. A text-book of electrical engineering;. ittle up to 60 miles per hour, or any higher i88 Electrical Engineering speed which may be practicable. Means are provided for preventing thebattery discharging through the dynamo at rest or at very low speeds. Forfurther information respecting this dynamo, the student should consult thearticles by Dr Rosenberg in the Electrotechnische Zeitschrift, 1905, p. 393;1906, p. 1035; and by Osnos, 1907, p. 197. A somewhat similar result is obtained by Osnos by the arrangementshown diagrammatically in Fig. 173. The field current of the main dynamoM is suppHed b
. A text-book of electrical engineering;. ittle up to 60 miles per hour, or any higher i88 Electrical Engineering speed which may be practicable. Means are provided for preventing thebattery discharging through the dynamo at rest or at very low speeds. Forfurther information respecting this dynamo, the student should consult thearticles by Dr Rosenberg in the Electrotechnische Zeitschrift, 1905, p. 393;1906, p. 1035; and by Osnos, 1907, p. 197. A somewhat similar result is obtained by Osnos by the arrangementshown diagrammatically in Fig. 173. The field current of the main dynamoM is suppHed by a direct coupled auxihary dynamo D. The latter has twosets of brushes and two sets of field magnet windings. If the vertical fieldbe Oj, an electromotive force Ey will be induced between the brushes BjB^,and the current I^ will be due to the difference between this and thatE of the auxiliary battery. Thus I^ =Oi(E- Ey) .{a). The electromotive force Ey is proportional to the vertical flux O,, and to thespeed, thus C5,.0,,.w .(6).. Fig- 173 The magnetic flux Oj, is produced by the current ly which flows through thearmature between the brushes B^ and B^ and through the compensatingfield winding shown below the armature; neglecting saturation the flux isproportional to this current, thus 0),, C^-Iy .(c). The exciting current ly is proportional to the electromotive force E^induced in the armature between the brushes B^ and B^ by its rotation inthe horizontal magnetic flux O^., thus ^a — ^4^0) Cs^x^ .{d). Finally, neglecting saturation, the flux Oj, is proportional to the current/„, thus <^. = o,h {e). Putting this value for O,. in equation {d), and substituting for I^ fromequation (a) and for Ey from equations [b) and (c), one obtains for /„ theequation a + ln^ where a and 6 are constants. 6i. Constant-current Variable-speed Dynamos 189 Above a certain value of the speed n the constant a may be neglected incomparison with bn^ and ly, which is the field current of t
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