Aircraft Electrically Powered Actuators

In the EMA type, hydraulic components are completely replaced. In contrast, in the EHA type, the pressurized hydraulic fluid is still used to transfer power. But the hydraulic fluid is not distributed by a centralized system as the traditional hydraulic system and kept local in the actuator. Considering many previous studies comparing (EHA) and (EMA), they summarized that EMA has many advantages over EHA. Due to the absence of an internal hydraulic system, it is lighter, smaller, and less complicated than an equivalent EHA. Also, due to the lack of hydraulic fluid in the load path, the EMA tends to be stiffer than an equivalent EHA. On the

Chapter 6: SRM-Based Electrical Actuators for Modern Aircraft Applications ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ ـــــــــــــــــــــــــــــــــــــــــــــــــ

100

other hand, the efficiency of the EHA is lower than the EMA due to the inefficiency of the pump or wind losses. Besides, the EMA system is more suitable for long-term storage because there is no leak potential. In Table 6.1, the main differences between Central Hydraulics, EHA, and EMA can be summarized [142], [143]. From the previous and recent studies, it can be concluded that the electrical actuator types, especially EMA, can be selected as the optimal approach. Therefore, choosing the right type of electric motor for electrical actuators is very important to ensure optimum actuator operation with high efficiency.

Table 6.1: The differences between EMA, EHA, and central hydraulics [142], [143]

Electrically Powered Actuators

Central Hydraulics

EMA EHA

Overall efficiency More than

90% From 50% to 70% Less than 50%

Tolerance (Stiffness &

Shock) High High High

Load Rating High High High

Maintenance Low High Very High

Lifetime cycle Long Generally, not long (Depends on maintenance)

Generally, not long (Depends on maintenance) Speed and

Acceleration High Medium Medium

Control and

Positioning Simple Complex Complex

Relative Space

Requirements low Medium Medium

Environmental Effect Negligible Fluid leaks and disposal Fluid leaks and disposal

6.2.1. Motor Technology for Aircraft Electrical Actuators Applications

Table 6.2 shows the acceptability of electric motors' different technologies for flight control actuators applications in terms of power density, robustness, damage tolerance, and controllability [144], [145], [146]. From this comparison, the reluctance motor technology can be considered the optimal type due to its robustness and fault tolerance.

Table 6.2: Acceptability of electric motor technologies for flight control surface actuators [142]

Induction Motors

DC Motors

Permanent Magnet Motors

Linear Motors

Reluctance Motors

Robustness * * *

Power Density * * * *

Controllability * * *

Damage

Tolerance *

Where “*” means acceptable.

101

Recently, much progress has been made in developing reluctance motors and using them in many applications and can compete with traditional types of motors. The reluctance motors are the lowest types of electrical machines in manufacturing cost due to their simple construction. They have an independent circuit for each phase as opposed to the induction motors and permanent magnet motors, which gives the motors fault tolerance and makes the rotor losses lower than the equivalent induction motors. In general, the reluctance motors' rotor can be a solid block of steel, which makes it extremely rugged compared to other types of machines, such as permanent magnet motors. Reluctance motors are very suitable for applications that require high torque because they have the highest torque and power density compared to other types. DC source ideally powers the reluctance motors, and it can be powered by an AC source as well, through an uncontrolled or simple rectifier. Concerning its control, the control of these types is complicated and is considered a significant challenge to use in many applications. However, the converter topologies of the drive circuit for the reluctance motors controller are clear and utilizes standard components found in other speed drive technology used nowadays.

There are different types of reluctance motor technologies; some of these technologies are well known, such as variable reluctance stepper motors and switched reluctance motors. Other types may also be used thanks to advances in semiconductor technology, such as cage-less synchronous reluctance motors and variant SR motor technology. Table 6.3 illustrates the differences between the types of reluctance motor technologies that would be relevant to this application in terms of maintenance, power density, position holding, position control, torque ripple controls, temperature, tolerance, shock tolerance, damage tolerance, and cost [147], [148].

Table 6.3: Reluctance motor technologies comparison [147], [148]

Synchronous Reluctance Motor

Stepper Motor

Flux Switching Motor

Switched Reluctance Motor

Power Density 2 5 3 5

Maintenance 5 3 5 5

Economically 5 2 2 5

Position Control 3 3 3 5

Position Holding 3 2 2 5

Torque Ripple

Controls 5 1 1 1

Shock Tolerance 5 3 5 5

Temperature

Tolerance 5 2 5 5

Damage Tolerance 2 2 2 5

Where “1” means Poor, and “5” means Excellent.

Chapter 6: SRM-Based Electrical Actuators for Modern Aircraft Applications ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ ـــــــــــــــــــــــــــــــــــــــــــــــــ

102

According to the comparison of reluctance motor types except for the torque ripple controls feature, the SRM can be considered the optimal type between all classes. Although the concept of the SRM was established in 1838, the motor was not used in a wide range until the power electronics revolution, particularly semiconductors, high-speed electronic control, and computer-aided electromagnetic design. The most significant challenge to be overcome, as it can be seen from the comparison, the torque ripples problem. This problem is very complicated and affected by many factors, and not easy to solve.