Hawkins Electrical Guide, Number One Part 26

Ans. It may be used for series arc lighting, series incandescent lighting, and as a _booster_ for increasing the pressure on a feeder carrying current furnished by some other generator.

=Ques. What is the effect of the series winding in the operation of the machine?=

Ans. Its characteristic is to furnish current at an increased voltage as the load increases. If sufficient current be drawn to overload the machine, the voltage will drop.

Since the armature coils, field magnets and external circuits are in series, any increase in the resistance of the external circuit lessens the power of the machine to supply current, because it diminishes the current in the coils of the field magnets and therefore diminishes the effective magnetism. Again, a decrease in the resistance of the external circuit will increase the voltage because more current will flow through the field magnets. Accordingly, when the external circuit has lamps in series (as is common in an arc light circuit) the switching on of an additional lamp both adds to the resistance of the circuit and diminishes the power of the machine to supply current. When the lamps are in parallel, the switching on of additional lamps not only diminishes the resistance of the circuit, but causes the field magnets to be further excited by the increased current, so that the greater the number of lamps put on, the greater becomes the risk of inducing too much current.

The series dynamo has also the disadvantage of not starting action until a certain speed has been attained, or unless the resistance of the external circuit be below a certain limit.

=Regulation of Series Dynamos.=--The series dynamo is ordinarily used for operating arc lamps connected in series. The current generally consumed is about 10 amperes, and it is necessary that it should remain at this strength to keep the lights burning steadily. If it increase, the lights will be too bright, and if it decrease, they will be too dim or flicker.

With all the lamps connected in series it is evident that the resistance of the circuit will vary widely as they are turned on or off, the resistance increasing as the lamps are turned on, and decreasing as they are turned off. It is necessary, therefore, that some means of regulation be provided to enable the dynamo to increase or decrease the voltage in proportion to the load. There are several methods of regulation, as by:

1. Variation of armature speed; 2. Variation of position of brushes; 3. Variation of field strength.

Whatever method be used the necessary regulation should be accomplished by automatic devices, as it would not be practical to station a man in constant attendance to regulate the voltage every time one or more lamps were thrown on or off.

=Ques. When is the first method of regulation used?=

Ans. It is only used in special cases, as for constant load; if the voltage be not just right to give the required current, it may be adjusted by changing the speed of the engine.

=Ques. What may be said of the second method?=

Ans. In both the "ring" and "drum" types of armature, rotating in a bipolar field, there are two points situated at opposite extremities of a diameter of the commutator, at one of which the potential is a maximum and at the other a minimum, and it is at these points that the brushes must be placed in order to obtain the greatest difference of pressure, the difference being less at other points. Hence, by rocking the brushes around the commutator the pressure at the terminals of the machine may be varied and regulated as required.

=Ques. What difficulty is experienced in rocking the brushes to regulate the voltage?=

Ans. Sparking takes place at the brushes when they are moved any considerable distance from the neutral position.

Special dynamos have been designed to overcome this objectionable feature, still this method of regulation is not extensively used.

=Ques. What may be said of the third method of regulation?=

Ans. The third method, that of variation of field strength, is the one in general use.

=Ques. How is the field strength varied?=

Ans. This may be done by the _two path method_, or by the _variable field coil method_.

=Ques. Describe the two path method of field regulation.=

Ans. An adjustable resistance or _rheostat_ is connected in parallel with the field winding as shown in fig. 191. This shunts more or less of the current from the field winding according to the amount of resistance made active by the lever, _L_.

Thus, if the current in the armature and main circuit be 10 amperes and the resistance of the field winding 10 ohms, a resistance of 40 ohms in parallel with the winding would cause the current to split in the ratio of 40 to 10, or 4 to 1; 2 amperes would pa.s.s through the resistance and 8 amperes through the field.

[Ill.u.s.tration: FIG. 191.--The two path method of regulating a series dynamo. The ends of the series winding are connected by a shunt containing a rheostat. The current induced in the armature, divides and flows through the two paths thus offered, the amount flowing through the shunt being regulated by the rheostat. In this way the field strength is easily regulated.]

