1134 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 47, NO.

5, OCTOBER 2000

Experimental Fault-Tolerant Control

of a PMSM Drive
Silverio Bolognani, Member, IEEE, Marco Zordan, and Mauro Zigliotto, Member, IEEE

Abstract—The paper describes a study and an experimental control to mitigate the effects of a sudden inverter failure. A
verification of remedial strategies against failures occurring in first effective example, applied to induction motors (IMs), can
the inverter power devices of a permanent-magnet synchronous be found in [11]. The strategy consists in reformulating the
motor drive. The basic idea of this design consists in incorporating
a fourth inverter pole, with the same topology and capabilities current references so that the rotating MMF generated by the
of the other conventional three poles. This minimal redundant armature currents do not change, even if one phase is open
hardware, appropriately connected and controlled, allows the circuited after a fault occurrence. For proper operation, the
drive to face a variety of power device fault conditions while neutral point of the motor has to be connected to the midpoint
maintaining a smooth torque production. The achieved results of the dc voltage link, created by the use of two capacitors. The
also show the industrial feasibility of the proposed fault-tolerant
control, that could fit many practical applications. technique, in principle quite simple, gets involved by the need
for preventing the capacitor midpoint voltage from drifting
Index Terms—Fault-tolerant control, permanent-magnet syn- from the correct point. A valid alternative that does not require
chronous motor drives.
the availability of the dc midpoint voltage is proposed in [14],
which deals with multiphase current-regulated IM drives. Other
NOMENCLATURE important aspects that heavily affect any remedial strategy
Motor phase currents. are the number of additional components with respect to the
Zero-sequence and neutral currents. standard drive, and the method used to isolate the faulty phase
from the rest of the drive. Again, for IM drives, a solution can
Transformed – currents.
Transformed – currents. be found in [10], in which a pair of back-to-back-connected
Phase-to-neutral voltages. SCRs is used to switch off the faulty motor phase current.
Stator resistance. After the fault, a phase remains permanently connected to
Stator direct, quadrature, and leakage induc- the midpoint of the dc voltage or, when insufficient voltage
tance. is available, the neutral is connected back to the midpoint,
Permanent-magnet flux linkage. that has to be derived by using series-connected capacitors.
Motor speed and position (electrical). Obviously, each SCR needs a proper gate circuit and must
bear the rated phase current. Analogous research topics may
be found for permanent-magnet synchronous motor (PMSM)
I. INTRODUCTION
drives [1], [2], [6], [7], which have an increasing market share

S EVERAL failures can afflict electrical motor drives

[1]–[7] and many different remedial techniques have
been proposed [8]–[11]. So far, redundant or conservative
due to their excellent dynamics and high torque-to-current
and torque-to-volume ratios. Actually, the investigation of
fault-tolerant control techniques for PMSM drives is arousing
design has been used in every application where continuity of lively interest, to extend their use to applications where high
operations is a key feature. Nevertheless, some applications reliability is a key feature, such as aircraft and automotive
accept short torque transients and even permanently reduced auxiliaries [8]. This paper is organized as follows. In Section II,
drive performance after fault, on condition that the drive still there is a list of the power stage faults which can be tolerated
goes on running. This is the clear case of home and civil by the proposed control scheme. The techniques suitable for
appliances, such as, for example, air conditioning/heat pumps, the detection of each particular fault are also illustrated. In
engine cooling fans, electric vehicles, laboratory stirrers, Section III, a survey of possible fault-tolerant drive schemes
but also some industrial loads, such as pumping plants and incorporating a four-leg inverter is discussed, pointing out
winders/unwinders, well tolerate such drive behavior. While advantages and drawbacks. In Section IV, a novel technique
current regulation has greatly improved the torque response for isolating the faulty pole of the inverter is illustrated and
of ac drives, an emerging technology aims to exploit current experimental verifications are presented. In Section V, a new
fault-tolerant torque control scheme is presented from a theoret-
ical point of view. In Section VI, the practical implementation
Manuscript received January 22, 1999; revised May 16, 2000. Abstract pub- is illustrated and the experimental results are given.
lished on the Internet July 1, 2000. This paper was presented at IEEE IECON’98,
Aachen, Germany, August 31–September 4, 1998.
The authors are with the Department of Electrical Engineering, Uni- II. INVERTER FAULTS AND RECOGNITION TECHNIQUES
versity of Padova, 35131 Padova, Italy (e-mail: bolognan@dei.unipd.it;
mauroz@dei.unipd.it). The failures that may involve the inverter power stage can
Publisher Item Identifier S 0278-0046(00)08845-6. take place either in the switches of the inverter or in their gate
0278–0046/00$10.00 © 2000 IEEE
BOLOGNANI et al.: FAULT-TOLERANT CONTROL OF A PMSM DRIVE 1135

