I have more stuff coming down the drains
And here it is.....
Here’s some data on an actual Rotary Converter for comparison, a Metropolitan-Vickers self-synchronous start machine of the 1920s:
1500kW rated for traction or lighting duty - 11,000V 3 Ø supply 50hz. [British] = 12 pole @ 500 rpm. 440/480vdc 3-wire lighting, or 550v dc for traction.
Regulation for load transfer was by shunt field regulation, the two voltage ratio ranges obtained by tapping switches on the transformer [not shown on the circuit drawing].
These were interlocked with the main isolator to prevent use under load. In addition, other taps of ±2½% and ±5% were provided on the HT side for voltage adjustment.
Transformer was an oil-filled 1650KW rated unit, naturally cooled construction, wired star/double star.
Quoted efficiencies of the Converter:
5/4 load 96% [ 2 hour maximum at this load ]
Full load 95.75%
¾ load 95%
½ load 93.25%
¼ load 88.75%
She could safely run 50% overloaded for 15 minutes and take a 100% overload momentarily. Approximately Unity Power-Factor, by transformer design, was at 460-550vdc Mechanical brush-oscillators were fitted at slip rings and commutator, to even-out wear and prevent grooves and flats.
This machine was self-synchronising. The pony motor was a very simple induction motor built with eleven stator-poles and a plain cast steel rotor. With slip, it ran at near enough the Converter’s operating speed, a considerable saving on a true synchronous motor, and was Metropolitan Vickers’ usual self-synchronous start technique. Here’s the circuit drawing, edited to show the essentials;Click for larger image
SELF SYNCHRONOUS STARTING- refer to circuit drawing.
The pony motor was direct coupled to the main shaft. A double throw switch, [see circuit], was provided for the motor. When this switch was closed in the top position, the motor could be used for running the rig at near normal speed for commutator / slip-ring grinding if required, [ with safety precautions taken, obviously ].
Starting was as follows: All switches at 'open'. The main 11,000v isolator and then the shunt field switch were closed. To excite the pony motor; the switch was put in it’s top position, and the machine started to run up. The dc side started to excite itself as an ordinary dynamo, and it built up the correct dc polarity due to pre-energising the field. Polarity was however checked on the center-zero voltmeter shown. A system for rectifying incorrect polarity existed which was only necessary in rare circumstances, such as human error, incorrect previous shutdown or a malfunction.
By the time full rpm approached, the dc side was at about half normal volts, and at this stage the starting motor switch was thrown to the second position. The Converter retained its correct polarity and the ac now injected into the armature windings came into relation with the fixed magnetic system poles. A synchronising torque thus resulted and the armature fell into sync, with a few fluctuations before stability was achieved.
As soon as this steady condition pertained, the dc volts rose to their maximum, and by adjusting the field rheostat a definite position was found for the near enough correct dc output voltage. In practice, the operator watched the pony motor voltage, as when this value was at a minimum, the dc volts were about right. The main low tension ac switch was then closed and the machine steadied into full operation within a few revolutions. The motor start switch was now opened, the pony motor freewheeled and the machine was ready to parallel its output into the dc side by final voltage adjustments on the rheostat. The dc volts needed to be equal each side before switching into service.
Three similar machines in a substation, except these are rated @ 600vdc :Click for larger image
As far as I can ascertain, the above or similar starting methods, using an ac pony-motor, was introduced in England about 1910, probably earlier in the US. Older machines can be seen in museums without ac starting-motors. By the twenties ac starting was the most common method. For a dc start, the machine was run up as a shunt-wound motor on full-field dc volts. It took longer to start, as the dc motor had to be run up incrementally, with a face-plate or other variable resistance/shunt type starter and the synchronisation required much skill on the part of the operator in smartly closing the switches.
Even when perfectly executed, the machine usually fell into step with a graunch due to currents flowing into the armature. To get the timing right one had to "take the phases". The early method was to use lamps wired across phases, not easy to interpret at a comparatively fast frequency and with but a faint indication of sync, little indication of voltage and no idea if motoring was fast or slow on the armature RPM.
The use of a 'Synchronoscope' was a far superior method for dc starts. This essentially comprised a tiny stator and rotor, one being connected across the bus bars, the other across the incoming machine, via transformers if necessary. By phase-splitting with an inductor or capacitor, one set of windings produced a rotating field. The other set was arranged so when its field coincided in frequency with the other set, no torque was produced and the rotor was stationary. A pointer needle was arranged in a dial-instrument to show the phase relationship, and when both sides were in sync, [ usually with the needle pointing up ], the switch was promptly closed. By the position of the needle left or right of center, one gauged if running was fast or slow and could set the field rheostat accordingly .
Usually the Synchonoscope was mounted alongside voltmeters to give the operator all the information required for the least traumatic switch-in.
By the 1930s, the use of totally automatic synchronised starts took all the fun out of the job!