Large TG rotors provide ventilating ducts in the rotor construction. The fans attached to the rotor discharge-cooling medium through these ventilation slots so that rotor windings which become hot due to passage of electrical currents, are kept cool up to a certain temperature. In one case of a large 200-MW TGs due to manufacturing defects of loose wedges over the rotor windings these cooling slots got blocked. As a consequence, when the TG was loaded to its fullrated load, the vibration levels (which were well within limits when the TG was rolled, excited, and not fully loaded) became excessive. To establish that the increase in vibration levels was because of a thermal bow, the excitation current was reduced in stages—in each stage of reduction of excitation the current reduction in vibration levels was noticed. The rotor then was sent back to manufacturing works for clearing the ventilation canals in the rotor and putting appropriately tight wedges and shop balancing. The repaired rotor subsequently never showed any signs of mechanical or thermal unbalance.
Similar 1 × n-type vibration problems can be experienced on a gas turbine when some of the combustors (spaced around) fail to fire. The resulting thermal unbalance will appear exactly similar to mechanical unbalance. The mechanical balancing trials in such cases invariably fail to reduce the vibration.
One of the gravest problems encountered on rotating machines (such as hydrogenerators, steam turbines, TG, motors) is 1 × n-frequency vibration problem caused by loose components such as blades, poles, and rotor windings. Mere spectrum analysis would indicate symptoms of unbalance yet the rotor fails to respond to balancing trials—in fact, due to loose components, the phase angle would never remain stationary and hence, positions of balance weights cannot be determined.
