SDG – Technologies

Frequently Asked Questions

Got a question? See some of our most commonly asked questions below, or alternatively contact us using our contact form below.

A start-up test needs to be performed, where the rotor currents are being recorded, to determine whether the concentration of the electrolyte is correct, and then the torques trace needs to be analysed.

Before replacing the electrolyte solution, the electrolyte needs to be inspected for contamination and conductivity measured to establish if the actual electrolyte corresponds with the nametag.

The service life of the electrodes and the electrolyte, depends on the quality of water used for the electrolyte solution and the contamination of the electrolyte.

Low-quality water and high contamination can reduce the service life of the electrodes to as little as 6-months and low contamination of the electrolyte can help to extend the life of the electrodes to more than 3 years.

Oil should be added to the electrolyte solution to reduce evaporation and contamination. Filters on the LRS panel louvers can reduce the ingress of dust and thus the contamination.

A conductivity meter may be used to measure the conductivity in mS/cm. The conductivity meter compensates the conductivity reading for the temperature, reading it as it would be at 25°C. The value given is therefore not a true reflection of the conductivity (as it varies with temperature), but rather of the total kg of chemicals (e.g. Sodium Bicarbonate) to make up the electrolyte in the tank.

Not all conductivity meters are equal – they should be able to handle the oil on the electrolyte.  If a conductivity meter is permanently installed in the LRS tank, it should be suitable for continuous immersion.

An immediate trip would normally be cause by electrolyte concentration that is too high, causing the starting current to be too high. A trip later in the start-up cycle can be caused by certain conditions in the LRS such as the bottom limit switch that did not make or the contactor not closing.

This is usually because the electrolyte concentration is too low. Therefor the resistance in the solution is too high, as it does not allow sufficient current flow and torque to accelerate the mill. Most of the time the mill stalls before the ore starts to tumble.

An LRS is basically a big resistor that prevents the electrical motor (that, e.g. drives a mill that crushes the ore), to start up too quickly. The electrical current travels between the electrodes in the electrolyte solution, putting energy into the liquid and this heats up the solution. On average, the temperature rises about 2°C to 3°C per start.

If the shorting contactor (used with permanent brush motors) accidentally opens while the motor is kept running, heat/energy will by induced into the LRS causing it to warm up and the liquid may start boiling within minutes.

Replacing electrodes should be done when the Turn Down Ratio is too low and or when the shorting spike is dangerously high.

The service life of the electrodes and the electrolyte, depends on the quality of water used for the electrolyte solution and the contamination of the electrolyte.

Low-quality water and high contamination can reduce the service life of the electrodes to as little as 6-months and low contamination of the electrolyte can help to extend the life of the electrodes to more than 3 years.

SDG Technologies provides a service called the Mill Start-up Evaluation test, which measures the current and voltages on the rotor circuit of the motor during the start-up cycle. By analysing the measurements, recommendations can be made whether to replace electrodes.

It has been proven that a visual inspection is not sufficient.

Electrodes should never be older than 10 years. (Helmke Liquid Resistance Starter) (ref 1: Helmke Liquid Resistance Starter. Helmke Orbis GmbH)

The electrodes must not be kept dry for more than one day, otherwise, the brown surface layer would lose its property of conductance and become isolating. (Helmke Liquid Resistance Starter)

It is recommended to use demineralized water for making up the electrolyte, as any minerals that are present will react to the electrode surfaces during current transfer. The result is that a layer with lower conductivity forms on the surface of the electrodes, which causes the final resistance be not as low as with new/clean electrodes.

The recommended water quality is as follows:

  • Total Hardness (mg/l as CaCO3) – to be less than 20
  • Conductivity in µS/cm – to be less than 50
  • Total Dissolved Solids at 105°C mg/l – to be less than 50

Reverse Osmosis water has been used with great success.

Replacing electrodes should be done when a Mill Start-up Evaluation test has been done and the TDR is too low and or when the shorting spike is dangerously high.

The only way to know if the LRS is performing as it should, is to do a test where the rotor currents are recorded during start-up and torque traces developed from it. SDG Technologies provides a service which is called the Mill Start-up Evaluation test, which measures the current and voltages on the rotor circuit of the motor during the start-up cycle. From the measurements, recommendations can be made to improve the LRS performance.

Make sure that the new motor is of the same size as the old motor? If not, confirm that the LRS’ performance is good enough to handle the larger motor.

The frequency of service is recommended based on general and normal plant conditions. For example, if there are more than 3 starts per week, on average, combined with high dust concentrations leading to dust accumulation in the electrolyte, the frequency of a Major service on an EPM type LRS will have to be increased to every two years or even more frequent. A minor service should be done at least once a year. Please refer to the maintenance manual (Downloadable document on the Homepage) for the difference between these services.

The main problem with the LRS heat is not that the temperature is too high, but that the temperature range is too wide. The temperature influences the conductivity of the electrolyte and therefore its performance. A wide temperature range corresponds to a wide range of starting torques.

SDG often advises installation of heaters or coolers or both, to address this issue. The aim is to limit the temperature variation to within a 10°C band.

It is the current measured and recorded during the start-up cycle at a very high sampling rate. The data is the processed and a torque curve is developed from it. The torque curve is representative of the current drawn during the start-up cycle. The values are normally presented in % of motor full load.

The absolute performance figure of a LRS is called the turn-down ratio (TDR). It is the ratio of the maximum resistance measured to the minimum resistance measured. The TDR remained at a typical value of 50:1 or worse for many years even though the mill power was inching up. New starters with 100:1 ratios are adequate for application to the bigger grinding mills.

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