 Open Access
 Total Downloads : 526
 Authors : Kiran George , Shinoy K. S. , Sija Gopinathan
 Paper ID : IJERTV2IS4406
 Volume & Issue : Volume 02, Issue 04 (April 2013)
 Published (First Online): 11042013
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Failure study on Increased Number of Phases for the Optimum Design of BLDC Motor
Failure study on Increased Number of Phases for the Optimum Design of BLDC Motor
Kiran George Shinoy K. S. Sija Gopinathan
Department of Electrical Engineering Sci. /Engr. Associate Professor
M A College of Engineering, Kothamangalam VSSC, Trivandrum Department of Electrical Engineering
M A College of Engineering, Kothamangalam
Abstract Faulttolerant capability of electrical motor drives is an essential feature in applications such as automotive, aeronautic, and many others. A multiphase permanentmagnet (PM) motor exhibits a high fault tolerant capability, as it can be designed to reduce the fault occurrence as well as to operate indefinitely in the presence of fault. With multi independent phases, in the event of failure of one or more, the remaining healthy phases let the motor to operate properly. This paper deals with the analysis of the behavior of a multiphase motor in case of some common faults, as open circuit of either one or two or three phases.
Index Terms multiphase, fault tolerant, Permanentmagnet motor

INTRODUCTION
In opposition to classical threephase machines, multiphase machines are well suited for high reliability drives, since the increase of the number of phases reduces the power per phase [1,4], and allows them to operate with the loss of feeding of one phase without any change in the power electronics and control architecture. Electromechanical Actuators (EMAs) take more space in aircraft day by day. From secondary applications (i.e. stairs, door actuation, door locking device, etc.) EMAs start to be adopted in more significant applications as brakes, spoiler or flap actuation moving with many R&D programs to primary aircraft application. The More Electric aircraft (MEA) concept intends the wider adoption of electrical systems in preference to established
hydraulic and pneumatic systems. Possible important improvements in machines and drives for the MEA applications can be obtained by introducing multiphase machines fed by multi phase power electronic converters, where both the motor and drive design must satisfy severe fault tolerant requirements. In facts, EMAs for aerospace applications must assure the maximum reliability and safe operation even in the occurrence of fault conditions until maintenance is possible [5,7]. The design foresees higher phase number to increase reliability and performance. Clearly, the increase of number of phases must be balanced against the increasing complexity of a high phase number and the inevitably greater chance of a failure. About the faulty mode operation, the following conditions have been investigated:

one phase open;

two phases open;

three phases open
When one, two or three phases are open, due to a device failure or a fault in the phase windings, a new set of currents for the healthy phases should be imposed in order to satisfy the torque values. This paper deals with the analysis of the behavior of five and seven phase motor and results for nine, eleven and thirteen phases, in case of some common faults, as open circuit of either one or two or three phases (the latter being either adjacent or not).


MOTOR DESIGN EQUATION
The EMF contributing to the electromagnetic power is:
= 1 2 (1)
where L is the active motor length, Dr is the rotor outer diameter, Nph is the number of phases, Nph 1 is the number of phases conducting simultaneously, Nt the number of turns per phase, kw is the winding factor, the airgap magnetic flux density and n the rotational speed in rev/s.
The electromagnetic torque is:
= 1
(2)
where, is the squarewave current amplitude[8,9].
Fig. 1. Nphase PM motor.
Fig. 2. nphase power converter
The electromagnetic power (P0) and torque are always positive because negative EMF times with negative current feeding gives a positive product. Here we can see that as the number of phases
increases the torque and emf increases by a factor
Nph 1.
We have
= (3)
and = (4)
Therefore (5)
Hence the effect of increased number of phases can be studied from the voltage equations.

