Currently, only one manufacturer I am
familiar with offers this MC-ASD technology
– MagnaDrive. The company was founded in
1999 and has more than 5,000 installations.
The drives can be mounted in configurations
that are vertical or horizontal, indoor or
outdoor, and from 5 HP to more than 5,000
HP. Talk about variety!
I first ran across MagnaDrives at
the Charter Street Heating Plant on the
University of Wisconsin-Madison campus,
where I was performing a plant survey as
part of a utilities master plan effort. The
company’s MC-ASD was installed on a
1,000 HP distribution pump and was stated
to be more economical to install than a
4,160 volt VFD. The facility staff swore by
this drive.
My reintroduction to MagnaDrives
came last year as we were looking for an
alternate solution to a 4,160 volt VFD for
use on a 500 HP vertical turbine pump. It
really saved the day since we did not have
the space for the medium-voltage VFD
( 9.8-ft long by 3-ft wide by 7-ft high); it
also offered operational savings.
Figure 1. Installation Costs of MagnaDrive Adjustable-Speed Drive vs. Variable-Frequency Drive Motors.
$70,000
$60,000
$50,000
$40,000
$30,000
$20,000
$10,000
$0
MagnaDrive
20
VFD
Source: Steve Tredinnick, Syska Hennessy Group, compiled with pricing from vendors and means cost data
published by Reed Construction Data.
25
50 100 150 200 250 300 350 400
Motor;Horsepower
Pros and Cons
From my research, it appears that
there are similar operating efficiencies
between MC-ASDs and VFDs. A test
comparing these drives was performed
by Pacific Northwest National Laboratory
as part of the U.S. Department of Energy’s
Office of Energy Efficiency and Renewable
Energy, and results were documented
in a June 2002 report (Technology
Demonstration: Magnetically-Coupled
Adjustable Speed Drive Systems; DOE
Contract DE-AC06-76RL01830). The test
found that MC-ASDs were 2 percent to
4 percent more efficient in the 80 percent
to 100 percent of full-speed range than
traditional VFDs. However, they were
substantially less efficient in lower-speed
(less than 50 percent) ranges and similar
in efficiencies in the mid-ranges.
Lower efficiencies at the slower
speeds are explained by the fact that
the coupling produces more slip at these
RPMs, since the rotors are farther apart.
So the winner of the efficiency battle really
depends on where you will be operating
the pump or fan the majority of the year.
Furthermore, as shown in figure 1,
MC-ASDs and VFDs have similar installa-
tion costs, especially exceeding 50 HP. The
MagnaDrive has a tremendous advantage
over VFDs based on the fact that standard
motors can be used and National Electrical
Manufacturers Association B motors rated for
VFDs don’t have to be used. Typically there
is at least a 30 percent premium for inverter-
rated motors. This is especially important
when considering a retrofit installation since
a new motor is not required; however, this
motor cost savings for the MC-ASD option
is not reflected in figure 1, since it represents
only the drive installation costs.
• electromagnetic interference is a factor
if the unit is disassembled
• not much is known about them in the
industry
The sweet spot for using MC-ASDs
is on larger pump and fan retrofits where
one does not want to change out the
motor to achieve operational savings of
varying the pump or fan speed. One fur-
ther note is that warranties typically are for
two years, and MagnaDrives require some
bearing lubrication every 40,000 hours of
operation, but that can be done on the
same schedule as motor lubrications.
So if you have some existing motors
that are more than 50 HP, and you are
looking for another flavor to spice up your
energy conservation efforts, take a peek at
MC-ASDs. The kilowatt-hour savings they
offer can be absolutely peachy!
Based in Madison, Wis.,
Steve;Tredinnick,;PE, is
vice president of energy
services for Syska Hennessy
Group, which has more
than 16 locations across
the U.S. He has more
than 26 years’ experience
related to building heating, ventilation and air-conditioning systems. The past 15 years of his
work have been focused on district energy systems. Tredinnick is a graduate of Pennsylvania
State University with a degree in architectural
engineering. He is a member of IDEA and
ASHRAE and is currently immediate past chair
of ASHRAE TC 6. 2 District Energy. Tredinnick
currently serves on IDEA’s board of directors. He
may be reached at stredinnick@syska.com.
