Investigation of Five-Phase Induction Motor Drive under Faulty Inverter Conditions M. I. Masoud1 and Sherif M. Dabour2 1
2 Sultan Qaboos University, Oman, Tanta University, Egypt
[email protected],
[email protected]
This paper concerns with the fault mode behavior of a voltage source inverter fed a five-phase induction motor drive. In this study, the machine internal faults are not considered. A number of faults have been identified. From these, the performance of the motor under a few selected fault modes have been introduced and analyzed, and then the predicted fault performance has been verified by simulation. The drive system is controlled by an open-loop scalar control method. The results can be applied for the other control methods. The results are useful for designing the protection system, calculation of post-fault operating conditions, and for fault tolerant control design. On the other hand, two fault-tolerant strategies for five-phase VSI are suggested.
Abstract—This paper introduces an investigation into a fivephase induction motor drive system in cases of fault occurrence within the inverter. This analysis deals with the different types of failure in inverter power electronic components, namely: one gating signal failure, one switch open, one-leg open and two-legs open. These faults can be applied to the system before or after steady-state operations. The study shows the effect of these faults on the motor performance in comparison with that of healthy conditions for no-load operation. The results showed that, for the faults under consideration, the motor is able to continue operation with the presence of torque pulsations as well as speed harmonic components without modifying the control scheme or the inverter topology. These results can be used to improving the performance of the system by using fault tolerant control or postfault control design. In order to overcome these effects, two faulttolerant strategies for five-phase voltage source inverter (VSI) are suggested.
II.
Generally, there are two different types of five-phase induction motors. One is called symmetrical five-phase motors; it uses distributed windings that produce a sinusoidal air-gap MMF. This type requires sinusoidal excitation voltages. The other one uses concentrated stator windings that generate a trapezoidal air-gap MMF. In this type torque production can be enhanced using stator current low order harmonic injection [9]. In this paper a symmetrical five-phase induction motor is utilized. Figure 1 shows the five-phase induction motor drive system based on five-phase VSI.
Keywords — five-phase; induction motor; fault-tolerant, voltage source inverter
I.
INTRODUCTION
The fault tolerant capability is one of the most distinct advantages of the multi-phase (more than three-phase) machines [1]. The knowledge about the fault mode behavior of voltage source inverter based multi-phase drives are very important from the viewpoint of improving the drive system design and the fault tolerant control strategies. The multi-phase drive system has to continue its operation under faulty operating conditions. Open-circuit faults in the motor windings (phase) and VSI switches (line) are common faults [2]. In the three-phase motor drive, if one phase is opened, the drive system requires a neutral line connected between the mid-point of the VSI and the motor to allow the current in the remaining phases to be controlled to produce a rotating MMF. Comprehensive research works have been reported on faulttolerant control of the three-phase motor drives under opencircuit fault conditions [3] –[7].
In the following subsections the machine and inverter models are introduced. It will be used in the simulation and modeling process. A. Machine Model The symmetrical five-phase induction motor can be modeled referred to stationary reference frame with the following voltage and flux equations [8]: The stator voltage equations are:
The five-phase induction motor is advantageous over the three-phase induction motor for fault-tolerant operation [1]. The dynamic and steady-state behavior of a five-phase induction motor under one and two-phase open-circuit are presented in [2], [8]-[10]. However, these works, consider only the faults after the motor operates in its steady-state region. Different fault tolerant topologies were reviewed and presented in [3], [6]-[7] for the three-phase drive system. These topologies can be extended to control the five-phase VSIs.
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FIVE-PHASE INDUCTION DRIVE SYSTEM
(1)
where the subscript qs, ds, xs and ys are the qd, xy-axis's of the stator respectively; Rs is the stator phase resistance and p is the d/dt operator. The rotor voltage equations referred to stator circuit are:
1
1
vdc
3 ia
a 6
0
ib
b 8
va
7
5 ic
c
2
10 vb
id
d
vc
five-phase induction machine phase-to-neutral voltages (van , vbn , … , vec) and inverter leg voltages (vao , vbo , … , veo) is given with:
9 ie
e 4
(7)
ve
vd
[
n
[
]
][
]
C. Decoupling Transformation To develop the complete model of the drive system a transformations between the abcde to the dqxyo variables are required. The transformation matrix for a five-phase system is given by [12]:
ωm Te
Fig. 1. Schematic diagram of symmetrical five-phase Induction drive
[ ]
(8)
[ ] where α = 2π/5. Application of (8) in conjugation with the inverter voltages yields the dqxyo-axes components of the motor terminal voltages, i.e.,
(2)
where the subscript qr, dr, xr and yr are the qd, xy-axis's of the rotor circuit respectively; Rr is the rotor phase resistance and ωr is the motor speed in r/s. The stator flux linkages equations are:
[ ]
(9)
[ ] [ ] Owing to the absence of the neutral line, the zero sequence voltage component of the inverter must equal to zero.
