The indirect method of distribution is almost exclusively used to-day. It alternating current dynamo, self or separately eicited, and one welder that is a transformer, with the necessary clamping and operating appliances. Theself-excitingdynamos used are 1000-20,000 watta output, and may be regulated by means of a reactive coil to give a varying F.M.P. never to exceed 300
They are substantially built to withstand sudden strains, and have self-oiling bearings; bruahea are never to be moved and perfectly spiritless, requiring no other attendance but cleanliness.
The winding on the transformer being one for all set, variation in the E.M.F. has to be obtained either by varying by hand the initial E.M.P. of dynamo, or to excite a dynamo to the lowest potential required, it being so compounded as to give with the maximum work the increased E.M.F., or finally to maintain a constant E.M.F. nt dynamo under all loads, and vary the E.M.F. at the transformer by interposing a variable resistance, or, preferably, self-induction. The transformer itself may be built so aa to give a different ratio of conversion, either by moving the primary and secondary relatively to each other, or by shunting lines of force by means' of an iron bridge between primary and secondary, or by altering the number of turns in primary and secondary by a switch.
When a single plant is used - viz. one dynamo and one welder, it is the dynamo generally which, if separately excited, is regulated by means of a rheostat in the exciter circuit or by a reactive coil in series with exciting coil on armature if self-excited. Fig. 133 shows the connections used with a separately excited machine. The only thing different from similar installations is the break-switch, which is operated by a foot treadle, and automatically opens the circuit when foot pressure is removed. This prevents any mishaps when operated by uninitiated persons, as all action ceases when one leaves the apparatus. The primary voltage never exceeds 300 volts, with 100 alternations or 50 complete cycles per second. Nothing but the very best insulation is used in the primary wiring. It is deemed necessary to protect customers, not only against any shocks, but even against the scare of one, and we recommend- to permanently ground the secondary, which in welding apparatus is virtually the table and pressure devices.
Fig. 134 shows the connections used for a self-excited composite dynamo, which has two windings on armature, a longer main and shorter exciting coil. Both are wound in the same direction, and currents generated therein pass in multiple commuted through the shunted field-magnet; after this through line No. 2, controlled by a break switch and split, one returning through reactive coil and line 3, the other through welder and line 1 to their respective windings. The exciting effects of each circuit add themselves. Reactive coil is conveniently placed near the welder. Dynamo is regulated from minimum to 300 volts, and excited for each weld anew.
For some work which requires to be done at great speed, the second method is resorted to; that is, to keep the field constantly excited by fixing a variable reactive coil in a given position just enough to produce about 150 volts, the lowest E.M.F. required. The proportion of the field-magnets, the E.M.F. of exciting coil, and the resistance of a shunt to field, are such as to produce an over-compounding of 100 per cent, in this case with largest current in primary.
This method of regulation is very nice, as it is absolutely automatic, responds quickly, the field of the dynamo not requiring to " build " every time from residual magnetism alone. We sometimes call it " cubic compounding," as it produces a constant heating effect for variable cross sections and variable lengths as well. With this modification the wiring is slightly altered, the break-switch breaking only the main circuit, leaving the exciting circuit permanently closed.
The methods so far described are only used when a single welding machine is to be operated. It was early recognised that if the process is at all to be used on a commercial basis, that at least the generating of the necessary currents should be limited to as few machines as possible. It required, in other words, a dynamo, giving a nearly constant E.M.F. of sufficient capacity to feed a number of welders while maintaining for each absolute independence.
Since all welding for a long while to come will probably be operated in isolated plants, the dynamo is to be placed in the hands of an engineer generally, not an expert in electrical matters, and has, therefore, to be easily attended and free from all the little faults and kinks which are at present the sole consolation of the dynamo tender. One of its prime requirements are close automatic regulation under all loads.
Constant potential, self - regulating dynamos are manufactured in various sizes, and are self-eiciting up to 30,000 Watts output. Dynamos with larger output are separately excited; but also self-regulating. Either constant potential or a percental increase with load can be obtained, and the regulation responds even if the load consists of scif-inductiva translating devices.
I lay some stress on the regulation of the dynamo for the reason that the generators are not to be placed in the hands of special attendants, as in case of a central station, but have to deliver their currents under constant pressure whatever ma; be the conditions of work they are subjected to; even an automatic regulator would hardly be able to follow the rapid variation of loud.
The following conditions influence the perfection of the work, and are variable with different materials and sizes: -
1. Projection of the abutted pieces in the path of the current.