ECM Motors in Fan Powered Terminal Units
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Krueger Corporate Headquarters
1401 N. Plano Rd. • Richardson, TX 75081
www.krueger-hvac.com
ECM MOTORS
The use of ECM motors can improve the efficiency of small fractional motors used in Commercial Air
Terminal applications today. The first fan-powered terminals, using conventional fractional hp motors, were
introduced to commercial buildings in 1974. They were immediately employed by a number of design
engineers due to their significant advantages in energy use when compared to the reheat options available
at that time. They also allowed designers to take advantage of building diversity through the recapture of
plenum heat, which was often wasted when returned to the air handler. Initially, a parallel design, with the
fan operating only when heat was required, was used. the series type, with constant fan operation, has
increased in use due to its constant air supply and sound levels, at a slight increase in energy use.
The issue of energy consumption by the low efficiency fan/motor, however, has concerned the industry
and has resulted in some negative press. In addition, the series fan box design’s somewhat non-constant
airflow, as it responds to inlet pressure changes, has made the units difficult to balance. These issues are
negligible compared to the several significant advantages offered by series-type fan powered terminals
(Contracting Business, 1997). The use of extra heat and ventilation air resulting from building diversity
more than overcomes the energy penalty and the balancing issue is a one-time concern.
Fan powered terminals typically employ direct-drive, fractional horsepower induction motors of the
permanent split / capacitor-start type. Many applications employ the use of an SCR speed controller on a
single or 3-speed motor. This is not the most efficient technology, as they are fundamentally fixed-speed
devices, but are very affordable from a first cost basis. Applications typically range from inefficient to very
inefficient depending on the combination of motor, speed controller and selected operation point.
It is complex and expensive to control both speed and torque in an induction motor. Without active
speed/torque control, fan outputs are highly pressure dependent, limited by a single fan curve. With fan-
powered boxes, the balancer therefore spends several trips to the unit as he balances both diffusers and
the unit’s SCR to achieve the desired airflow. This then changes as the inlet side is loaded and unloaded
in response to the thermostat’s changing load requirements, and resultant changes in varying primary air
on the inlet side of the fan. This can be a noticeable nuisance for air balance personnel especially if the air
intake port on the fan-powered box is too small.
Upon an increase of primary air into the fanbox
cavity, the cabinet is increasingly relieved of
negative static pressure, therefore increasing
fan performance. The resulting output error is
referred to as fan shift and can be as much as
10-25% change in fan output depending on the
difference between min and max air flow
settings of the primary air valve. Balancing time
has been reported to be as high as an hour per
unit. Remote SCR setting, through the DDC
system, often adds $100 or more to the first cost
of a unit if fan airflow verification is included. If a
three speed motor is used, the efficiency at less
than full speed (or torque) is always reduced,
often significantly, over full rpm / torque
applications.
While there has been an increasing call from the industry over the last several years to solve these
problems, there are few options that can solve both the set-up and energy use options. The use of a
digitally controlled motor, developed for the residential industry, has demonstrated excellent reliability,
energy reduction and control performance, and is responsible for a significant increase in energy ratings.
Unfortunately, residential applications are either 115 or 220 VAC, where 80% of all commercial