Physics Part 52

[Ill.u.s.tration: FIG. 293.--DeLaval multi-stage turbine and gear driving 750-kw., 750-r.p.m., 600-volt direct-current generator.]

A practical dynamo, however, has many coils upon its armature with a corresponding number of segments upon the commutator. (See Figs. 289 and 293.) As each coil and commutator segment pa.s.ses a brush, it contributes an impulse to the current with the result that armatures with many coils produce currents that flow quite evenly. (See Fig. 292, 3.)

The current represented in Fig. 292 (2) is called a _pulsating_ current.

[Ill.u.s.tration: FIG. 294.--A wire carrying a current across a magnetic field is pushed sideways by the field.]

=305. The electric motor= is a machine which transforms the energy of an electric current into mechanical energy or motion. The _direct current motor_ consists of the same essential parts as a direct current dynamo, viz., the field magnet, armature, commutator and brushes. Its operation is readily comprehended after one understands the following experiment:

Set up two bar electromagnets with unlike poles facing each other about an inch apart. A wire connected to a source of current is hung loosely between the poles as in Fig. 294. The circuit through the wire should contain a key or switch. If a current is sent through the electromagnets and then another is sent through the wire, the latter will be found to be pushed either up or down, while if the current is reversed through the wire it is pushed in the opposite direction. These results may be explained as follows:

Consider the magnetic field about a wire carrying a current (See Fig.

295.) If such a wire is placed in the magnetic field between two opposite poles of an electromagnet (Fig. 296), the wire will be moved either up or down. The reason for this is shown by the diagram in Fig.

297. Here a wire carrying a current and therefore surrounded by a magnetic field pa.s.ses across another magnetic field. The two fields affect each other causing a crowding of the force lines either above or below the wire. The wire at once tends to move sideways across the field away from the crowded side. In the figure, the wire tends to move downward.

[Ill.u.s.tration: FIG. 295.--The magnetic field about a wire carrying a current.]

[Ill.u.s.tration: FIG. 296.--The magnetic field between two unlike poles.]

[Ill.u.s.tration: FIG. 297.--The crowding of the lines of force above the wire, pushes it downward.]

In a practical motor, the wires upon the armature are so connected that those upon one side (see Fig. 298), carry currents that pa.s.s in, while on the other side they pa.s.s out. To represent the direction of the current in the wires, the following device is employed; a circle with a cross (to represent the feather in the tail of an arrow) indicates a current going away from the observer, while a circle with a dot at its center (to represent the tip of an arrow) indicates a current coming toward the observer.

[Ill.u.s.tration: FIG. 298.--The crowding of the lines of force causes the armature to revolve in a clockwise direction.]

In Fig. 298 the north pole is at the left and the south pole at the right. The field of the magnets therefore pa.s.ses from left to right as indicated in the figure. Now in the armature the currents in the wires on the left half of the armature are coming toward the observer while those on the right move away. Applying the right-hand rule, the magnetic lines will crowd _under_ the wires on the left side of the armature while they will crowd _over_ the wires on the right side. This will cause a rotation up on the left side and down on the right, or in a _clockwise_ direction.

[Ill.u.s.tration: FIG. 299.--View of a one-half horse-power motor.]

If the current in the armature is reversed (in on the left and out on the right), the lines of force will crowd the armature around in the opposite direction or _counter clockwise_. The rotation of the armature will also be reversed if, while the current in the armature is unchanged in direction, the poles of the magnet are changed thus reversing the magnetic field.

The motorman of a street car reverses the motion of his car by reversing the direction of the current in the _armature_ of the motor.

[Ill.u.s.tration: FIG. 300.--The frame and electromagnet (at left), front bracket and brush holder (at right) of the motor shown in Fig. 299.]

[Ill.u.s.tration: FIG. 301.--The armature of a motor.]

=306. Practical motors= have many coils upon the armature with a corresponding number of segments upon the commutator. A large number of coils and commutator segments enables some one of the coils to exert its greatest efficiency at each instant, hence a steady force is provided for turning the armature which causes it to run smoothly. Fig. 299 represents a 1/2 horse-power motor ready for use while Fig. 300 shows the frame and poles and the front bracket and brush holder, and Fig. 301 represents the armature.

Important Topics

1. The dynamo, four essential parts, action (a) for alternating currents, (b) for direct currents.

2. The electric motor: (a) essential parts, (b) action.

Exercises

1. _Why_ is an alternating current produced in the armature of a dynamo?

2. _How_ is this current produced? Give careful explanations.

3. What is the result of Lenz's law as applied to the dynamo?

4. Apply the first two laws of electromagnetic induction to the dynamo.

5. What is the power of a dynamo if it produces 40 amperes of current at 110 volts?

6. How much power must be applied to this dynamo if its efficiency is 90 per cent.?

7. A motor takes 10 amperes of current at 220 volts; what is the _power_ of the current in _watts_? If this motor has an efficiency of 95 per cent., how many horse-power of mechanical energy can it develop?

