Aerospace and Electronic Systems Magazine April 2017 - 18

Feature Article:

DOI. No. 10.1109/MAES.2017.150281

Electromechanical Actuator Fan Failure Analysis and
Safety-Critical Design
W. Wu, Y. R. Lin, L. C. Chow, University of Central Florida, Orlando, FL, USA
E. Gyasi, J. P. Kizito, North Carolina A&T State University, Greensboro, NC, USA
Q. H. Leland, Air Force Research Laboratory, Wright Patterson Air Force Base, OH,
USA

INTRODUCTION
The aim of this study is to use fault tree analysis to gain a better
overall understanding of how to improve the reliability of a fan
cooling system used to prevent electromechanical actuation systems (EMAs) from overheating. It is critical to manage the waste
heat generated by EMAs on aircraft during all flight missions
because EMA failure due to overheating could lead to an aircraft
catastrophic failure. Heat generation by high-power-density power
electronic devices and EMA motors has reached a heat flux density that thermal management of EMA by natural convection is
not feasible, forced air cooling is needed. Since EMA motors and
power electronic drives must operate at various altitudes and temperature throughout a flight mission, there is a need to develop a
variable-speed fan that can operate efficiently and reliably over a
wide range of pressure and temperature. To prevent EMAs from
overheating and bring EMA failure probability to an acceptable
level, a sturdy cooling fan which can operate under variable body
forces due to aircraft maneuvering is required.
Cooling fans can fail electronically and mechanically. The
mechanical parts of a fan include bearings, lubricant, shaft, fan
blades, and housing. The electronic parts include control circuitry,
motor windings, power supply, and power drives. A fan can have
either "hard failures" in which the fan is nonfunctional (jamming)
or "soft failures" (performance degradation) such as slower revolutions per minute (rpm), increased input current, or higher noise level. To have a full understanding of fan reliability, all possible failure mechanisms and failure modes should be taken into account.
Bennett et al. [1] used fault tree analysis to study the failure
probability of EMAs. EMA fans have similar components: power
supply, controller and inverter, control signals, motor bearings, and

Authors' current addresses: W. Wu, Y. R. Lin, L. C. Chow,
University of Central Florida, MAE, 4000 Central Florida Blvd.,
Orlando, FL 32826 USA, E-mail: (wuwei98@gmail.com). Q. H.
Leland, Air Force Research Laboratory, Wright Patterson Air
Force Base, OH 45433 USA.
Manuscript received December 17, 2015, revised April 18,
2016, April 20, 2016, and ready for publication April 23, 2016.
Review handled by M. Jah.
0885/8985/17/$26.00 © 2017 IEEE
18

motor windings. There could be significant differences in failure
probabilities of the five similar components between EMA and
EMA's fan because a 100 W fan motor and a 20 kW EMA motor
might have different mechanical and electrical components and designs. It would be ideal if fan manufacturers break down and publish the failure probabilities of each individual component for their
fan products. In the absence of the values for these failure probabilities, one may assume the relative reliability of the individual
components for a fan and an EMA are similar, so the failure probabilities cited by Bennett [1] for EMA can be used to compare the
relative reliability of the individual components for a fan motor.
A fan with a fault probability of 2 × 10−5 means the probability
of failure is 0.002% per hour of flight time. To analyze the fault
probability of a fan, the failure mechanisms are grouped into two
types, mechanical and electrical failures. Reliability improvements
by utilizing dual windings, dual bearings, and dual fans for fault
tolerant strategies are investigated and the results are compared.
The safety driven design process for EMA fan is discussed with
reliability calculations presented for all proposed fan components
to show where fault tolerant design can be enhanced to improve
reliability.
An EMA typically has an EMA motor, drive electronics, gear
box, and drive train. It is assumed that the EMA motor and drive
electronics are directly fan-cooled with integral heat sinks. The
drive train and gear box are cooled by the recirculating air within
the aircraft or wing bay. The fan motor can be structurally integrated with the EMA motor, but is powered and controlled independently with temperature sensors so that the fan is turned on whenever a certain temperature of the EMA is exceeded. The fan speed
can also be controlled as a function of motor temperature saving
fan power. One advantage with forced air convection is the entire
bay surface area can be treated as a heat sink, thus avoiding the
need to mount a condenser or cold plate onto the bay wall. Given
the different geometry of the motor and electronics, specific heat
exchangers are required. To effectively cool a cylindrical motor,
tall flat fins are attached radically to the motor housing, as shown
in Figure 1. Enclosing the fins with an outer shell creates an annular duct for air to pass through. The fan can provide the necessary
amount of air speed through the annular duct, thus creating effective heat transfer. Fan performance including static pressure head
and volume flow rate at various rotational speeds, fan diameter, air

IEEE A&E SYSTEMS MAGAZINE

APRIL 2017



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