Inside
Insights
Magnetic Couplings:
A peachy ‘new’ flavor for ASDs?
Steve Tredinnick, PE, Vice President of Energy Services, Syska Hennessy Group
Editor’s Note: “Inside Insights” is a column designed to address ongoing issues
of interest to building owners, managers
and operating engineers who use district
energy services.
Spring has just arrived, and with sum- mer on the way, my thoughts turn to days at the beach, brats on the
grill and a cold brewski in hand – or perhaps a more G-rated treat, like a good,
old-fashioned ice cream cone. Speaking of
which, have you ever tried Cherry Garcia
or Peachy Paterno (the latter vended at the
Penn State Creamery in honor of my alma
mater’s beloved football coach)? They happen to be my favorites, both tasty alternatives to plain vanilla – kind of like magnetic
couplings in lieu of a variable-speed drive.
Allow me to explain: In these tough
economic times when we are all looking
to pinch pennies or stretch the few that
we have, there may be opportunities to
do so right before our eyes, but we miss
them. One example is adding an adjustable-speed drive (ASD) to a pump or fan
motor to reduce electrical consumption at
part load. Varying the speed of a pump or
fan takes advantage of the affinity laws
where power is proportional to the cube
of the shaft speed.
ASDs come in two flavors: those
that cause the motor to rotate at varying
speeds and those that act as a clutch to
introduce slip in the system. The majority of ASDs are variable-frequency drives
(VFDs), which by definition use electronics
to vary the frequency of the alternating
current to the motor, thereby varying the
motor speed – pretty much the vanilla
flavor. For a more exciting twist, there
are the ASDs that keep the motor speed
constant and are more mechanical-drive
than electronic. These are more the Peachy
Paterno-variety ASDs.
Variety and Savings
One such ASD class is magnetically
coupled ASDs (MC-ASDs), which have no
physical connection between the motor shaft
and the load portion of the shaft. For those
of you who are old enough to remember:
The MC-ASD is sort of an updated version
of the Eddy-current clutch of years gone by.
A change in the air gap between the
magnetic surface and magnets offers precise
torque transfer and therefore speed control
from 0 percent to 100 percent of the load.
The MC-ASD has two rotor plates
mounted on drive shafts separated by an
air gap. A high-energy rare earth magnetic
rotor is mounted on the load (pump or fan
impellor) side and a conductor rotor on the
motor side. Similar to the magnetic effect
that microbrews have on me (like a carrot
on a stick to a horse, or a ring in a bull’s
nose), the two rotor plates interact with
one another. The closer they are together,
the stronger the attraction and the less slip;
hence, more torque is transferred magnetically from the constant-speed motor to the
variable-speed pump shaft.
Conversely, the farther away, the
more slip, the less torque is transmitted,
and the load side slows down. The magnetic coupling is nonenergized, but motive
force is required to move the two plates
back and forth – typically an electric or
pneumatic actuator.
In these tough economic times we
are all looking to pinch pennies or
stretch the few that we have.
Photo Steve Tredinnick.
At the University of Wisconsin-Madison’s Charter Street Heating Plant, a MagnaDrive magnetic coupling
is used as an adjustable speed drive on a 1,000 HP, 4,160 volt chilled-water distribution pump.
