Text 8. Powerhouse auxiliary motors

A complete description of the many and varied motor applications found in modern steam station is almost the description of the station itself. Every phase of power generation requires some closely associated auxiliary equipment, which, in a modern power plant, id driven almost exclusively by electric motors. Indicative of the large number of water applications in the steam stations, a resent 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 generations.

No two generating stations are identical. It is impossible to state exactly the motor size and types that will be present in a steam generating station of the particular size. The requirements are governed by such factors as type of fuel, heat cycle, source of water, and anticipated station loading cycle.

Approximate size 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 bellow.

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 on 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 oil0ring 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 indication motor have made it the almost universal choice for powerhouse applications.

Powerhouse auxiliary motors range in size from less than one horse power, 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 auxiliary 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.