DYNAMOS AND ELECTRIC MOTORS.

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Chapter IX. DYNAMOS AND ELECTRIC MOTORS.

A simple dynamo—Continuous-current dynamos—Multipolar dynamos—Exciting the field magnets—Alternating current dynamos—The transmission of power—The electric motor—Electric lighting—The incandescent lamp—Arc lamps—"Series" and "parallel" arrangement of lamps—Current for electric lamps—Electroplating.

IN previous chapters we have incidentally referred to the conversion of mechanical work into electrical energy. In this we shall examine how it is done—how the silently spinning dynamo develops power, and why the motor spins when current is passed through it.

We must begin by returning to our first electrical diagram (Fig. 50), and calling to mind the invisible "lines of force" which permeate the ether in the immediate neighbourhood of a magnet's poles, called the magnetic field of the magnet.

Many years ago (1831) the great Michael Faraday discovered that if a loop of wire were moved up and down between the poles of an electro-magnet (Fig. 66) a current was induced in the loop, its direction depending upon that in which the loop was moved. The energy required to cut the lines of force passed in some mysterious way into the wire. Why this is so we cannot say, but, taking advantage of the fact, electricians have gradually developed the enormous machines which now send vehicles spinning over metal tracks, light our streets and houses, and supply energy to innumerable factories.

The strength of the current induced in a circuit cutting the lines of force of a magnet is called its pressure, voltage, or electro-motive force (expressed shortly E.M.F.). It may be compared with the pounds-to-the-square-inch of steam. In order to produce an E.M.F. of one volt it is calculated that 100,000,000 lines of force must be cut every second.

The voltage depends on three things:—(1.) The strength of the magnet: the stronger it is, the greater the number of lines of force coming from it. (2.) The length of the conductor cutting the lines of force: the longer it is, the more lines it will cut. (3.) The speed at which the conductor moves: the faster it travels, the more lines it will cut in a given time. It follows that a powerful dynamo, or mechanical producer of current, must have strong magnets and a long conductor; and the latter must be moved at a high speed across the lines of force.

A SIMPLE DYNAMO.

In Fig. 67 we have the simplest possible form of dynamo—a single turn of wire, w x y z, mounted on a spindle, and having one end attached to an insulated ring c, the other to an insulated ring c1. Two small brushes, b b1, of wire gauze or carbon, rubbing continuously against these collecting rings, connect them with a wire which completes the circuit. The armature, as the revolving coil is called, is mounted between the poles of a magnet, where the lines of force are thickest. These lines are supposed to stream from the N. to the S. pole.

In Fig. 67 the armature has reached a position in which y z and w x are cutting no, or very few, lines of force, as they move practically parallel to the lines. This is called the zero position.

In Fig. 68 the armature, moving at right angles to the lines of force, cuts a maximum number in a given time, and the current induced in the coil is therefore now most intense. Here we must stop a moment to consider how to decide in which direction the current flows. The armature is revolving in a clockwise direction, and y z, therefore, is moving downwards. Now, suppose that you rest your left hand on the N. pole of the magnet so that the arm lies in a line with the magnet. Point your forefinger towards the S. pole. It will indicate the direction of the lines of force. Bend your other three fingers downwards over the edge of the N. pole. They will indicate the direction in which the conductor is moving across the magnetic field. Stick out the thumb at right angles to the forefinger. It points in the direction in which the induced current is moving through the nearer half of the coil. Therefore lines of force, conductor, and induced current travel in planes which, like the top and two adjacent sides of a box, are at right angles to one another.

While current travels from z to y—that is, from the ring c1 to y—it also travels from x to w, because w x rises while y z descends. So that a current circulates through the coil and the exterior part of the circuit, including the lamp. After z y has passed the lowest possible point of the circle it begins to ascend, w x to descend. The direction of the current is therefore reversed; and as the change is repeated every half-revolution this form of dynamo is called an alternator or creator of alternating currents. A well-known type of alternator is the magneto machine which sends shocks through any one who completes the external circuit by holding the brass handles connected by wires to the brushes. The faster the handle of the machine is turned the more frequent is the alternation, and the stronger the current.

CONTINUOUS-CURRENT DYNAMOS.

An alternating current is not so convenient for some purposes as a continuous current. It is therefore sometimes desirable (even necessary) to convert the alternating into a uni-directional or continuous current. How this is done is shown in Figs. 69 and 70. In place of the two collecting rings c c1, we now have a single ring split longitudinally into two portions, one of which is connected to each end of the coil w x y z. In Fig. 69 brush b has just passed the gap on to segment c, brush b1 on to segment c1. For half a revolution these remain respectively in contact; then, just as y z begins to rise and w x to descend, the brushes cross the gaps again and exchange segments, so that the current is perpetually flowing one way through the circuit. The effect of the commutator is, in fact, equivalent to transposing the brushes of the collecting rings of the alternator every time the coil reaches a zero position.

