Rotor Shaft Design
The generator rotor shaft is typically forged from high-strength alloy steel with good magnetic permeability. A central axial hole is incorporated for quality inspection and for routing excitation conductors. Evenly distributed transverse grooves along the shaft improve stiffness balance between orthogonal axes, helping to reduce vibration at harmonic frequencies during operation.
Rotor Winding Structure
Rotor windings are installed in parallel slots with axial ventilation channels and inclined slot ends. This design allows cooling hydrogen to flow evenly through each ventilation branch. Each winding turn consists of two silver-bearing copper conductors arranged to form internal ventilation ducts. Class F insulation is commonly applied, with PTFE sliding layers included to accommodate thermal expansion and contraction of copper conductors. Damping copper bars are embedded beneath aluminum slot wedges to enhance resistance to negative-sequence currents.
Retaining Ring Configuration
Rotor coil ends are supported by high-strength, non-magnetic retaining rings manufactured from stress-corrosion-resistant steel alloys such as 18Mn18Cr. In large turbine generators, suspended embedded retaining ring structures are often used, where the ring is secured directly onto the rotor body to improve mechanical stability.
Ventilation and Cooling System
Modern generators typically adopt a combined cooling method consisting of:
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Water internal cooling for stator windings
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Hydrogen internal cooling for rotor windings
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Hydrogen cooling for stator cores and structural components
Segmented air gaps and oblique flow ventilation improve heat dissipation. Hydrogen circulation is driven by single-stage axial flow fans installed at both ends of the rotor. Heat absorbed by hydrogen is transferred to cooling water via hydrogen coolers before recirculation.
Hydrogen Cooler Design
Hydrogen coolers use finned nickel-copper alloy tubes with thin copper fins to achieve high heat transfer efficiency and low flow resistance. These coolers are pressure-tested before delivery and designed to allow partial withdrawal while maintaining safe generator operation. Adjustable cooling water flow enables temperature control under varying load conditions.
Sealing System
Single-flow ring oil seal structures are commonly used to prevent hydrogen leakage along the shaft. The sealing system maintains hydrogen purity inside the generator while preventing air ingress. Expansion tanks, float valves, and oil return systems are designed to ensure safe separation of oil and gas during operation.
Fan and Guide Vane Assembly
Due to reduced airflow requirements in hydrogen-cooled generators, rotor fans are designed as single-stage axial flow units with forged aluminum blades. Each blade undergoes overspeed testing prior to installation. Guide vane seats are installed to optimize airflow paths and improve cooling efficiency.
Neutral Point Grounding
To protect the stator core during single-phase ground faults, generators adopt specific neutral grounding methods, including ungrounded systems, arc suppression coil grounding, or resistance grounding. Proper grounding limits capacitive ground currents and reduces the risk of insulation damage and internal short circuits.
Industry Application
These structural and cooling design principles are widely applied in large thermal, hydro, and nuclear power generators. Proper understanding of rotor structure supports effective testing, maintenance, and condition monitoring throughout the generator service life.
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