Novel Winding Concept for MEA Actuators

03/02/2015
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Novel Winding Concept for MEA Actuators

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application/pdf Novel Winding Concept for MEA Actuators Puvan Arumugam, Tahar Hamiti, Chris Gerada
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Novel Winding Concept for MEA Actuators

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Novel Winding Concept for MEA Actuators Puvan Arumugam, Tahar Hamiti, Chris Gerada Power Electronics Machine and Control Group, Faculty of Engineering, University of Nottingham, Nottingham, UK email: puvan.arumugam@nottingham.ac.uk Abstract This paper proposes a novel winding concept for Permanent Magnet (PM) machines used for more electric aircraft actuators where reliability is a concern. The proposed winding concept is called vertical winding that has inherent fault current limiting capability than conventional round structure arrangement. A 12 slot, 10 pole surface mounted, concentrically wound PM machine particularly designed for rotorcraft swashplate actuation is used for the analysis. The impact of the winding arrangement for such machine is investigated with a focus on turn-turn Short-Circuit (SC) faults. Implications of the SC fault and methods to restrain the resulting SC current are discussed. Introduction Permanent magnet machines are gaining a considerable attention in more electric aircraft technologies such as actuators, fuel pumps and starter-generators. This is due to their higher torque and consequently higher power density. This is an important advantage in the aircraft industry because a reduction in weight increases fuel efficiency and significantly impacts emissions and cost. One of the key issues with PM machines is their reliability, especially in safety-critical applications. The main challenge with PM machines stems from the fact that the field cannot be de-excited in an event of a fault. In such applications the necessary reliability and safety levels can be achieved in two ways. One approach is to design the machine such that it can tolerate faults; the other is to design the machine in such a way that the likelihood of faults occurring is reduced up to an acceptable level. Both approaches have their respective disadvantages and this paper will illustrate the application of these methodologies to two different environments. It will be noted that for different application performance requirements either of the two approaches might be more suitable. Adopting a machine with Fault Tolerant (FT) features [1-3] that facilitate a degree of operation under a fault condition is the first option. The second is to reduce the probability of failure through operation with less stressful conditions (for example lower winding operating temperature) as well as to implement prognostics and health monitoring techniques to reduce the probability of a sudden failure to acceptable levels. Machines adopting fault tolerant features can be designed to tolerate various faults such as winding open circuit faults [4], phase to phase Short-circuit (SC) faults and phase to ground SC fault [5]. Winding turn-turn SC faults however remain problematic [2]. This paper addresses an approach to deal with internal winding failures by adopting a novel winding concept. For the investigation a 12 slot/10 pole machine with concentrated tooth windings used for a rotorcraft swashplate actuator is considered. Analytical tools are adopted for the analysis. The performance implications during both healthy and faulty conditions of utilising the fault tolerant winding will be modelled and discussed. It is shown that the proposed winding concept has inherent fault limiting capability. Fig 1: A cross sectional view of a 12-slot 10-pole FT- PM machine FT-PM Machine for rotorcraft actuators This section describes a FT-PM machine (Fig.1) designed for a rotorcraft swashplate actuator. This being a primary flight control actuator, very high reliability is required. Failing to move the swashplate to a desired position or holding it in a particular position can be catastrophic for the rotorcraft [6]. Thus, the machine was designed to satisfy the fault tolerant criteria to avoid catastrophic damages in the system at the event of single fault. Where, 1. Dual star windings supplied through different converter units were adopted to introduce redundancy under an event of fault. 2. Concentrated windings are used to provide physical and magnetic isolation between the phases. 