Voltage U1 In the latter case the associated phase current is always equal
which, in turn, is an indication of a disconnected IGBT 26a. to Zero, regardless of the chronologically varied Voltage,
For the systematic evaluation of the series of measure 55 while in the first mentioned case the corresponding phase
ments of the phase currents l1, 12 on the basis of the above current is equal to Zero only if it depends from a faulty
explained principle, the following actions are performed: IGBT. In this case the faulty behavior of the IGBT can also
The detected current maxima and current minima (each have its source in the associated logical control device.
occurring between a maximum and a Zero-crossing, or a The said errors in the frequency converter 2 on the one
minimum and a Zero-crossing, of the underlying electrical 60 hand, and the drive motor 1 on the other, can also be
voltage) are evaluated to determine whether they represent distinguished from a defect of one of the two current sensors
an actual current maximum or current minimum, or whether 41, 42 (“current sensor stuck at full deflection'), since in
merely the maximum or minimum of the noise had been case of a defect in one of the current sensors 41, 42 it can
measured, while the current is actually Substantially equal to be expected that the current sensor always Supplies the same
Zero. For this purpose the values of the measured current 65 current value, regardless of any of the Voltages.
maxima or current minima are compared with the maximally An explanation will now be given by the representation in
measured current value limess, using an evaluating factor k. the form of a table in FIG. 17 as to how the test for a short
 US 7,355,436 B2
13 14
circuit between two phases of the drive motor 1 mentioned exactly at the time at which the respectively other IGBT of
in the description of the entire method by the flow chart in the same IGBT pairs is switched on.
FIG. 2 is performed. This test is always performed when in The detection of a short circuit in one of the upper or
the course of the measurements of the phase current 11, 12 an lower IGBTs leads to an appropriate error message 408 or
overcurrent has occurred, see FIG. 2. 411. A separate error message 412 is issued if neither a
A total of three measurements is performed for the test for line-to-ground fault nor a short circuit in one of the IGBTs
a phase short circuit, namely at angles of current flow of 90°, was detected during the performance of the test. This can
210° and 330°. At these angles of current flow, respectively also indicate an error in the performance of the test itself.
two of the phases 11, 12, 13 are connected to an identical Further exemplary embodiments exist within the scope of
potential, while the third phase is connected to another, 10 the present invention besides the described examples.
different potential. Thus, a corresponding first voltage value, We claim:
for example -U/2, is present at the two first phases, and a 1. A method for error detection in a drive mechanism,
another voltage value differing therefrom, for example U, is having a multiphase electric motor and a converter con
present at the third phase. nected upstream thereof, wherein said converter controls
If a short circuit, corresponding to an overcurrent in at 15
Voltages of individual phases of said electric motor, and
least one of the IGBTs, is observed in the course of one of individual phase currents in said individual phases of said
the three measurement steps, it cannot have occurred electric motor each extend periodically, the method com
between the two phases connected to an identical electric prising:
potential, since with an identical electrical potential even measuring a phase current of said electric motor at a
short-circuited lines do not generate a current flow. An predetermined point of a respective period;
appropriate decision matrix is represented in FIG. 17. simultaneously varying a Voltage that is associated with
Accordingly, in the three measurement steps the three said measured phase current;
voltages U1, U2, U3 of the phases 11, 12, 13 of the drive evaluating a measured value of said measured phase
motor 1 are varied, as explained above, wherewith respec current as a function of said Voltage that is associated
tively one of the phases is connected with the potential “U”. 25
with said measured phase current, wherein said evalu
and the two other phases with the potential "-U/2. Depend ating allows distinguishing between an error in said
ing on the distribution of the values “0” and “1” in the error converter and a disruption of current flow in a phase of
output “ERR’ it can then be determined whether there is no said electric motor that corresponds to said measured
short circuit, or possibly which one of the three phases 11, phase current; and
12, 13 is short-circuited. 30
wherein in case of a disruption in a phase of said electric
If the test for a short circuit between the phases 11, 12, 13 motor, said measured phase current is equal to Zero,
of the drive motor 1 terminates with the result that in all
three measurement steps the value “0” appears at the error regardless of a value of said associated Voltage, and in
output “ERR', a check is made in accordance with FIG. 2 case of an error in a converter valve of said converter,
whether the cause of a previously detected overcurrent in an 35 said measured phase current is equal to Zero only for a
IGBT is a result of a line-to-ground fault in one of the phases portion of said value of said associated Voltage.
11, 12, 13 of the drive motor 1. Alternatively, there can be 2. The method in accordance with claim 1, wherein said
a short circuit in one of the IGBTs themselves. disruption of current flow is caused by a line break or an
In accordance with FIG. 18, for performing the test 400 open electrical contact.
for a line-to-ground fault, first all PWM signals PWM1, 40 3. The method in accordance with claim 1, wherein said
PWM2, PWM3 are fixed at the value “0” (401). This means error in said converter is caused by a faulty converter valve.
that respectively the lower IGBT 26b, 27b and 28b of the 4. The method in accordance with claim 3, wherein said
three IGBT pairs of the frequency converter is switched on. converter valve is in the form of a transistor or a thyristor.
A switch-off 402 because of an overcurrent occurring in the 5. The method in accordance with claim 1, wherein said
course of this is stored as “Erro'. Subsequently, the three 45 measured phase current is equal to Zero at the point of said
PWM signals PWM1, PWM2, PWM3 are set to the value period at which said measuring is performed.
“1”, so that the upper IGBTs 26a, 27a and 28a are switched 6. The method in accordance with claim 1, wherein said
on (403). An overcurrent measured in the course of this will individual phase currents are phase-shifted with respect to
be stored as “Err1 in the next step 404. each other by a defined amount, and said measuring takes
Subsequently a check is made in a further step 405 50 place at a defined point of one of the periods of said
