Determining Electric

Motor Load Factor

M ost likely your operation's
motors account for a large
part of your monthly elec-
Reasons to Determine Motor Loading
Most electric motors are designed to run at 50 to 100 percent of rated
trical bill. Make certain your mo- load. Optimum efficiency is at 75 percent of rated load. Thus, a 10 horse-
power (hp) motor has an acceptable load range of 5 to 10 hp; peak effi-
tors are properly loaded. This ciency is at 7.5hp (see Figure 1).Surveys of installed motors indicate
will increase their reliabiZity and that a large portion of motors in use are improperly loaded.
efjciciency-saving you money
and energy, and helping you Underloaded motors, those loaded below 50 percent of rated load, are
' avoid potential problems. This ineffiaent and exhibit low power factor (see Figure 2). Power factor is a
measurement of the phase angle lag between electrical voltage and cur-
Technology Update enables rent, with 100%as optimum. (See Technology Update, Reducing Power
you to determine the load factor Factor Cost for more information.) Low power factor results in in-
and efficiency of your electric mo- creased electrical distribution system losses. For this reason an increas-
tors-information you can use to ing number of utilities charge a penalty for low power factor. Replacing
identify motors that should be re- underloaded motors with correctly sized motors improves efficiency
placed with a different size or in- and raises power factor.
creased efficiency model.
I a
0
r
Motor Part Load Efficient
Y
as a Functlon of % Full Load E ficiency

.-: s o %
G
i;i
u 8oo/c
m
0

--
J

LL
3
40%
Lad Ranger:

c
c
Q1 20%
$
p.

0% 20% 40% 60% 80%

e
Percent Full Load

0 - 1 P .....lO..hP . ?!:SP. hP
P
1.5-5 hp ..........
15-25 hp I ------
75-100 hp

Toll-free Hotllne 1-800-872-3568 FAX 1-800-872-3882 Electronic Bulletin Board 1-800-762-3319

Bonnwille
POWER ADMINISTRATION
 Figure 2 rotates under its full rated load
Motor Power Factor and is usually about 3 to 5 per-
%O%j as a Function of % Full Load Amperage cent slower than synchronous
speed. Most electric motors have
a full load RPM rating stamped
on their nameplate.
Slip is characteristically linearly
proportional to load (see Figure
3). For example, a motor running
with 50 percent load factor has a
slip halfway between full load
and synchronous RPM. By using
a tachometer to measure actual
motor speed, it is possible to cal-
d a t e motor loads.
First, determine fullload slip
and measured load slip. The
formulas are listed in Figure 4.
i5% 45% !k'o i5% kyo 85% k0/, 1do%
Percent Full Load Amperage Then calculate load using the
formula in Figure 5.
Overloaded motors can overheat maintain motor loads within the
and lose efficiency. Many mo- design range.
tors are designed with a service
factor that allows occasional
overloading. Service factor is a
multiplier that indicates how
Determining if your motors are
properly loaded enables you to
make informed decisions about
Motorlat
Rep acement
Annual Savings and
"9
much a motor can be over- when to replace motors and
loaded. For example, a 10 horse- which replacements to choose. Simple Payback
power motor with a 1.15service Measuring Motor Loads is rela- The "Motor Load and Efficiency
factor can handle an 11.5 horse- tively quick and easy when you Analysis" form (on page 4) pro-
power load for short periods of use the worksheet provided in vides a stepby-step process for
time without incurring signifi- this Technology Update. You should determining motor load and effi-
cant damage. Although many perform a motor load and effi- ciency, and calculating the po-
motors have service factors of ciency analysis on all your major tential annual dollar savings
1.15,running a motor above the working motors as part of your and resulting simple payback
rated load reduces efficiency, preventative maintenance and en-
ergy conservation program.
and if done frequently, reduces
service life. I Figure 3
If your operation uses equip Percent Motor Slip
ment with motors that operate Determining Loads as a Furctlan of M
ob Load

for extended periods under 50

percent load, consider making A relatively simple way to deter-
modifications. Sometimes mo- mine the load on a working mo-
tors are oversized because they tor is by measuring "slip." Slip
must accommodate peak condi- refers to the difference between
tions, such as when a pumping the motor's synchronous speed
system must satisfy occasionally and loaded speed. Synchronous
high demands. Options avail- RPM is the speed of the motofs
able to meet peak condition rotating electric field, usually 0
needs include two-speed mo- either 3,600,1,800,1,200,or 900
tors, adjustable-speed drives, RPM. An unloaded motor ro- ND
Load
Full
Load

and management strategies that tates at approximatelyits syn- Load

chronous speed. Full load FWM
is the speed that the motor
2
full load = motor HP x (measured running slip/full load slip)
load Slip = synchronous speed (RPM) - rated fullload speed (RPM)
measured Example:
load Slip = synchronous speed (RPM)- measured load speed (RPM)
-
Example:
10 HP 1,800 RPM synchronous motor

