DESCRIPTION
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Jan. 1927 A GRAPHICAAL DETERMINATION OF AMPERE-TURNS 57
The reverse current adjustment is primarily for the purpose of insuring that all relays function alike when opening on small values of reverse current.
A relay was recently developed fpr use on a two-phase, five-wire network. Two two-pole network breakers will be used with each bank of transformers, one being connected in each phase. The relay just discussed could have been used for this application, but since two relays would have been required to control each breaker and a's space was an important item, it was decided to design a relay so that only one would be required to control each breaker. This relay, Fig. 11, commonly referred to as a two-phase re'ay, is really a single-phase device and is very similar in construction and operating characteristics to the present standard network relay. It differs from the standard, however, in that it has two phasing and two current circuits similar to the first double contact network relay. One phasing and one current circuit are connected across each pole of the network breaker.
Experience with the design and operation of network relays all points toward the desirability of having a relay which has operating characteristics that make it applicable to any type of network system. Enough work has been done along this line to show that it is
possible to produce such a universal network relay having adjustable operating characteristics. Such a relay would have to be adjusted to fit the characteristics of the system on which it was to be installed. These adjustments, instead of simplifying the relay, will add somewhat to its complication. Because of this, the ideal network relay would be one having fixed characteristics of such a nature as to make it universally applicable to network systems. Such characteristics are theoretically possible and considerable work has already been done which indicates that such a relay is entirely practical.
The automatic network relay is now past the experimental stage, and there are at the present time approximately 1000 network relays, such as have been described in this paper, installed and operating satisfactorily on a number of network systems. From the above discussion it can be seen that the operating characteristics of the automatic network relay and the characteristics of the system on which it is to be installed are very closely related. This relation must be fully understood and appreciated both by the designers of low-voltage, a-c. network systems and the designers of network relays in order to secure the most satisfactory operation of the system as a whole.
A Graphical Determination of Ampere-Turns for Trapezoidal Teeth Sections
BY J. F. CALVERT 1
THIS method applies to rotating electrical machines having rectangular slots and trapazoidal teeth sections. If any apparent tooth density at the widest
section be assumed, then a quick and reasonably
_̂ _ ^ __ ^
AT t t AT.t A'Itt AT l b
AMPERE TURNS PER INCH
FIG. 1
accurate determination of the corresponding no-load field ampere-turns may be made.
The proof of the method is as follows: For the tooth pitch under the center of the pole, it
is assumed that the surfaces at right angles to the flux lines are cylindrical and coaxial with the center line of the shaft.
In Fig. 1, curves are plotted for one inch in an axial
1. Design Engineer, Westinghouse Electric and Mfg. Co.
direction. Flux is plotted as ordinates, ampere-turns per inch as abscissa, and the following notation is used:
The first subscript t = in the iron (or tooth) s = in the non magnetic parts (or.slot) T = total in the iron and non magnetic parts at
any section.
The second subscript t = at the tip section m = at the mid section b = at the base section ap = apparent (at the widest section).
The main symbols $ = flux B = flux density in the iron AT = ampere turns per inch W = tooth width a = cross sectional area.
Primes indicate the stator. Bap = apparent tooth density at the base of the
stator's teeth Bap = apparent tooth tip density for the rotor.
58 A GRAPHICAL DETERMINATION OP AMPERE TURNS Journal A. I. E. E.
At any section
$t = Bt at
$ s = 3.19 as AT (1) The area, as, wiU be calculated at the mid section and
assumed to be the same at all sections. The ampere-turns per inch at the tip, mid and base
sections are shown in Fig. 1 as A Tu, A Ttm, and A Ttb, respectively. If the ordinates of the curves in Fig. 1 are all divided by att, then new values are obtained so that for the rotor
3> j
= B0j
= B
A = 3 . 1 9 AT An att
<$>tm Wt
= H att
= B
Wtt
Wtb
Wt,
(2)
(3)
(4)
(5)
(6)
If instead of the actual curves indicated by equations (3) to (6), a new set of curves are plotted as
AMPERE TURNS PER INCH
FIG. 2—FOR THE STATOR
B' 4>t atb
= 3.19 A s A T atb atb
FOR THE ROTOR
att
att
3.19- A T att
shown in Fig. 2, then the ampere-turns may be calculated as follows.
For the rotor Assume Bap, and draw Bap — E so that according
to the scales used the ordinate divided by abscissa
as
==3.19 . Extend Bap—E only to the curve att
Wtb indicated by — ^ — which corresponds to the actual
case under consideration. Average the ampere-turns per inch, m — n — o — p, and multiply by the length of the tooth in inches to obtain the ampere-turns for the tooth.
For the stator Calculate Bap', draw Bap — D', average m' — nf
— o' — p' and multiplying by the length of the tooth. When one value has been found, it is only necessary
to assume new apparent densities and to shift a triangle parallel to Bap — E or Bap —9Dr to obtain the new values of ampere-turns per inch.
Bibliography The Reluctance of Some Irregular Magnetic Fields, John F. H.
Douglas, TRANS. A. I. E. E. 1915. Vol. 34, p. 1067. The Reluctance of the Teeth in a Slotted Armature, W. B.
Hird. Jour. Institute of Electrical Engineers. (Brit.) (1900). Vol. X X I X , p. 933.
A Simple Method of Calculating Magnetic Conditions in Electrical Machinery, C. MacMillan. General Electric Review, May 1925. Vol. XXVIII, No. 5, pp. 302.
A Method of Calculating the Ampere-Turns for Driving Magnetic Flux Through Wedge Shaped Teeth, Boris Worohoff, JOUR. A. I.E.E.,Nov. 1923, p. 1171.
RESULTS^ ATTAINED THROUGH ENLARGED ELECTRIFICATION
In earlier publications of the department, such as "Super Power Studies in Northeastern Section of the United States/ 7 great emphasis was laid upon the possibilities of the elimination of waste which lay in the transformation of the power industry through the discoveries in the science of long-distance transmission and their application by large central generating plants feeding large system and their interconnection with each other. Such savings lay in the greater economy in power production by saving fuel and labor by the larger central plants; the reduction of the amount of reserve equipment required; a better average load factor, and thus less equipment, through pooling of the daily and seasonal fluctuations, together with wider diversification in use; more security against interruption; better utilization of water power by applying it to base loads while making steam carry the peaks; utilization of secondary power from the seasonal flow of streams to the partial relief of steam; savings in industry by replacement of factory steam plants, the increased day load being supplied by the same generating equipment as night load for cities; the larger application of power in replacement of factory labor; and likewise the economies in the household and farm application of power.
All of these anticipated values have been realized in an extraordinary degree through the initiative and genius displayed in the electrical industry. The electrical generating capacity in the country has increased from 14,280,000 to 23,840,000 kilowatts in five years, an increase of 67 per cent. Although 66 per cent of our energy output is from fuel, the development of water power has been most active.—(From the Annual Report of the Secretary of the Dept. of Commerce.)