Comparison between Induction and PM Machine for High Speed Starter-Generator Applications D. Papaoikonomou*, M. van der Geest*, H. Polinder* *Delft University of Technology,Delft, The Netherlands,
[email protected] for the realization of the MEA/AEA [7]. Not only would the implementation of such a technology result in higher efficiency, improved fuel usage and reduced environmental impact but also combining the starter and generator into one device could potentially lead to a lighter system reducing ultimately the cost of ownership [8].
Keywords: Starter/generator, induction machine, machine comparison, high speed aerospace applications, optimization
Abstract An integrated starter/generator is a single device that provides both the electrical starting and alternating operations essential for combustion engines. Starter/generator applications’ requirements include a low speed-high torque motoring mode followed by a wide high speed-constant power region. These conflicting requirements in combination with strict rules and regulations in terms of safety, size, weight, efficiency and power factor constitute the main challenges this application entails. This paper describes a procedure followed to design a high speed starter/generator based on an induction machine drive. Design optimization is performed as a way to emphasize the trade-offs to be made between the performance at different modes of operation and the weight of the machine. Eventually, a comparison with an already available permanent magnet machine designed specifically for the same application is attempted.
The objective of this paper is to compare a permanent magnet (PM) and an induction machine (IM) functioning as a S/G for high speed aerospace applications. For that purpose, the design of the IM according to analytical methods is made and validated through two dimensional transient finite element (FE) simulations. The dimensions of the candidate IM will be optimized with particle swarm optimization (PSO) and the performance of the machines will be analyzed with analytical calculations due to the excessive amount of time required by FE simulations especially when skewing is considered. Optimization results are presented in the form of optimal pareto fronts which are subsequently used to compare the induction S/G with an already available PM machine designed and optimized for the same application [8]. This paper is organized as follows. The application requirements are introduced in section 2. Section 3 presents a qualitative comparison between the two machine types functioning as a S/G based on published work up to present. The following section 4 shortly describes the PM S/G design whereas section 5 explains in detail the methodology developed for the optimization of the IM as well as the evaluation of its steady state performance. Eventually an extensive section discussing the optimization results used for the comparison between the two machine types follows, before conclusions are reached.
1 Introduction Conventional aircraft systems are decomposed into two main categories: the propulsive and the non-propulsive. The latter are typically driven by a combination of different power sources that employ electrical, pneumatic, hydraulic and mechanical power [1]. The necessary non propulsive power is harnessed from the main engine by different disciplines. For instance, pneumatic power commonly extracted by bleeding compressed air is utilized by turbine motors that provide the engine starting functions whereas an electrical generator is additionally connected in order to supply power to a number of secondary subsystems. A considerable amount of debate regarding these non-propulsive conventional systems has been expressed in literature owing to their complexity and their interactions which may result in a reduced overall efficiency [2,3]. Furthermore, climate change policies stress the need for a radically modified aircraft, since these goals cannot be sufficiently accomplished by the optimization of the current aircraft subsystems and components [4]. The aforementioned facts constitute the main incentives that gave birth to the concept of the More/All Electric Aircraft (MEA/AEA) which is envisioned to be the future trendsetter in the aerospace industry by a high number of researchers [1,5-7]. Previous studies to date, strongly point out that the integrated starter/generator (S/G) is a key subsystem required
2 Target specifications During the engine starting cycle the starter accelerates the engine until fuel ignition takes place and the combustion temperature is adequate for the engine to rotate without support. At this point, the starter is disconnected and fuel flow is adjusted according to idle. During the generator mode mechanical energy is extracted in order to supply power for the aircraft electrical system. Table 1 demonstrates the application specifications. The limited available room space and the necessity for a light design, impose a number of technical issues that need to be met in order to satisfy the requirement of a starting torque equal to five times the generating torque.