Figs. 71 and 72 give end views in section of the coil and the commutator, with the coil in the position of minimum and maximum efficiency. The arrow denotes the direction of movement; the double dotted lines the commutator end of the revolving coil.

PRACTICAL CONTINUOUS-CURRENT DYNAMOS.

The electrical output of our simple dynamo would be increased if, instead of a single turn of wire, we used a coil of many turns. A further improvement would result from mounting on the shaft, inside the coil, a core or drum of iron, to entice the lines of force within reach of the revolving coil. It is evident that any lines which pass through the air outside the circle described by the coil cannot be cut, and are wasted.

The core is not a solid mass of iron but built up of a number of very thin iron discs threaded on the shaft and insulated from one another to prevent electric eddies, which would interfere with the induced current in the conductor. Sometimes there are openings through the core from end to end to ventilate and cool it.

We have already noticed that in the case of a single coil the current rises and falls in a series of pulsations. Such a form of armature would be unsuitable for large dynamos, which accordingly have a number of coils wound over their drums, at equal distances round the circumference, and a commutator divided into an equal number of segments. The subject of drum winding is too complicated for brief treatment, and we must therefore be content with noticing that the coils are so connected to their respective commutator segments and to one another that they mutually assist one another. A glance at Fig. 73 will help to explain this. Here we have in section a number of conductors on the right of the drum (marked with a cross to show that current is moving, as it were, into the page), connected with conductors on the left (marked with a dot to signify current coming out of the page). If the "crossed" and "dotted" conductors were respectively the "up" and "down" turns of a single coil terminating in a simple split commutator (Fig. 69), when the coil had been revolved through an angle of 90° some of the up turns would be ascending and some descending, so that conflicting currents would arise. Yet we want to utilize the whole surface of the drum; and by winding a number of coils in the manner hinted at, each coil, as it passes the zero point, top or bottom, at once generates a current in the desired direction and reinforces that in all the other turns of its own and of other coils on the same side of a line drawn vertically through the centre. There is thus practically no fluctuation in the pressure of the current generated.

The action of single and multiple coil windings may be compared to that of single and multiple pumps. Water is ejected by a single pump in gulps; whereas the flow from a pipe fed by several pumps arranged to deliver consecutively is much more constant.

MULTIPOLAR DYNAMOS.

Hitherto we have considered the magnetic field produced by one bi-polar magnet only. Large dynamos have four, six, eight, or more field magnets set inside a casing, from which their cores project towards the armature so as almost to touch it (Fig. 74). The magnet coils are wound to give N. and S. poles alternately at their armature ends round the field; and the lines of force from each N. pole stream each way to the two adjacent S. poles across the path of the armature coils. In dynamos of this kind several pairs of collecting brushes pick current off the commutator at equidistant points on its circumference.

EXCITING THE FIELD MAGNETS.

Until current passes through the field magnet coils, no magnetic field can be created. How are the coils supplied with current? A dynamo, starting for the first time, is excited by a current from an outside source; but when it has once begun to generate current it feeds its magnets itself, and ever afterwards will be self-exciting, owing to the residual magnetism left in the magnet cores.

Look carefully at Figs. 77 and 78. In the first of these you will observe that part of the wire forming the external circuit is wound round the arms of the field magnet. This is called a series winding. In this case all the current generated helps to excite the dynamo. At the start the residual magnetism of the magnet cores gives a weak field. The armature coils cut this and pass a current through the circuit. The magnets are further excited, and the field becomes stronger; and so on till the dynamo is developing full power. Series winding is used where the current in the external circuit is required to be very constant.

Fig. 78 shows another method of winding—the shunt. Most of the current generated passes through the external circuit 2, 2; but a part is switched through a separate winding for the magnets, denoted by the fine wire 1, 1. Here the strength of the magnetism does not vary directly with the current, as only a small part of the current serves the magnets. The shunt winding is therefore used where the voltage (or pressure) must be constant.

A third method is a combination of the two already named. A winding of fine wire passes from brush to brush round the magnets; and there is also a series winding as in Fig. 77. This compound method is adapted more especially for electric traction.

ALTERNATING DYNAMOS.

These have their field magnets excited by a separate continuous current dynamo of small size. The field magnets usually revolve inside a fixed armature (the reverse of the arrangement in a direct-current generator); or there may be a fixed central armature and field magnets revolving outside it. This latter arrangement is found in the great power stations at Niagara Falls, where the enormous field-rings are mounted on the top ends of vertical shafts, driven by water-turbines at the bottom of pits 178 feet deep, down which water is led to the turbines through great pipes, or penstocks. The weight of each shaft and the field-ring attached totals about thirty-five tons. This mass revolves 250 times a minute, and 5,000 horse power is constantly developed by the dynamo. Similar dynamos of 10,000 horse power each have been installed on the Canadian side of the Falls.

