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"Motor
Control a la Nola -- A Durable Technology "
With
trilingual summary
The Nola
device, named for its inventor who received a patent in 1977, acts to reduce
voltage applied to an a-c motor whenever the driven machine demands less power.
Using semiconductors to control voltage, the circuit senses the motor power
factor. As in-phase current decreases with reduced shaft torque, a corresponding
decrease in motor voltage maintains the same power factor. Nola's invention was
therefore widely known as a power factor controller. "Voltage controller" is
perhaps more appropriate.
Since
then, several variations have been developed. All, however, produce the same
result: lower motor voltage to suit reduced load. Because the motor's
voltage-dependent core loss then dominates, the result is an energy saving. The
smaller the motor, and the larger the number of poles, the more pronounced is
the effect.
Regardless of motor design, however, such control is cost-effective only for
motors subject to long periods of operation at half load or less. Typical
applications are industrial sewing machines, moving stairways, or certain
machine tools, for which frequent starting and stopping is impractical. Some
loads, such as mixers, may exhibit wide swings in load, but predicting the cost
saving from voltage control may be quite difficult.
With the
advent of solid-state starters, little extra cost is involved in adding voltage
control circuitry to the basic starter electronics, and such additions are
commonly made.
Although
exaggerated claims of energy savings were made for many of the early Nola
controllers, the principle remains sound. As with any energy-saving measure, the
motor user needs to consider both motor and load characteristics carefully
before making an economic decision.
From “Motor
control a la Nola,"
in the
Electrical Apparatus December 2007 issue
. Visit our online webstore to
order
copies. © 2007
Barks Publications, Inc. All Rights Reserved.
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