Fig. 1. Fault-tolerant drive schemes.

command circuitry. The following permanent faulty situations detected and fixed before it causes, in turn, other damage. In
are considered in this paper: particular, it is supposed that power device faults do not damage
1) open circuit of one power device; the control system, which will still work properly. The general
2) open circuit of both power devices of an inverter leg; fault-tolerant drive scheme is drawn in Fig. 1(a).
3) short circuit of one power device; On the left-hand side is represented a conventional three-
4) short circuit of both power devices of an inverter leg. phase inverter whose poles can be completely isolated from the
Indeed, the fourth fault needs a very rapid hardware in- dc bus in case of faults by the intervention of isolating devices,
tervention to avoid a destructive shoot-through fault. Even sketched as fuses in the figure. Details of the isolating technique
in this case (that usually triggers a complete drive stop) the will be given in the next section. Actually, the isolation devices
technique presented in Section III manages the complete could be put in the motor terminals at the inverter output, but
isolation of the faulty pole and the beginning of the remedial the resulting structure would not bear the short circuit of both
mode. Various fault-recognition techniques can be adopted. In the power devices in the inverter leg (fault #4). Such a fault
[13], a sophisticated procedure is proposed. However, simpler would put out of service the whole inverter. The right side of the
and cost-effective solutions can be envisaged, avoiding any figure shows the additional fourth leg. According to the motor
special transducers or dedicated devices. For example, a smart winding configuration and the adopted remedial technique, dif-
strategy compares the voltages at the device outputs and checks ferent switching patterns can be generated for the fourth leg. De-
if they correctly follow the corresponding triggering signals. pending on the selected control strategy, the isolating devices
Another method takes advantage of the power devices desat- ID and ID [Fig. 1(a)], and the connecting devices (CDs)
uration voltages. Both strategies must carefully consider the [Fig. 1(b) and (c)] can be incorporated or not. Some possible
unavoidable delays in the switching instants and the necessary remedial techniques are discussed hereafter.
electronics must be properly tuned according to the inverter
performance in order to distinguish a device failure from an A. Three-Terminal Motor Winding
overload condition. In the analysis carried out in this paper, the This is the case of delta-connected or star-connected wind-
remedial strategy has been tested considering a failure in phase ings with isolated neutral stator windings [Fig. 1(b)]. With
; of course, similar actions apply when failures affect phase these configurations, a remedial strategy consists in replacing
or phase . the faulty pole with the fourth one. As a consequence, only
the three conventional inverter legs have to be equipped with
the isolating devices. The faulty pole is isolated and replaced
III. FAULT-TOLERANT DRIVE SCHEMES
by the fourth one, turning on the related CD (e.g., a TRIAC
As mentioned above, the proposed fault-tolerant drive or a pair of back-to-back thyristors). No modification of the
presents a four-pole inverter with the capability of isolating digital control code is required, apart from the deviation of the
one faulty pole, while activating (if not active yet) the fourth switching commands from the faulty pole to the fourth one.
auxiliary pole. Of course, a fault in a power device has to be For safety reasons, a null torque reference is delivered from
1136 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 47, NO. 5, OCTOBER 2000

TABLE I
CHARACTERISTICS OF DIFFERENT REMEDIAL TECHNIQUES

the rising edge of the fault detection signal, for the whole time
duration of the faulty inverter pole replacement.