MULTIPHASE DRIVE TOPOLOGY
Consider an nphase twopole PM machine, as shown in fig. 1, where phase 1 is under fault condition. The polarities of the back EMFs, induced in the stator windings, and the direction of the excitation currents are also shown in this figure. This helps in understanding the case where Nph 1 phase are conducting simultaneously. The rotor angular speed is .
Fig. 3.five phase voltage wave form
Opencircuit in a power switch causes asymmetric feeding of motor phase by the healthy leg of HBridge. This condition
needs rapid detection and turning off of all the power switches of the faulty module. The topology we use here is Hbridge inverter topology with number of legs equal to the number of phases shown in fig.2. The constant torque is generated by
feeding the motor phases with constant current in constant backEMF wave region as expressed in (2). The fivephase voltage wave form obtained is shown in fig.3.

ANALYSIS ON FIVE AND SEVEN PHASE TOPOLOGIES
The analysis is done for the study of various fault condition of different phase during a time period. In order to study the effects of the faults we have considered the five phase voltage equation. The conduction period for a threephase machine is 120,where as it is 144 for a fivephase machine and for a seven phase machine it is 155Â°. Therefore
expected with square wave drive and perfect commutation of the phases [10]. The importance of the backEMF wave shape for minimum torque ripple with square wave drive is clearly seen. For a five phase system the output dc voltage has a fundamental frequency of 10 times the input frequency and that in a seven phase is 14 times the fundamental frequency and which was 6 times in three phase system. The average EMF for a five phase can be found out by,
= 10 2 + 4 +
10 5 5
+ 4 + + 2 (11)
to deliver the same energy in each revolution, the 5 5
threephase generator has to deliver larger magnitude pulses of energy leading to higher torque ripple at the shaft and with a lower mechanical frequency hence greater vibrations and fatigue stress. Improvements in torque ripple provided by the multiphase machine are important features of renewable energy generation systems, where vibration and fatigue offer significant challenges to the industry. By giving a proper control algorithm it is possible for a multiphase machine to provide a constant torque. The five phase voltage equation is given by,
= (6)
The amplitude of the wave obtained is a factor that contributes to the total torque produced as per (5).Hence it is necessary to study the effect of fault of each phases on the net torque produced in order to make a proper design of the machinery and to provide a proper control circuitry .
The feasible fault conditions in phase windings of a fivephase machine and seven phase machine under analysis can be categorized in the following five groups:

Singlephase

Nonadjacent doublephase

Adjacent doublephase
= 2 (7)
5

Nonadjacent triplephase

Adjacent triplephase
5
5
= 4 (8)
5
5
= + 4 (9)
5
5
= + 2 (10)
Considering the period to that is for duration
10 10
of 36Â° the active phases contributing to the
electromagnetic torque is b phase cphase dphase and e phase. The fig.4 shows the effect of commutating the back EMF waveforms of all the five phases with a square wave drive. The commutations occur every 36Â°during which a square wave drive would force a constant current
A ) SinglePhase OpenCircuit Fault
The key advantage of multiphase machines is that it can operate even when Nph 2 phases gets faulty. Under the operating condition mentioned above if for a five phase machine, the phase B gets faulty due to open circuit, there is a considerable reduction in torque produced. This reduction can be overcome by providing an appropriate control circuit to keep the torque constant. The average EMF contributing to the electromagnetic torque for a five phase is given by,
against the backEMF. With constant current drive the resulting torque wave form would follow the
= 10
10
4
5
+ +
4 +
5
shape of the backEMF waveform, and therefore this figure gives an idea of the inherent electromagnetic torque ripple that would be
+25 (12)
B) NonAdjacent DoublePhase Fault
The choice of the faulty phases is completely arbitrary;
Here as already mentioned it is seen that aphase is forced to operate so that more reliability can be obtained for the multiphase system. The average EMF contributing to the torque is given by (15).
= 10 + 4 (15)
10 5
9
8
7
6
5
4
3
Fig.4 Factor which contributes to torque in a five phase machine 2
1
0
however, to simplify the mathematical notation c
phase and ephase are chosen in this case. The analysis is done for both five phase and seven phase and the average EMF contributing
to the torque is computed. Considering the same
5 phase
7 phase
9 phase
11 phase
13 phase
period of operation of the motor, for the two non adjacent phases cphase and ephase under fault the average EMF is computed by (13).
Fig.5 average EMF contributing to the torque by different number of phases under different fault conditions
E ) Adjacent TriplePhase
= 10 4 + + 2
(13)
The three adjacent phases considered are b
10 5
5 phase, cphase, dphase. The average EMF contributing to the torque is given by (16). For
C ) Adjacent DoublePhase
The two adjacent phases we have considered for the analysis are bphase and cphase. Again we can
higher number of phases more faults can be tolerated. TABLE I gives the factor contributing to the output, for different operating conditions.
see that the fundamental component contributing to the torque has been reduced. The average EMF contributing to torque in this case is given by (14). For a five phase machine in the considered time
= 10
10
+ +
2
5
(16)
period as per the current situation it is not possible to tolerate a fault in three phases. Hence it is necessary to force the aphase operating in this interval.
= 10 + 4 + + 2


