Ques. Why should a starting box be used with a motor?
Ans. If the line voltage should be applied directly to the terminals of the armature when not running, an excessive flow of current will result, on account of the low resistance. Accordingly, to prevent injury to the winding, a variable resistance or starting box is inserted between one supply terminal and the armature so that the pressure may be applied gradually while the motor is coming up to speed.
Fig. 495.—Starter with no voltage release for a shunt motor. The terminals of the motor are at M, M', m, and those of the starter at S, S', s. The lever SA is shown in the "on" position. The current enters the motor at the terminal M, and there divides, part going through the field coil F, and the main current through the motor armature A. The armature current enters the starter at the terminal S', and traversing the lever SA, leaves by the terminal S. The field current enters the starter at the terminal s, traverses the coil of the magnet E (which holds up the armature a linked to the lever) and thence completes its journey through the whole of the resistance R, and through the lever SA, to the terminal S. When the supply is cut off by opening Sw, or should the field circuit be accidentally broken, the magnet E will release a and the lever, which will thereupon fly to the "off" stop O. It should be noticed that when SA is off, A and F form a closed circuit with the resistance R and magnet E. The inductance of F has consequently no chance of causing destructive sparking when the current is shut off. In starting the motor, Sw is first closed, and then, as the lever is slowly moved, the resistance R, which at first is all in circuit with A, is gradually transferred from A to F. The resistance of R is too small to affect appreciably the current in F, which necessarily consists of a comparatively large number of turns of fine wire. The arrangement is adopted to render the breaking of the shunt circuit unnecessary and is rendered clearer by the diagram fig. 496. It should be noted that E may be provided with a short circuiting key or push if required.
Fig. 496.—Simplified diagram of the connections of [fig. 495].
Fig. 497.—Starter with no voltage release and overload release connected to a compound motor. With a shunt motor, the only difference in the diagram would be that the series winding Se would be absent, and the armature A would then be connected straight across between the main terminals M and M'. When switch Sw is closed, the current will enter the starter at its terminal S, and pass through the magnet coil m' of the overload release to the switch lever L, which is shown in the off position. As soon as L is moved up to make contact with the first contact S the current divides; part going through the resistance R and the terminals S' and M' to the series coil Se (if a compound motor) and armature A; and part through the no voltage magnet E to the shunt winding Sh. As the lever L is moved up toward E, the effect is to take R out of the armature circuit and put it into the shunt circuit. When the iron armature a, fixed on the switch lever, comes against the poles of E, the laminated copper brush C bears against the blocks B, B, and so affords a better path for the current than through the spindle s. Should the supply voltage fail, either temporarily or permanently, E will release a, and L will fly off under the tension of a helical spring coiled round s. If there should be an overload on the motor, tending to pull it up and cause an excess of current to flow through the armature; this excess current, passing through m', will make it attract its armature, so bringing two contacts together at K which will short circuit E, and allow the switch to fly off. The connections between E and m' are not shown in the figure, but they are indicated at C in [fig. 498], which is a simplification of [fig. 497], and which should be carefully compared therewith. When only the normal current is flowing, the attraction between m' and its armature is not sufficient to pull the latter up. The actual forms and arrangement of parts on the starters are well shown in some of the figures.
Ques. What attachments should be provided on a starting box?
Ans. An overload release, and a no voltage release.