Braking device for a high-inertia rotor

The segments (1) of the braking track (112) fixed to the rotor of an alternator driven by a water turbine are cooled by circulating water by means of a thermosiphonic effect from tanks (5) which are also fixed to the rotor. Application to the security of safety water-driven turbo-alternator units.

BACKGROUND OF THE INVENTION 
The invention relates to a braking device for a high-inertia rotor. 
A high-power water-driven alternator is usually mechanically braked by 
friction between a steel track integral with the rotor against brake 
linings moved by the pistons of stationary cylinders. 
The friction forces which appear when the brakes are applied are designed 
to be sufficient to overcome both the angular momentum of the rotor and 
the residual motor torque of the turbine due to leakage from its inlet 
manifold even when in the closed position, in order to slow the rotor down 
in a time which is compatible with the operation of the machine. The 
quantity of heat produced by friction is proportional to the product of 
the friction forces multiplied by the distance traveled by a point of the 
track of the rotor relative to a stationary observer. 
The dissipation of this quantity of heat sets problems which are tricky to 
solve mainly on very powerful machines in which the rotor inertia is 
considerable and in which the water inlet manifold which feeds the turbine 
allows a high leakage flow to pass even when it is required to close the 
manifold completely to stop the alternator. The difficulty resides in the 
fact that heat is produced very much more rapidly than it can be 
dissipated naturally, in which case if suitable precautions are not taken, 
there occur local heating, deformation, excessive wear, and pollution of 
the machine by powdered material worn off the track and off the linings of 
the brake cylinders. 
This is due to the fact that the coefficient of heat exchange between the 
metal of the track and the air is low. Also the heat emitted on the 
friction surface is stored in the mass of the track and flows only slowly 
in the ambient air. 
In most known devices the tracks are constituted by solid steel segments 
and heat accumulates in their mass, but since the heat conductivity of 
steel is poor, the distribution of the temperatures is not homogenous. 
This limits the uses to which such tracks can be put. 
Preferred embodiments of the present invention provide a braking device for 
a high-inertia rotor by which an increased braking capacity can be 
obtained without danger of damage. 
SUMMARY OF THE INVENTION 
The present invention provides a braking device for a high-inertia rotor 
which rotates in a predetermined direction about an axis wherein the 
braking device comprises: 
a circular braking track fixed coaxially on said rotor and composed of 
successive water-cooled metal segments, each segment constituting a closed 
chamber with an inlet and an outlet for the water; 
a succession of stationary brake cylinders disposed adjacent to said track 
in a position suitable to thrust brake linings against said track; 
a water tank fixed on the rotor for each segment or hydraulically 
interconnected group of segments, said tank being disposed radially nearer 
to the axis of the rotor than its corresponding segment and being oriented 
angularly to the rear of said segment relative to the direction of 
rotation of the rotor, whereby both centrifugal force and the deceleration 
force due to braking tend to urge water from said tank towards said 
segment; and 
inlet pipes and outlet pipes which connect said tank respectively to said 
inlet and to said outlet of said segment to form a segment cooling circuit 
and to allow water to circulate in this circuit by the thermosiphonic 
effect which results firstly from the rise in temperature of the water in 
the segment during braking and secondly from the presence of centrifugal 
and deceleration forces. 
DESCRIPTION OF KNOWN PRIOR ART 
U.S. Pat. No. 4,013,148 (Kobelt) describes a hollow braking disk provided 
with fins inside it which are cooled by circulating a current of water 
which comes from the outside via connections. Also, published German 
patent application No. 1,288,377 (Messerschmitt) describes a hollow 
braking disk which contains a store of cooling water and is provided with 
safety valves which allow the removal of vapour in the case of high 
overpressure. However, such hollow disks are not adapted to braking a 
rotor of such high inertia as that of a large hydraulic alternator whose 
braking track is constituted by a succession of segments and is capable of 
absorbing braking energy exceeding 100 Mj, e.g. 750 MJ or more, in normal 
braking, and 5,400 MJ in emergency braking.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The braking track can be stationary or moving. 
The example described corresponds to a segmented moving track which makes 
it possible to brake a rotor 102 having a vertical axis 104 in an 
alternator whose stator is referenced 106. A stationary spider 108 
supports the bearings 110 of the rotor which is made to rotate by a water 
turbine (not illustrated) disposed coaxially beneath the spider. 