B. Four-Terminal Motor Winding

This is the case of star-connected windings with accessible
neutral [Fig. 1(c)]. Two different operating conditions are pos- Fig. 2. Topology for isolating a faulty inverter pole.
sible with this motor windings configuration.
1) Fourth Pole Always Connected: Fourth pole inverters
for three-phase loads have been already presented [13], [14]
to get improved system performance. If such a configuration
is adopted for the healthy drive, the fourth leg is permanently
connected to the neutral. No CD is present in the drive. A fault
in one of the inverter legs is simply remedied by isolating the
leg itself. IDs are, therefore, incorporated also in the fourth
pole. If the fault occurs in the fourth pole, after the remedial
intervention the motor is supplied by a conventional three-leg
inverter. Conversely, if the fault occurs in one of the inverter
legs connected to the motor phases, then the motor has to be
supplied as described in the following technique in Section
III-B-2. Four digital pulsewidth modulation (PWM) outputs
(Section III-B-2) are required for these drives. A proper modi-
fication of the digital control code has to be performed during
the remedial intervention to adapt the control to the modified
inverter configuration which applies after the fault. However,
the time duration of the remedy intervention is limited by the
faulty pole isolation only. Fig. 3. Fuse current and capacitor voltage during pole isolation.
2) Fourth Pole Connected in Case Of Fault Only: In this
case, the fourth pole is connected to the motor neutral through call for a proper fuse protection of each inverter leg. A scheme
a CD activated together with the fourth pole itself at the fault that can be profitably used is reported in Fig. 2.
occurrence. No ID is incorporated in the fourth pole. Once a failure indicating a short or an open circuit of a switch
As illustrated in Section V, a slight modification of the digital is sensed, the whole inverter pole is disconnected by firing the
control code is required and the fourth pole is driven by the two SCRs, which, in turn, blow up the pole series-connected
PWM commands which were previously driving the faulty pole. fuses. Capacitors avoid a dc path through the SCRs, allowing
Table I summarizes the characteristics of the above-discussed their turn-off. The value of is chosen to have an energy
remedial techniques. The remedial solution in Section III-B-2 transfer from the main dc-link capacitor sufficient to blow the
is considered and proposed in this paper. fuses within a very short time. The scheme of Fig. 2 has been
realized and tested apart from the whole fault-tolerant drive.
IV. FAULTY POLE ISOLATION In Fig. 3, the current flowing through one of the fuses and the
As a first countermeasure after any fault detection, the pro- voltage across the series capacitor are represented.
posed remedial strategy forces the damaged inverter pole to be This test has been done for two purposes:
electrically isolated from the dc bus in order to eliminate its in- • to size the capacitor;
fluence over the drive behavior. The disconnection should be • to evaluate the current magnitude during phase disconnec-
rapid, for a prompt start of the remedial algorithm and for a tion.
consequent limited torque transient. In addition, it has to be ad- As concerns the first point, a minimum capacitor size is re-
equate to face either positive or negative faulty phase current, quired in order to overcome the fuse characteristic. In fact, if
as well as to be able to interrupt unidirectional current, as can the capacitor is too small, the voltage across its terminals rises
happen during some faults [6]. Such requirements make inad- immediately to the dc-bus value. The current flows through the
equate any electromagnetic switch or thyristor component, and fuse for a very short time, and its thermal limits could not be
BOLOGNANI et al.: FAULT-TOLERANT CONTROL OF A PMSM DRIVE 1137

in which is the rotor electrical angle. From (2), it is evident

that, also, the components in the – plane have to remain un-
changed after the fault. In this situation, the stator zero-sequence
current component is found to be

(3)

as results substituting in the – – to – – transfor-

mation, given by (4).

(4)

The new current references can be calculated by substituting

(3) in (4), obtaining

Fig. 4. Current vector loci in healthy and faulty drive.