(3)
III.
CONVERTER FAULTS
The five-phase voltage source inverter shown in Fig. 1 can develop various fault types that may be classified as follows:
where the Ls and Lls are the self and leakage inductances of the stator circuit respectively; Lm is the magnetizing inductances. The flux linkages of the rotor circuit equations are:
One-leg open-circuit fault, Two-leg open-circuit fault, Gating signals failure, One switch open-circuit fault, and One switch short-circuit fault, Faults may also occur inside the induction motor. These faults are not employed in this paper. Also the possibilities of multiple faults occur at the same time are very rare, and therefore are not considered.
(4)
where the Lr and Llr are the self and leakage inductances of the rotor circuit respectively. The motor electromagnetic torque, Te can be expressed in terms of phase variables as follows:
IV. EXPERIMENTAL STUDY UNDER FAULTS CONDITIONS
(5)
To investigate system performance during faulty inverter conditions, an experimental evaluation has been developed. The experimental results are given for both normal (healthy) and an open-circuit condition while the short-circuit case destroys many parts in the motor drives, mainly the five-phase inverter switches and over current protection takes action in this case.
Mechanical equation of the motor is: (6) where P determines the number of pole pairs, J is the moment of inertia, B is the friction constant and TL is the load torque. B. Inverter Model The inputs to the five-phase induction motor are the fivephase voltage supply generated from the VSI. The power circuit topology of this inverter is shown in Fig. 1. The inverter is controlled by Space Vector PWM (SVPWM) technique to obtain a constant V/f ratio [11]. The relationship between the
A. Experimental Setup The inverter power circuit is realized by 10-power MOSFET (IRFP460A). The system demands 6 isolated supplies and 10 gate-driver circuits. The employed MOSFET has the following characteristics; voltage blocking capability is
2
500V, current capacities is 20A, integral freewheel-diode, no need for snubber circuit, lower switching losses, and a total turn on and turn off times 77 and 168 ms respectively. The gate driver circuit is based on a high speed optocoupler device (6N137) with a typical 50 and 12 ns rise and fall time respectively. The control system is based on the DS1104 controller and the motor was operated in open-loop scalar control with space vector modulated five-phase VSI. The inverter voltage transfer ratio is adjusted to 0.577 at a frequency 20 Hz. Furthermore, it has been implemented using Matlab/Simulink and then, compiled to real time system. Measurements are obtained using a Tektronix MSO2024B mixed signal oscilloscope and a current sensor LA25-P. The voltage measured by TERCO-Differential Probe MV1971 and scaled by X100. All experimental results have been obtained with the experimental rig shown in Fig. 2 using a switching frequency of 1.5 kHz and sampling time of 100μsec. The fivephase IM is originally a 36 slots, 2-pole three-phase IM, whose stator has been rewound to provide a five-phase IM. B. Normal Operation Fig. 3 shows the motor currents experimental waveforms at steady-state conditions for normal/healthy operations. These results will be taken as base results to be compared with the faulty results. C. Analysis of Fault Modes In this part, the following faults types will be considered for our paper.
Five-Phase VSI
Five-phase IM
Switch
Current transducers
Currents
Power Supply
DS1104
Speed
PC
Fig. 2. Test rig scheme for the case of a constant dc-voltage supply
ia ib ic id
1) One-leg open-circuit fault If the leg-a of the inverter is opened (S1 and S6 are opened) after the starting process, the inverter terminals still connected to the remaining four phases (b, c, d, and e). The motor speed and fault-tolerant currents under these conditions are shown in Fig. 4. The phase currents are initially identical to the healthy condition currents, but when the fault condition is introduced, the stator current in the faulty phase (phase-a) is zero. In addition, the currents in remaining phases are increased by about 48.5% from its steady-state value at healthy conditions. Moreover, the motor speed is reduced by a slight dip of 0.5% from its final value. When free-wheeling diodes in the faulty leg-a switches are still operating the current in phase a equals the free-wheeling diode conduction current as shown in Fig.5. If the motor is accelerated with four-phase excitation (leg-a of the inverter is opened before the starting process), the voltage is applied to the remaining four phases (b, c, d, and e). The motor starts with a long time (about 30% from starting process time of healthy operation) to get the steady-state speed. The fault-tolerant currents under these conditions are shown in Fig. 6. The stator current in the faulty phase (phase-a) is zero. While Fig. 7 shows the stator voltage waveform of the faulty phase, this voltage is due to the induced e.m.f in the phase-a. This voltage is proportional to the motor speed.
