The paper develops theoretical methods of calculating the variation of torque and load angle of a synchronous machine connected to a fixed supply voltage. In order to supplement and explain the results of recent full-scale tests the following conditions are considered:(a) Condition after a sudden short-circuit.(b) Condition after switching in a reactance between the generator and the supply.(c) Asynchronous operation.(d) Resynchronization after asynchronous operation.In a companion paper experimental equipment for measuring the variations of torque and load angle of a small synchronous machine is described. The equipment has been used to verify the theoretical results for each of the above conditions. The machine tested was a model alternator or ‘micro-alternator’ which simulates a typical large machine. The special laboratory equipment is of great value for studying in detail the factors governing the performance of large alternators.