Design of an Integrated Inductor for 45kW Aerospace Starter-Generator

A close physical and functional integration of passive components is required to make an efficient and power dense overall system. Such power dense systems are a prerequisite in aerospace and marine applications. This paper presents a design of an integrated rotor-less inductor for the application of a 45kW aerospace starter-generator. The impact of high current density inductor is investigated and compared with traditional EE core inductor in terms of total weight and volume. Both inductors are sized using area product approach and its design parameters are validated using finite element analysis. Comparative analysis between the traditional EE core and the integrated inductor has shown a significant reduction in total weight and volume. The total weight ofthe integrated inductor is reduced by 55.4% whereas the total volume is reduced by 52.7% when compared to traditional air cooled EE core inductor.


INTRODUCTION
assive filter components such as filter inductors and capacitors occupy a substantial amount of space in electric motor drives which add the penalties of increased system losses and its associated weight. In a conventional approach, the filters are designed and introduced separately after the drive system components have been defined. This leads to discrete sub-systems which require a functional and structural integration of each sub-system in order to make an efficient and power dense overall system. Such power dense system is vital in aerospace and marine applications [1][2][3][4][5]. In order to overcome these drawbacks, the integration of passive components need to be introduced both from functional and physical point of view [1][2][3][4][5].
There are many possibilities in aircraft drives system to integrate the passive components. The integration of passive components in such systems offer many benefits such as power dense design, reduction in cost, mass, size and eases manufacturing process. Thus, applications where high power density is needed, integrative approach seems to be the best solution. This paper will look into a design of an integrated rotor-less inductor which was proposed in [2][3][4][5] for a 45kW aerospace starter-generator. This paper investigates the impact of high current density on the inductor design, effectiveness of integration and compares its design with traditional core aircooled inductor in terms of its weight and volume. Both integrated and traditional inductors are modelled and analysed through finite element analysis (FEA). Fig. 1 shows the traditional shapes and designs of the filter inductor. In common practice, different type of cores has been used for fabricating inductors which includes: tape wound, powder and laminated cores [6]. Air core does not use the steel material. The coils are wound on non-magnetic formers such as plastic or ceramic. The drawback of air-cored inductor is that they are less permeable than steel material. However, they are often adopted in high frequency applications where core losses need to be avoided which are dependent on frequency squared [6][7][8].

A. Traditional Filter Inductors
Tape wound cores are manufactured by winding the copper tape/wire around a mandrel. A magnetic material in the form of preslit tape as shown in Fig. 1(a) and (b). The benefit of using this type of core is that flux is parallel with the direction of P Toroidal Core Construction Wa Ac Wa rolling of the magnetic material, which allows to setup maximum flux with minimum field strength. Tape wound core can be constructed with , or toroid cores [6].
Powder cores are very unique as they have inherent airgap which is evenly distributed throughout the core material. This acts in a similar way as a core with airgap which reduces the core saturation at higher levels of current. They come in a variety of materials and are very stable with temperature. They can be toroid, or in construction [6].
The laminated cores are one of the most commonly used cores in power electronics and motor drive applications. Laminated core consists of pressed steel sheets with the coating of insulation on the surface. The insulation coating reduces the eddy currents between the sheets. The laminated cores can also be , , , and toroid in construction as shown in Fig.  1(c)-(g) [6].

B.
Integrated Filter Inductors Recently, the passive integration has been a prime focus in power electronics and motor drive application that has resulted in an overall compact and power dense system. In [2], a novel approach to integrate the inverter output filter inductor is presented for PMSM motor drives. The proposed motor uses the inherent motor magnetics as a filter inductance instead of using an external filter inductor. This leads to the elimination of power losses and its associated weight and volume.
The author of [3][4][5] introduced the novel options to integrate the passive filter inductors within the housing of the electrical machine as shown in Fig. 2. The novel options include: motorshaped rotational inductor and motor-shaped rotor-less inductor. Both inductors are integrated axially on the same shaft, inside the motor housing which results in a shared cooling system and hence, eliminates the requirement of a separate cooling system. The rotor of the rotational inductor rotates at the synchronous speed of the stator magnetic field to minimize the magnetic losses in the rotor. On the other hand, rotor-less inductor is having the similar structure without rotor which makes it suitable for DC-Link smoothing inductors, grid input filters and isolation transformers. In contrast, the rotational inductor can only be adopted for the applications of high speed inverter motor drives.
In [9][10][11][12], the perspectives on the integrated filter inductors are presented, that motivates the drive integration on a system level. The design of integrated filter inductor for power factor correction application is presented in [9]. The paper modified the stator laminations to increase the stator back iron which acts as an integrated filter inductor. This modification increases the outer diameter of the motor.
In [10][11], the entire stator back iron is utilized as a magnetic part for one or more discrete inductors by integrating toroidal winding which pushes the alternating magnetic flux in the complete loops through the back iron of the stator core. If the stator back iron of the machine is operating linearly then the presence of ring flux due to toroidal winding will not affect the main flux. But in physical prototype because of nonlinearity the back iron thickness has to be increased.
The principle of electromagnetic integration is used for integrating capacitor in the same magnetic component as that of the inductor. The integrated filter is the planar integrated L-C winding, which consists of a dielectric substrate with conductor windings directly deposited on both sides, thus resulting in a distributed inductance and capacitance structure. Moreover, different equivalent circuits can be achieved by connecting the terminals of integrated LC structure in an appropriate manner [12][13][14][15][16][17][18].
The same principle of [12][13][14][15][16][17][18] is applied for C-core EMI inductor in [19][20]. The distributed capacitance is implemented in the conventional way, whereas the inductor is implemented by utilizing the cathode and anode foils of the capacitor to form the windings. The windings are then enclosed in a can, which has a hole in the middle for the magnetic core.