reached. In Fig. 3, equal to 470 F is used, while is 3300

F charged to 300 V. In order to reduce the capacitor cost, a
greater capacitance with lower rated voltage may be used. For
instance, a of 8800 F is charged up to 15 V only during the
isolation process. The current peak gives the higher bound for (5)
the circuit sizing. Fig. 3 refers to an ultrafast 12.5-A fuse with 30
A s as the value. With respect to Fig. 3, a maximum current where and are the unchanged – and – current
of 750 A and a residual voltage of 180 V are assumed. For references. The reference for the neutral current becomes
this application, a 450-V capacitor has been used for the sake of
(6)
safety. After the long pulse current which blows the fuse, an arc
current remains for a short time, slightly increasing the voltage The neutral current (6) flows through a connection between the
across . motor star center point and the fourth active inverter pole, as
shown in the proposed fault-tolerant scheme of Fig. 5.
V. FAULT-TOLERANT TORQUE CONTROL The complete drive also includes outer proportional plus in-
From the point of view of the control strategy, the current ref- tegral (PI) speed and – current loops, omitted in Fig. 5 for the
erences, expressed in a synchronous – rotating frame fixed to sake of clarity. The current loops produce the voltage references
the rotor, do not have to be affected by the faulty condition, since and , that in healthy conditions are given as input to the
they represent the torque and flux demanded by the speed loop. algorithm that implements the space-vector modulation (SVM)
Therefore, it is easy to understand that, to preserve the drive per- technique [6]. Once the fault is sensed, the control algorithm
formance after the disconnection of one motor phase (remedial isolates the damaged inverter pole and starts the active control
strategy in Section III-B-2), the currents in the remaining two of the fourth inverter pole connected to the motor star center. It
healthy phases have to produce the same – current compo- is worth noting that the additional pole commands can be
nents that were flowing before the fault and, thus, also the same given by the microcontroller output first devoted to the faulty
– currents in the stationary reference frame. A zero-sequence pole , provided that an appropriate external switch logic is
component arranged; the PWM algorithm remains exactly the same. Since
the number of available PWM outputs is strictly related to the
(1) microcontroller, the mentioned possibility preserves it from the
need of expensive and often unacceptable modifications to ex-
of the stator current must arise, and the current vector trajectory
isting hardware. An important issue that is worth highlighting
in the faulty drive departs from the – plane, where it moves
refers to the rise of sinusoidal components in the – reference
during healthy operation, to the – plane, as shown by the cur-
frame during operation after fault. With the symbols defined in
rent loci in the three–dimensional (3-D) space of Fig. 4.
the Appendix, the steady-state – motor equations are
This is an obvious consequence of the null phase current. To
obtain the same performance, the current locus after a fault has
to be an ellipse, whose projection on the – plane coincides
with the healthy current circle. In fact, the produced torque and (7)
flux have to remain unchanged after the fault occurrence. The
flux- and torque-related motor currents are linked to the re-
spective – components by Neglecting the resistive terms, which are small compared to
(2) the inductive ones, and adopting first the inverse of transfor-
1138 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 47, NO. 5, OCTOBER 2000

Fig. 5. Proposed fault-tolerant drive scheme.

mation (2) and then (4), the phase-to-neutral voltages of the current loop performance. In order to compensate for this
and can be obtained undesired effect, a feedforward term is added, indicated by the
“feedforward compensation” block in Fig. 5. The feedforward
components are obtained from (9) as

(10)

where

(11)

The compensation causes the current loop to produce

constant voltage reference (Fig. 5) at steady state,
(8) avoiding the tracking of the sinusoidal components, thus real-
After the fault, the reference voltage vector produced by izing the best PI controllers working conditions. Fig. 6 reports
the inverter and applied to the healthy phases ( , and neutral) the PI outputs and the voltage references with the feedforward
differs from the voltage vector that was applied to the motor compensation at rated speed (coordinate scales are expressed
phases before the fault. Essentially, the difference is in p.u. of rated quantities).
proportional to , properly transformed in the – reference
frame. Using the inverse of transformation (2) and the first of VI. EXPERIMENTAL RESULTS
(8), the voltage vector can be written as A laboratory prototype has been realized for the full-digital
implementation of the drive presented above. The control
hardware is based on a Texas Instruments C31 floating-point
(9) digital signal processor (DSP) evaluation board, completed
with software tools for program trace and debug. Control
It is evident from (9) that, under faulty conditions, additional software has been mainly written in C language, with some
sinusoidal components appear in the – voltage references, at low-level subroutines written directly in assembly language.
twice the frequency of the stator quantities. These quantities are The drive is completed with a 1-kW anisotropic PMSM with
normally produced by the current PI controllers, to the detriment sinusoidal electromotive forces (EMFs), fed by a four-pole
BOLOGNANI et al.: FAULT-TOLERANT CONTROL OF A PMSM DRIVE 1139

Fig. 9. Locus of the – currents during the fault occurrence.

Fig. 6. PI outputs and voltage references.

Fig. 10. Transformed d–q currents during the fault occurrence.

Fig. 7. Phase currents i ;i , and i and forth pole current i during the fault
occurrence.

Fig. 8. Transformed – currents during the fault occurrence.