The braking track is in the form of a circular ring 112 disposed coaxially 
with the rotor and fixed to the lower surface thereof on the wheel disk 3 
which connects the shaft 116 to the windings 118. The braking track has an 
inside diameter of 9500 mm and an outside diameter of 10300 mm, for 
example. A suitable number, e.g. 24, of vertically oriented brake 
cylinders 114, are fixed on the spider 108 beneath the braking track. The 
piston head of each brake cylinder has a brake lining 115 which the piston 
applies against the track 112 when the rotor is to be stopped. 
FIG. 3 illustrates a few segments 1 of the brake track 112. The complete 
track is constituted by 30 identical segments disposed side by side around 
the same circumference. Each segment is made of cast steel and has three 
radially aligned groups of four cavities 10 which communicate with one 
another via holes 12. The partitions which separate the cavities maintain 
the shape of the segment despite local heating and they increase the 
surface area of heat exchange with water. The upper (non-braking) surface 
of each segment is closed by closing metal sheets 14 welded all around the 
segment and to the partitions which separate the cavities. A water inlet 
16 is situated at the trailing end of the segment relative to the 
direction of rotation and on its inside diameter. A water outlet 18 is 
situated at the leading end of the segment and on its outside diameter. 
FIG. 2 illustrates the installation which corresponds to each segment. 
The segment 1 is supported by studs 2 fixed by means of bolts 120 on the 
lower wheel disk 3 of the rotor shim 4 is inserted between the disk and 
the stud to compensate for defects in the planeness of the disk. The studs 
disposed in the middle of the length of the segment have cotters, not 
illustrated, which enter the segment to fix its position rigidly. The 
other studs allow the segment to expand circumferentially. 
In operation, when rotating at rated speed, the water is centrifuged 
outwardly. All the water in a tank 5 is driven outwardly such that the 
segment 1 and pipes 6 and 7 connecting the segment to the tank 5 are 
completely filled with water, remaining in the tank 5 only at its outer 
end. 
When the unit is to be stopped, the supply manifold is closed and the rotor 
slows down. Inertia deforms the shape of the water surface in the tank 5, 
but providing axis of the tank is suitably orientated to take this 
component into account i.e. is not strictly radial, the segment and the 
pipes 6 and 7 remain full of water. 
If, as illustrated, the pipe 7 is connected substantially to the leading 
edge of the tank ahead of the pipe 6, inertia causes water to circulate in 
the circuit formed by the pipes 6 and 7, the tank 5 and the segment 1. 
The rotor speed decreases and reaches the speed at which the mechanical 
brakes are designed to be applied. 
The brake linings on the pistons then come into contact with the track 
which heats up and heats the water which it contains. A thermosiphonic 
circulation effect is set up due to the reduction in specific gravity of 
the water in the segment as this water is heated to a higher temperature 
than that of the water in the tank. This thermosiphonic effect accelerates 
the already started circulation of the water. As the temperature rises, 
the pressure rises in the tank. The heat produced by friction is then 
stored in the mass of the segments and in the mass of the water. The rotor 
is completely stopped before the temperature and the pressure reach their 
design limit values. 
Valves 8 remain closed and the system does not lose any water. The braking 
system must be allowed to cool before further braking. 
In the case of accidental emergency braking (e.g. with the brakes applied 
when the rotor speed is too high), everything happens initially as 
described, above but the rotor does not stop when the temperature and the 
pressure reach their design limit values. The valves 8 open and let steam 
escape. As long as all the water is not transformed into steam, the 
temperature remains stable due to the fact that a large quantity of heat 
is absorbed by the evaporation of the water. 
The track can thus absorb a large amount of energy (e.g. three times more 
than during normal braking). This gives it a wide safety margin. Water is 
brought into the tank after cooling to make up the normal level. 
Of course, a single tank can be used for a group of several successive 
segments hydraulically connected in series for this group to constitute a 
functional equivalent of the previously described segment 1. 
The advantages of the invention are as follows: 
proper distribution of the temperature in the thickness of the track, this 
avoiding detrimental deformation; 
a high heat capacity which allows a very high inertia rotor to be braked, 
the energy which can be absorbed being, for example, 750 MJ during normal 
braking and 5400 MJ during emergency braking; 
no intervention is necessary after normal braking, since the pressure in 
the tank remains less than the valve release pressure and there is no loss 
of water; 
the circulation of water is entirely natural and requires neither pump nor 
auxiliary units; and 
in the case of accidental braking at too high a speed or with too high a 
residual motor torque, the track does not overheat as long as all the 
water is not evaporated, which leaves a wide safety margin (but it is then 
necessary to refill the track with water and possibly to dry the machine).