3. To limit the SC current to a safe value as well as to minimize the resulting breaking torque at operation under the fault, the phase winding is designed to have an appropriate inductance [2]. 4. The machine is oversized to handle the increased current loading and thus provide required torque during faulted operation. The specifications of the machine that satisfy the abovementioned FT criteria are presented in Table I. The machine is capable of operation after any type of winding failure (open-circuit/SC fault) with a continuous torque of 1.25 Nm and a continuous speed of 180 rpm. In addition a peak torque of 8 Nm and a maximum speed of 5250 rpm are also needed to be delivered under fault conditions. Rated current (rms) 5.6 A Back-emf at 180rpm (rms) 32.5 V Number of phase 3 (with dual channel) Phase inductance 1.25 mh Phase resistance 306 mΩ Airgap 1.2 mm Axial length 99.5 mm Outer Diameter (OD) 58.0 mm Magnet height 3.2 mm Table I: Specification of 12-slot 10-pole PM machine Concept of vertical winding As previously mentioned turn-turn faults are the most difficult to handle in PM machines both from a timely detection point of view and from a machine design point of view if the machine is to tolerate such failures. Indeed, if an inter-turn fault is left undetected and uncorrected, the resulting current can be excessively high due to the low impedance of the SC turn. This may in turn lead to further uncontrolled failure modes eventually resulting in lack of motor output, possibly a motor jam or excessive motor temperatures or fire. To avoid this failure mode, these faults must be detected with adequate speed so that the drive system can be reconfigured before the initial fault causes collateral damage and hence results in secondary faults. To accommodate this SC fault several post-fault control methods can be adopted within the system. The most common post-fault methods are: faulted phase/machine terminals shorting [7], current injection [8], mechanical shunts and sleeves [9] and electrical shunts/ special wire [10] techniques. Amongst them phase/machine terminals shorting is popular since it can be implemented easily via the power converter terminals without requiring any additional arrangements. This method forces all the turns in the phase to share the net winding magneto motive force (mmf) under the turn-turn SC fault. As a result the current in the shorted turn reduces. However, it has been reported in [1] that this method is not effective for all single turn SC fault cases. This is due to the variation of the shorted turn inductance with respect to its position in the slot; consequently the magnitude of the SC current varies according to the fault location. Thus, a significantly high fault current compared to the rated current may still persist [2]. Fig 2: Proposed vertical winding To overcome this, a winding concept known as vertical conductors winding [2], placed along the slot height can be employed (Fig 2). This winding concept is a solution to position-dependent fault current and allows for safe operation under SC faults. The next two subsections present an analytical study of the effectiveness of such a winding when compared to the conventional winding (using round conductors) for the swash plate actuator motor. Analysis of the SC fault current In order to investigate the implication of the SC current under fault an analytical model proposed in [2] is adopted. The method evaluates the flux linking each turn within the slot and subsequently the inductances that determine the SC current. It is worth noting that the model is only valid for FT machines where mutual coupling between the phases are negligible since the phase windings are segregated from each other. Under this assumption steady-state SC fault current (Is) before and after application of post-fault remedial stagey (terminals shorting) can be estimated using equations (1) and (2), respectively [2]. The detailed modelling process can be found in [2]. ) ( ) ( 1 2      I L j R L j L j R e I s s m s s s      (1) 2 2 2 1 2 2 . ) ( ) ( . ) ( ) ( e L R L R j L L L R R R L j e L R L R j L L L R R L j I h s s h h s m h s h h h s s h h s m h s m s                  (2) Where, - E2: electromotive force (emf) in the shorted turns (Ns); - E1: emf in remaining healthy turns (Nh); - I1: phase current; - Is: induced current in the Ns turns; - Lh: self-inductances of the Nh turns; - Ls: self-inductances of the Ns turns; - Lm: mutual inductance between the Nh and the Ns turns; - Rh: resistances of the Nh turns; - Rs: resistances of the Ns turns; - Ω: angular velocity. Table II shows the obtained SC current for different fault locations (Fig.3) in the slot of the vertical conductors winding compared to the round conductors winding. From the results it is obvious that the vertical conductors winding limits the SC current inherently due to the arrangement of the conductors in the slots in a way that all the conductors share the slot-leakage flux almost equally. This is in contrary to the case for the round conductors winding where turn- turn SC faults located closer to the slot opening region exhibit potential SC currents up to six times the rated value. 