1
Parameter Generator power Generator speed Starting torque (0-40% speed) Starting duration Dimensions (dia×length) Cooling
whereas the ability to sustain high rotor temperatures and the straightforward way to apply field weakening are additional desirable features. Furthermore, despite the mutual inductance of the distributed windings, the presence of the inherently passive rotor makes this kind of machine fail-safe and thus suitable to be used as a S/G. On the other hand, although the rotor currents reduce the efficiency to values below the PM machine, efficiency in the generator mode can be relatively high attributed to the low iron losses in the field weakening region. The main problem with this type of machine is the lagging power factor resulting in over sizing of the power converter [9,18].
Requirement 6 kW 6,000-12,000 r/min 30 Nm 15-25 s max 134×110 mm Forced air
Table 1: S/G application requirements
3 Qualitative comparison Various types of electric machines have been considered suitable to meet such specifications [9,10]. Wound-field synchronous machines have been widely used for electric power generation in aerospace applications [1,8,9]. A great deal of attention has also been placed on the switched reluctance generator which fault tolerant operation on account of its salient features and the magnetic and electric independence of the machine phases, is guaranteed [8-12]. Furthermore, PM and IM are two machine types candidate to be used in aerospace S/G systems. Main criteria that would define the performance of each type as a S/G include power density, overload capability and fail-safety.
4 Permanent Magnet S/G Work published in [8] included the optimization of a few types of radial flux inner rotor PM machines for the same application. Despite the enhanced overloading capability of the interior PM machine (IPM), surface PM (SPM) machines were preferred since IPMs are penalized at high speeds due to the need for thicker iron bridges [19]. Machines with different slot/pole combinations were optimized, in order to determine which specific combination depicts the best performance. Optimization was performed utilizing a PSO method and the performance characteristics of each design were evaluated with FEA which allowed saturation during starting and magnet eddy current losses to be considered in the analysis. One of the objectives was the minimization of 3 phase fault losses, ensuring that the optimization progresses towards designs with an adequate degree of fail-safety. Ultimately, a SPM machine with 12/8 slots/poles was selected as a design that can meet both the performance and safety requirements. Detailed information on how the design and optimization were accomplished is included in [8] and thus a more extensive analysis is out of the scope of this paper.
3.1 Permanent Magnet Machine This type is characterized by high power density on account of the loss-free excitation offered by the utilization of magnets. Interior magnets are usually considered due to the wide range of the constant power zone. Examples of PM machines designed to function as an S/G are available in literature [8,10]. Despite the large value of the d axis current (Id) required at high speeds, efficiencies reached are the highest. On the other hand, poor safety aspects are derived from the lack of ability to disable the field. Fail-safety in a PM machine can be achieved by special design aiming in utilizing a one per-unit inductance and no mutual coupling between the phases [8,13]. In this way, the fault current will be limited to nominal values in case of single phase short circuit [8]. This strategy however results in an increase of the inverter VA-rating and required DC-bus voltage [8]. Another considerable problem with this type of machine is the possibility of a loss of the power electronics control in case for example of an inverter undesirable turn-off during high speed operation [9]. Such a scenario would require the presence of additional protection systems for the isolation of the machine in order to avoid further damages [9].
5 Induction S/G The sizing of a variable speed (frequency) IM is done on a selected base speed of 4,800 r/min up to which the machine operates at the constant flux region whereas field weakening is adopted at higher speeds. The torque capability in the full flux operation should be high enough to insure that the required power of 6 kW can be delivered at the maximum speed. 5.1 Optimization The IM design is considered as a multivariable constrained optimization problem. The machine dimensions to be optimized are depicted in Figure 1. The range of the variables was wide enough to consider any design that would lead to a physically consistent geometry. Two additional optimization variables used, are the stack length and the pull-out torque at the base frequency. The latter is used in order to define with constant number of turns, the voltage at the base point above which field weakening takes place. Table 2 presents the parameters held constant during the optimization which is described in a subsequent section.
