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Copper, Gold, Platinum and Molybdenum mines require processing of large quantities of ore for extracting these precious metals. Large capacity grinding mills are used for processing the ore. Using one large mill is more economical than using a number of smaller mills. This requires a very large capacity electric drive to rotate these mills.
The mills themselves are large rotating cyclinders, ten to fifteen meters in diameter, grinding hundreds of tons of ore per hour. For effective grinding, these mills have to rotate at low speeds. The motor capacities are as high as 15 to 20 MW for such an application.
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Conventional motors run at high speeds requiring drastic speed reductions using very high torque gear trains. Often a hydraulic coupling is required to facilitate smooth starts. The load transmitted by the gear trains will be very high, leading to enormous mechanical stress on the shafts and gears. This also requires an elaborate lubrication system. The load carrying parts are subject to a high level of wear and tear. All this lead to reduced availability, affecting production and the process.
The alternative to the use of these large capacity motors is to use gearless motor drives or GMD’s. Gearless motors up to 22 MW are in service in many of these ore processing plants. Gearless motor drives are also known as “ring motors” or “wrap around motors.”
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The motor consists of a large number of magnets placed circumferentially on the shell of the grinding mill. This is similar to the rotor of a conventional motor. The stationary part, the electromagnetic coils, is a stationary ring encompassing the rotating magnets. This makes a gigantic coil around 18 to 20 meters in diameter. The mill shell with the fixed magnets is the rotor. The large numbers of magnets produce a multi-pole machine that by itself reduces the rotational speed of the rotor.
Cycloconverters, which are high power rectifier inverters, convert the incoming three phase 50 Hz power supply to a lower frequency power supply. By using power electronics, the output frequency can be reduced to as low as 50% of the input frequency. Power Electronic and associated software give a very good control of the output of the cycloconverters which results in a very good speed control of the driven machine.
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Together the cycloconverter and the multi-poles produce a high torque, low speed drive without many rotating parts.
The low speeds reduce the air friction loss in the coils, and the lower frequency also reduces copper losses. The efficiency achieved is in the order of 95%, which is considerably higher than mechanical gear drives.
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The maintenance requirements are considerably less. There are no separate bearings, gear reducers or lubrication systems.
This also helps in soft starting and stopping the mills. It is possible to inch and creep at speeds less than one rpm. It is also possible to operate the motor in both directions, creating an oscillation that can shake off stagnant or frozen charges inside the mill. All these considerably reduce plant outages.
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Improvements in the quality and reliability of power electronic components have made Gearless Motor Drives the preferred method for these applications. The deterrent is only the high initial cost and the non standard nature of the drive.