Abstract: A hydraulically driven welding machine employs a closed loop control system for maintaining a constant speed of a generator. A feedback device produces a controlled frequency proportional to the actual generator frequency. The controlled frequency is compared with a reference frequency preset in the machine. Any error between the controlled and referenced frequencies is used to control a proportional flow control valve to regulate the flow of hydraulic fluid through a hydraulic motor that rotates the generator to maintain a constant generator speed under varying welding conditions. The source of the hydraulic fluid may be a hydraulic system of a motor vehicle. In an alternate embodiment, an adjustable orifice produces a pressure drop in the hydraulic system. A change in generator speed changes the pressure drop. The pressure drop is used to control a valve that diverts the proper amount of fluid flow to the hydraulic motor to maintain a constant pressure drop and thus a constant generator speed.