3.2 Induction Machine Rapid progress in power electronics [14] has made the cagetype IM a considerable candidate for high speed S/G systems. Research that studied the design of an IM S/G resulted in concepts utilizing either an integrated [10,15,16] or a beltdriven structure [17]. Motivation for this work was provided by well known characteristics of the IM, particularly beneficial in the application of the S/G. Robustness and simplicity make IMs suitable for transient overloads [8]
2
Attribute Base speed Number of poles Stator outer diameter Stator inner diameter Number of stator slots Stator winding Stator slots/pole/phase Stator slot fill factor Turns per phase Stator tooth width Number of rotor slots Rotor tooth width Rotor skewing Rotor cage Steel type
5.2 Evaluation of IM performance
Value 4,800 r/min 4 134 mm 83 mm 36 Two layer chorded 7/9 3 40% 36 3.5 mm 28 4 mm One stator slot pitch Al Cobalt-iron alloy
A methodology based on the IEEE-recommended equivalent circuit in Figure 2 is developed in order to evaluate the IM steady state performance. The equivalent circuit parameters are initially approximated via available formulas [23-25]. For the calculation of the phase leakage inductances contributions from the slot, the end connections (stator), the end ring (rotor), the harmonic (airgap) and zig zag leakage are considered. Additionally, skewing as a source of flux of leakage character is taken into account. Skewing leakage flux is shifted from the magnetization flux with an angle which is a function of axial position and load (slip). Hence, this flux is not compensated and since it remains inactive for zero rotor current, this leakage inductance contribution is exclusively added to the rotor [23,25].
Table 2: Design overview. The approach adopted is based on a PSO heuristic algorithm widely used in literature for multi objective electrical machines optimization problems [8,20-22]. In a PSO system, a number of particles that constitute a swarm, fly around in a multidimensional search space modifying their position in every step according to their own and their neighboring particles’ experience. 20 particles and 175 steps were selected based on literature and past experiences. Three optimization objectives are used:
Figure 2: IM single phase equivalent circuit In practice, the resistances and inductances of the equivalent circuit vary with load. As the slip increases, the magnetization current Im decreases whereas there is a tendency for the leakage magnetic field path to saturate. When open slots are considered this effect is negligible, but, in case of semiopen or semiclosed slots, the tooth tops tend to become saturated [23]. A simple way to account for this is by increasing the slot opening by the tooth top length divided by the relative iron permeability.
Maximize generator mode efficiency Minimize weight Minimize copper losses during starting
While the first two objectives are obvious, the low frequency during starting makes copper losses dominant and therefore minimizing those is a key objective regarding the limitation of temperature rise and power required to be delivered by the inverter and source during motoring.
Fundamental distribution of airgap flux density Saturated distribution of airgap flux density Fundamental mmf wave 0
30
60
90
120
150
180
Figure 3: Fundamental airgap flux density in saturation conditions (reconstructed from [26]). Furthermore, the airgap flux waveform is distorted on account of magnetic saturation, as shown in Figure 3. A mathematical way described in [26] was applied in order to determine the fundamental airgap flux density as a function of the fundamental MMF (Fmax,fund). The magnetizing current can then be estimated by equation (1) [26]. In (1), p1 is the pole
Figure 1: Optimization parameters for the IM S/G.
3
pairs, Kw1 the fundamental stator winding factor and Wph the number of stator conductors in series per phase.
(1)
Skin effect in the rotor bars is additionally taken into account using the elementary layer method (ELM) experimentally validated for arbitrary defined slot shapes in [27]. Lastly, stray load losses caused by space harmonics are considered in the analysis since they are expected to play a more decisive role for higher speeds and frequencies. Main contributions considered are the surface core losses and tooth flux pulsation losses estimated according to approximate equations found in [23]. Both the skin and saturation effects are treated iteratively as presented in Figure 4 in order to calculate by applying corrections on the equivalent circuit adjustable parameters, the machine steady state performance.
