ASHRAE Journal - July 2014 - 21

Technical FeaTure
Fea

electrical savings are not as high as
presented in the feasibility study
due to two major factors: measured
horsepower and load management.

Other Electrical Savings Factors
As previously stated, the energy
savings are further reduced if one
takes into consideration the losses
associated with the VFD. Some
manufacturers of VFDs publish
associated loss values. Although
the motors discussed in this article
are nominal 2400 V, it is worth noting to some that AHRI established
Standard 1210, Performance Rating
of Variable Speed Drives, for less than
and equal to 600 V and up to 75 hp.
In general, the VFD for motors 200
hp (14 kW) and over has a 2% to 3%
frequency conversion loss as heat in
the VFD.2
Also, motors have part-load efficiencies. However, the performance does
not show a substantial change until
the motor load drops below 40% of its
nominal rating.3 As stated earlier, the
VFD savings software takes motor, fan,
and drive part load efficiencies into
consideration. If the owner does not
have access to such software, one will
need to account for these efficiency
losses for accuracy in calculating savings. Under the load management
scheme, each motor operates with a
higher boiler steam flow compared
to the current operating scheme as
shown in the feasibility study.

Simple Payback
Using the consultant's estimated
initial cost of the VFD drives
(installed), the simple payback
Equation 2 is used.
Simple Intial Investment Cost
Payback =
Savings

(2)

Simple payback is defined as the
time it takes a project or investment
to pay for itself in the absence of a
life-cycle cost analysis (LCCA). In
comparing the three schemes, Table
2's Columns C and E, present simple
payback values between 13 and 16
years for the entire plant (assuming
the construction estimate is correct
as stated in the feasibility study).
The discussion of the construction estimate is sensitive and
beyond the scope of this article;
therefore, the estimate is assumed
to be correct but one should
ensure the accuracy of the construction estimate since it greatly
impacts the simple payback.
Under Federal Program Guidelines,
the project may not be installed if
the simple payback is greater than 10
years. The VFD installation should be
analyzed using a LCCA because the
project requires an environmental
cooling system and enclosure for
the dissipated heat from the drives;
hence, the system presents an outlay
of additional energy and equipment
maintenance.

this article provides guidance to
infrastructure owners in the area
of energy conservation, specifically VFD for industrial boilers. As
a result, the proper analyses and
operational schemes may avoid
unnecessary capital investment in
equipment that leads to an increase
in ownership costs, e.g., VFD, environmental cooling system, etc.

References
1. U.S. Department of Energy. 2010. "Operation and Maintenance Best Practices, A
Guide of Achieving Operating Efficiency,
Release 3.0."
2. Electrical Construction and Maintenance (ECM). 2009. "The Basics of VariableFrequency Drives." http://tinyurl.com/
klrybhy.
3. U.S. Department of Energy. 2008.
"Improving Motor and Drive System Performance: A Sourcebook for Industry."

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Spanish-
Refrigeration
Handbook
2010 ASHRAE Handbook-
Refrigeration CD, Spanish Edition

Conclusions
To ensure the proper estimating
of energy savings and costs, the
owners and energy consultants
need to consider four main factors; (1) use measured motor power
instead of nameplate ratings, (2)
ensure proper load management
operational procedures, (3) consider losses associated with the
variable frequency drives (VFD),
and (4) account for motor and fan
part load efficiencies. To manage
infrastructure that consumes large
amounts of energy presents annual
increases in capital and operational
budgets that are already large,
J u ly 2 0 1 4

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ASHRAE Journal - July 2014

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