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Fig. 39.—How to Wire Rotary Converter Charging Outfit, With Ammeter Rheostat and Voltmeter,
Showing Front and Rear of Switchboard.
field-coil rheostat is shown at Fig. 37. A shunt-wound generator is employed, and the main leads from the armature brushes are connected to the lower poles of a double-pole knife-switch. The battery is connected to the upper portion of the switch, an amperemeter being placed in circuit as indicated. The hinges of a small double-throw, double-pole switch are connected with a voltmeter. The amperemeter and voltmeter should be of the permanent type. Before throwing in the charging switch it is possible to read the voltage of the battery, and also by throwing the switch to read that of the charging generator so that it may be adjusted to a slightly greater voltage. The main switch is then closed and the rheostat used to raise the voltage sufficiently to drive a suitable charging current through the battery. With a system of this kind a circuit breaker or automatic overload switch should be included in the main line to protect the apparatus in case of accidental short circuit. An underload circuit breaker should also be provided to shut off the battery if the current falls to such a point that the battery will discharge through the generator. These are not shown in the simplified wiring diagram, neither are the fuses that prudence dictates should be used.
The Westinghouse Vibrator Rectifier is an inexpensive form of apparatus to charge small batteries from ordinary lighting circuits. The device, which is shown at Fig. 42 A, reduces the voltage of the lighting circuit to the proper value by the use of a small double step-down transformer, and rectifies this reduced voltage to the uni-directional voltage necessary for battery charging by electrically operating switching mechanism. The transformer serves the double purpose of diminishing the line voltage for the battery to be charged and also for providing a return path for the direct current. The charging current flows from one end of the secondary winding, and after passing through a regulating resistance passes through a pair of contacts, which are closed automatically and at the proper time, and out from the center point of the armature to the battery, from which it returns to the neutral point of the transformer. During the next half cycle the voltage in the transformer secondary is reversed in direction and the other pair of contacts is closed and the voltage is applied to
the battery from the half of the secondary that has previously been idle. As the current flow is in the same direction as that previously supplied, the battery is charged exactly the same as if uni-directional current from a generator was used.
The element upon which the success of the outfit depends is the vibrating mechanism, upon which devolves the duty to reverse connections in synchronism with the voltage and also exactly in step with the transformer secondary voltage in such a manner as to open the current character circuit at the instant of zero current and prevent injurious wear of the contacts by sparking. The following description of the action of this rectifier is repro
duced from the Electric Journal, and the action of the device may be understood by studying the wiring diagram shown at Fig. 42 B. "Two small laminated iron magnets, marked A.C. magnets, are connected in series across one-half of the transformer secondary, connections being made so that the corresponding ends of the magnet are of the same magnetic polarity. A direct-current magnet, polarized by shunt current from the battery, is so placed as to bring its ends within the effective field areas of the A.C. magnets. Since the ends of the D.C. magnet are of opposite polarity, they are forced at any instant in opposite directions by the fields of the A.C. magnets and one pair of contacts is closed. During the succeeding half cycle the A.C. magnets are reversed in polarity, while the D.C. magnet is not, the impelling force is reversed, and the armature takes such a position as to close the other pair of contacts. One side of the battery is thus connected alternately to the opposite ends of the secondary of the transformer in synchronism with the alternating voltage, while the other side is permanently connected to the center point. Exact timing to insure sparkless operation, by
Fig. 41.—Devices for Rectifying Alternating Current. A-Vibrator
Type Rectifier. B-Small Rotary Converter Set.
breaking the current-carrying circuit at the time when the battery and transformer voltages are equal and opposite and no current is flowing, is secured by connecting in series with the A.C. magnets, a resistance which alters the power-factor of the current in the magnets without affecting that of the load current in the transformer. This change in power-factor translates in time the impelling force, with respect to the current in the contacts, and secures the result of sparkless operation. This phase-controlling resistance is made variable, in order that the outfit will be applicable on circuits of which the wave form is not a true sine wave, and on circuits on which the voltage is not of normal value. The condensers connected around the contacts reduce to a negligible amount the unavoidable slight sparking, due to fluctuations in the line voltage, variation in wave form and change in battery voltage.
"The regulating resistance, which is connected in each side of the secondary circuit between the transformer and the stationary contact, is for the purpose of giving the outfit high or low regulation, in order that the change in battery voltage, as the charge progresses, will make only a small change in the current delivered. The standardization of lighting batteries in general use has resulted in the selection for the commercial form of this apparatus of such transformer voltage and resistance value as to make the charging current under normal conditions approximately 8.5 amperes at the start of charge and 6.5 amperes at the finish. The features above mentioned result in an outfit which can be connected to an ordinary alternating-current lighting circuit and to a battery, without attention to polarity, owing to the polarization of the D.C. magnet by the battery, and which will then, after a single adjustment of the phase-controlling resistance, give a full charge to the ordinary lighting battery without further attention. The cost of power for such a charge at the common rate of 10 cents per kilowatt hour is roughly 6.5 cents, as compared to the ordinary charge of 75 cents to $1.25 per charge by a public garage.
Battery-Charging Apparatus.—The apparatus to be used in charging a storage battery depends upon the voltage and character of the current available for that purpose. Where direct current can be obtained the apparatus needed is very simple, consisting merely of some form of resistance device to regulate the amperage of the current allowed to flow through the battery. The internal resistance of a storage battery is very low, and if it were coupled directly into a circuit without the interposition of additional resistance, an excessive amount of current would flow through the battery and injure the plates. When an alternating current is used it is necessary to change this to a uni-directional flow before