ANALYTICAL RESULTS AND FUTURE SCOPE
Numerical analysis is done for different number of phases and the values obtained are shown in a comparing manner as shown in fig.5, shows the average EMF, as a factor which contributes to the
5
5 torque in the motor. In future it is beneficial to
10
(14)
D ) NonAdjacent TriplePhase
In this case for the analysis purpose we have chosen arbitrarily are bphase, dphase, ephase.
consider this factor so as to optimize the design of the machine for better performance and reliability Hence as a result of the analysis, the torque and EMF equations can be modified replacing the factor Nph 1 by the average of the EMF which actually the component is contributing to the
output power.

CONCLUSION
This paper shows the advantage of multiphase together with the enhancement in the torque by using multiphase. The paper gives the actual factor which contributes to the output regardless to the theoretical approach. This helps in developing a more reliable design of a machine.
REFERENCES

Nicola Bianchi, Silverio Bolognani, Michele Dai PrÃ©, Strategies for the FaultTolerant Current Control of a FivePhase PermanentMagnet Motor, IEEE transactions on industry applications, vol. 43, no. 4, July/August 2007.

L. Parsa and H. A. Toliyat, Faulttolerant fivephase permanentmagnet motor drives, in Proc. IEEE IAS Annu. Meeting, Oct. 37, 2004, CD ROM.

L. Parsa and H. A. Toliyat, Fivephase permanentmagnet motor drives,IEEE Trans. Ind. Appl., vol. 41, no. 1, pp. 3037, Jan./Feb. 2005.

L. Parsa, On advantages of multiphase machines, in Proc. IEEE Ind.Electron. Soc. Annu. Conf., Nov. 2005, pp. 15741579.
 [5] M. Villani, M. Tursini, G. Fabri, L. Castellini, FaultTolerant PM Brushless DC Drive for Aerospace Application, XIX International Conference on Electrical Machines – ICEM 2010, Rome.

Marco Villani, Marco Tursini, Giuseppe Fabri, and Luca Castellini, High Reliability Permanent Magnet Brushless Motor Drive for Aircraft application, IEEE transactions on industrial electronics, vol. 59, no. 5,
May 2012

Xiaoyan Huang, Andrew Goodman, Chris Gerada, Youtong Fang, Qinfen Lu,Design of a FivePhase Brushless DC Motor for a Safety Critical Aerospace Application, IEEE transactions on industrial electronics, vol. 59, no. 9, September 2012.

Jacek F. Gieras, Rong Jie Wang and Maarten J. Kamper. Axial Flux Permanent Magnet Brushless Machines. Springer Science + Business Media, Inc. Kluwer Academic Publishers, 2005.

J. R. Hendershot Jr., T.J.E. Miller. Design of Brushless Permanent magnet motors. Magna Physics publishing and Clarendon press Oxford 1994.

Suman Dwari, Leila Parsa,FaultTolerant Control of FivePhase PermanentMagnet Motors With Trapezoidal Back EMFIEEE transactions on industrial electronics, vol. 58, no. 2, February 2011.