Alternating-current motors

Alternating-Current Motors, Wound-Rotor Induction. 29-7... 

Within these basic principles there are many types of electric motors. Each has its own individual operating characteristics peculiarly suited to specific drive applications. Equations (29-1) through (29-9), presented in Table 29-1, describe the general operating characteristics of alternating-current motors. When several types are suitable, selection is based on initial installed cost and operating costs (including maintenance and consideration of rehability). 

TABLE 29-1 Useful Formulas for Alternating-Current Motors... 

Synchronous Alternating-Current Motors These motors run in exact clock synchronism with the power system. For most modern power systems, these are truly constant-speed motors. 

Alternating-current motors are classified as induction motors or synchronous motors. Faraday found that a stationaiy wire in a magnetic field produced no current. However, when the wire continues to move across magnetic lines of force, it produces a continual current. When the motion stops, so does the current. Thus Faraday proved that electric current is only produced from relative motion between the wire and magnetic field. It is called an induced current—an electromagnetic induction effect. 

This chapter gives primary attention to the commonly used general-purpose alternating current motors, although others are noted to acquaint the engineer with the variety of motors, motor enclosures, and mechanical drive turbines available and sometimes used in the chemical and petrochemical industries. Because direct-current motors are not frequently used in the process industries (other than in heavy-duty processes), they are not examined further in this chapter. 

Alternating current motors are either single-phase current or polyphase (three-phase current). The polyphase motors are... 

This is the most common kind of alternating current motor used by the industry. The term squirrel cage vs usually... 

A general-purpose alternating-current motor is an induction motor, rated 200 horsepower and less, which incorporates all of the following ... 

Pull-up torque For an alternating current motor, this is the minimum external torque developed hy the motor during the period of acceleration from rest to the speed at which breakdown torque occurs. For motors that do not have a definite breakdown torque, the pull-up torque is the minimum torque developed up to the rated speed. 

Breakdown torque For an alternating current motor, this is the maximum torque that it will develop with rated voltage applied at rated frequency, without an abrupt drop in speed. 

Alternating-current motors are of three general types, induction, synchronous, and series, and are defined as follows ... 

Induction Motors. An induction motor is an alternating-current motor in which a primary winding on one member (usually the stator) is connected to the power source and a polyphase secondary winding or a squirrel-cage secondary winding on the other member (usually the rotor) carries induced current. There are two types ... 

Power Factor. The power factor of an alternating-current motor or generator is the ratio of the kilowatt input (or output) to the kVA input (or output) to the kVA input (or output) and is usually expressed as a percentage. 

Service Factor of Aiternating-current Motors. The service factor of an alternating-current motor is a multiplier that, when applied to the rated horsepower, indicates a permissible horsepower loading that may be carried under the conditions specified for the service factor (see MG 1-14.35). 

Locked-Rotor Current. The locked-rotor current of a motor is the steady-state current taken from the line with the rotor locked and with rated voltage (and rated frequency in the case of alternating-current motors) applied to the motor. 

A steam turbine is a machine with an ancient genealogy. It is a direct descendant of the overshot water wheel, used to kick off the industrial revolution in England, and the windmill still used in Portugal. Turbines are widely used in process plants to drive everything from 2-hp pumps to 20,000-hp centrifugal compressors. They are versatile machines, in that they are intrinsically variable-speed devices. Electric motors are intrinsically fixed-speed machines. It is true that there are a variety of ways to convert AC (alternating-current) motors to variable speed, but they are all expensive and complex. 

But, we are working with an ordinary AC (alternating-current) motor—which is a fixed-speed device. There are then three methods available to control the temperature in the evaporator ... 

Most of the pumps we see, both at home and at work, are centrifugal pumps. And most of these pumps are driven by constant-speed, alternating-current motors. 

MOTOR SHALL BE PROVIDED IN ACCORDANCE WITH DATA GIVEN ON STANDARD DATA SHEET FORM E242P1 ELECTRICAL ALTERNATING CURRENT MOTORS. 

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