Fig. 11. Transformed – voltages at steady state, after the fault.
insulated gate bipolar transistor (IGBT) voltage inverter and an
interface board. Figs. 7–10 show the first experimental results, After the fault, which occurs at the time , the faulty
measured on the prototype during the fault occurrence. During pole is promptly disconnected by the circuit shown in Fig. 2.
the tests, the fault occurrence was emulated by forcing the Consequently, current suddenly drops to zero, while a neutral
active state of a digital input. The same line, in the real case, current begins to flow, with an amplitude equal to the sum of
should be driven by a proper fault detection circuit, which can and . For and A, one can observe that the
be properly arranged. Fig. 7 reports the phase currents phase currents are sinusoidal with a phase displacement of 60 ,
and and the current of the fourth inverter pole . and amplitude of 1.21 A, according to (5). Fig. 8, taken under the
1140 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 47, NO. 5, OCTOBER 2000

Transformed d–q voltages at steady state, after the fault.

Transformed d–q currents, after the fault.
Fig. 12.
Fig. 15.

This is confirmed also by Fig. 9, in which the locus of the

phase current vector is drawn in the – plane. One can recog-
nize a circle with a radius of 0.7 A, sometimes disturbed by the
electromagnetic noise that afflicts the current measurement. The
good motor current control after the intervention of the remedial
technique is pointed out also in Fig. 10, where the actual direct
and quadrature motor currents are given as results by elaborating
the actual phase currents. They remain constant and at the same
level they had before the fault as requested by the current refer-
ences, thus avoiding any long torque transient. The combination
of balanced phase EMFs with the unbalanced voltage drops due
to the phase currents cause unbalanced voltages to be applied to
the motor phases. The transformed – components, at steady
state and at 30% of the rated speed, are shown in Fig. 11.
The unbalanced motor phase voltages reflect on the trans-
formed – voltages that contain a sinusoidal component at
Fig. 13. Healthy phase currents i and i and fourth pole current i , after the
fault. twice the frequency of the phase current, as shown in Fig. 12.
This agrees with the theoretical results given by (9).
Figs. 13–15 show the steady-state time behavior of the currents
at rated torque and higher speed, under fault conditions,
confirming the results already discussed.

VII. CONCLUSIONS
This paper has proposed a hardware and software remedial
strategy for PMSM drives affected by faults in the inverter
power devices. Experimental validations were given for the
faulty phase isolation technique and the torque control of the
faulty drive. The achieved results show both the industrial
feasibility of the proposed fault-tolerant control and the prompt
recovery from a fault occurrence, which could fit many prac-
tical applications.

APPENDIX
PMSM DATA
Fig. 14. Transformed – currents, after the fault.
Nominal torque 1.25 N m;
peak torque 3.75 N m;
same test conditions of Fig. 7, proves that the transformed – nominal speed 3600 r/min;
phase currents remain unchanged in spite of the asymmetrical nominal current 2 A rms;
feeding of the motor after the failure. This guarantees a smooth peak current 6.5 A rms;
torque and a regular overall operation of the faulty drive. pole pairs 4.
BOLOGNANI et al.: FAULT-TOLERANT CONTROL OF A PMSM DRIVE 1141

REFERENCES Marco Zordan was born in Padova, Italy, in 1970.

He received the degree in electronic engineering
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[2] A. M. Oliveira, A. G. Badan Pahlares, A. H. Kumakura, G. Win-
For his electronic engineering thesis, he took part
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[6] N. Bianchi, S. Bolognani, and M. Zigliotto, “Analysis of PM syn-
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[7] N. Bianchi, S. Bolognani, M. Zigliotto, and M. Zordan, “Influence of Mauro Zigliotto (M’98) was born in Vicenza, Italy,
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in 1963. He received the degree in electronic engi-
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Silverio Bolognani (M’98) is a native of Trento

Province, Italy. He received the Laurea degree in
electrical engineering from the University of Padova,
Padova, Italy, in 1976.
In 1976, he joined the Department of Electrical
Engineering, University of Padova, where he was
involved in the analysis and design of thyristor
converters and synchronous motor drives. He also
founded the Electrical Drives Laboratory and carried
out research on brushless and induction motor drives.
He is presently engaged in research on advanced
control techniques for motor drives and motion control and on design of ac
electrical motors for variable-speed applications. He has authored more than
80 papers on electrical machines and drives. His teaching activity was first
devoted to electrical circuit and electromagnetic field theory and, later, to
electrical drives and electrical machine design. He is currently a Full Professor
of Electrical Drives.