Fig. 3. Experimental motor currents under normal operation
Fault
ia ib ic
Fig. 4. Experimental results under leg-a is opened after the starting process
ia ib ic
2) Fault on Double-leg For a five-phase motor, there are two different cases of double-phase fault. In the first case, the double-phase opencircuited are the two adjacent phases (for example phases a and b) and in the second case, the fault may occur in two nonadjacent phases (for example phases a and c).
id
Fig. 5. Motor current under faulty phase-a considering free-wheeling diodes
3
ia
Fault
ib ic id
Fig. 6. Experimental motor currents under leg-a is opened at starting
Fig. 9. Experimental motor currents under leg-a and c are opened after steady state
If the motor is accelerated with three-phase excitation (for example leg-a and leg-c of the inverter are opened before the starting process), the voltage is applied to the remaining three phases (b, d, and e). The motor starts with a long time (about three-times from starting process time of healthy operation) to get the steady-state speed. The fault-tolerant currents under these conditions are shown in Fig. 10. The stator current in the faulty phases (phase-a and phase-c) are zero.
van
3) Transistor Gating Signal failure The inverter switches are normally controlled by isolated gate driver circuits. If the gate signal is lost, the corresponding transistor is opened. Suppose, transistor Q1 is now inefficient, the phase-a of the motor is connected to the positive rail of the dc-supply through the anti-parallel diode D1. The voltage of phase-a is then determined by the polarity of current and the switching pattern of transistor Q4. Figure 11 shows the simulated motor currents under these conditions
Fig. 7. Experimental faulty phase voltage under leg-a is opened at starting
If leg-a and leg-b are opened (S1, S6, S3 and S8 are opened) after the starting process, the voltage is applied to the remaining three phases (c, d, and e). The phase currents are initially identical to the healthy condition currents, but when the fault condition is introduced, the stator current in the faulty phases (phase-a and phase-b) are zero as shown in Fig. 8. In addition, the currents in remaining phases are increased by about 69% from its steady-state value at healthy conditions. Moreover, the motor speed is reduced by a slight dip of 0.58% from its final value. If leg-a and leg-c are opened (S1, S6, S5 and S10 are opened) after the starting process, the voltage is applied to the remaining three phases (b, d, and e). The phase currents are initially identical to the healthy condition currents, but when the fault condition is introduced, the stator current in the faulty phases (phase-a and phase-c) are zero as shown in Fig. 9. In addition, the currents in remaining phases are increased by about 36% from its steady-state value at healthy conditions. Moreover, the motor speed is reduced by a slight dip of 0.5% from its final value.
ia ib ic id
Fig. 10. Experimental motor currents under leg-a and c are opened at starting 0.4
ia
0.3
Fault
ib
ic
id
ie
0.2
ia (A)
0.1
ib
0 -0.1
ic
-0.2 -0.3
Fig. 8. Experimental motor currents under leg-a and b are opened after steady state
0
0.01
0.02 Time (Sec.)
0.03
0.04
Fig. 11. Simulated motor currents under gate signal failure of transistor Q1.
4
From these results, it can be concluded that the five-phase motor is able to operate with a four or three-phase excitation without any modifications in the modulation technique with the presence of torque pulsations as well as speed harmonic components. In order to overcome these effects, two different fault tolerant strategies are suggested in the following section. V.
Redundant leg
vdc
FAULT-TOLERANT STRATEGIES FOR FIVE-PHASE VOLTAGE SOURCE INVERTERS
0
a
c
b
r
e
Isolating Thyristors
va
The first step of most fault-tolerant solutions is the physical fault isolation, especially in the case of short-circuit fault. The fault–isolation unit (usually isolating thyristors) force damaged converter switches or legs to be electrically isolated from the system to eliminate its influence over the system behavior [13]. Then, the post-fault reconfiguration can be activated. Similar principles of three-phase VSI fault-tolerant strategies [6] can be applied in the five-phase case. This section suggests two faulttolerant control strategies for five-phase VSIs. These strategies are:
d
vb vc vd ve Redundant leg inserting thyristors
ωm Te
n
(a)
a) Redundant Leg topology b) DC-Bus Mid-point Connection topologies
C
vdc
The circuit topologies of Fig. 12 give the proposed faulttolerant five-phase voltage source inverter. A conventional five-phase inverter consists of only five legs. In the first strategy, the fault-tolerant inverter has one leg as redundant. The redundant leg has not been used when the conventional five legs are working without any fault. The isolated back-toback thyristors are connected between the inverter output terminals and the corresponding motor phases [14]. These thyristors are used as isolating switches of faulted leg. Additional five thyristors (redundant leg, inserting thyristors) are connected between the mid-point of redundant leg and the motor phases as shown in Fig. 12-a. These thyristors are used for inserting the redundant leg in the place of faulted phase. This strategy can be used for tolerance of all the aforementioned faults except the phase-leg short-circuited.
