Aerospace and Electronic Systems Magazine April 2017 - 23
Wu et al.
and there is no need to improve it unless
there are significant improvement in the
reliability of electronics .
DUAL WINDINGS EMA FAN
Figure 6 shows our current effort on
dual-winding design. In the fault tree
analysis in Bennett's paper  to estimate the reliability of EMA, the failure probability of five components are:
power supply 5.4 × 10−5, electrical controller and inverter 8.55 × 10−5, control
signals 1.3 × 10−5, motor bearings 6.6
× 10−7 and motor windings 1.38 × 10−8.
Bennett came up with an overall failure Figure 7.
probability of 1.4 × 10−4. This estimate Fault tree analysis on simplified fan failure analysis and dual-winding system.
agrees reasonably well with the life span
of 50,000 hours for fans (from many fan manufacturers' product
specific sheets). If the mechanical failure probability of 6 × 10−7
is mainly due to the failure of bearings, one can deduct this from
the overall fan failure probability of 2 × 10−5 to get the electronics
failure probability of 1.9 × 10−5. Thus, this simple analysis shows
that dual-winding fan will achieve the failure probability of about 2
× 10−5. Thus dual-winding approach could not improve the overall
reliability of a fan cooling system (see Figure 7).
From Figure 7, it also can be seen that even though dual-winding system could have a low failure probability of 1.1 × 10−8 ,
the overall loss of output still remains at 2 × 10−5. So, there is no
improvement in the overall reliability. This is mainly because the
motor windings have a failure probability of ∼O(10−8) and there is
no need to improve it.
DUAL-MOTOR WINDINGS AND DUPLICATED ELECTRONIC
The system overall reliability can be improved by using dualwinding system plus a duplicated electronics system . Our
analysis shows that this combined redundancy could have a low
failure probability of 6.1 × 10−7 (see Figure 8). This is two orders of
magnitude of improvement compared to Case 1 and Case 2 where
the overall failure probability was at ∼O(10−5).
Even though that dual-winding system could have a low failure
probability of 1.1 × 10−8 and duplicated electronics systems could
have a low failure probability of 3.6 × 10−10, but the overall loss
of output is mainly determined by the mechanical system failure
which leaves the mechanical system (or say the bearings) to be the
decisive factor in overall reliability.
Two set of blades, running in the opposite rotation direction, seven-blade and five-blade on two independent shafts (see Figure 9)
are shown as a dual-fan design. Counter rotating motors (each motor has an independent shaft and rotating in opposite directions)
Fault tree analysis on dual windings plus duplicated electronic system.
Our new design on dual-fan with seven and five blades running in opposite direction (front and back).
are used in our design. The shafts are aligned in middle housing.
The fan system can achieve a failure probability about 4 × 10−10. It
is possible to improve the life of fan by a dual lane fault-tolerant
electric drive. The selection of such an optimum configuration and
fault tolerant control schemes is as follows: A dual-fan system with
IEEE A&E SYSTEMS MAGAZINE