 For a.c.
winders a great advance was the introduction of dynamic braking, which
extended their range to powers of 3000 hp or so, much higher than
was otherwise possible. Loads were usually lowered under the control
of the mechanical brakes, and in order to relieve them expensive hydraulic
brakes of the dynamometer type were being installed on high-power
deep shaft winders. Dynamic braking provided an alternative method
that was both reliable and easily controlled. When braking, the motor
is isolated from the supply, and its stator connected to a d.c. exciter;
by varying the d.c. excitation the braking characteristic can be made
to follow the load requirements. The control of a.c. winders thus
becomes as easy as that of WardLeonard equipments, and the M-V system,
first used in 1937 on the service winder at the new Comrie colliery
in Fife, was subsequently adopted by all manufacturers of a.c. winders,
both in this country and America.
In
1937 the British mining regulations regarding landing speed for
men were made more stringent, and the Company developed a compound
braking system for use by the makers of the mechanical parts. The
traditional British and American methods employed a single-power
vertical braking engine, which could cause objectionable and sometimes
dangerous shocks to the equipment and also to the men-in the cage;
the use of damping devices involved a sacrifice of speed in the
application of the brakes and depended too much on proper adjustment
and maintenance. The M-V compound brake system applies a moderate
braking force gently and rapidly (by means of tension springs under
independent control) and retains the advantage of deadweights as
the main source of braking power.
Some
years earlier another M-V development had made it possible to employ
synchronous motor generator sets on Ward-Leonard winder equipments.
This was the use of an hydraulic slip coupling between motor and
flywheel, controlled in such a way that the speed of the motor remained
constant while that of the flywheel and generator fell on increased
load, thus allowing the flywheel to give up its stored energy. In
this way power factor correction was made available without recourse
to the Scherbius system, a complicated matter of many machines and
elaborate brushgear. The first installation was put in at the
Roan Antelope copper mines in Northern Rhodesia in 1930, and most
of the large equalized winders in Rhodesia and Australia are now
equipped with hydraulic couplings.
At
the same time a system of automatic winding control was being developed
chiefly for use on skip winders in metalliferous mines overseas.
For Ward-Leonard equipments it was a comparatively simple step from
supervisory control by cams to semi-automatic or indeed fully automatic
control, though the need for exact decking with varying skip loads
presented some tricky problems. An automatic winding equipment,
the first in the world, was installed in 1931 in a zinc and silver
mine at South Broken Hill, Australia, and is still in satisfactory
operation. It was followed by other sets in Australia and, in later
years, at the copper mines of Northern Rhodesia and the gold mines
of the Witwatersrand; the skip winding plant at Bickershaw colliery
in Lancashire is similarly equipped. Automatic control was also
developed for a.c. winders, and in 1935 an installation was put
in at Lynemouth colliery, which is believed to possess the only
completely automatic a.c. hoist.
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