Switches for elevator service.

three Se switches and also to the top Sm switch.

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5

4

From the left-side upper terminal of the S, switch a connection. runs to the wattmeter W, and from this a connection runs to the Se switches and to the top Sm switch. Thus it will be seen that through switch S, and wattmeter W the g and f busses feed the Se switches and one of the Sm switches. The street mains, coming in at the right side of the board, lead through switch S to the three Se switches, and also to the three Sm switches; hence, the circuits controlled by these switches can be fed from the generators or from the street mains. The circuit controlled by the lower Sm switch is provided with a circuit breaker, C B, and the three circuit breakers C B5, C B, C B, protect the circuits controlled by the Se switches. These latter switches are in circuits that operate passenger and freight elevators.

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4

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If the switch S, is opened, there is no connection between the d e and f g busses. Therefore, under this condition the three generators can be connected all with one set of busses, or one with one set and the remaining two with the other set of busses. If the S, switch is closed, bus g will be connected with bus e, and bus f with bus d, while the two equalizing busses, i and h, will be connected through the center blade. With this connection the three generators will feed into the whole system and the two sets of busses will become practically one set. In each set of busses there is an equalizing connection, so that two or more generators may be connected with that part of the system.

Under ordinary circumstances it is desirable to keep the power and light circuits separate, as the sudden changes in the current taken by the motors tend to un

Power and light circuit.

steady the voltage too much for the successful operation of the lights, but by having two parts of the system so arranged that they can be joined into one, it becomes possible to operate the entire plant with greater economy at times when the full capacity is not required. Suppose that at some time a small number of lights were in use on all the various branches of the system, but not enough in the aggregate to absorb more than a portion of the capacity of one generator, and at the same time suppose that the demand in the power circuits were also very low, then if the two parts could be connected in one, a single generator might be capable of doing the whole work, but if each part had to remain separate, two machines would have to be kept in service.

In this installation the wires leading in from the street are only two in each circuit, owing to the fact that the lamps in use are of the 220-volt type; hence, they can be fed from the outside wires of the three-wire street system. As all the S L, Se and Sm switches are of the double-throw type, any one of the branch circuits can be fed from the street mains or from the generators, and if the switches S, S., S, are kept constantly closed, the change can be effected instantly.

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The ammeters 1, 2, 3 are connected with the three generators and are of the shunt type. The ammeter Am is so arranged that it can be cut into any of the circuits leading from the SL switches, and thus render it possible to measure the current in these branches at any time. The shifting of this instrument to the various circuits is accomplished by the aid of the switch Ass, the switch proper being back of the board, the part seen being simply the handle. The ammeter Am, is arranged so as to be con

Recording instruments.

nected in the power circuit or the light circuit, and is controlled by the double-throw switch As. The two voltmeters are so arranged that one will indicate the voltage of the power circuit and the other that of the light circuit. When a generator is cut into service one of the voltmeters is used to indicate the voltage of the busses with which it is to be connected, and the other connects with the generator terminals. Thus the two voltages can be equalized before the generator switch is closed.

BA and B V are recording instruments, the first being an ammeter and the second a voltmeter. These instruments trace a line upon a cardboard dial, just like a recording steam gauge. The wattmeters W1, W2, W 3, W 4 are sometimes called recording wattmeters, but they are more properly called integrating meters, for they indicate the sum total of the electrical energy that has passed through them up to the time of reading. The Ls switches are for the purpose of turning on or off the lights that illuminate the switchboard. The voltmeters are controlled by the switches whose handles are shown at Vs Vs. The field regulators are shown at R1, R., R. The switch seen below each regulator is for the purpose of opening the circuit through the shunt field of the generator, but as there is nothing to be gained by being able to open this circuit, and as it may produce serious results, the introduction of this switch cannot be recommended.

Construction of Switchboards.

Switchboards designed for plants of small capacity have no particularly interesting constructive features, owing to the fact that small conductors are capable of conveying the current, and these can be made of wire which is readily bent into any desired shape. As a rule the cross section of conductors is estimated so that the current density will be from 600 to 1,000 amperes per square inch. If the capacity of the plant is not over two or three hundred amperes it can be readily seen that the various connections can be easily made by running two or three copper wires of large size side by side. If the electromotive force is 220 volts, which is the pressure most commonly used, the power may be estimated roughly as equal to one horse-power to every four amperes; hence, 200 amperes would represent fifty horse-power. If the capacity of the plant is 500 horse-power, the amperes will be about 2,000, and the cross section of the conductors will have to be from two to three square inches. In manufacturing establishments 500 horse-power is not considered a large plant; in fact, it is smaller than many, the capacity in some instances running up to several thousand. From this it will be seen that in electrical installations such as are required for large manufacturing establishments the currents handled range all the way from, say, 1,000 amperes up to 15,000 or 20,000, and in some cases may go far beyond this point, if the voltage is very low, as is frequently the case where the current is used for