One cause of power system voltage instability is the loss of short-term operation equilibrium after a disturbance. This paper proposes an active power control method for preventing short-term voltage instability. The active power control is implemented by means of an adjustable speed machine, which is primarily used as a synchronous condenser, but includes a flywheel for short-term active power injection into the system. This adjustable speed machine can support the power system to avoid voltage collapse or excessive voltage dips by compensating both active and reactive power supply to the system immediately after a disturbance. The focus of this paper is a proposed active power control method, illustrated with simulation results. A power system consisting of a synchronous machine, a hybrid-load, and an adjustable speed condenser is used as an example.