Техническое чтение для энергетиков. Бухарова Г.П. - 72 стр.

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74
power generation requires some closely associated auxiliary equipment, which, in a
modern power plant, is driven almost exclusively by electric motors. Indicative of the
large number of motor applications in a steam station, a recent power plant compris-
ing two 75,000-kw turbines required over 700 auxiliary motors. In a typical plant the
auxiliaries consume approximately 6 percent of the total power output and have a
total horsepower rating of from 12 to 15 percent of the kilowatt rating of the main
turbine generators.
No two generating stations are identical. It is impossible to state exactly the
motor sizes and types that will be present in a steam generating station of a particular
size. The requirements are governed by such factors as type of fuel, heat cycle, source
of water, and anticipated station loading cycle.
Approximate sizes of the major auxiliary motors are given later as percentages
of the nominal rating of the turbine-generator unit. The figures are average values
based on a survey of steam stations with turbine-generator units of 100 megawatts
and below.
Characteristics of Powerhouse Auxiliary Motors.– The primary
characteristics to be considered in selecting auxiliary motors are size, speed, motor
type, torque requirements, operating conditions, class of insulation, and type of
enclosure. In addition, motors for central-station service must have special features
that insure reliability and ease of operation, features such as special moisture-resistant
insulation, adequate provision for oil-ring inspection on motors with sleeve bearings,
easy accessibility of the bearings and windings for servicing and inspection, and
adequate terminal boxes. The reliability, efficiency, and simplicity of installation and
control of the squirrel-cage induction motor have made it the almost universal choice
for powerhouse applications.
Powerhouse auxiliary motors range in size from less than one horsepower, used
to open and close valves, to several thousand horsepower, used to pump water into
the boiler. They usually have drip-proof enclosures with class A insulation, and are
designed to have low starting current and normal starting torque. However, some
auxiliaries require special torque or speed characteristics, or present unusual service
conditions such as excessive dirt, moisture, abrasive flyash, or high temperature; or
the plant may be an outdoor installation. Motors for such applications must have
special characteristics to satisfy these requirements.
Pump Motors.– Pumping is one of the major duties performed by powerhouse
auxiliary equipment, and usually the largest motors in the station are those that drive
the boiler-feed pumps. In a typical station the total horsepower rating of the boiler-
feed-pump motors is between five and six percent of the kilowatt rating of the
associated turbine. At least two and usually three boiler-feed pumps of equal rating
are used. These pumps operate against a very high head of water and require 3,600-
rpm driving motors.
The output of the boiler-feed pumps is controlled by throttling or by varying
the speed of the pump. The latter method is attractive because of reduced operating
cost. Variable-speed control, when used, is achieved with a variable-speed coupling
or by using a wound-rotor motor and a liquid rheostat.

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