Aerospace and Electronic Systems Magazine April 2017 - 20

Electromechanical Actuator Fan Failure Analysis and Safety-Critical Design
From (1), the reliability of a fan motor can be significantly improved by using two identical fans with independent
power drives. Fault-tolerant design,
health monitoring, and other redundancy improvement measures can be taken
and the failure probability can be calculated by using (1) and (2) to achieve the
desired level of reliability [2].

CONSIDERATION OF BLADE
DESIGN
It is best to select a fan that will give
an operating point toward the high flow,
low pressure end of the performance
curve to maintain fan blade efficiency.
A high-speed Ametek Propimax-II
(82 mm diameter) fan (see Figure 3)
was tested extensively. Also shown in
Figure 3 are slightly wider blades fabricated with 3D printing. To obtain a
sufficiently smooth surface of blade, the
blade surface was submerged in 60° C
acetone for 120 seconds immediately Figure 3.
after the 3D printing. These blades were Ametek Propimax 2 fan blade (top) compared with UCF 3D printed about 10% wider fan blade (bottom).
mounted on a brushless DC axial fan for
performance evaluation.
The Ametek Propimax-II and the modified fan were tested in a
closed test loop following the International Organization for Standardization 5801 standards. For various chosen ambient pressures
and rotational speeds, the fan static pressure head, volumetric flow
rate, and flow velocity were measured. The fan flow rate and static
pressure were recorded at various rotational speeds and ambient
pressure. Each fan curve is obtained by carefully adjusting the flow
control valve to change the overall flow resistance inside the fan
test loop. Figure 4 compares the two-blade printed fan results with
the Ametek fan results at 25,000 rpm and ambient pressure of 0.2
atm. It can be seen the wider blade design yields much better performance. At the design operating point of 1.1 m3/min, the pressure
Figure 4.
heads for the wider blades and the Ametek blades are 85 and 45
25,000 rpm Fan Curve Results at 0.2 atm.
Pa, respectively. This means the Ametek fan blade design should
of potential failure mechanisms can be found in microelectronics
be modified to accommodate low pressure operation. More blades
packages within BLDC drives.
(three or more) with larger width should be fabricated and tested
Before discussing the potential failure mechanisms in a BLDC
to see if similarly significant improvement in performance can be
drive, it has to be mentioned that diffusion is a critical failure
realized.
mechanism in electronic packages. Diffusion is time-dependent
migration of an atomic, molecular, or ionic species within another
FAN FAILURE CAUSES ANALYSIS
material. Failure in electronic packages is a primary failure mechanism in mechanical, civil, and aerospace engineering applications.
According to the definition of a fan, correctly shaped blades and
Dendritic growth is another electrochemical process. Dendrites are
electric motor are core components that help a fan achieve its dedeveloped when metal ions migrate from the anode to the cathsired function. Table 1 lists the major fan failure modes/causes.
ode regions. When dendrite forms a complete bridge between the
Life estimate can be made from mechanisms such as interdiffutwo electrodes, a short circuit occurs due to the increase of the
sion, dendritic growth, corrosion, and fatigue. A BLDC drive is
leakage current across the bridge. Electronic packages in transiscomposed of multiple subcomponents including transistors, retors and ICs could also develop an open circuit by interdiffusion
sistors, capacitors, and ICs on a printed circuit board. A number
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IEEE A&E SYSTEMS MAGAZINE

APRIL 2017

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine April 2017