Motor Torque Reqs

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

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MOTORS AND DRIVES

Centrifugal pump example

Continued from page 45

Screw down actuator example
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Motor Torque Reqs