TRANSMISSION OF POWER.

Alternating current is used where power has to be transmitted for long distances, because such a current can be intensified, or stepped up, by a transformer somewhat similar in principle to a Ruhmkorff coil minus a contact-breaker (see p. 122). A typical example of transformation is seen in Fig. 79. Alternating current of 5,000 volts pressure is produced in the generating station and sent through conductors to a distant station, where a transformer, b, reduces the pressure to 500 volts to drive an alternating motor, c, which in turn operates a direct current dynamo, d. This dynamo has its + terminal connected with the insulated or "live" rail of an electric railway, and its – terminal with the wheel rails, which are metallically united at the joints to act as a "return." On its way from the live rail to the return the current passes through the motors. In the case of trams the conductor is either a cable carried overhead on standards, from which it passes to the motor through a trolley arm, or a rail laid underground in a conduit between the rails. In the top of the conduit is a slit through which an arm carrying a contact shoe on the end projects from the car. The shoe rubs continuously on the live rail as the car moves.

To return for a moment to the question of transformation of current. "Why," it may be asked, "should we not send low-pressure direct current to a distant station straight from the dynamo, instead of altering its nature and pressure? Or, at any rate, why not use high-pressure direct current, and transform that?" The answer is, that to transmit a large amount of electrical energy at low pressure (or voltage) would necessitate large volume (or amperage) and a big and expensive copper conductor to carry it. High-pressure direct current is not easily generated, since the sparking at the collecting brushes as they pass over the commutator segments gives trouble. So engineers prefer high-pressure alternating current, which is easily produced, and can be sent through a small and inexpensive conductor with little loss. Also its voltage can be transformed by apparatus having no revolving parts.

THE ELECTRIC MOTOR.

Anybody who understands the dynamo will also be able to understand the electric motor, which is merely a reversed dynamo.

Imagine in Fig. 70 a dynamo taking the place of the lamp and passing current through the brushes and commutator into the coil w x y z. Now, any coil through which current passes becomes a magnet with N. and S. poles at either end. (In Fig. 70 we will assume that the N. pole is below and the S. pole above the coil.) The coil poles therefore try to seek the contrary poles of the permanent magnet, and the coil revolves until its S. pole faces the N. of the magnet, and vice versâ. The lines of force of the coil and the magnet are now parallel. But the momentum of revolution carries the coil on, and suddenly the commutator reverses its polarity, and a further half-revolution takes place. Then comes a further reversal, and so on ad infinitum. The rotation of the motor is therefore merely a question of repulsion and attraction of like and unlike poles. An ordinary compass needle may be converted into a tiny motor by presenting the N. and S. poles of a magnet to its S. and N. poles alternately every half-revolution.

In construction and winding a motor is practically the same as a dynamo. In fact, either machine can perform either function, though perhaps not equally well adapted for both. Motors may be run with direct or alternating current, according to their construction.

On electric cars the motor is generally suspended from the wheel truck, and a small pinion on the armature shaft gears with a large pinion on a wheel axle. One great advantage of electric traction is that every vehicle of a train can carry its own motor, so that the whole weight of the train may be used to get a grip on the rails when starting. Where a single steam locomotive is used, the adhesion of its driving-wheels only is available for overcoming the inertia of the load; and the whole strain of starting is thrown on to the foremost couplings. Other advantages may be summed up as follows:—(1) Ease of starting and rapid acceleration; (2) absence of waste of energy (in the shape of burning fuel) when the vehicles are at rest; (3) absence of smoke and smell.

ELECTRIC LIGHTING.

Dynamos are used to generate current for two main purposes—(1) To supply power to motors of all kinds; (2) to light our houses, factories, and streets. In private houses and theatres incandescent lamps are generally used; in the open air, in shops, and in larger buildings, such as railway stations, the arc lamp is more often found.

INCANDESCENT LAMP.

If you take a piece of very fine iron wire and lay it across the terminals of an accumulator, it becomes white hot and melts, owing to the heat generated by its resistance to the current. A piece of fine platinum wire would become white hot without melting, and would give out an intense light. Here we have the principle of the glow or incandescent lamp—namely, the interposition in an electric circuit of a conductor which at once offers a high resistance to the current, but is not destroyed by the resulting heat.

In Fig. 80 is shown a fan propelling liquid constantly through a pipe. Let us assume that the liquid is one which develops great friction on the inside of the pipe. At the contraction, where the speed of travel is much greater than elsewhere in the circuit, most heat will be produced.

In quite the early days of the glow-lamp platinum wire was found to be unreliable as regards melting, and filaments of carbon are now used. To prevent the wasting away of the carbon by combination with oxygen the filament is enclosed in a glass bulb from which practically all air has been sucked by a mercury pump before sealing.

