### Designing Efficient Fans for Electronics Cooling Applications

```for Electronics Cooling Applicotions Introduction
where Pout (W) is the fan airpower and can be
expressed as :
In modern day equipment racks, the power
consumed by the cooling fans represents a
significant percentage of the overall system power
budget. With the advent of data centers and their
large energy costs, the issue of "cooling power" is
exacerbated. It is, therefore, becoming crucial to
design and implement methods for reducing data
centers' power consumption . As an example, a
large data center contains about 400,000 servers
and consumes 250 MW of power [1] . It has been
estimated that about 20 % of the total power
supplied to a high end server is consumed by fans.
for reducing energy consumption through increasing
fan efficiency. Three main ways of improvement
by optimization will be discussed: motor and
electronic driver optimization, fan aerodynamic
optimization and surrounding inlet/outlet equipment
optimization .
Background
A fan is an energy converter, so its efficiency can
be expressed as the ratio of input and output power
[2]:
(1)
_ ___,r;; .
p out =pV
(2)
where p is the air pressure (Pa) and V is the air flow
(m 3/s) at the fan outlet.
Pin (W) is the fan electrical input power:
PIn =U·l
(3)
where U is the voltage (V) and I (A) the current
drawn by the fan .
To date, a number of power management
techniques have been adopted in the electronics
industry in order to achieve the desired level of
cooling with minimum fan energy consumption .
For example, fan speed and power are controlled
to the minimum level necessary for cooling. Since
fan power consumption is proportional with (RPM)3,
significant savings can be achieved through fan
speed control. In another example, the speed of
a brushless DC fan is controlled by varying the
input DC voltage. Variable-speed fan controllers
for computers usually use Pulse Width Modulation
(PWM) . An AC induction motor driven fan can be
speed controlled by varying its input voltage and
frequency.
Beyond fan speed control, there are more savings
available through optimization of the motor and
electronic driver, increased fan efficiency and proper
air system design .
Motor/Electronic Driver Optimization
DC Fans
For a brushless DC motor driven fan, one way
to improve efficiency is by using more complex
three-phase electronic drivers and motors,
replacing the industry standard two- phase
design. Figure 1 presents the schematics and
practical implementations for these two concepts .
Component count is 2 to 3 times greater for the
three-phase design, but produces significant energy
savings .
Cost and Complexity Increase for 3 Phase Drive
motor. Th is higher frequency PWM switching results
in magnetic hysteresis losses which, combined with
motor iron losses, can reduce overall fan efficiency
by 5 to 15 %. As an example, Figure 2 compares
the phase voltage and current waveforms for a fan
operating under natural commutation and under
PWM, wh ile del ivering the same air performance. By
using PWM switch ing, the input power increased by
12% (from 47 W to 53 .5 W) .
PWM Losses
Pha ' e vollage aud current
"uaturn l counmttatiou"
24 0 RPM Fre e Deli ve oy
4 7 wa tt nn input
Phase voltage aud cmTeut
" PWM 17Kh z CUITent limil"
2450 RJ'M Free Dclivc•y
52 .5 watts nn inpu t 12% increase l
Figure 2. Phase Voltage and Current Waveforms for a Fan
Operating under Natural Commutation and PWM [1]
L ow o t tandard 2 Pba e
High Elllcieucy Fu ll B1idge 3 Phase
A tradeoff should be sought between employing
PWM at low voltages (and therefore achieving full
performance) and operating the fan at full voltage,
with the continuous associated losses . In certain
cases, the use of PWM may not be justified.
Figure 1. Standard 2-Phase and 3-Phase Driver Designs [1] Test results comparing a 120 mm round axial fan
(7 .6 mm H20, 140 CFM) equipped with a standard
two phase electron ic driver and a three phase driver
indicate an energy savings of 9.8 W, or 30 %of the
electrical input power [1]. Additionally, the reduced
internal electrical losses decrease the motor
temperature with a positive effect on fan reliabil ity.
PWM heating is another source of motor losses,
occurring when a DC fan is operated to its current
limit as opposed to the natural commutation
frequency of the brushless permanent magnet
AC Fans
Oftentimes, commercially available inverters are
used to control the speed of an induction motor
driven fan . Sign ificant losses can result from the
mismatch between the fan and the inverter [1].
Table I summarizes the performance comparison for
a motor/fan operated under pure 3-phase sine wave
versus with a 3-phase inverter. The inverter driven
system consumes 14 % more power, mostly due
to deterioration of motor efficiency. As a result, the
stator temperature is 17 oc higher. The additional
losses are due to harmon ics contained by the
distorted current waveform as well as PWM losses.
An ideal inverter would synthesize a pure sine
wave w ith no PWM imposed on the phase current
waveform .
