In recent tests made in Britain by the National Coal Board on the newest types of continuous mining machines, operated from conventional transformers sited at considerable distances from the machines, it was shown that the fall in voltage between the transformer and the machines at the coal-face was over 100 V; the 550 V from the trans- former becoming only 450 V at the terminals of the machine motors. This poor supply of electricity to the continuous mining machines is being remedied by the systematic installation of dry-type transformers which, being flameproof, can be employed by the side of the machines they are supplying at the coal-face.
The distribution of electrical power in coal mines has had to be reorganized due to the greater number of electric motors that are now used in coal recovery. Only a few years ago, when the standard method of coal mining consisted of the use of a power-driven coal cutter to undercut the seam, only one motor was used. Filling or loading was done manually and the only electrical power needed was that for the 40 HP motor. At the present time, filling or loading is often done by machines, and these need a power supply of a magnitude greater than that for the cutter motor.
Whereas the cutting machine was the
only unit using current at
the coal-face, with the advent of the combination cutter and loader,
together with the operation of conveyors at the coal-face: the electrical requirements in that area have increased by as much as 300 per
cent. To meet these extra demands for electrical power, the underground transformer capacities had to be increased. A coal cutting machine can be supplied adequately by the use of a 150 kVA transformer, but the same machine when operating with mechanical loading and conveying will have to be supplied from a transformer of much higher capacity. The starting current of the motors operating the machines may be at least four times that of the current required
to operate the motors under normal load; the transformers must,
therefore, be of sufficient capacity to cope with not only the normal
motor loadings but with the additional power required to start the
These considerations practically rule out the possibility of employing sufficient oil-filled transformers in the confined spaces underground, particularly as these conventional transformers would have to be, for reasons of safety, at some distance from the coal-face. The modern idea is to make the transformer part of the coal-getting plant and to operate it alongside the machines at the coal-face. Another inhibiting factor in the use of the oil-filled transformer is the accentuated voltage-drop which becomes more serious as the electrical HP at the coal-face increases. This was illustrated by a test carried out by the Nalional Coal Board in connection with a Dosco continuous mining machine, which is operated by two 75 HP motors. An oscil-logram made during the test showed that the transformer was supplying current normally at 560 V, but the current reaching the machine some distance away was only a little over 400 V. This means that no matter how efficient the modern mining machine may be, it cannot be operated to the greatest advantage when the transformer has to be a considerable distance from the machine.
The dry-type transformer, which solves the problem, is being
adopted by the National Coal Board, and to make the unit completely safe in the gaseous atmosphere at the coal-face, it has to be sealed in a steel casing, and the latter kept as cool as possible on the
outside. Even so, here is a considerable rise in temperature inside
the casing; but the design and assembly of the transformer enable it to
withstand this temperature. All the insulation used is of the high temperature variety and consists of an ingenious assembly of glass, asbestos, mica and silicon preparations.
By A.E.Williams, "Electrical Journal"
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