TEXT 18. POWERHOUSE AUXILIARY MOTORS
A complete description of the many and varied motor applications found in a modern steam station is almost a description of the station itself. Every phase of 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 comprising 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.
The torque requirements of boiler-feed pumps and most other pumps are satisfied by motors with low starting current and normal starting torque. Most boiler-feed-pump motors are rated for a temperature rise of 40 degrees С above ambient and have class A insulation. Where the ambient temperature is above 40 degrees, class В insulation is used.
Although drip-proof construction is usual, special enclosures are sometimes used to reduce the noise level of the motor or to protect the motor from flyash and other unfavourable atmospheric conditions. Noise can be reduced by using pipe- or base-ventilated motors in which the inlet and exhaust for cooling air are at a remote location. In particularly dirty locations enclosed motors are used. Air for such motors is cooled by either an air-to-air or an air-to-water heat exchanger. Since outside air is never drawn into the motor, the windings are protected from contamination.
In addition to boiler-feed pumps, numerous other pumps are associated directly with the water cycle of the plant or perform auxiliary functions. These include pumps for handling circulating water, condensate, drain water, raw water, water-purification chemicals, ash, flood water, water for fire protection, sump water, lubricating oil, and station water supply. Usually the largest of these are the circulating-water pumps. In a typical station, there are two circulating-water pumps per turbine with a total horsepower slightly less than one percent of the turbine rating. The remaining pumps range in size from a fraction of a horsepower for small chemical-feed pumps to 100 to 300 horsepower for some raw-water, ash, and fire pumps. The size of the driving motor in a particular application is determined by the head and capacity requirements, which are influenced by the nature of the water source.
Air-and Coal-Handling Motors.— Fans supplying air for combustion are usually driven by squirrel-cage motors. Where variable speed is desired for control and to reduce fan wear, wound-rotor induction motors or squirrel-cage induction motors with hydraulic or magnetic couplings are used. Frequently, two-speed squirrel-cage induction motors are used to take advantage of the reduced fan wear at the slower speed permissible during light-load periods.
In a typical station, induced-draft fans have a total horsepower of 2.5 percent of the turbine rating, while forced-draft fans have a total horsepower of approximately 1.2 percent. Usually two forced-draft fans and two induced-draft fans are used per boiler. Forced- and induced-draft fans are often installed in locations with abnormally high ambient temperatures. For such applications the motors must have class В insulation. With unfavourable atmospheric conditions, totally enclosed, fancooled motors are used for the fan drives.'
Most stations using coal for fuel burn the coal in pulverized form. Pulverizers present the problem of a high starting torque, high-inertia load. The starting torque requirements vary with the type of pulverizer, the amount of moisture in the coal, and the duration of shutdowns. Motors are selected that can start the mill with favourable coal conditions after a momentary shutdown and have sufficient thermal capacity to allow two consecutive normal starts at full voltage. The motors are either high-torque or low-torque, drip-proof, squirrel-cage induction machines with class A insulation.
Motors driving coal-handling machinery are subject to particularly severe operating conditions and must possess special characteristics. They operate in dust- and moisture-laden areas; therefore, totally enclosed or totally enclosed fan-cooled motors are necessary.
Notes
powerhouse['pauahaus] - електростанція
exclusively[iks'klu:sivli] - виключно
bearing['beani]] - навантаження
to throttle['sratl] - регулювати
sump[sAmp] - брудний
ambient['aembiant] - оточуючий
to lubricate['lu:bnkeit] - мастити
squirrel['skwiral] cage - замкнутий
synchronous ['sinkranas] - одночасний
combustion[kam'bstсn] - згоряння
consecutive[kan'sekjutiv] - послідовний, наступний
favourable['feivarabl]-сприятливий
shutdown -закриття
equipment - обладнання, устаткування
rpm = revolution(s) per minute – обертання за хвилину
Comprehension questions:
1. What are the main requirements of power generation?
2. Discuss with your partner characteristics of powerhouse auxiliary motors.
3. What is the intention of pump motors?
4. What did you learn from the text about air-and coal-handling motors?