Analysis of peak torque of line-operated synchronous machines subjected to symmetrical voltage sags

Voltage sags and voltage interruptions are serious power quality problems for many industries. Apart from eventual tripping, voltage sags cause high torque peaks, which may cause damage to the shaft or equipment connected to the shaft. This paper shows how the torque depends on both the duration and the magnitude of voltage sags. First a simple theory is presented, and then to verify the theory, simulations of a blast furnace blower located at the steel plant SSAB Oxelosund Sweden are performed. A line operated salient-pole synchronous machine powers the blower, which feeds the blast furnace with air. The conclusion is that voltage sags with durations equal to multiples of a whole cycle generate minimum torque. peaks, and the maximum torque peaks are generated by voltage sags with durations equal to multiples of a whole cycle plus half a cycle.