LCIs and synchronous motors applied to roller mills [load commutated invertors]

A cement company concerned about high power-factor penalty costs requested an evaluation of the feasibility of using synchronous motors starting through LCIs to drive three new roller mills at a cement plant. The prior experience with roller mills centered about the application of wound rotor induction motors with either cascaded secondary resistance or liquid rheostat. By utilizing synchronous motors, there was the possibility of providing leading power factor to reduce the reactive power demands seen by the utility feeding the cement plant. It was theorized that, if sufficient power factor correction could be obtained, the payback period of the incremental additional cost of the synchronous motor drive system would be very short. After payback of the initial investment, all future energy savings gravitate directly to the bottom line as additional revenue. Several key concerns had to be addressed in evaluating the use of synchronous motors for these relatively high starting torque requirement applications. This paper discusses the electrical and mechanical evaluation leading up to the decision to proceed with this new application of synchronous motors started through an LCI