1 46 23 (a) (b) Fig 3. Illustration of the fault location of (a) the round conductor and (b) the vertical conductor Round conductor Vertical Conductor Fault location SC current (A) Fault location SC current (A) 1 22.51 1 12.55 5 15.45 6 13.31 9 50.91 11 13.48 10 55.85 16 13.94 19 22.48 21 14.21 23 14.02 23 14.55 27 50.91 31 14.02 28 37.97 36 13.57 39 12.95 41 13.21 46 22.34 46 12.62 Table 2: Magnitude of turn-turn SC current vs fault location Eddy current loss computation Although the SC fault current can be limited to a safe value by adopting vertical conductors, winding eddy current losses (proximity effect and skin effect) associated with this winding will be considerably higher. On the positive side, the winding effective radial thermal conductivity is very high [1]. It was sown that even though the loses were high when compared to a round conductor winding, the vertical conductors winding experienced similar temperature hot spots as for the round conductors winding although its copper losses were twice as much as those of the round conductors. To ensure these losses are within acceptable margins, a method for their computation in a flexible way and reasonable time is proposed hereafter. An analytical model presented in [11] is adopted here. Two different formulations of the electromagnetic problem are considered since the conventional winding is less sensitive to skin effect than the vertical conductors. To calculate the losses in the round conductor winding the magnetic vector potential in the slot is estimated using the sub-domain field model itself based on the separation of variable techniques. The following Poisson’s equation is used in the computation of the vector potential in the slot domain: ( )       j e A J C t t (3) 2 2 2 2 2 1 1             A A A J r r r r (4) Where, A represents the magnetic vector potential, J is the current density and µ is the permeability of free space and r and θ are the radius and angle associated to the polar coordinates. The detailed derivation for estimating the magnetic vector potential can be found in [12]. Hence, the eddy current density (Je) in the conductors and the associated copper losses (P) in the round conductors can be estimated using (4) and (5) respectively.   2 2 1 1 2 2 0 . 2             c c rm c c r stk e r l P J J r d t d d r (5) Where, lstk is the axial length, σ is the conductivity and rc1, rc2, θc1 and θc2 are the radial and tangential coordinates delimiting the cross-sectional area of interest. In a similar way the losses are estimated in the vertical conductors, however the current penetration is accounted for by adopting a complex representation. Thus Helmholtz’s equation is used to solve the AC field distribution in the slot domain. 2 2 2 2 2 1 1 0 j j j o j A A A j A r r r r               (6) The detailed process of solving (6) can be found in [11]. It is worth highlighting here that the problem is solved in the slot domain while assuming an equivalent current sheet at the slot-opening region. This current sheet is obtained using Amperes’ theorem. Hence, the total current density in the conductors can be expressed from complex magnetic vector potential by a ctu a l j J j A     (7) Thus, the total copper loss in a conductor from the complex current density (7) can be re written as     2 2 1 1 * . 2 c c c c r stk a ctu a l a ctu a l r l P J J r d d r        (8) Where, the first term within the integral represents the complex current density in a considered conductor and the second one is its conjugate. Fig. 4 compares the losses obtained for both round and vertical conductors using (5) and (8), respectively. It is worth noting that in the computation a fill factor of 0.45 is assumed in both cases so as to have equal DC copper losses for both windings. From the results, it can be seen that the losses in the vertical conductors at rated speed (180 rpm) are almost equal to the round conductors’ ones whilst they are significantly (3 times) higher at maximum speed (5250 rpm). However, it is worth highlighting that this application requires continues operation at low speed and therefore the use of vertical conductors is plausible. If this machine had to operate continuously at peak speed the proposed winding structure is difficult to justify. A design with modified slot geometry could considerably enhance the performance and fill factor for the vertical strip winding which would allow for improved efficiency [13]. 