Electrical noise can be a major source of problems, especially when using VFDs and high horsepower motors in
close proximity to the encoder. Differential line drivers and
properly shielded twisted-pair cabling help to minimize these
headaches.
Environmental issues are also a very important consideration when selecting the right encoder. Because industrial
pumping systems are rarely used in a clean environment, an
electronic device such as an encoder needs to be properly protected. Contaminants like water, grease, oil, and particulate
matter can interfere with the proper operation of the encoder.
Published NEMA and/or IP ratings are a help here when
selecting the right encoder for a pumping application. An
environmentally-sealed encoder suitable for pump use will
usually be rated at NEMA 4/IP65 or greater, depending upon
application.
Figure 1 is a good example of a rugged motor-mount
encoder suitable for use in a pumping application.
On this type of encoder, IP67 shaft seals and an unbreakable code disc make for a reliable platform. Differential line
driver outputs mean high noise resistance and compatibility
with most VFDs on the market. Field-serviceable connectors
with ½-inch NPT input should be used to eliminate the need
for soldering – a big plus with maintenance departments.
A dual isolated output option can make this type of unit
even more of a reliability-centered item because of the “built
in spare” that the second output will provide. This type of
encoder should also have an insulated bore to help eliminate
those grounding issues described earlier.
Really big applications with large motor shafts usually
require a different type of encoder, such as the unit shown in
Figure 2.
This kind of encoder can accommodate large shafts up to
4.5-in in diameter. Mounting is similar to the smaller hollow-shaft units, using a torque arm or swivel ball tether to prevent
the encoder body from rotating. However, this type of encoder
Figure 1. This hollow-shaft encoder is an example of an optical
encoder for pump-motor speed control feedback.
utilizes magnetic technology to generate pulses.
Magnetic encoders are typically more robust than optical
units because they are more resistant to contamination from
water, debris, and some chemicals. These kinds of magnetic
encoders also have field-replaceable sensors. If one sensor gets
zapped by a current spike, it can be removed on the fly – with
the motor still running – for a quick replacement, meaning
low downtime.
Bottom Line
As plants and mills become more reliability-centered, anything
done to increase reliability and boost productivity is a welcome
addition. The recent advances in drive technology make the
up-front costs of converting to a fully speed-controlled system
a bit more reasonable for the budget-conscious user. Couple
this with the energy savings that can quickly result, and the
return on investment is attractive.
P&S
Figure 2. This is a typical magnetic encoder suited for use with pump
motors in dirty environments.
Allen Chasey is the marketing manager for DynaparTM,
1675 Delany Road, Gurnee, IL 60031-1282, 678-817-
4945, Fax: 678-817-4949, allen.chasey@dynapar.com,
www.dynapar.com.
