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178 Gas Turbine Handbook: Principles and Practices
There are three general types of converters:
• DC Link Converter
• High Frequency Link Converter
• Cycloconverter
A list of microturbine and power converter manufacturers is
provided in Appendix C-15. Power electronic converters operate either
in voltage mode or current mode. Current mode is used when
the microturbine is connected to the grid. Voltage mode is used when
the microturbine is in the “island” or stand alone mode.
Bearings
Because operating conditions are very demanding at the high
rotational speed (up to 100,000 rpm), bearings are a critical component
of the microturbine. Conventional hydrodynamic and anti-friction bearings,
employing a pressurized lube oil system (including a pump and an
oil cooler) are still employed by some manufacturers. However, advances
have been made into the use of “air,” “gas” of “film” bearings.
This type of bearing uses a thin film of pressurized air to support
the shaft. Fluid film bearings utilize a thin film of air between
the rotating shaft and the stationary housing and this film transfers
the forces from one to the other. The fluid film is accomplished by
delivering airflow through the bearing compartment. One design
technique creates the air film with an orifice. Another design technique
delivers the air through a porous medium thereby ensuring
uniform pressure across the entire bearing area.
The result is that air bearings provide a frictionless load-bearing
interface between surfaces that would otherwise be in contact
with each other. Since air bearings are non-contact, they avoid the
traditional bearing-related problems of friction, wear, and the need
for a lubricant.
Air bearings are typically about 60% efficient. Therefore the
lifting capability can be determined by the following equations:
and
Maximum Theoretical Lift ≡ L MT = P B x B A
where P B = Air pressure in psia
B A = Bearing area in sq inches
Actual Lift ≡ L A = 60% x L MT