SDG – Technologies




Tharisa Minerals experienced mill gearbox and motor failures at their Voyager Plant near Mooinooi,  Rustenburg in South Africa.  

Typical incidents

  1. The LRS’s overheated as a result of the shorting contactor opening during normal mill running.  This caused loss of production time as it took several hours for the overheated LRS to cool down before the mill could be started again.
  2. Mills were started with the LRS in the wrong state (too hot or the dippers not at the top)  This caused high starting spikes and eventually a gearbox failure.
  3. Arcing on the motor slip rings and motor rotors caused a number of motor failures.


SDG were requested to integrate their SafeStart Condition Monitoring and Protection System into the existing LRS to prevent the problems they experienced.

SDG proposed that the existing LRS’s be replaced with ALRS’s, with factory integrated SafeStart Condition monitoring, Protection and Control Systems. This proposal was accepted.


Three ALRS’s were installed at the Tharisa Minerals Voyager Plant during 2022/2023.


The new ALRS will provide the following advanced protection and control features:

  1. The SafeStart System will detect LRS and Rotor arcing and issue a MV breaker trip to prevent the arcing (Flashing) from escalating in more serious faults, motor failure, LRS damage and downtime.
  2. The LRS Status will be monitored and status faults will be detected .
  3. Advanced Rotor and LRS protection  prevents the mill drive train from experiencing adverse starting torque spikes and conditions.
  4. Lock Charge Protection will be provided.
  5. The startup torque will be monitored and controlled to ensure it is kept within acceptable limits.
  6. The condition of the LRS will be evaluated during each startup and reported to the maintenance personnel via SMS and email.
  7. The time consuming traditional mill barring can be done with the MillSafeStart , by using the Mill Motor to lift the ore and checking if the ore tumbles. If it does not tumble the mill motor will be tripped and the process repeated. 

Mill motor barring greatly reduce barring time.

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LRS & Safe Start Documents

Datasheet for Advanced LRS
Advanced LRS Protection Overview
Testing of an LRS on a grinding mill – Typical Results
Safer Starting – Engineering Background
LRS Maintenance Manual
Datasheet for Cooler
List of installations