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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-alternatorÂ¿ 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.