Application Note

4 Fluke Corporation Optimizing electric-motor efficiency – Part II
Correcting insulation resistance can be
achieved by de-rating the motor in accordance
with NEMA standards, if it will not too greatly
impact production. Under any circumstances, gen-
erate a work order to replace the motor as soon as
possible with a high-efficiency motor, if operating
conditions warrant the upgrade.
Potential savings and ROI
To determine whether insulation degradation
merits a motor replacement, try using these tools:
•
Use MotorMaster+ software.
•
Calculate potential savings from motor rated
horsepower (hp), load factor, annual operating
hours, average energy costs, efficiency of the
motor being replaced and the efficiency rating
of the new motor. (See the sample calculation in
the Current Unbalance section in Part I of this
application note.
4
)
Recall that premium efficiency motors are about
one percent more efficient than standard efficiency
motors, and the energy savings will typically
result in a payback period of less than 18 months.
Inrush
If you’ve replaced older motors with high-effi-
ciency models, you may encounter inrush
problems. Granted, the older models being
replaced were probably out of spec, worn, and
didn’t have any compensating electronics. How-
ever, high-e models can sometimes generate
substantially more initial start-up current—or
inrush current—than their operating or steady-
state current.
While all motors draw inrush current, it is
greater in high-efficiency motors. In a three-
phase motor, for example, inrush current generally
lasts about 100 milliseconds with a current spike
between 500 and 1200 percent. While short
lived, this surge can create problems, the most
annoying of which is tripping the over current
protection device (OPCD).
Inrush current can be five-to-twelve times the
normal running current, depending on the type
of motor. For example, if the running current of a
motor is 8 A and the starting current multiplier is
five times the running current, your inrush mea-
surement should read about 40 A, even though
the circuit breaker is rated for 20 A.
The reason the breaker or overload unit does
not trip is because both of these devices have
a time versus current curve that indicates how
much current for how long a period they will pass
without opening the circuit. Tripping can also
be based on temperature—temperature can build
up slowly during continuous operation, or more
quickly during startup, but when the maximum
temperature threshold is reached, the circuit trips.
Measurements
Start by measuring inrush current at startup. Use
a tool specifically designed for inrush, such as the
Fluke 337 Clamp Meter, that will automatically
synchronize your measurement starting point with
the motor’s start point. Your tool’s inrush function
will take a large number of sample measurements
during the startup period, filter them, and calcu-
late the actual starting current.
Note: Don’t try to use Min/Max to capture inrush. Max current
can measure a momentary spike in power, but an Inrush feature
measures the same energy delivered to the motor that the APCD
measures and trips on. Max current measurements used to work on
old models, but more advanced electronics mean more noise on sig-
nals, too much for an accurate inrush measurement.
Correcting Inrush
If the running current or temperature is too close
to the continuous rating of the circuit breaker, then
the motor will usually cause the breaker to trip
on starting almost all of the time. Since inrush is
always factored in for breaker sizing before instal-
lation, when upgrading to new more efficient
motors, it may be necessary to upgrade the motor
circuit protection to a newer type motor rated cir-
cuit breaker with adjustable instantaneous trip
settings.
Notes
1
The information in this section can also be found in the source document, Motor Tip
Sheet #9 (June 2008), “Improve Motor Operation at Off-Design Voltages,” an Energy
Tips – Motor document written for DOE’s Industrial Technologies Program.
2
DOE’s fact sheet: “Optimizing Your Motor-Driven System.”
3
www.fluke.com/insulationtestguide
4
www.fluke.com/electric-motors
Fluke Corporation
PO Box 9090, Everett, WA 98206 U.S.A.
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
For more information call:
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Web access: http://www.fluke.com
©2009 Fluke Corporation.
Specifications subject to change without notice.
Printed in U.S.A. 12/2009 3511717B A-EN-N
Modification of this document is not permitted
without written permission from Fluke Corporation.
Fluke. Keeping your world
up and running.
®