whether both the error “Erro” and “Err1 are present. An individual phase currents.
error message 406 will be output in this case, so that a 7. The method in accordance with preceding claim 1,
line-to-ground fault is indicated, since in case of a line-to wherein a ratio of said converter control voltages which are
ground fault an overcurrent occurs in the respective phase not equal to Zero during the respective current measurement
11, 12 or 13 of the drive motor 1 regardless of which one of 55 remains constant during the measurement, and the Sum of
the two respectively associated IGBTs is switched on. said converter control Voltages is equal to Zero during the
If, however, the inquiry at 405 is negative, a check is measurement.
made in two further steps 407 and 409 whether one of the 8. The method in accordance with claim 1, wherein said
two errors “Erro” or “Err1 is present. In the first case there Voltage during said measuring said phase current is periodi
is a short circuit in one of the upper IGBTs 26a, 27a, 28a, 60 cally varied during said measuring.
by which the motor phases U1, U2 and U3 can be switched 9. The method in accordance with claim 8, wherein said
to the upper intermediate circuit voltage UZ/2. In the other Voltage passes through exactly one period during said mea
case there is a short circuit in one of the lower IGBTs 26b, Suring.
27b, 28b, by which the lower intermediate circuit voltage 10. The method in accordance with claim 9, wherein said
-UZ/2 can be applied to the respective motor phases U1, U2 65 Voltage has a Zero-crossing that extends sinusoidally.
or U3. The reason for this lies in that with a short circuit in 11. The method in accordance with claim 8, wherein said
one IGBT a Switch-off because of an overcurrent takes place evaluating is performed at a maximum or a minimum of said
 US 7,355,436 B2
15 16
measured current located between a maximum or minimum converter comprises linking different Voltage signals gener
and a Zero-crossing of said Voltage associated with said ated at said input of said converter with said output signal of
measured phase current. said controller to each other.
12. The method in accordance with claim 1, wherein a 24. The method in accordance with claim 23, wherein said
value of said measured phase current is stored in the form of 5 linking is performed by at least one logical gate.
a table for evaluating said measured phase current. 25. The method in accordance with claim 1, further
13. The method in accordance with claim 1, wherein said comprising checking whether a short circuit exists between
multiphase electric motor is embodied as a three-phase two phases of said electric motor in case of an appearance
motor.
of an overcurrent during said measuring.
14. The method in accordance with claim 1, characterized 10 26. The method in accordance with claim 25, wherein said
in that several measurements of said measured phase current checking is only performed when an overcurrent has
are performed for error detection at a respectively predefined occurred in a converter valve of said converter.
point of the period.
15. The method in accordance with claim 14, further 27. The method in accordance with claim 25, wherein said
comprising measuring other phase currents associated with 15 checking comprises connecting two phases of said electric
the remaining phases of said electric motor other than said motor to a common electrical potential.
phase associated with said, said measuring other phase is 28. The method in accordance with claim 27, wherein said
performed simultaneously with said measuring a phase checking comprises connecting said two phases of said
current and said measured phase current and said other electric motor one after the other to said common electrical
phase currents are each measured a number of times equal potential in order to determine between which phases said
to the number of phases of said electric motor. short circuit has occurred by evaluating phase currents
16. The method in accordance with claim 15, wherein occurring during said connecting.
during said measuring of said phase current and said other 29. The method in accordance with claim 27, further
phase currents, one of said phase currents has a value equal comprising determining said Voltage of said measured phase
tO Zero. 25 current by determining an angle of current flow of said
17. The method in accordance with claim 1, wherein said measured phase current.
evaluating comprises evaluating said measured current at 30. The method in accordance with claim 1, further
points of a respective current path which is defined by a path comprising checking whether a line-to-ground fault exists in
of said Voltage associated with said measured phase current. a phase of said electric motor that corresponds to said
18. The method in accordance with claim 1, further 30 measured phase current when an overcurrent in a converter
comprising determining behavior of a second phase current valve of said converter exists.
based on a value of said measured phase current. 31. The method in accordance with claim 30, wherein said
19. The method in accordance with claim 1, further checking comprises charging individual circuits of said
comprising generating an input signal of said converter as a converter connected upstream of said phases of said electric
function of an output signal of a controller, wherein said 35 motor with identical input signals.
controller generates pulse width modulated signals. 32. The method in accordance with claim 31, wherein
20. The method in accordance with claim 19, further each of said individual circuits is assigned to a correspond
comprising checking said output signal at an interface ing one of said phases of said electric motor.
between said controller and said converter for an error in
said output signal by an evaluation circuit. 40 33. The method in accordance with claim 32, wherein
21. The method in accordance with claim 20, further each of said individual circuits have transistors or thyristors
comprising: arranged in pairs.
varying said output signal in accordance with a preset 34. The method in accordance with claim 33, wherein
pattern; and each of said electrical circuits cause one of two possible
measuring and evaluating a Voltage signal generated at an 45 Voltages to be applied to a respective phase as a function of
input of said converter as a function of said output said input signal of said converter.
signal. 35. The method in accordance with claim 31, wherein said
22. The method in accordance with claim 21, wherein said checking comprises setting each Voltage that is associated
error is considered to have been detected if an expected with said individual phase of said electric motor to an
Voltage signal based on said output signal does not appear. 50 identical value.
23. The method in accordance with claim 21, wherein said
evaluating said Voltage signal generated at said input of said
 UNITED STATES PATENT AND TRADEMARK OFFICE
CERTIFICATE OF CORRECTION

