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Classification of Systems according to Source of Electricity-Transmission
at Constant Pressure—Motors mechanically governed-Self-Regulating Motors— Transmission at Constant Current—Difficulty of Self-Regulation
- Motor for Constant Current made Self-Regulating—Application to Transmission over large Areas—Continuous Current TransformatorTransmission between two Distant Points-Loss of Current by LeakageTheory_Commercial Efficiency-Conditions for Maximum Commercial Efficiency-Self-Regulation for Constant Speed—Practical Example.
It will be necessary to distinguish between different systems of electric transmission of energy, according to the source of electricity. An almost endless variety of cases may present themselves in different applications of electrical transmission, but three systems are of special interest, because most frequently occurring in practice. These are the following :
1. Transmission of energy from primary or secondary batteries at short distances to one motor only.
2. Transmission of energy from one or several dynamos to a number of motors placed upon the same circuit, but working independently of each other.
3. Transmission of energy between two distant points by means of one generator and one motor.
We may also make another classification according as the motors are intended for a constant or variable load, or a constant or variable speed. Generally speaking, the systems of transmission coming under heading 1) are not required for a constant load, nor is it of any great impor
tance that the speed should remain constant under a variable load. We shall not enter into a minute description of these cases here, as the investigation of electric tramways and railways, worked by accumulators, will afford ample opportunity of entering into details.
System 2) is that presenting most difficulties on account of the condition that all the motors must be independent of each other. The case is further complicated by the requirement that each motor should run with the same speed when empty or loaded. A moment's consideration will show that the last condition is an absolute necessity if we would make the electric transmission of energy of real practical use to small domestic industries. The artisan or small manufacturer would have his motor connected to a common system of service leads, and whenever he required power he would switch the current on to his motor. In doing In doing so he must not disturb any
other work which, at the same time, may be done elsewhere from the same service mains, such, for instance, as lighting or working other motors; and further, his motor should always run at the same safe speed, whether it is giving him little or much mechanical energy.
Most operations requiring the use of tools as turning, planing, &c., can only be properly performed at a certain fixed rate of speed, and the machinery must be kept going at that rate at all times.
System 3) presents difficulties of a different nature. Since we have to deal only with one generator and one motor, it is easier to make each fit the other, and as a rule the load is fairly constant, so that regularity of speed is not difficult to obtain. In this case the difficulty lies more in the necessity of proper insulation of line and machinery. Generally speaking, the system is required for long distance transmission, and to obtain an economical arrangement, both as regards first cost and commercial efficiency, the use of a high electro-motive force is necessary. This entails some danger to human life, and some difficulty in maintaining an efficient insulation. Both these points can, however, be satisfactorily dealt with, if proper care is used in the design and execution of the work. As regards the danger to human life involved in the use of electric currents of high pressure, this is generally greatly overrated. It is quite possible for a man who with both hands should touch the positive and negative wires in a non-insulated part, to be killed or severely injured if the pressure is over two or three thousand volts, but the accident can be rendered almost impossible if due precaution is taken. A circular saw if only lightly touched whilst revolving will cut a man's finger off, and what can be more dangerous than a pair of powerful spur wheels? Yet we have found means of
protecting life very effectually from destruction by purely mechanical means, and shall, without much difficulty, find means for protecting it from the electrical danger.
System 2) is best described as an electrical transmission and distribution of energy from one central station to several distant points. Now this distribution can be made on the parallel or on the series system. In the first case the electro-motive force (or pressure) between the positive and negative mains must be kept constant, and the motors are connected all in parallel from the mains ; in the second case the current passing through the mains must be kept constant, and each motor, when at work, is traversed by the same current. The pressure at the station must be the greater the greater the number of motors at work. In the first case the pressure is kept constant, but the current delivered into the mains must be the greater the greater the number of motors at work. We have thus to distinguish between distribution at constant pressure and distribution at constant current. The first system is represented—though as yet on a more restricted scale than was anticipated by the promotersby the Edison system in New York and Berlin, the latter by the Thomson-Houston system.
Electric Distribution of Energy at Constant Pressure.
We must now inquire into the theoretical conditions of this case. It will be evident at the outset that for economical reasons any attempt to obtain constancy of speed by the use of artificial resistances can only lead to a partial and not very satisfactory solution of the problem, and had better not be made if other means are at hand. This, happily, is the case in the present instance. We have two means by which we can without waste regulate the power of the motor to the work and yet keep it running at a constant speed. First, we may apply a mechanical device by which the power of the electric current is cut off in proportion as the work is cut off, and, secondly, we may apply an electrical device in the shape of special winding of the field magnets of the motor by which the torque exerted by the armature is automatically regulated so as to correspond to the mechanical load. As regards the first system, which applies equally well to the distribution at constant pressure, and to that at constant current, Professors Ayrton and Perry have in a paper on Electro-Motors and their Government shown how this can be done. They say: “ The method of cutting off the
1 « Journal of the Society of Telegraph Engineers and Electricians,” No. 49, vol. xii, 1883.
power as hitherto employed has this serious defect, that instead of the power cut off being directly in proportion to the work cut off, the arrangements have been such that either all power was cut off or none, so that the motion of the motor was spasmodic, just as in an ordinary gas-engine, which suffers from the same defects, that full charge of gas or no charge are the usual only alternatives. An electro-motor governor of this type,
of this type, which may be called a “spasmodic governor,' consists merely of a rotating mercury cup into which dips a wire, which makes in this case contact with the mercury, and so completes the circuit when the speed is slow, but which, on account of the hyperbolic form assumed by the surface of the mercury as the speed rises, ceases to dip into the mercury at high speeds, and so breaks contact." Later on the inventors say: “ The first improvement we made in governing consisted in replacing the “spasmodic governor' by aʻperiodic governor.' With our periodic governor the power is never cut off entirely for any length of time, but in every revolution power is supplied during a portion of the revolution, the proportion of the time in every revolution during which much power is supplied to the time during which less is supplied depending on the amount of work the motor is doing Our periodic governor, then, differs from the spasmodic governor in the same way that a good loaded steam-engine governor differs from the ordinary governor of a gas-engine. One of the ways of effecting this result is as follows: a brush, A, Fig. 64, lies on the rotating piece, B K, the cylindric surface of which is formed of two conducting portions connected with one another through any resistance, and the brush, A, is moved along the cylinder B K under the action of the governor balls. When the brush A is touching the contact part B, the