8. Explain why reversing the current in the armature of a motor reverses the direction of rotation.

9. Find the cost of running a was.h.i.+ng machine using a 1/2-horsepower motor 2 hours if the cost of the electricity is 10 cents a kilowatt hour.

10. A 1/8-horse-power motor is used to run a sewing machine. If used for 3 hours what will be the cost at 11 cents a kilowatt hour?

(3) THE INDUCTION COIL AND THE TRANSFORMER

=307. The Induction Coil.=--Practically all electric currents are produced either by voltaic cells or by dynamos. It is frequently found, however, that it is desirable to change the E.M.F. of the current used, either for purposes of _effectiveness_, _convenience_, _or economy_. The _induction coil_ and the _transformer_, devices for changing the E.M.F.

of electric currents, are therefore in common use. _The induction coil_ (see Fig. 302) consists of a _primary_ coil of coa.r.s.e wire _P_ (Fig.

303) wound upon a core of soft iron wire, and a _secondary_ coil, _S_, of several thousand turns of fine wire. In circuit with the primary coil is a battery, _B_, and a current interrupter, _K_, which works like the interrupter upon an electric bell. The ends of the secondary coil are brought to binding posts or spark points as at _D_.

[Ill.u.s.tration: FIG. 302.--An induction coil.]

The current from the battery flows through the primary coil magnetizing the iron core. The magnetism in the core attracts the soft-iron end of the interrupter, drawing the latter over and breaking the circuit at the screw contact, _K_. This abruptly stops the current and at once the core loses its magnetism. The spring support of the interrupter now draws the latter back to the contact, _T_, again completing the circuit. The whole operation is repeated, the interrupter vibrating rapidly continually opening and closing the circuit.

[Ill.u.s.tration: FIG. 303.--Diagram showing the parts of an induction coil.]

=308. The Production of Induced Currents in the Secondary Coil.=--When the current flows through the _primary_ it sets up a magnetic field in the _core_. When the current is interrupted, the field disappears. The increase and decrease in the field of the core induces an E.M.F. in the secondary coil, in accordance with the first law of electromagnetic induction. The E.M.F. produced depends upon (a) the number of turns in the secondary, (b) the strength of the magnetic field and (c) the rate of change of the field. The rate of change in the field is more rapid at the break than at the make. When the circuit is closed it takes perhaps 1/10 of a second for the current to build up to its full strength while at a break the current stops in perhaps 0.00001 of a second, so that the induced E.M.F. is perhaps 10,000 times as great at "break" as at make.

To increase the suddenness of the "make" and "break," a condenser is often connected in the primary circuit, in parallel, with the interrupter. (See Fig. 303, _C._) This condenser provides a place to hold the rush of current at the instant that the interrupter breaks the circuit. This stored up charge reinforces the current at the make producing a much more sudden change in the magnetic field with a corresponding increase in the E.M.F. The induced currents from induction coils are sometimes called _faradic currents_ in honor of Faraday who discovered electromagnetic induction. They are used to operate sparking devices upon gas and gasoline engines and in many devices and experiments in which high-tension electricity is employed.

[Ill.u.s.tration: FIG. 304.--The transformer has a closed core; the induction coil, an open core.]

[Ill.u.s.tration: FIG. 305.--The laminated iron core of a transformer.]

[Ill.u.s.tration: FIG. 306.--Cross-section of the transformer shown in Fig.

305 showing the magnetic field around the primary and secondary coils.]

=309. The Transformer.=--This is like the induction coil in that it uses a _primary_ and a _secondary_ coil, and an iron core to carry the magnetic field. (See Fig. 304.) They differ in that the transformer has a _closed_ core or one forming a continuous iron circuit, while the induction coil has an _open_ core, or one in which the magnetic field must travel in air from the north to the south poles of the core. The transformer must always be used with an _alternating_ current while the induction coil may use either a direct or an alternating current.

Further, the _induction_ coil always produces a higher E.M.F. while the transformer may produce an E.M.F. in its secondary coil that is either higher or lower than the one in the primary. The former is called "_step-up_" while the latter is a "_step-down_" transformer. The alternating current in the primary coil of the transformer produces an _alternating magnetic flux_ in the iron core. This iron core is _laminated_ (see Fig. 305) to prevent the heating that would result if a solid core were used. The alternating magnetic flux induces in the secondary coil an E.M.F. in accordance with the following rule. The ratio of the _number_ of _turns_ in the _primary_ to the _number_ of the _turns_ in the _secondary_ coil equals the ratio of the electromotive forces in these respective coils. If the secondary coil has 8 turns while the primary has 4, the E.M.F. of the secondary will be just twice that of the primary. Or, if in the primary coil of the transformer Fig.

306 is an E.M.F. of 110 volts, in the secondary will be found an E.M.F.

of 220 volts.

[Ill.u.s.tration: FIG. 307.--A commercial transformer.]