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to enlarge and reorganize dynamo test; by the end of 1920 two new
pits had been provided, one for turbo-generators and one for rotary
converters and induction motors, and plant rated at 25,000 kVA could
be tested. Further world's records for their speeds were set up in
1925 by generators rated at 43,750 kVA 1500 r.p.m. and 15,625 kVA
3600 r.p.m. for Japan, and in 1926 by a 38,500-kVA 3000-r.p.m. machine
for Russia. A 62,500-kVA 1500-r.p.m. turbo-generator built for Portishead
in 1928 was the largest that had been produced in any British factory.
 These
rapid developments required testing accommodation in which the rotors
could be run at over-speed, and in view of their size a special
test house was built in 1926. It was made of concrete lined with
timber and sandbags almost enough to resist a bomb explosion, and
the motor and gear box driving the rotor were placed outside the
building. Overspeed tests are controlled by one man from a safe
distance.
One
of the most important technical developments was closed circuit
ventilation. The air filters used on the early machines gave serious
trouble, and in 1920 the Company's engineers decided to adopt a
closed circuit system, in which the same air was used continuously
and passed over water tube coolers. This system with subsequent
improvements is still the standard method of ventilating large turbogenerators
and similar machines.
Improvements
in generator construction concerned the transposing of parallel
conductor strips in the stator windings to reduce eddy currents,
for which a method still in use was patented by J. A. Kuyser in
1920, and also the supporting and construction of rotor coil-retaining
rings. The rings became more liable to move with the increasing
length and speed of rotors, and trouble from the consequent damage
to the insulation was cured in 1922 by the adoption of a sleeve-supported
retaining ring. The use of non-magnetic steel for the retaining
rings started in 1928 and had the effect of reducing the stray losses
and also the intensity of stray magnetic fields, which caused local
overheating at the ends of the stators.
For
turbo-generator stators the Company introduced in 1924 the practice
of bakelizing together the parallel straps of the slot conductors
in order to provide a solid foundation on which to wrap the main
insulation, thus avoiding insulation breakdown owing to movement
Of the individual copper straps. Soon afterwards silver soldering
by electric brazing was substituted for the soft soldering in general
use on stator winding joints.
A three-part
construction for turbo-generator rotors was adopted in 1927 to replace
solid steel forgings, which were unobtainable in the sizes required
for the largest 1500-r.p.m. machines. In the M-V design a central
cylinder and two shaft ends were held together by links of high
tensile steel, shrunk into slots at each end of the rotor; by measuring
the length of each link before and after shrinking, thenstresses
are accurately known. This construction has been used on generators
totalling over 1,000,000 kVA in capacity.
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