[Ill.u.s.tration: FIG. 192.--Regulation of series dynamo by variable field. A multipoint switch is provided with connections to the field winding at various sections, thus permitting more or less of the field winding to be cut out to regulate its strength.]

=Ques. Describe the variable field coil method of field regulation.=

Ans. This consists in dividing the field winding into a number of sections and throwing the sections in and out of circuit as shown in fig. 192.

Since the strength of any magnet depends on the number of ampere turns in its field winding, reducing or increasing the number of turns will respectively reduce or increase the field strength, the current being kept constant.

=Ques. What is the objection to this method?=

Ans. This arrangement is undesirable for magnets of large size, because of the tendency to flashing at the contacts of the regulating switch.

[Ill.u.s.tration: FIG. 193.--Shunt wound dynamo for parallel circuit incandescent lighting, and for mill and factory power. The coils of the field magnet form a shunt to the main circuit; they consist of many turns of fine wire and consequently absorb only a small fraction of the current induced in the armature. The characteristic of the shunt dynamo is that it gives practically constant voltage for all loads within its range. If overloaded the pressure will drop and the machine cease to generate current.]

=The Shunt Dynamo.=--The shunt wound dynamo differs from the series wound machine, in that an independent circuit is used for exciting its field magnet. This circuit is composed of a large number of turns of fine insulated copper wire, which is wound round the field magnets and connected to the brushes, so as to form a shunt or "by pa.s.s" to the brushes and external circuit, as shown in fig. 193. Two paths are thus presented to the current as it leaves the armature, between which it divides in the inverse ratio of the resistance. One part of the current flows through the magnetizing coils, and the other portion through the external circuit.

In all well designed shunt dynamos, the resistance of the shunt circuit is always very great, as compared with the resistance of the armature and external circuit, the strength of the current flowing in the shunt coils being very small even in the largest machines.

=Ques. For what service is the shunt dynamo adapted?=

Ans. It is used for constant voltage circuits, as in incandescent lighting.

=Ques. In the operation of a shunt dynamo what is its characteristic feature?=

Ans. The voltage at the dynamo remains practically unchanged, and the current varies according to the load.

=Ques. Does the voltage remain constant for all loads?=

Ans. There is a certain maximum load current that the shunt dynamo is capable of supplying at constant voltage; beyond this, the voltage will decrease, the machine finally demagnetizing itself, and ceasing to generate current.

=Ques. Why does the voltage not remain constant for all loads?=

Ans. Because there is a drop in the voltage in forcing the current through the armature windings which increases with the load.

=Ques. What is the usual method of regulation for shunt dynamos?=

Ans. The method of varying the current through the field coils by means of a rheostat inserted in series with the field winding as shown in fig.

194.

Moving the lever L of the rheostat to the right increases the resistance in series with the field winding, and this reduces the amount of current in that winding, thus reducing the strength of the magnet and consequently the voltage at the brushes. The contrary movement of the lever, by cutting out the resistance, produces the opposite effect.

=The Compound Dynamo.=--This cla.s.s of generator is designed to automatically give a better regulation of voltage on constant pressure circuits than is possible with a shunt machine. It possesses the characteristics of both the series and shunt machines, of which it is in fact a combination.

[Ill.u.s.tration: FIG. 194.--Regulation of shunt dynamo by method of varying the field strength. A rheostat is placed in series with the field coils, and by varying the resistance, more or less current will flow through the coils, thus regulating the field strength.]

The field magnets of the compound dynamo, as shown in fig. 195, are wound with two sets of coils, one set being connected in series, and the other set in parallel, with the armature and external circuit. The purpose of the series winding is to strengthen the magnets by the current supplied from the armature to the circuit, and thus automatically sustain the pressure. If the series winding were not present, the pressure at the terminals would fall as the load increased. This fall of pressure is counteracted by the excitation of the series winding, which increases with the load and causes the pressure to rise. The number of turns and relative current strengths of the series and shunt windings are so adjusted that the pressure at the terminals is maintained practically constant under varying loads.

With respect to the ratio between the number of turns of the two field windings, the dynamo is spoken of as:

1. Compound; 2. Over compounded.