a
c
b
d
e
C
ve vd
Isolating Thyristors
vc vb va DC-Bus Midpoint inserting thyristors
Similar to three-phase inverters, if one phase fails, the remaining two phases can maintain continuous operations. Two typical fault-tolerant topologies with additional switches employed in motor applications are presented in [6]. These topologies have been proposed for the five-phase inverters as a strategies b and c as shown in Fig. 12 (b) and (c). The first fault-tolerant topology shown in Fig. 12 (b) forces the faulty phase to connect to the mid-point of the dc-link via the additional dc-bus mid-point inserting thyristors. After faults, the reconfigured system is similar to the structure where only four switches are used to drive a three-phase machine [15]. The maximum balanced line-to-line output voltage in post-fault operations is reduced to half of its nominal value; this is the main drawbacks of this strategy. Moreover, this approach is only applied in situations where the mid-point of dc-link capacitors can be accessed.
ωm Te
n
(b)
C
vdc
a
c
b
d
e
C
Isolating Thyristors
vb
vc
vd
va
ve
DC-Bus Midpoint inserting thyristor
n
(c)
The last method connects the neutral point of the five-phase motor to the dc-bus mid-point via a dc-bus inserting thyristor as shown in Fig. 12(c). Note that only one thyristor is added for the fault tolerance.
ωm Te
Fig. 12. Fault tolerent strategies for five-phase voltage source inverter (a) switch redundant topology, (b) and (c) dc-bus mid-point connections
5
VI.
[11] Iqbal, A; Moinuddin, S., "Comprehensive Relationship Between Carrier-Based PWM and Space Vector PWM in a Five-Phase VSI," Power Electronics, IEEE Transactions on , vol.24, no.10, pp.2379,2390, Oct. 2009 [12] Parsa, L., "On advantages of multi-phase machines," Industrial Electronics Society, 2005. IECON 2005. 31st Annual Conference of IEEE , vol., no., pp.6 pp.,, 6-10 Nov. 2005 [13] Mirafzal, B., "Survey of Fault-Tolerance Techniques for Three-Phase Voltage Source Inverters," Industrial Electronics, IEEE Transactions on , vol.61, no.10, pp.5192,5202, Oct. 2014 [14] Errabelli, Rammohan Rao, and Peter Mutschler. "Fault-Tolerant Voltage Source Inverter for Permanent Magnet Drives", IEEE Transactions on Power Electronics, 2012. [15] Zhang, W.; Xu, D.; Enjeti, P.N.; Li, H.; Hawke, J.T.; Krishnamoorthy, H.S., "Survey on Fault-Tolerant Techniques for Power Electronic Converters," Power Electronics, IEEE Transactions on , vol.29, no.12, pp.6319,6331, Dec. 2014
CONCLUSION
This paper introduces the effect of different types of fault in a five-phase VSI on an induction motor drive that uses an open-loop scalar control (V/f=constant) method. The internal faults in the machine are excluded from this paper. Some important faults are indicated in the beginning, then it is followed by a simulation study to the other faults. The aim of fault performance of any drive system is very important to determine the semiconductor devise stress, to optimize the protection system design, and to predict the post-fault drive operating capability. The results showed that, for the faults under consideration, the motor is able to continue operation with the presence of torque pulsations as well as speed harmonic components without modifying the control scheme or the inverter topology. These simulation and experimental results can be used to improve the system performance by using fault tolerant control or post-fault control design. Moreover, two strategies of fault-tolerant control for five-phase VSIs are suggested to overcome these effects. VII. ACKNOWLEDGEMENT This paper was made possible by NPRP grant # NPRP 4-2502-080 from the Qatar National Research Fund (a member of Qatar foundation) – The Statements made herein are solely the responsibility of the authors. VIII. REFERENCES [1]
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