PATENT NO. : 7,355.436 B2 Page 1 of 1

APPLICATIONNO. : 10/633275
DATED : April 8, 2008
INVENTOR(S) : Georg Zehentner et al.
It is certified that error appears in the above-identified patent and that said Letters Patent is
hereby corrected as shown below:

In the Claims

In column 15, in claim 15, line 4, after “associated with delete “Said, Said' and
Substitute-Said measured phase current, Said-in its place.

Signed and Sealed this

Twentieth Day of January, 2009

WDJ
JON. W. DUDAS
Director of the United States Patent and Trademark Office
 UNITED STATES PATENT AND TRADEMARK OFFICE
CERTIFICATE OF CORRECTION

PATENT NO. : 7,355.436 B2 Page 1 of 1

APPLICATIONNO. : 10/633275
DATED : April 8, 2008
INVENTOR(S) : Georg Zehentner et al.
It is certified that error appears in the above-identified patent and that said Letters Patent is
hereby corrected as shown below:

In the Claims

In column 15, in claim 15, line 17, after “associated with delete “said, said
and Substitute-Said measured phase current, Said-in its place.

This certificate supersedes the Certificate of Correction issued January 20, 2009.

Signed and Sealed this

Seventeenth Day of February, 2009

4 (O-e-
JOHN DOLL
Acting Director of the United States Patent and Trademark Office