VFD for Mining & Mineral Processing

Mining is the hardest duty a variable frequency drive can be asked to survive. A shovel bites into ore and the load jumps from no load to locked rotor in milliseconds; a downhill conveyor pushes power back into the drive instead of drawing it; a mill has to be brought up to speed against the inertia of hundreds of tonnes of steel. On top of that the supply is often a weak remote feeder, the ambient is dust and heat, and the machine is expected to run for years between overhauls.

The result is that mine drives are not chosen by kW alone. A 630 kW hoist and a 630 kW ball mill need completely different drives, because one has to hold torque at half a revolution per minute and the other has to survive a slow ramp against enormous inertia. This guide works through the four machine families found on a mine site, the duty each one puts on the drive, and the settings and options that make the difference between a drive that lasts and one that trips.

A mine hoist is the only machine on site whose failure is measured in people as well as tonnes. It moves ore, waste, materials and miners, so the drive has to hold the conveyance still at standstill with the brake released, follow a speed reference smoothly enough that the rope never goes slack, and know the position of the cage at all times.

That rules out open-loop control. A hoist drive needs encoder feedback — a SinCos encoder for a permanent-magnet machine, because the motor needs to know its rotor position before it is allowed to develop torque — plus a mechanical brake sequence interlocked with the drive's own run signal, and a speed reference profile that is checked against the actual speed, not assumed.

Rope length changes the load, and the same cage is heavier when it is loaded than when it is empty. The drive therefore has to be commissioned in a fixed order: learn the encoder offset with the conveyance empty, run the system dry, then wind the rope on and trim the parameters under load while recording speed command, actual speed, torque and current together. Only when the recorded crawl speed matches the design duty can the hoist be handed over.

A loaded belt conveyor is a constant-torque load that has to be broken away from rest. The belt may have sat still overnight with ore on it, the idlers are cold and stiff, and the drive has to develop between 150 % and 200 % of rated torque before anything moves. Once it moves, the acceleration has to be gentle: jerking a belt spills material at the transfer points and, on a long installation, sends a tension wave down the whole line.

Long belts are usually driven from both ends or from the head and an intermediate pulley, and those two drives have to agree on what they are doing. Without load sharing, one drive takes more than its share, runs hotter and wears its belt cover faster. Sharing can be done through a master and follower relationship over a communication link, or through droop control, where each drive lets its speed fall slightly as torque rises until the two settle at a common operating point.

Downhill sections reverse the energy flow. When gravity pulls harder than friction, the motor is driven by the belt and the drive has to send that energy somewhere: into a braking resistor and chopper, or back onto the network through a regenerative front end. Sizing that path correctly is what stops an overvoltage trip at the moment the belt starts to run away.

Ball and rod mills are inertial loads where the drive is bought as much for what it stops as for what it starts. A direct-on-line start draws five to seven times rated current, hits the gearbox with the full starting torque and makes the local voltage dip; a controlled ramp removes the inrush, protects the girth gear and the bearings, and gives the plant a speed setpoint it can trim to ore hardness. Because the mill is often the largest single load on site, the drive also has a role in cutting the monthly peak-demand charge.

Dewatering and slurry pumps are the opposite problem: ordinary variable-torque loads in an environment that destroys ordinary equipment. Run-dry protection is not optional, because a slurry pump asked to run without liquid destroys its own seals within minutes. Long cable runs between a remote substation and a borehole or a sump make the motor terminal voltage behave very differently from the drive output, so the drive either has to compensate for cable impedance or be fitted with an output filter.

Finally, the site itself has to be engineered for. Ambient above 40 °C, altitude above 1,000 m and heavy conductive dust all reduce what a drive can deliver, and all three have published derating curves that must be applied before the frame size is fixed. Enclosure choice — IP54 or a dedicated cabinet — follows from how close the drive sits to the crusher, not from what is cheapest.