Advantages of low speed drive solutions

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The choice of drive type for ring-geared mills has been influenced by major trends in the operating environment.

These trends include increasing mill sizes, requiring drives that can supply higher power while avoiding stress to the network and mechanical equipment; rising energy costs, focusing attention on the need for energy savings; and greater awareness of the opportunities for process optimisation, which has highlighted the advantages of variable speed operation.

Taken together, these factors have boosted demand for low speed drives using synchronous motors and frequency converters.

To meet the high torque requirements of today's large ring-geared mills, low speed drives generally use synchronous motors.

The motors are usually rated in the range 3.5 to 9 MW and operate at a nominal speed of approximately 200 rpm.

Compared to the large 30 to 40 pole synchronous motors typically used in fixed speed drives, synchronous motors for variable speed operation have only 8 to 12 poles and are less expensive, more compact and lower in weight.

They are therefore easier to transport to remote locations, have a smaller footprint within the mill and can use lighter foundations than fixed speed motors. As a result, installation of variable speed motors tends to be easier.

Low speed synchronous motors with frequency converter control can generate sufficient torque to start a mill turning without needing an air clutch.

This is a major advantage of variable over fixed speed drives, because eliminating the clutch from the system reduces maintenance needs.

Variable speed operation also provides smooth starting, avoiding the stresses imposed on the mill machinery and electrical network when a fixed speed system starts up. 

As the grinding process is not highly dynamic, low speed synchronous motors in mill drives can use brushless excitation. This eliminates wearing parts, helping to reduce maintenance, and keeps the requirement for AC/AC excitation power low.

The brushless exciter is a separate rotary transformer mounted on the motor shaft, and the motor supply, excitation control and protection are all provided by the frequency converter.

This compares favourably with the wound rotor motors sometimes used in high fixed speed solutions: these more complex motors need frequent maintenance due to wearing of the brushes and carbon dust build-up.

Other disadvantages of wound rotor motors are that they put significant stress on the network and mechanical components during start up, and that gear reducers are needed.

As low speed synchronous motors generate high torque at low speeds, they need efficient cooling. A TEWAC (Totally Enclosed Water to Air Cooled) enclosure is the optimal solution in dusty environments, however air cooled motors are also an option. 

Low speeds mean that the motors can use self-lubricated sleeve bearings with hydraulic jacking pumps. As a result they can be operated at around 5 to 10 percent of nominal speed during starting and servicing.

The bearings are either flange mounted on the end shields of the motor or, for higher power ratings, pedestal mounted.

Motors with 'integrated pedestal bearings' are as easy to mount and align as those with flange mounted bearings, as they are delivered completely assembled and so no on site assembly is needed.

Low speed operation also avoids the mechanical problems that affect high speed mill drives. The weakest point in high speed drive solutions tends to be the gear reducer which couples the motor to the pinion.

This additional component increases the amount of equipment and the required maintenance, and it also decreases the reliability and efficiency of the complete mill. 

In many cases low speed synchronous motors can be tailored to suit the application or customer specifications. For dual pinion mills, for example, customers often specify that the terminal boxes and cooling systems should be located centrally so that the cables and cooling water supply, if used, can be brought to a single point.

Motors for dual pinion mills can be designed to reduce the need for spares. By building the motors so that cooling units can be turned and the terminal boxes swapped from one side to the other, a single spare motor can be used to replace either the left or right motor in a dual pinion system. 

Delivering flexibility

The industry has been quick to adopt frequency converter solutions using low speed synchronous motors because they permit soft starting and controlled stops, while at the same time enabling the grinding process to be optimised.

The operator can easily react to changes in ore characteristics and throughput, which is ideal for ores with varying grinding properties, and mills are able to achieve higher throughput for the same energy input.

Process optimisation can also reduce variability in the output particle size and decrease utilisation and wear of grinding media.

Frequency converters with direct torque control (DTC) deliver precise and effective speed and torque control during start up and when the material is cascading. DTC can reduce wear and tear to extend gear life by up to 30 percent, helping to ensure high availability and minimise backlash.

The ring-geared mill drive solutions developed by the ABB minerals unit, for example, have a dedicated integrated mill controller, permitting smooth starts and controlled stops, reducing stress on the equipment and providing critical monitoring during the starting period to protect against frozen charges.

In dual pinion systems, torque and load sharing between motors is very accurate, improving the lifetime of the pinions.

The mill drive controller also provides service operating modes such as creeping and positioning, so the drives do not need additional equipment for maintenance purposes.

Creep mode can be used to slowly rotate the mill for visual inspections or grinding out. Fast and automatic positioning of the mill based on angle or liner reference reduces the downtime needed for changing liners.

These factors have encouraged many mill operators to upgrade from fixed to variable speed operation based on low speed synchronous motors and frequency converters.

Many SAG mills already use variable speed drives, and variable speed is increasingly the preferred option for ball mills, too. 

[Anu Tuomaala and Jouko Virta are with ABB Oy Motors and Generators; Tatiana Ravani von Ow is with ABB Switzerland.]