III. AREA PRODUCT APPROACH
The voltage induced on the inductor terminals can be obtained by referring to Fig. 3 while assuming the terminal voltage and current through the inductor is sinusoidal. The expression for induced voltage is, , f and are waveform factor, cross-sectional core area, magnetic flux, peak flux density of the magnetic core, source frequency and number of turns per phase respectively.
The phase turns of the inductor for a given window and conductor strand area can be determined by, is the window fill factor which is defined by the ratio of copper area to the window area.
is the window area and is the conductor area.
In practice, for inductor, window fill factor typically varies from 0.4 to 0.6 to provide enough space for wire insulation, bobbins, slot liner and air space between the insulated wire turn. By substituting the Eq. 3 in Eq. 2, we have, By multiplying the current through the inductor on both sides, we have, Solving for the area product ( . ) we have, Where Jrms is the current density of the conductors which is limited by thermal losses in the windings. For three phase inductor, the area product is different from the one indicated in Eq. 6. Since the window utilization is half in the 3phase core for each coil, therefore, the area product changes to, From Eq. 6, it can be seen that factors, such as peak flux density, current density and fill factor have a strong influence on the area product. The right-hand side shows the electrical parameters whereas left-hand side of Eq. 6 indicates the physical core dimensions. The iron core area relates the flux permeance capabilities whereas the window defines the current conduction capabilities of an inductor which is limited by the conductor's thermal characteristics [4][5][6]21].
It is important to note that the area product does not depend on the fundamental supply frequency. However, the core losses are proportional to the frequency squared. Therefore, while sizing an inductor for high frequency ( ) applications, it is required to consider the core flux density lower compared to that of the low frequency ( ) applications [21][22].

IV. 45KW AEROPSACE STARTER-GENERATOR
The radial cross-section and required torque-speed characteristics of the starter-generator are depicted in Fig. 4 and Fig. 5 respectively. Its parameter details are shown in Table I.
The machine works as a motor during engine start and is needed to produce a maximum constant torque from standstill to an engine firing speed of 8000 . Between the speeds of 8000 ( ) to 20,000 ( ), the machine supplies the constant power to accelerate the engine. Once the engine reaches its steady state region, the machine acts as a generator between the speeds of 20,000 ( ) and 32,000 ( ). In generating mode, the machine generates a maximum power of 45kW up to a maximum speed of 32000 ( ). Since the phase inductance of the starter-generator is low (99 µH), an additional inductance is required to increase the motor side inductance by twice. This increase in inductance will reduce the magnitude of the inverter generated switching ripple by half. Moreover, doubling the motor inductance will also ease the control system design of the starter-generator. ) is chosen to be designed at current density of 18 A/mm 2 (which is same as the current density of the starter-generator) whereas, the core inductor is designed for natural convection cooling system. The core reason of choosing is to limit the overall volume of the end-windings which was the strict guideline from the starter-generator's point of view.
To size the core and the integrated inductor, the area product approach is used. Both inductors are sized by specifying the required synchronous inductance, peak magnetic flux density in the core, fill factor, current density of the conductor and the type of magnetic core material, the details of which are shown in Table II and Table III respectively. However, the current density through the inductor is different for both inductors.
While sizing the inductor, the following design ratios were considered for both integrated and core inductors.  Window to Core Area Ratio, / = 0.7  Window Length to Height Ratio, / = 3  Stack to Limb Length Ratio, The author of [6] has suggested to set a low window-to-core area ratio in order to keep the fringing effect at a minimum level. Also, the window length-to-height ratio is selected based on the information provided by the manufacturer in [6]. However, the stack-to-limb ratio is chosen based on the outer diameter limitation of the starter-generator for the integrated one and the same ratio was maintained for core inductor.

