Enhancing the reliability of pumped storage hydroelectric system is mandatory for building the stable network. This paper experimentally analyses the electrical and mechanical effects of voltage unbalance on doubly fed synchronous (fixed speed) machine and doubly fed asynchronous (variable speed) machine operating under motoring mode. The input electrical quantities such as stator current, rotor current, input power and losses are analyzed under normal and abnormal conditions. As well, the three-directional vibration of both the machines are analyzed under normal and abnormal operating conditions. The practical importance of this comparative analysis is for developing the robust system with the countermeasures for avoiding voltage collapse. © 2016 IEEE.