An alternating-current (ac) motor which operates at a fixed synchronous speed proportional to the frequency of the applied ac power supply. A synchronous machine can be operated as either a generator or a motor (and sometimes as a source of reactive power, fulfilling the function of a capacitor), depending only on its applied shaft torque (whether positive, negative, or zero) and its excitation. There is no fundamental difference in the theory, design, or construction of a machine intended for any of these roles, although certain design features are stressed for each of them. In use, the machine may change its role from instant to instant. As a result, it is preferable to set up a common general theory for synchronous generators, motors, and capacitors. The distinction between generator and motor is merely a difference in the direction of the currents and the sign of the torque angles.