The manufacture of glow-lamps is now an important industry. One brand of lamp is made as follows:—First, cotton-wool is dissolved in chloride of zinc, and forms a treacly solution, which is squirted through a fine nozzle into a settling solution which hardens it and makes it coil up like a very fine violin string. After being washed and dried, it is wound on a plumbago rod and baked in a furnace until only the carbon element remains. This is the filament in the rough. It is next removed from the rod and tipped with two short pieces of fine platinum wire. To make the junction electrically perfect the filament is plunged in benzine and heated to whiteness by the passage of a strong current, which deposits the carbon of the benzine on the joints. The filament is now placed under the glass receiver of an air-pump, the air is exhausted, hydro-carbon vapour is introduced, and the filament has a current passed through it to make it white hot. Carbon from the vapour is deposited all over the filament until the required electrical resistance is attained. The filament is now ready for enclosure in the bulb. When the bulb has been exhausted and sealed, the lamp is tested, and, if passed, goes to the finishing department, where the two platinum wires (projecting through the glass) are soldered to a couple of brass plates, which make contact with two terminals in a lamp socket. Finally, brass caps are affixed with a special water-tight and hard cement.

ARC LAMPS.

In arc lighting, instead of a contraction at a point in the circuit, there is an actual break of very small extent. Suppose that to the ends of the wires leading from a dynamo's terminals we attach two carbon rods, and touch the end of the rods together. The tips become white hot, and if they are separated slightly, atoms of incandescent carbon leap from the positive to the negative rod in a continuous and intensely luminous stream, which is called an arc because the path of the particles is curved. No arc would be formed unless the carbons were first touched to start incandescence. If they are separated too far for the strength of the current to bridge the gap the light will flicker or go out. The arc lamp is therefore provided with a mechanism which, when the current is cut off, causes the carbons to fall together, gradually separates them when it is turned on, and keeps them apart. The principle employed is the effort of a coil through which a current passes to draw an iron rod into its centre. Some of the current feeding the lamp is shunted through a coil, into which projects one end of an iron bar connected with one carbon point. A spring normally presses the points together when no current flows. As soon as current circulates through the coil the bar is drawn upwards against the spring.

SERIES AND PARALLEL ARRANGEMENT OF LAMPS.

When current passes from one lamp to another, as in Fig. 82, the lamps are said to be in series. Should one lamp fail, all in the circuit would go out. But where arc lamps are thus arranged a special mechanism on each lamp "short-circuits" it in case of failure, so that current may pass uninterruptedly to the next.

Fig. 83 shows a number of lamps set in parallel. One terminal of each is attached to the positive conductor, the other to the negative conductor. Each lamp therefore forms an independent bridge, and does not affect the efficiency of the rest. Parallel series signifies a combination of the two systems, and would be illustrated if, in Fig. 83, two or more lamps were connected in series groups from one conductor to the other. This arrangement is often used in arc lighting.

CURRENT FOR ELECTRIC LAMPS.

This may be either direct or alternating. The former is commonly used for arc lamps, the latter for incandescent, as it is easily stepped-down from the high-pressure mains for use in a house. Glow-lamps usually take current of 110 or 250 volts pressure.

In arc lamps fed with direct current the tip of the positive carbon has a bowl-shaped depression worn in it, while the negative tip is pointed. Most of the illumination comes from the inner surface of the bowl, and the positive carbon is therefore placed uppermost to throw the light downwards. An alternating current, of course, affects both carbons in the same manner, and there is no bowl.

The carbons need frequent renewal. A powerful lamp uses about 70 feet of rod in 1,000 hours if the arc is exposed to the air. Some lamps have partly enclosed arcs—that is, are surrounded by globes perforated by a single small hole, which renders combustion very slow, though preventing a vacuum.

ELECTROPLATING.

Electroplating is the art of coating metals with metals by means of electricity. Silver, copper, and nickel are the metals most generally deposited. The article to be coated is suspended in a chemical solution of the metal to be deposited. Fig. 84 shows a very simple plating outfit. a is a battery; b a vessel containing, say, an acidulated solution of sulphate of copper. A spoon, s, hanging in this from a glass rod, r, is connected with the zinc or negative element, z, of the battery, and a plate of copper, p, with the positive element, c. Current flows in the direction shown by the arrows, from z to c, c to p, p to s, s to z. The copper deposited from the solution on the spoon is replaced by gradual dissolution of the plate, so that the latter serves a double purpose.

In silver plating, p is of silver, and the solution one of cyanide of potassium and silver salts. Where nickel or silver has to be deposited on iron, the article is often given a preliminary coating of copper, as iron does not make a good junction with either of the first two metals, but has an affinity for copper.

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