15 10 5
0.1
0.2
0.3
0.4
0.5 Slip [-]
0.6
0.7
0.8
0.9
1
6 Optimization Results The adoption of a multiobjective optimization approach allows the analysis of the trade-offs between the different targets to be visualized as pareto optimal fronts. Figures 6 and 7 present the optimal fronts obtained between the three objectives, namely the copper losses during starting, the generating efficiency at 12,000 r/min and the weight of the machine. The red lines indicate the maximum performance between two targets without consideration of the third objective. 9
Calculation of the equivalent cicuit parameters as influenced by skin and on load saturation effects Copper Losses [kW]
8
Calculation of new values of currents and PF
Convergence Question
20
Figure 5: Torque speed characteristic at a stator frequency of 160 Hz (synchronous speed of 4,800 r/min) - comparison between results obtained analytically and with FE simulations.
Solve the equivalent circuit Cacluation of currents and PF
YES
25
0
Calculation of the initial (constant) equivalent circuit parameters
End
30
-5 0
IM Geometry
Voltage, Frequency Load Torque (Starting mode) Output Power (Generating mode)
2D FEM (skewing neglected) Analytical (skewing neglected) Analytical (with skewing)
35 Electromagnetic Torque [Nm]
2 p1 I m ,rms 3 Wph 2 K w1 Fmax, fund
40
7 6 5 4 3 2
NO
1 0
Figure 4: Iterative algorithm developed for the calculation of the S/G performance.
4
4.5
5
5.5
6 6.5 Weight [kg]
7
7.5
8
8.5
Figure 6: Pareto optimal front between copper losses during starting and weight of the machine.
Results obtained via the described analytical method agree to a satisfying degree with 2D FE simulations in Figure 5. The torque capability is according to FE simulations significantly higher than the one calculated when skewing is taken into account. Although this is a considerable difference, rotor skewing cannot be omitted since it constitutes the most effective way of reducing torque ripple and space harmonics. However, this relatively large difference as well as the high amount of time required for the simulations makes the 2D FE model not suitable for precise performance evaluation. Therefore, the optimization is performed using the developed analytical model.
96 95
Efficiency [%]
94 93 92 91 90 89 88 87
4
4.5
5
5.5
6 6.5 Weight [kg]
7
7.5
8
8.5
Figure 7: Pareto optimal front between generating efficiency and weight of the machine.
4
The optimization results grouped according to copper losses during starting are presented in Figure 8. Designs that combine both good starting and generating functions are penalized for higher weights. Such designs are characterized by increased pull-out torque in the constant flux region which in turn reduces the constant power slip at maximum speed and therefore increases efficiency. At the same time, the high torque capability does not result in saturation during motoring thanks to the higher weight and thus copper losses remain low. For lighter machines the performances between the two modes compete more notably. Light designs with high efficiency depict high copper losses since a good starter is a less saturated one and thus has a lower torque capability. The decreased torque at the pull-out point affects negatively the alternating efficiency as the constant power slip is approaching the pull-out slip at maximum speed.
characterized by even higher rating. Designing a fail-safe PM machine requires high per unit inductance which constitutes a contradicting practice when compared with the need to supply the machine with a high amount of current during starting. Moreover, the Id=0 control strategy selected for the PM S/G increases efficiency during generating at the cost however of a higher required inverter voltage rating and a lower than unity power factor. The simple way to apply field weakening constitutes the main advantage of IMs over PM machines for applications at which wide constant power regions exist. Furthermore, although IMs heavier than 6.5 kg perform well in both modes of operation these machines actually lead to a waste of material since they do not fully exploit the core high saturation level. Longer stacks lead to lower level of saturation during starting as well as lower required torque producing current and thus reduced copper losses. On the other hand, heavy designs’ efficiencies do not surpass the one obtained from the light PM S/G. This behavior can be attributed not only to the presence of the additional rotor copper losses but also to core and stray load losses being proportional to the weight provided that frequency and flux density level remain unchanged. Therefore, the improvement regarding the maximum efficiency obtained is not as notable as expected for heavier machines.
96 95
Efficiency [%]
94
1.00kW
