ASHRAE Journal - December 2021 - 20

TECHNICAL FEATURE
FIGURE 7 Measured controller, motor, impeller and system static efficiencies of
Fan System 1 along the system line.
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
Controller Efficiency
Motor Efficiency
Impeller Static
Efficiency
Static System
Efficiency
System Line
100
90
80
70
60
50
40
30
20
10
500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000
Airflow (cfm)
contribute to wire-to-air efficiency equally.
To demonstrate the net effect of these efficiencies,
consider the following example. At
6,000 cfm (2832 L/s) and 4.5 in. w.c. (1.1 kPa)
static pressure, both Fan System 1 and Fan System
2 have a wire-to-air efficiency of 62% (Figure 4).
At this test point, the measured combined motor
and controller efficiency is 88% for Fan System 1
and Fan System 2, which results in an impeller
efficiency of 70% (per Equation 2). Hypothetically,
if the combined motor and controller efficiencies
of both Fan System 1 and Fan System 2 were
increased to 100% (maximum possible but unfeasible),
the resulting wire-to-air efficiency would
only be 70% using the same impeller. To achieve
the same wire-to-air efficiency, assuming combined
motor and controller efficiency is left at
88%, the impeller efficiency would need to be improved
to a feasible 80%. This demonstrates diminishing
returns for improving motor and controller efficiency
without considering the impeller design or selection.
Impeller efficiency has more impact on the wire-to-air
efficiency, though maximizing efficiency of all components
will reduce overall energy use.
Influence of Motor Technologies along the System Line
On Wire-to-Air Efficiency
Fan System 1 (direct drive) and Fan System 2 (EC) efficiencies
(fan static, motor controller and wire-to-air)
were evaluated along a given system line as demonstrated
in Figure 9. It is observed that both the combined
motor and controller efficiency and the " wire-to-air "
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ASHRAE JOURNAL ashrae.o rg D ECEM BER 2 0 2 1
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
FIGURE 8 Measured impeller static, controller plus motor and system static efficiencies
of Fan System 2 along the system line.
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
Controller and Motor Efficiency
Impeller Static
Efficiency
Static System
Efficiency
System Line
1,000
100
90
80
70
60
50
40
30
20
10
2,000
3,000
4,000
Airflow (cfm)
FIGURE 9 Comparison of efficiencies of Fan System 1 and Fan System 2 along the system line.
Motor + Controller h2
Impeller Static h2
Impeller Static h1
Wire-to-Air h2
Wire-to-Air h1
System Line 2
System Line 1
1,000
2,000
3,000
4,000
Airflow (cfm)
efficiency of Fan System 1 and Fan System 2 are the
same (within 1%), similar to the single-speed analysis
(Figure 6). It is also noted that the calculated impeller
static efficiency of both the systems remain constant
along the system line.
While VAV systems do not behave according to this type
of curve, the variation in impeller efficiency is small
across a majority of the operating load, assuming the
impeller is selected near peak efficiency. Regardless, the
impeller efficiency would be the same for Fan System 1
and Fan System 2. This confirms that the two motor
technologies do not significantly differ in terms of wireto-air
efficiency, even when turning down the system
from full- to part-load conditions.
This plot clearly shows that to improve the off-load
5,000
6,000
Motor + Controller h1
100
90
80
70
60
50
40
30
20
10
7,000
5,000
6,000 7,000
Static Pressure (in. w.c.)
Static Pressure (in. w.c.)
Efficiency (h) (%)
Static Pressure (in. w.c.)
Efficiency (h) (%)
Efficiency (h) (%)
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ASHRAE Journal - December 2021

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