Diagnostics and testing of rotating electrical machines - DNV Kema
Diagnostics and testing of rotating electrical machines - DNV Kema Diagnostics and testing of rotating electrical machines - DNV Kema
08 ENERGY Non-destructive testing & plant diagnostics Rotor dynamics and electromagnetic fields A powerful but dangerous combination A rotor is exposed to a complex spectrum of forces of both mechanical origin (rotation) and electrical origin (electromagnetic fields). Furthermore, a rotor is an assembly of numerous components that have to operate in perfect balance and fully insulated from one another. The main components of a turbo generator rotor are: ■■ Rotor body ■■ Windings ■■ End winding retaining rings ■■ Rotating rectifier at very high speed (typically 3,000/3,600 rpm). Furthermore, although the rotor experiences great mechanical stress and high temperatures (in some cases up to 300°F/150°C) while subjected to electrical voltage and current, it is expected to function in this manner for years without failure. Each component has its own particular properties and therefore requires specialist testing and analysis. The generator rotor The rotor can be visualized as a large rotating electromagnet with north and south poles. The generator rotor represents an excellent combination of electrical, mechanical and manufacturing skills in which the field coils are well insulated, supported and ventilated in a compound structure rotating
Non-destructive testing & plant diagnostics ENERGY 09 The three design constraints limiting the size and life of generator rotors are temperature, mechanical force and electrical insulation. Degradation of the generator field can be caused by a number of factors, including a breakdown in insulation due to time and temperature and mechanical wear. To understand the intricacies of the field winding design, we must remember that the basic function of the rotor is to produce a magnetic field of the size and shape necessary to induce the desired output voltage in the stator. The winding and components should be designed to require little maintenance during the 30 or more years of expected operation, which is the typical lifetime for a base-loaded power station. Rewinds may be more frequent under extreme conditions such as an open ventilated gas turbine generator in a dirty environment, or frequent start/stops or load cycling. As a generator rotor ages, its insulation can be affected by temperature, mechanical wear and operating incidents. Rotor forging and other rotor components are also at risk. The most common problems occurring with generator rotors are shorted winding turns and breakdown in groundwall insulation. Rotating diode bridge assembly There are two types of excitation systems which can be employed for generators, the brushless excitation system and the static excitation system. With the brushless excitation system the exciter consists of two basic component assemblies, the exciter stationary field assembly and the exciter rotor comprising the rotating exciter armature and a rotating rectifier bridge assembly. A turbo generator rotor mainly consists of the following components ■■ Rotor body ■■ Windings ■■ End winding retaining rings ■■ Rotating rectifier
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08 ENERGY Non-destructive <strong>testing</strong> & plant diagnostics<br />
Rotor dynamics <strong>and</strong><br />
electromagnetic fields<br />
A powerful but dangerous combination<br />
A rotor is exposed to a complex spectrum <strong>of</strong> forces <strong>of</strong> both<br />
mechanical origin (rotation) <strong>and</strong> <strong>electrical</strong> origin (electromagnetic<br />
fields). Furthermore, a rotor is an assembly <strong>of</strong> numerous<br />
components that have to operate in perfect balance <strong>and</strong> fully<br />
insulated from one another. The main components <strong>of</strong> a turbo<br />
generator rotor are:<br />
■■<br />
Rotor body<br />
■■<br />
Windings<br />
■■<br />
End winding retaining rings<br />
■■<br />
Rotating rectifier<br />
at very high speed (typically 3,000/3,600 rpm). Furthermore,<br />
although the rotor experiences great mechanical stress <strong>and</strong> high<br />
temperatures (in some cases up to 300°F/150°C) while<br />
subjected to <strong>electrical</strong> voltage <strong>and</strong> current, it is expected to<br />
function in this manner for years without failure.<br />
Each component has its own particular properties <strong>and</strong> therefore<br />
requires specialist <strong>testing</strong> <strong>and</strong> analysis.<br />
The generator rotor<br />
The rotor can be visualized as a large <strong>rotating</strong> electromagnet<br />
with north <strong>and</strong> south poles. The generator rotor represents<br />
an excellent combination <strong>of</strong> <strong>electrical</strong>, mechanical <strong>and</strong><br />
manufacturing skills in which the field coils are well insulated,<br />
supported <strong>and</strong> ventilated in a compound structure <strong>rotating</strong>
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