The Standard Electrical Dictionary Part 47

Current, Wattless.

Whenever there is a great difference in phase in an alternating current dynamo between volts and current, the true watts are much less than the product of the virtual volts and amperes, because the the watts are obtained by multiplying the product of the virtual volts and amperes by the cosine of the angle of lag (or lead). Any alternating current may be resolved into two components in quadrature with each other, one in phase with the volts, the other in quadrature therewith, the former is termed by S. P. Thompson the Working Current, the latter the Wattless Current.

The greater the angle of lag the greater will be the wattless current.

Curve, Arrival.

A curve representing the rate of rise of intensity of current at the end of a long conductor when the circuit has been closed at the other end.

In the Atlantic cable, for instance, it would require about 108 seconds for the current at the distant end to attain 9/10 of its full value. The curve is drawn with its abscissa representing time and its ordinates current strength.

Curve, Characteristic.

A curve indicating, graphically, the relations between any two factors, which are interdependent, or which vary simultaneously. Thus in a dynamo, the voltage increases with the speed of rotation, and a characteristic curve may be based on the relations between the speed of rotation and voltage developed. The current produced by a dynamo varies with the electro-motive force, and a curve can express the relations between the electro-motive force and the current produced.

A characteristic curve is usually laid out by rectangular co-ordinates (see Co-ordinates). Two lines are drawn at right angles to each other, one vertical, and the other horizontal. One set of data are marked off on the horizontal line, say one ampere, two amperes, and so on, in the case of a dynamo's characteristic curve.

For each amperage of current there is a corresponding voltage in the circuit. Therefore on each ampere mark a vertical is erected, and on that the voltage corresponding to such amperage is laid off. This gives a series of points, and these points may be connected by a curve. Such curve will be a characteristic curve.

The more usual way of laying out a curve is to work directly upon the two axes. On one is laid off the series of values of one set of data; on the other the corresponding series of values of the other dependent data. Vertical lines or ordinates, q. v., are erected on the horizontal line or axis of abscissas at the points laid off; horizontal lines or abscissas, q. v., are drawn from the points laid off on the vertical line or axis of ordinates. The characteristic curve is determined by the intersections of each corresponding pair of abscissa and ordinate.

169 STANDARD ELECTRICAL DICTIONARY.

Variations exist in characteristic curve methods. Thus to get the characteristic of a commutator, radial lines may be drawn from a circle representing its perimeter. Such lines may be of length proportional to the voltage developed on the commutator at the points whence the lines start. A cut giving an example of such a curve is given in Fig. 125.

(See Curve of Distribution of Potential in Armature.)

There is nothing absolute in the use of ordinates or abscissas. They may be interchanged. Ordinarily voltages are laid off as ordinates, but the practise may be reversed. The same liberty holds good for all characteristic curves. Custom, however, should be followed.

Synonym--Characteristic.

Fig. 120. CHARACTERISTIC CURVE OF A DYNAMO WITH HORSE POWER CURVES.

Curve, Characteristic, of Converter.

The characteristic curve of the secondary circuit of an alternating current converter. It gives by the usual methods (see Curve, Characteristic,) the relations between the electro-motive force and the current in the secondary circuit at a fixed resistance. If connected in parallel a constant electro-motive force is maintained, and the curve is virtually a straight line. If connected in series an elliptical curve is produced.

170 STANDARD ELECTRICAL DICTIONARY.

Curve, Charging.

In secondary battery manipulation, a curve indicating the increase of voltage as the charging is prolonged. The rise in voltage with the duration of the charging current is not uniform. In one case, shown in the cut, there was a brief rapid rise of about 0.1 volt; then a long slow rise for 0.15 volt; then a more rapid rise for nearly 0.40 volt, and then the curve became a horizontal line indicating a cessation of increase of voltage. The charging rate should be constant.

The horizontal line is laid off in hours, the vertical in volts, so that the time is represented by abscissas and the voltage by ordinates of the curve.

Fig. 121. CHARGING CURVE OF A SECONDARY BATTERY.

Curve, Discharging.

A characteristic curve of a storage battery, indicating the fall in voltage with hours of discharge. The volts may be laid off on the axis of ordinates, and the hours of discharging on the axis of abscissas. To give it meaning the rate of discharge must be constant.

Curve, Electro-motive Force.

A characteristic curve of a dynamo. It expresses the relation between its entire electromotive force, as calculated by Ohm's Law, and the current intensities corresponding thereto. To obtain the data the dynamo is driven with different resistances in the external circuit and the current is measured for each resistance. This gives the amperes. The total resistance of the circuit, including that of the dynamo, is known.

By Ohm's Law the electro-motive force in volts is obtained for each case by multiplying the total resistance of the circuit in ohms by the amperes of current forced through such resistance. Taking the voltages thus calculated for ordinates and the corresponding amperages for abscissas the curve is plotted. An example is shown in the cut.

171 STANDARD ELECTRICAL DICTIONARY.

Curve, External Characteristic.

A characteristic curve of a dynamo, corresponding to the electro-motive force curve, except that the ordinates represent the voltages of the external circuit, the voltages as taken directly from the terminals of the machine, instead of the total electro-motive force of the circuit.

The dynamo is run at constant speed. The resistance of the external circuit is varied. The voltages at the terminals of the machine and the amperages of current corresponding thereto are determined. Using the voltages thus determined as ordinates and the corresponding amperages as abscissas the external characteristic curve is plotted.

This curve can be mechanically produced. A pencil may be moved against a constant force by two electro-magnets pulling at right angles to each other. One must be excited by the main current of the machine, the other by a shunt current from the terminals of the machine. The point of the pencil will describe the curve.

Fig. 122. CHARACTERISTIC CURVE OF A DYNAMO.

Curve, Horse Power.

Curves indicating electric horse power. They are laid out with co-ordinates, volts being laid off on the axis of ordinates, and amperes on the axis of abscissas generally. The curves are drawn through points where the product of amperes by volts equals 746. On the same diagram 1, 2, 3 .... and any other horse powers can be plotted if within the limits. See Fig. 120.

Curve, Isochasmen.

A line drawn on the map of the earth's surface indicating the locus of equal frequency of auroras.

172 STANDARD ELECTRICAL DICTIONARY.

Curve, Life.

A characteristic curve showing the relations between the durability and conditions affecting the same in any appliance. It is used most for incandescent lamps. The hours of burning before failure give ordinates, and the rates of burning, expressed indirectly in volts or in candle-power, give abscissas. For each voltage or for each candle-power an average duration is deducible from experience, so that two dependent sets of data are obtained for the construction of the curve.

Curve, Load.

A characteristic curve of a dynamo, expressing the relation between its voltage and the amount of excitation under a definite condition of ampere load, at a constant speed. The ordinates represent voltage, the abscissas ampere turns in the field, and the curves may be constructed for a flow of 0, 50, 100, or .. , or any other number of amperes.

Fig. 123. LOAD CURVES.

Curve, Magnetization.

A characteristic curve of an electromagnet, indicating the relation of magnetization to exciting current. Laying off on the axis of ordinates the quant.i.ties of magnetism evoked, and the corresponding strengths of the exciting current on the axis of abscissas, the curve can be plotted.

It first rises rapidly, indicating a rapid increase of magnetization, but grows nearly horizontal as the iron becomes more saturated. The effect due to the coils alone, or the effect produced in the absence of iron is a straight line, because air does not change in permeability.