Thermal Unbalance Behaviour of Turbogenerator Rotors

Turbine generator sets (gas and/or steam turbine plus generator) operating at their rated speed under non-steady-state conditions normally show transient changes of shaft and bearing vibration. These vibration changes are generally associated with thermal variations of the rotor support structure or thermal effects within the rotor shaft line. For the turbogenerator rotor the changes of shaft and bearing vibrations may be due to the increase of rotor field current during initial loading or due to a change of rotor field current during load changes. To achieve that on-site vibration behaviour of the generator rotor is acceptable Alstom has introduced the so-called heat run test in the mid 1960s. As part of the internal factory acceptance tests in the spin pit, the heat run test is carried out after the balancing of the rotor by injecting direct current into the rotor field winding at rated speed and monitoring of the winding temperature and the first order shaft and bearing vibrations.