full load speed = 1,740

full load slip = 60
synchronous speed = 1,800 RPM
measured load speed = 1,778 RPM
full load speed (f" nameplate) = 1,740 RPM measured load slip = 22 RPM
full load slip = 1,800 - 1,740 = 60 RPM
10 x 22/60 = 3.7 HP load, or 37%o hull load
measured load speed (bytachometer) = 1,778 RPM
-
measured load slip = 1,800 1,778 = 22 RPM

from replacing a significantly

oversized and underloaded mo- to rounding errors and vari-
tor with a more efficient, prop- ations. For example, full load
erly sized model. (Simple H The slip technique should not ratings are often rounded to
payback is a measurement of be used for rewound motors the nearest 5 RPM, and a spe-
how long it will take for the effi- or motors operating at other cific motor may have an ac-
ciency savings to compensate for than design voltage since full tual speed slightly different
the increased cost of the more ef- load RPM nameplate data is than the nameplate value es-
ficient motor .) no longer accurate. tablished for that model.
These differencescan lead to
We recommend that you survey H This analysis is designed for significant variation and un-
and test all motors operating over motors operating under certainty in this analysis,
, OO hours per year. Using the
1O steady load conditions. If which should be considered
analysis results, divide your m e loads vary, you'll need to per- when evaluating results.
tors into the following categories: form a separate analysis for
each portion of the load m e 4 Many utility energy conser-
H Motors that are significantly to obtain annual energy sav- vation programs offer rebates
oversized and underloaded- ings. for the purchase of energy
replace with more efficient, efficient motors and other
properly sized models at the 4 This analysis may be inappro- industrial conservation activi-
next opportunity, such as priate for motors driving con- ties. Contact your utility con-
scheduled plant downtime. veyors or crushers, since servation department for
oversizing may be required information.
Motors that are moderately due to startup torque require-
oversized and underloaded- ments, transient loads, or ab-
replace with more efficient, normal operating conditions.
properly sized models when
they fail. 4 This analysis requires use of a
tachometer to measure motor
Motors that are properly RPM, and a Multimeter to
sized but standard effi- measure voltage and current.
ciency-replace most of these Such monitoring is poten-
with energy-efficient models tially hazardous and requires
when they fail. The cost effec- proper equipment calibration;
tiveness of an energy-efficient it should only be performed
motor purchase depends on by trained personnel.
the number of hours the mo-
tor is used, the price of elec- 4 Several of the values used in
tricity, and the price premium this analysis, including rated
of buying an energy-efficient full load RPM, measured
motor. RPM, and current are subject

3
Company Plant Datemime

Building Dept. Application

Motor Shop Number Phase and HZ

Make Frame size

Model/Type Insulation class

Serial number Efficiency rating

Service factor NEMA torque type

Enclosure type Temperature rise

A) Full load HP

B) Volts N) Running slip (E-K)

C) Amperes P) Per cent load (N/F)(~00%)

D) Full load speed Q) HP Output (A)(P)(l 00%)

E) Sync. speed R) kW output (cq(o.746)

2 pole = 3,60O,4 pole = 1,800, 6 pole = 1,200)

F) Full load slip (E-D) S ) Eff. per cent (w~)(loo%)

T) kVA input (G)(H)(1.732)/(1000)

G) Average volts U) Power factor (J)/(r)(i 00%)

H) Average amperes v) kW IOSSeS (J-R)

J) Average kW W) $/Year operation (J)(L)(M)

K) Operating speed, RPM X) $/Year losses (v)(L)(M)
L) Full load operating hours Y)' Annual energy savings due to changeout with a h p
high-efficiency motor (R)(L)(M)(lOO/S - 1WIEFF)

W) Average electricity price including demand charges Z)* Replacement motor cost
($/kWh)

Simple payback, years (Z)/(Y)

@

EFF Is the efflclency (%) of a mpiacement premium efflclency motor at the appropriate load factor.
!Costis the total cost of purchaslngand installing an optlmaiiy slzed, high-efficiency motor

4
V Electric Ideas Clearinghouseis a
comprehensive information Bibliography
source for and indus- McCoy, G., T.Litman, and J. Douglas. Energy-Efficient Electric Motor Selec-
trial energy users. It is operated tion Handbook. Prepared for the Bonneville Power Administration by the
by the WashingtonState Enflu Washington State Energy Office. DOE/BP-34623-3. October 1991.
Office and is part of the Electric
Ideas technology transfer pro-
gram sponsored by participating
utilities and the Bonmille
Power Administration.
V Neither the United States nor the
Bonneville Power Administra-
tion, the state of Washington, the
WashingtonState Energy Office,
nor any of their contractors, sub-
contractors, or their employees
make any warranty, expressed or
implied, or assume any legal re-
sponsibility for the accuracy, com-
pleteness, or usefulness of any
information,apparatus, product,
or process disclosed within the
publication or through the Clear-
inghouse Bulletin Board System.
V Technology Update CH-17
'
I Toll-free Hotline:
1-800-872-3568
Fax: 1-800-872-3882
Electronic Bulletin Board:
1-800-762-3319

DOYBP-1768
lune 1992
15C