8/10/2019 Motor Torque Reqs
1/245Electricity Today Issue 3, 2003
AC motors are used worldwide inmany residential,
commercial,industrial and utility applica-
tions. Motors transform electrical ener-gy into mechanical
energy. An AC motormay be part of a pump or fan, or con-nected to
some other form of mechanicalequipment such as a winder, conveyor
ormixer. AC motors are found on a varietyof applications from those
that require asingle motor to application requiringseveral
motors.
A number of factors effect the correctselection of a motor for a
specific appli-cation. One of these factors is whetherthe specific
torque capabilities of amotor meet the torque requirements ofthe
applications load.
AC motors running on an AC lineoperate with a constant flux ()
becausevoltage and frequency are constant.Motors that operate with
constant fluxare said to have constant torque. Actualtorque
produced, however, is deter-
mined by the demand of the load.
Matching AC Motors to the LoadOne way to evaluate whether
the
torque capabilities of a motor meet thetorque requirements of
the load is tocompare the motors speed-torque curvewith the speed
torque requirements of
the load.To find the torque characteristics, a
table, similar to the partial one shownabove, can be used. NEMA
publicationMG 1 is one source of typical torquecharacteristics.
Note: NEMA(the National ElectricalManufacturers Association)
sets stan-dards for a wide range of electrical prod-ucts, including
motors. NEMA is pri-marily associated with motors used inNorth
America. These standards can befound in NEMA Standard
PublicationNo. MG 1. Some large AC motors may
not fall under NEMA standards and areusually motors built to
meet the require-ments of a very specific application.
Calculating load torqueThe most accurate way to obtain
torque characteristics of a given load isto obtain them from the
equipment man-ufacturer. A simple experiment can beset up to show
how the torque of a givenload can be calculated. In the
illustrationbelow, a pulley is fastened to the shaft ofa load that
a motor is to drive. A cord iswrapped around the pulley with one
endconnected to a spring scale.
The torque can be calculated bypulling on the scale until the
shaft turnsand noting the reading on the scale. Theforce required
to turn the shaft, indicated
by the scale, times the radius of the pul-ley equals the torque
value.
It must be remembered that theradius is measured from the center
of theshaft. If the radius of the pulley and shaftwere 1 foot, for
example, and the forcerequired to turn the shaft were 10pounds, the
torque requirement is 10 Lb-Ft. The amount of torque required to
turnthe connected load can vary at differentspeeds.
MOTORS AND DRIVES
ENSURING THE MOTOR YOU SELECT MEETS
YOUR LOADS SPECIFIC TORQUE REQUIREMENTS
IS CRITICAL FOR OPTIMIZING OPERATING CONDITIONS
One way to evaluate whether the torque capabilities of a motor
meet the torque require-ments of the load is to compare the motors
speed-torque curve with the speed torquerequirements of the load.
To find the torque characteristics, a table, similar to the
partialone shown above, can be used. NEMA publication MG 1 is one
source of typical torquecharacteristics.
A simple experiment can be set up to show how the torque of a
given load can be calcu-lated. In the illustration above, a pulley
is fastened to the shaft of a load that a motor is todrive. A cord
is wrapped around the pulley with one end connected to a spring
scale. Thetorque can be calculated by pulling on the scale until
the shaft turns and noting the read-ing on the scale. The force
required to turn the shaft, indicated by the scale, times the
radius
of the pulley equals the torque value.
Continued on page 48
8/10/2019 Motor Torque Reqs
2/2Electricity Today Issue 3, 200348
Centrifugal pumpWhen a motor accelerates a load
from zero to full-load speed, the amountof torque it can produce
changes. At anypoint during acceleration and while themotor is
operating at full load speed, the
amount of torque produced by the motormust always exceed the
torque requiredby the load.
In our first example, a centrifugalpump has a full-load torque
of 600 Lb-Ft. This is equivalent to 200 HP. Thecentrifugal pump
only requires approxi-mately 20 per cent of full-load torque
tostart. The torque dips slightly after it isstarted and then
increases to full-loadtorque as the pump comes up to speed.This is
typically defined as a variabletorque load.
A motor has to be selected that canstart and accelerate the
centrifugalpump.
By comparing a 200 HP NEMA Bmotor curve to the load curve, it
can be
seen that the motor will easily start andaccelerate the
load.
Screw down actuatorIn our second example, a screw
down actuator is used. The startingtorque of a screw down
actuator isapproximately 200 per cent of full-loadtorque. Comparing
the loads require-ment (200%) with the NEMA design Bmotor of
equivalent horsepower, it canbe seen that the loads starting
torque
requirement is greater than the motorscapability (150%). The
motor, therefore,will not start and accelerate the load.
One solution would be to use ahigher horsepower NEMA B motor.
Aless expensive solution might be to use aNEMA D motor of the same
horsepowerrequirements as the load. A NEMA Dmotor would easily
start and acceleratethe load.
The motor selected to drive the loadmust have sufficient torque
to start,accelerate, and run the load. If, at anypoint, the motor
cannot produce therequired torque the motor will stall orrun in an
overloaded condition. This willcause the motor to generate excess
heatand typically exceed current limits caus-ing protective devices
to remove themotor from the power source.
If the overload condition is not cor-
rected, or the proper motor installed, theexisting motor will
eventually fail.
With material from the SiemensTechnical Education Program
bookletAC Motors.ET
CLASSIFIEDS
For 21 years weve specialized in placing people whotest, repair,
maintain or sell electrical apparatus, elec-tronic or electric
power equipment. Nationwide. All feescompany paid. Call or send
confidential resume toLISA LINEAL:
46 Copper Kettle Road, Trumbull,CT 06611TOLL FREE 877-386-1091
Fax: 203-386-9788
Email: [email protected] www.lineal.com
RecruitingLINEALServices
Power UPS NETA Certified
Field Service Engs Techs Mgrs
Sales Design Vibration, Motor Repair,
Pred/Prev Maintenance
Looking for an inexpensive way
to market yourproducts/services to thousands
of North American electricalindustry professionals?
Contact:
The Electricity ForumBrian Wallington
[email protected]
Barbara John
315-789-8323
[email protected]
MOTORS AND DRIVES
Centrifugal pump example
Continued from page 45
Screw down actuator example
mailto:[email protected]://www.lineal.com/http://www.lineal.com/mailto:[email protected]:[email protected]://www.cyme.com/mailto:[email protected]:[email protected]://www.cyme.com/mailto:[email protected]:[email protected]:[email protected]://www.lineal.com/mailto:[email protected]