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Power
Input
Amps
Inverter Onve
3 Phase 480 vac/400 Hz
Increase In Power
(Electrical Losses)
RPM
4120
11 .035 RPM
61 Deg.
3550
t1 ,045RPM
44 Oeg C
I
6.50
(PWM sellO 4khz)
Pure Sone Wave
3 Phase 466 vacJ388 Hz
Stator
Temperature
RJse Deg. C
Watts
584
0.66
I
I
1 570 ' Watts
(14 %)
Equal AJr
Petformance
c
17DegC
Table 1. PWM and Harmonic Losses for an Inverter
Driven Induction Motor [1]
Fan Optimization
Fan efficiency varies dramatically with the
performance plot for a standard 120 mm diameter
axial fan, with the efficiency curve plotted together
with the usual pressure-airflow curve [2].
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It can be observed that the peak efficiency occurs
at a pressure value about one third of the maximum
pressure. As a general rule, fan efficiency increases
with blade diameter and rotational speed.
200 in-depth articles, researched and written by
veteran engineers. They address the most critical
30%
areas of electronics cooling, with a wide spectrum
1.0
25 %
of topics and thorough technical explanation.
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Figure 3. Pressure and Efficiency as a Function of Flow
for an Axial Fan [2]
An illustration of fan optimization is offered in [1]
for a standard 120 mm axial fan . The optim ization
is performed for the performance point of 50 CFM
and 1.45 inch Hp.
Figure 5. Standard and Optimized Fan [1]
ClligiMI Prodootion
120 mm Tube A:6al fiiio
Opefiitlng
Point
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Figure 4 . Performance Comparison between Standard and New Designs [1] PO'IY'er at opel'il1•ng po1nt
Speed (RPMl
23.6 Usee (50 CFM ).
Redesigned Propeller
120 mrn Tuboo Axial Fan
3U mm HJO (1.45 m ~0)
23.6 Usee (50 CFM), 36.8 mm
Hp (1.45 1n H2 0 )
5
11
Hull: 38 •1 24.13 mm (0.95')
Tip: -30' I 24.13 mm (0.95" )
Hub. 2&> 1 8.13 mm (0.32' )
Trp: 18<> I 10.67 mm IOAZ')
57 Watts
26 Watts
6700 RPM
7700 RPM
Table 2 . Performance Comparison of Fan Designs [1]
The optimized fan requ ired a new propeller
a higher operating speed . Figure 4 compares the
performance of the standard fan with that of the
optim ized design, while Figure 5 illustrates the old
and new designs, side by side .
Note : The basic motor design and the electronic
driver remained the same.
Power consumption of the new fan at the operating
po int was reduced by more than 50 %, from
57 watts to 26 watts (Table II). The cost of this
increase in efficiency is 6 more blades and 1000
more RPM.
A CFD snapshot output (Figure 6) showing velocity
vectors for the two designs near the operating
point also uncovers a stall region (for the standard
design), just under the blade. This stall has been
eliminated in the redesign process.
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Despite these advantages, high efficiency fans
can be costlier than older fan types, a significant
economic deterrent. Also, it should be mentioned
that any fixed-geometry fan has a single peak
efficiency point. In practice, the system operating
point must fall with in fan's high-efficiency operating
range [3].
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Figure 6 . Results from CFD Analysis [1]
System Optimization
Additional energy savings can be gained by
designing the fan within its operating environment
rather than isolated, running on an air chamber. Fan
performance and efficiency are both impacted by its
air inlet and outlet configurations . The equivalent
of inlet fairings and outlet diffusers can be built
into the customer equipment to significantly boost
overall air performance [1].
Conclusions
Fan power consumption is traditionally reduced
by controlling the motor speed to produce
only the airflow required for adequate cooling,
rather than operating continuously at full speed.
Significant energy savings can be achieved beyond
this technique through fan efficiency increase.
Optimizing the motor and electronic driver,
increasing fan aerodynamic efficiency through
careful redesign, and optimizing fan-system
integration are three ways of achieving this.
Additionally, using high efficiency fans has a
positive cascade effect on the entire system design.
Power suppl ies can be reduced in size and the fans'
power loss can be minimized. Also, the load and
losses for power conversion equipment (ACto DC)
are reduced.
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References:
C l)e.Jfl'l~ P ein t 1. Smith, N.," High Efficiency Electronic
Cooling Fans", 25th IEEE SEMI-THERM
Symposium, 2009.
2. http://www.nmbtc.com/fans/white­
papers I Fa n_Efficiency_Important_
Selection_Criteria.html.
3. Lawless, P.B., " Advanced Aerodynamics
for Electronics Cooling Fans", Electronics
Cooling Magazine, February 2013 .
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