0 50 100 150 200 250 300 350 400 450 0 50 100 150 200 250 300 350 Frequency (Hz) Winding losses (W) Round conductor Vertical conductor Fig 4: Losses comparison between the vertical and round conductor Another way of minimising the eddy current losses and thus, enable the use of this vertical winding arrangement in high speed applications is to use rectangular multi-stranded bundle conductors or Litz conductor arrangement. This would drastically reduce the copper eddy current losses. As previously mentioned, to adopt this winding structure a concentrated winding arrangement is however required. Conclusions In this paper a novel winding concept for the PM machines used for more electric aircraft actuator has been proposed. The behaviour of the proposed winding structures was investigated with respect to potential turn-turn Short-Circuit (SC) faults. Implications of the SC fault and methods to restrain the resulting current or predict such faults were discussed. In addition influences of the windings structure on losses were investigated and the ways of minimising excessive eddy current losses in such winding were discussed. References [1] P. Arumugam, T. Hamiti, C. Brunson, and C. Gerada, "Analysis of Vertical Strip Wound Fault-Tolerant Permanent Magnet Synchronous Machines," Industrial Electronics, IEEE Transactions on, vol. 61, pp. 1158- 1168, 2014. [2] P. Arumugam, T. Hamiti, and C. Gerada, "Modeling of Different Winding Configurations for Fault-Tolerant Permanent Magnet Machines to Restrain Inter-turn Short-Circuit Current," Energy Conversion, IEEE Transactions on, vol. 27, pp. 351-361, 2012. [3] B. C. Mecrow, A. G. Jack, J. A. Haylock, and J. Coles, "Fault-tolerant permanent magnet machine drives," Electric Power Applications, IEE Proceedings -, vol. 143, pp. 437-442, 1996. [4] F. Baudart, B. Dehez, E. Matagne, D. Telteu-Nedelcu, P. Alexandre, and F. Labrique, "Torque Control Strategy of Polyphase Permanent-Magnet Synchronous Machines With Minimal Controller Reconfiguration Under Open-Circuit Fault of One Phase," Industrial Electronics, IEEE Transactions on, vol. 59, pp. 2632-2644, 2012. [5] Urresty, J.; Riba, J.; Romeral, L.; Saavedra, H., "Analysis of demagnetization faults in surface- mounted permanent magnet synchronous with inter- turns and phase-to-ground short-circuits," Electrical Machines (ICEM), 2012 XXth International Conference on , vol., no., pp.2384,2389, 2-5 Sept. 2012. [6] M. Rottach, C. Gerada, T. Hamiti, and P. W. Wheeler, "Fault-tolerant electrical machine design within a Rotorcraft Actuation Drive System optimisation," in Power Electronics, Machines and Drives (PEMD 2012), 6th IET International Conference on, 2012, pp. 1-6. [7] J. A. Haylock, B. C. Mecrow, A. G. Jack, and D. J. Atkinson, "Operation of fault tolerant machines with winding failures," Energy Conversion, IEEE Transactions on, vol. 14, pp. 1490-1495, 1999. [8] A. J. Mitcham, G. Antonopoulos, and J. J. A. Cullen, "Implications of shorted turn faults in bar wound PM machines," Electric Power Applications, IEE Proceedings -, vol. 151, pp. 651-657, 2004. [9] K. D. e. al, "Method and apparatus for controlling an electric machine," 7443070, Oct 28, 2008. [10] C. Gerada, K. J. Bradley, M. Sumner, P. Wheeler, S. Pickering, J. Clare, et al., "The results do mesh," Industry Applications Magazine, IEEE, vol. 13, pp. 62- 72, 2007. [11] P. Arumugam, T. Hamiti, and C. Gerada, "Estimation of Eddy Current Loss in Semi-Closed Slot Vertical Conductor Permanent Magnet Synchronous Machines Considering Eddy Current Reaction Effect," Magnetics, IEEE Transactions on, vol. 49, pp. 5326- 5335, 2013. [12] T. Lubin, S. Mezani, and A. Rezzoug, "2-D Exact Analytical Model for Surface-Mounted Permanent- Magnet Motors With Semi-Closed Slots," Magnetics, IEEE Transactions on, vol. 47, pp. 479-492, 2011. [13] Arumugam, P.; Hamiti, T.; Gerada, C., "Fault tolerant winding design — A compromise between losses and fault tolerant capability," Electrical Machines (ICEM), 2012 XXth International Conference on , vol., no., pp.2559,2565, 2-5 Sept. 2012.