Page 282 - Steam Turbines Design, Applications, and Rerating

Transmission Elements for High-Speed Turbomachinery 261

VORECON requires the usual external input signal (for example, 0/4
to 20 mA). An electronic logic unit handles the internal control of the
VORECON. The logic unit is speed-dependent. Instrumentation for
monitoring bearing and oil temperatures as well as oil pressure is pro-
vided as standard. It is designed so that it can be incorporated in a
plant-monitoring system.

13.5.1 Function of the multistage
variable-speed drive
The operation of the hydrodynamic drive can be divided into two
ranges. In the first range the power is transmitted by the variable-
speed coupling directly through the planetary gear. The speed is con-
trolled by changing the level of oil in the coupling using the adjustable
scoop tube. The regulating range is from 0 to approximately 80 percent
speed. The torque converter is drained and therefore has no function in
the first range. The hydrodynamic brake is filled and generates the
counter torque for the planetary gear, which is designed as a superim-
posing device. A hydrodynamic rather than a mechanical brake was
chosen because it allows the planetary gear to rotate continuously at
slow speed, preventing brinelling or chatter marking of the gear teeth.
In the second range the impeller and turbine wheel of the hydraulic
coupling are locked together by the clutch, bridging the input and output
of the variable-speed coupling so that the drive motor is now coupled
mechanically to the driven machine.The regulating range from 80 to 100
percent speed is covered using the torque converter. Speed regulation is
effected by changing the guide vane position of the torque converter.
An additional drive via the planetary gear fixed is superimposing the
speed of the revolving planetary gear. The hydrodynamic brake is
drained.

13.5.2 Design and operating details
As depicted in Fig. 13.19, variable-speed turbo couplings can often be
combined with one or more gear stages in a common housing. The bot-
tom part of this compact unit forms an oil sump. From the basic concept
consisting of a speed-increasing gear followed by a variable-speed tur-
bocoupling, other models have been derived to provide stepless speed
control for both high-power, high-speed machines such as boiler feed
pumps and compressors and with a speed-reducing gear low-speed
machines such as coal mills, ID fans, and crude oil pumps. The Voith
turbocoupling is a hydrodynamic fluid coupling. Power developed by the
prime mover is converted into kinetic energy in the impeller (primary
wheel) of the turbocoupling and converted back into mechanical energy
in the turbine wheel (secondary wheel) which is connected to the driven