Present limits of speed and output of single-shaft turbo generators

Output is determined broadly by rotor or stator dimensions. With speeds of 1200 rev. per min. and lower, the stator is the limiting member, while with higher speeds, the rotor is the limiting member. The most effective rotor diameter is not necessarily the largest diameter. To obtain maximum output at a given speed the rotor proportions must be chosen to properly balance mechanical stresses, rotor ampere turns and flux. American design practise has established 400 ft. per sec. as an upper limit of rotor peripheral speed. The maximum length of core is determined by such factors as ventilation, bearing temperatures, critical speed and limits to weight imposed by forging and transportation facilities. Fig. 4 shows present limits to kv-a. rating at speeds from 3600 to 900 rev. per min. These limiting values are given as indicating present boundaries to knowledge and experience, rather than as real physical or other limits that cannot be exceeded. Mechanical forces due to short-circuit current, and damage caused by armature winding failures, are no greater in the very large generators indicated by Fig. 4 than in present day 20,000 and 30,000 kv-a. units. No opinion is expressed as to the wisdom of installing very large single-shaft units. If operating engineers desire units of 50,000 to 100,000 kw., there is no question but that such generators can be conservatively designed and constructed.