January 17, 1913.

THE COLLIERY GUARDIAN.

123

a plain hollow iron box casting cooled with water- circu-
lation, the cooling water entering through a pipe at the
bottom of the lower half of the yoke and rising through
the pipes which break the horizontal division, discharging
through the pipe at the top, and in doing so carrying
away the heat generated by the eddy currents in the
inner face of the yoke. In order to supply exciting
current to the brakes, a 100-kw. motor generator set has
been supplied for each group of three hoists; it was
originally proposed that these sets should be equipped
with flywheels of sufficient capacity to allow of braking
three lowering trips in the event of the failure of the
main supply, but this scheme was ultimately abandoned
in favour of two storage batteries of somewhat greater
capacity, which under normal circumstances float across
the terminals of the direct-current generators. The
power braking system is therefore quite independent of
the main supply, as lowering trips on all the hoists can
be completed at ordinary speeds with perfect safety,
whether the supply is available or not; in fact, if

Fig. 3.—View of Converted Whiting Hoist, Equipped with
Eddy Current Brakes.

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Fig. 4.—3,000-Horse Power Hoist Equipped with
Eddy Current Brake.



necessary, lowering trips may be continued until the
storage battery is exhausted, as the full unbalanced load
is sufficient to accelerate the hoist.

The eddy current brake controllers are of the standard
tramway type with magnetic blow-outs, and have five
braking notches, the resistances, which are of the cast
grid type, being proportioned to give 20, 40, 60, 80 and
100 per cent, of the excitation.

The writer does not propose to enter into a discussion
of the theoretical features of this class of brake as
these also have been fully dealt with by Dr. Rosenberg
in the paper above referred to; the theory advanced by
him regarding the as-it-were automatic adjustment of
the relations between the speed (or rather periodicity)
of the brake and the resistance of the path in which the
eddy currents circulate due to the use of cast iron is
extremely interesting and well worth perusal.

The curves in fig. 2 show the braking effect of these
particular brakes at speeds varying from zero to full

speed, with 100 per cent, and 75 per cent, of full excita-
tion respectively. The tests were made by driving the
motor against the brake without the ropes on the
drums, the readings of input to the motors were taken
on a polyphase indicating wattmeter, and the braking
torques calculated from them, the efficiencies of the
motors at various loads being known. The curves of
braking torques must, however, only be taken as
approximate, but they are probably near enough for
practical purposes, especially in view of the fact that with
the ropes on the additional rope shaft and headgear
sheave, friction will more than compensate for any errors
in the assumed efficiencies at low speeds, and this is borne
out by the result obtained when allowing an unbalanced
load of 5 tons to run down the shaft against the fully
excited brake, the rope speed of 150/180 feet per minute
obtained under these conditions necessitating a retarding
torque of about 41 tons, which agrees very closely with
the curve. Whilst the braking effect with 75 per cent,
excitation begins to flatten at a rope speed of 3,000 ft.









per minute, it will be noted that with full excitation the
torque is still rising rapidly at 3,500 ft. per minute ; it
is very desirable that tests should be made above this
speed, but, unfortunately, this is impossible with an
A.C. motor. From tests which have been made experi-
mentally with models, however, it appears that with full
excitation the torques would continue to rise up to a
speed equivalent to a rope speed of at least 6,000 ft. per
minute, and would probably not develop a drooping
characteristic under a speed equivalent to a rope speed
of 10,000 ft. per minute, if then.

For all practical purposes, therefore, there does not
appear to be a critical point with this type of brake, and
with a normal speed of 3,500 ft. per minute, there is an
enormous margin of safety, as the faster a runaway the
greater the braking torque will be, which is, of course,
the ideal condition.

The operation of the brake is extremely smooth and
the retardation perfect; the writer recently made a

succession of trips in order to obtain a personal impres-
sion of the effects of high-speed braking on this system.
The first trip was made with a lowering speed of
2,200 ft. per minute when the current was cut off and
the brake fully applied; the rope speed dropped to
500 ft. per minute in 11 seconds, in a distance of 245 ft.
The second trip was made hoisting at the same speed
with practically the same result. The third trip was
made with a lowering speed of 3,200 ft. per minute (the
maximum obtainable), and the rope speed dropped to
500 ft. per minute in 13 seconds in a distance of 315 ft.
A fourth trip was similarly made hoisting. Neither the
writer nor his two companions suffered the slightest
discomfort, although in the third and fourth trips the
retardation must have reached the high maximum value
of 10 ft. per second per second, although there was an
exaggeration of the feelings of bodily heaviness and
ascension during the retardation period, and vice versa,
which are usual with high-speed winding.

With an A.C. hoist equipped with an eddy current

brake the driver has three methods of braking:—

(ct) Eddy-current braking,

(&) Counter-current braking,

(c) Mechanical braking,

all of which can be applied simultaneously under normal
conditions, and the failure of any one of these methods
still leaves him in a position to bring his hoist to rest
by means of the other two ; in the improbable (though
not impossible) contingency of a failure of the
mechanical brakes, due to a breakage of parts or
accidental jamming of the levers, by reducing the

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Fig. 5.—Eddy Current Brake on 3,000-horse
Power Hoist.

speed to a low value with the eddy current brake he
can bring the hoist to rest, and hold it there with such
a small amount of counter-current that the risks
I attending this class of braking, which have been referred
I to previously, would for all practical purposes be
eliminated. So convenient in application is this form
of braking, that although it was originally only intended
for lowering purposes, in practice it will be found that
drivers prefer to use it in all classes of winding, particu-
larly when hoisting rock rapidly, as not only can the
length of the trip be shortened by decreasing the
retardation period by using the brake instead of letting
the hoist run itself out, but the skip can be taken almost

up into the tip on the brake without any application
of the mechanical brake except the final one.

In the writer’s opinion it is very desirable, with hoists
equipped with these brakes, that drivers should be
instructed to make occasional trips at regular intervals
with counter current braking, in order to keep their
hands in, as it is always objectionable for a man to be
called upon to perform an unfamiliar operation in an
emergency; the very fact that a failure of the eddy
current or the mechanical brakes is so remote makes it
desirable that in applying counter current the driver
should do so with perfect confidence, and such confidence
will depend upon his familiarity with the operation ; if
a definite instruction on this point be not given and
enforced, drivers will probably never use counter current
at all.

The A.C. hoists which are fitted with these brakes
have been authorised for man winding up to full speed—
viz., 3,500 ft. per minute, both for hoisting and lowering,