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There are a number of advantages in using a direct drive system. The elimination of the intermediate gearbox gives the following advantages :
1) Reduced Component Count
2) Reduced jamming probability thus increasing reliability 3) Increased System Efficiency 4) Reduced Inertia
This may potentially lead as well to a reduced actuator weight and volume. Increased system efficiency due to the elimination of the intermediate gearbox will reflect in a lower power rating of the machine. As regards to the inertia, this may be of a major advantage depending on the actuator acceleration requirements and on the duty cycle characteristics. In the developed system there was a fivefold improvement in the inertia of the motor and gear train in going to a direct drive system rather than using a gearbox.
III. MID SPOILER ACTUATION SYSTEM
The actuator considered in this study is that for a mid spoiler flight surface for a large civil aircraft. An EMA using a highspeed motor coupled to the roller screw through a 6:1 speed reduction gearbox has already been developed. The design of a machine able of the same performance at the roller screw end without the gearbox is considered and compared with the existing actuator.
The Spoiler surface serves a dual purpose in flight. It can, as its name suggests, spoil the flow of air over the surface of the wing thereby reducing lift and it can also be used in conjunction with the ailerons to assist the roll function. In the spoiler function large movements of the actuator are seen in order to fully extend the surface and ‘spoil’ the lift. In its use for roll assist, smaller movements are seen with a high dynamic response to match the aileron movements.
This dual role of the spoiler surface makes the actuator performance requirements quite demanding. The duty cycle of the spoiler varies during the flight with higher duty during the landing periods and a lower one during the cruise. Full movement of the spoiler is only required at landing for descent and post touch down. During cruise there are only occasional small movements of the surface for roll assistance.
The actuator itself is normally acting to hold the surface down as the aerodynamic forces over the wing attempt to lift the surface. By incorporating the roll assist requirement to the spoiler surface, the duty on the actuator becomes more onerous. The dynamic requirements for the actuator become highly demanding such that a hold down brake is unfeasible when the actuator is stowed, then a steady state background load has also to be dealt with. These two elements result in the spoiler case being very similar as to that for the primaries where efficiency becomes key to the actuator performance.
Other demanding requirements on the actuator occur in emergency scenarios.
One is when there is the case of an aborted take off and the other is an emergency descent in the event of cabin depressurisation. This last case is the most demanding requirement for a spoiler as it requires full deployment for a period of minutes.
IV. MACHINE DESIGN
The machine has to meet all the transient and steady state specifications keeping the machine size to a minimum. The size of the machine is mainly limited by physical dimension constraints, the loading characteristics and the temperature range the actuator should be able to operate in. The internal rotor diameter is constrained by the roller screw nut whilst the outside stator diameter is constrained by the maximum package size. A surface mount magnet machine was opted for due to high torque requirement throughout the operating speed range and due to the relatively low centrifugal forces experienced by the magnets. A) Number of Poles
As the number of poles goes up the copper packing factor is reduced if the number of slots per pole per phase is left unchanged. In order to reduce the number of slots and maximise the slot fill factor, as well as to achieve the necessary isolation between the phase windings, modular wound machines were considered. These are machines which have coils wound around consecutive or alternate teeth and have a number of slots per pole per phase which is less than 1. The two machines which are considered for this work are a 20 and 22 pole machines, both having a 24 slot stator. Fig.1 shows a cross-section of the 20 pole machine. The number of poles was chosen as a compromise between, the restriction of the electrical supply frequency, the minimum rotor internal diameter so as to fit the nut of the roller screw in the centre and the outside stator diameter limited by the physical space the actuator has to fit in. The number of slots was chosen so as to have a high fundamental winding factor, isolation between the phase windings and a high slot fill factor. B) Winding Configuration
For a traditional, distributive wound machine, the minimum number of slots for a 3phase 20 pole machine would be 60. This results in a machine with large end winding length, low copper packing factor and with windings highly coupled both physically and magnetically. Going for a concentrated wound motor, having the number of slots close to the number of poles is inductive to a high fundamental coil pitch factor as well as a high frequency, low magnitude cogging torque. The main disadvantage of these types of windings is the high harmonic content of the stator MMF.
Fig. 1. 24 slot 20 pole motor geometry
a)24 slot 20 pole all teeth wound b)24 slot 22 pole all teeth wound
c)24 slot 20 pole alternate teeth wound d)24 slot 22 pole alternate teeth wound
Fig. 2. Coil EMF phasor diagram
Table 1. Winding factors
All teeth wound Poles Stator Slots Dist. Fac. Coil P Fac. Wind. Fac. 20 24 0.966 0.966 0.933156 22 24 0.958 0.991 0.949378 Alternate wound teeth Poles Stator Slots Dist. Fac. Coil P Fac. Wind. Fac. 20 24 1 0.966 0.966 22 24 0.966 0.991 0.957306 Figures 2a and 2b represent the fundamental emf phasors for each coil round every single tooth for the 20 and 22 pole motors respectively. In both cases there is
symmetry between the north and south poles of the machine hence not having any even harmonics in the MMF distribution or in the phase EMF. Defining the fundamental field as being a 2 pole field with a wavelength of ?D, then, for the 3 phase 24 slot 20 pole machine stator MMF harmonics occur at 2, 10, 14, 22,26,…,
whilst for the 24 slot 22 pole machine MMF harmonics occur at 1,5,7,11,13,… These harmonics are likely to cause a number of undesirable effects, such as saturation, acoustic noise, and excessive eddy current losses in the magnets, rotor iron and any conductive retaining sleeve. Having a large airgap, surface mount magnet motor, the extent of all these parasitic effects is relatively small.
In order to have a physical separation between phases an alternative winding arrangement has to be adopted. This is basically achieved by halving the number of coils, winding them on alternate teeth and doubling the number of turns. The resulting coil EMF phasor diagram for such windings is shown in Fig. 2.c for the 20 pole motor and in Fig. 2.d for the 22 pole machine. These machines will have the same stator MMF harmonic content as for the case when all teeth are wound as listed above. As can be observed from comparing the phasor diagrams, the fundamental distribution factor for both machines increases as detailed in Table.1. This will also be the case for the parasitic fields. C) Design Optimization
The main design goal was to achieve the set performance requirements with lightest possible machine. The machine split ratio is a very important design parameter in order to change the ratio of the electrical to the magnetic loading as well as having a sufficiently large inductance to limit short circuit currents. The motor will be most of the time counter-acting a background load in order to keep the spoilers stowed in. On the other hand, the most demanding operating condition on the actuator during normal operating conditions is that of holding torques at zero speed, during landing. During abnormal operation, an emergency descent would require a high torque at zero speed to hold the actuation surface fully deployed as described in section III. This pushes the design to minimize copper loss while heavily loading the magnetic material, within the limits of still achieving the required inductance and torque linearity. The figure below shows the variation of the copper loss against stator internal diameter for fixed stator outer dimensions, a fixed inner rotor diameter and the motor producing maximum required torque.