A. EE Core Inductor
Once the area product is estimated using Eq. 7, the core length ratio ( / ) and window aspect ratio ( / ) are then chosen to set the stack and limb dimensions and the window dimensions of the core respectively. The number of turns per phase is calculated based on the specified voltage across the inductor. The airgap is fixed to get a required synchronous inductance. So, the number of turns and the airgap length can be determined using,

B. Integrated Rotor-less Inductor
The area product of the integrated rotor-less inductor (Fig. 6) is estimated based on the current density as illustrated in Table III. At first, the windows area is fixed to that of EE core inductor along with the identical number of turns. The tooth width is then selected as the limb length ( )of core inductor. This is correct for 6 slots inductor however, for higher number of slots, the tooth width needs to be adjusted in proportion to the total number of slots. The back iron width is adjusted to keep the identical flux density in the core and the slot opening height is increased to keep the uniform flux density throughout the stator slots. Since it is a rotor-less inductor, phase inductance is only controlled by the slot opening unlike integrated rotational inductor in [3,4]. Fig. 7(a) and Fig. 7(b) shows the cross-section and flux distribution of core and integrated rotor-less inductor respectively.

A. Weight and Volume Comparison
Both core and integrated inductors are compared in terms of their total weight and volume. The total weight and volume includes: iron core and copper including the endwindings. The end-windings length is calculated using the method described in [4][5]. The comparison of the sizing parameters between the core and the integrated inductor is shown in Table IV, whereas its design parameters are illustrated in Table V. Significant reduction in total weight and volume is achieved by sizing the integrated inductor at the same current density of the starter-generator (as expected). As a result, the weight of the integrated inductor is reduced by 55.4%, while its volume is reduced by 52.7% as compared to the traditional core inductor.  The phase resistance of the EE core inductor is 8.2 times lower than that of the integrated inductor due to lower current density. The associated thermal losses are managed by the existing cooling system of the starter-generator which is forced oil cooling (engine oil). The inductance values are validated using the simulations. The synchronous inductance of 90µH is obtained through for both core and the integrated inductor. The required inductance of 99µH is achieved by adjusting their stack length of the iron core as indicated in Table V.

B. Total Loss, Volume and Weight Comparison
In this section, the total losses, volume and weight of the filter inductor is combined with that of the starter-generator for comparing traditional and integrated systems. The core and copper loss of the EE core inductor, integrated inductor and the starter-generator has been investigated and compared using models at 8000 . This is due to the fact that the copper losses are maximum at this speed which is also a dominant loss component compared to the iron loss.
From Table VI, it can be seen that both volume and weight of the combined system are significantly reduced. The combined volume is reduced by 29.1% while the combined weight is reduced by 25.5% for the integrated inductor as compared to the system with EE core inductor. However, this reduction comes at the expense of extra 0.9 kW loss which is absorbed effectively by the existing cooling system of the starter-generator. The thermal behaviour of the integrated inductor can also be predicted by looking at its losses. Since the integrated inductor is placed axially with the starter-generator, the loss of 1.57KW can easily be handled by the existing cooling system of the starter-generator.

VII. CONCLUSION
An integrated option is adopted to realize a physical integration of passive filter inductor which was required to smooth out the switching ripple component from the current waveforms of the starter-generator. This paper presented a design of an integrated rotor-less inductor for the application of a 45kW aerospace starter-generator. The effect of high current density inductor was investigated and compared with traditional core inductor in terms of their total weight and volume. Both the inductors were sized using the area product approach and its design parameters were validated through finite element analysis.
The comparative analysis between the traditional core and the integrated inductor has shown a significant reduction in total weight and volume of the system. The total weight of the integrated inductor is reduced by 55.4% whereas the total volume is reduced by 52.7% as compared to a traditional inductor. The total losses, volume and weight were combined with that of the starter-generator for comparing traditional and integrated systems. The combined volume was reduced by 29.1% while the combined weight was reduced by 25.5% when compared to the system with EE core inductor. However, this reduction comes at the expense of an extra 0.9 kW losses which is absorbed by the existing cooling system of the startergenerator.

VIII. FURTHER WORK
The thermal behaviour of the combined integrated inductor and starter-generator system has not been discussed in this paper. A full CFD of the integrated inductor at different operating points would be ideal to predict its thermal behaviour.