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Shunt machines in parallel.
that the variation in pressure will be the same, practically, in all as the current strength changes, no one machine will be likely to take a portion of the load differing from its proper proportion by more than a few per cent.
The way in which two shunt-wound generators are connected is shown in Fig. 15, only the generators and
The Derry Collard Co
field regulators being represented, so as to avoid complication. Three, four or any other number of shunt machines would be connected in the same manner; that is, with all the a brushes connected with one buis bar, and all the b brushes with the other bus. Shunt-wound generators, however, are seldom used owing to the fact that they cannot maintain as uniform a voltage as the com
To equalize strength of current.
The. Derry Collard, Co.
Figure 16 Connected to equalize strength of current in series of coils of both machines. pound type. When two or more generators of this latter type are connected in parallel it is necessary to so connect the series coils that the strength of current passing through them will be the same in all the machines. This
10 R Figure 17
The Derry, Collard Co Two generators in parallel with instruments connected in place. Apparatus required.
result is attained by the type of connection shown in Fig. 16. By examining this diagram it will be seen that the lower ends of the two series coils are connected with each other, and that this is also the case with the upper ends. As m and m' are of the same resistance, or nearly so, in generators of the same size, the current coming through wire N will divide upon reaching n, one half passing to m, by wire a, and the other half to m', by wire a'. Through wires b and b’ the current will reach the short wire c, and from here it may, and it may not, pass in equal amounts by the wires d and d' to the two armatures. If the wires b b', d d' and c were not provided, and the current from armature A passed directly to coil m, and that from armature A' to coil m', then the current flowing in these coils would be of the same strength as that in the respective armatures. Under these conditions any tendency for the generators to run unequally would be aggravated, for the armature developing the strongest current would be acted upon by the strongest m coil, and the effect of this would be to still further increase its current in consequence of the increase in voltage due to the increased strength of the field. When the wires d d', b b' and c are added, which are called the equalizing connection, the current generated by the two machines is equally divided between the two series coils m and m', and the result is that the difference in the strength of the currents generated in the two armatures cannot be very great.
For the operation of two generators in parallel the instruments and apparatus required and the connection of the circuits must be as indicated in Fig. 17. This diagram shows the various parts without any reference to systematic arrangement. We have, as in the case of the
single generator, a field regulator for each machine, and also an ammeter and a voltmeter; these being shown at Am and at V. At f and f' are shown the circuit breakers, and at s a switch for opening the circuit of the leftside generator, while at s' is a similar switch for the rightside generator. In this diagram the wire e is the equalizing wire and takes the place of the wires d d', b b' and c, in Fig. 16.
If we were to open switch s, the left-side generator would not be entirely disconnected from the other one, for through wire e the current of armature A' could pass to and through armature A and to line wire P; thus shortcircuiting armature A' through armature A. To be able to entirely disconnect the two generators it is necessary, therefore, to provide the additional switch s". When the generators are running, it is not safe to open the switch s" in the equalizing wire before the s switch is opened, and when it is desired to cut the second machines into the circuit, it is not safe to close the s" switch first. To avoid accidents by the improper manipulation of these switches, it is necessary to provide switches that combine s and s" in one, so that by the movement of a single handle both circuits are opened. Such switches are called two-pole switches. Even with this arrangement we would not be able to entirely disconnect either one of the generators from the circuit; for, as will be noticed, if the switches s and s" are opened, the armature A will still be connected with the line wire P; hence, if A were grounded, both generators would become useless. For a complete separation of the two machines we must have another switch in the wire running from line wire P to armature A.