ASHRAE Journal Supplement - AMCA InMotion - November 2024 - 27

Mo orMotor
Mo orMotor
Gearbox
ox
Bllades
ad
Blades
Blades
FIGURE 1. Typical
gear-driven LDCF
assembly.
FIGURE 2. Typical
direct-drive LDCF
assembly.
infl uencer of airfl ow performance and energy use. It also
is a substantial driver of the cost of a fan. To drive more
informed product selections, this article will highlight
similarities and differences between the two types of
motors used in LDCF: direct-drive and gear-driven.
Motor Types
Of the two motor types, gear-driven (Figure 1) is the more
traditional solution, with a maximum speed of approximately
1,800 rpm (depending on the model) and gears that
reduce the gearbox output shaft to LDCF speeds. Typically,
this is anywhere from 10 rpm to 200 rpm, depending on
the speed setting and fan-blade diameter.
Direct-drive motors run at much lower speeds, as
they lack the conventional drivetrain. In the absence
of a gearbox, the output shaft of the motor is coupled
directly to the fan blades, which requires the motor to
run at the exact desired revolutions per minute of the fan
blades, still typically from 10 rpm to 200 rpm (Figure 2).
The airfl ow produced by a fan is not entirely dependent
on motor type. Whether attached to a direct-drive
motor or a gear-driven motor, a set of fan blades will create
similar airfl ow profi les, provided fan-blade speed is the
same (tables 1 and 2). For this reason, some manufacturers
use remarkably similar components between fan models
regardless of motor type.
Gear-Driven Motor
Speed Motor RPM Gear Ratio Fan RPM
1
2
3
1,000
1,500
2,000
24:1
24:1
24:1
42
63
83
Speed
1
2
3
TABLES 1 AND 2. Comparison of gear-driven and direct-drive motors.
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2024 AMCA inmot ion
27
LDCF Selection
With airfl ow performance being similar, specifi ers and
purchasers may wonder when and why they should
choose a specifi c motor technology. To answer that, we
need to look at product cost, installation, maintenance,
and energy effi ciency. There also is the matter of noise,
which typically favors direct-drive motors. LDCF noise,
however, is the combination of motor and blade-assembly
noise, as the operation of fan blades often is quite audible.
A fan assembly should be thought of holistically in terms
of sound.
Cost. Direct-drive motors tend to cost more than
their gear-driven counterparts. First, their design tends
to be more complex, incorporating sensors for gathering
the high-resolution feedback required for operation.
Additionally, because of their fewer revolutions per
minute, low-speed versions of direct-drive motors are
more " niche, " with less utilization across industries and,
thus, less volume.
Installation. While direct-drive motors may be
assumed to weigh more than their gear-driven counterparts
because of the copper and magnets inherent in
their design, signifi cant weight is saved with the lack
of a gearbox. The bottom line is the weight of a motor
is determined primarily by the manufacturer's design
choices, rather than whether the motor is direct-drive or
gear-driven. Ultimately, differences in weight usually are
not signifi cant enough to affect the equipment or procedures
required for fan installation.
Maintenance. Regular inspection and external
cleaning are recommended for both motor types. With
a gear-driven motor, however, the gearbox is a point of
potential failure. Internal seals can fail. Oil pressure can
spike because of conditions such as overheating. A pressure
vent may be left closed because of improper installation
or, in rare cases, leak oil. How often such failures
Direct-Drive Motor
Motor RPM
42
63
83
Fan RPM
42
63
83
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ASHRAE Journal Supplement - AMCA InMotion - November 2024

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