CHAPTER 7
GENERATORS
ELECTRICITY FROM MAGNETISM

The oldest and simplest known source of electricity, or emf, is the static charge. It is possible to build high static potentials of many thousands of volts. But these charges cannot be used as a source of electric power, because they have no RESERVE of ENERGY to call upon to keep the electrons flowing. With static charges, the potential falls almost to zero once the spark has jumped-the-gap.

Static charges can be created continuously in a number of different ways, but the RATE OF BUILDING is TOO SLOW to make the charges of practical use as a source of emf.

For a long time, primary cells were the only source of emf to power such devices as the telegraph and telephone. Electric motors, heating coils, and 101 types of heavy current consuming equipment, are common today. But in the early days, batteries alone could not furnish the

The development of the principle of obtaining electricity from magnetism is relatively new-within the last 100 years-and is credited to a Danish scientist, Oersted. Since the appearance of Oersted's first machine, rapid progress has been made. GENERATORS have been enlarged until now millions of kilowatts of power are delivered to homes and industry daily.

And you know the tremendous power of those big ship's generators. Battleships and carriers have generators capable of supplying a city of moderate size with enough electric power to run its factories, operate its street cars, and to supply all the other electric power requirements of the community.

The story of how a generator works starts with the principle of electromagnetic induction.

INDUCTION

Here's the process of induction in a nutshell. WHENEVER A CONDUCTOR CUTS ACROSS THE FLUX OF A MAGNETIC FIELD, AN EMF IS PRODUCED IN THE CONDUCTOR. If the two ends of the conductor are connected to an outside circuit, the induced emf causes current to flow in the circuit.

In figure 57, the GALVANOMETER, an instrument that shows DIRECTION and AMOUNT OF CURRENT, is connected to a CONDUCTOR. When the conductor is thrust DOWNWARD into the FIELD between the north and south poles of a magnet, the meter needle is DEFLECTED, indicating a FLOW OF CURRENT in the conductor.

Moving the conductor UPWARD (figure 58) causes the needle to move in the OPPOSITE DIRECTION, indicating that the direction of current flow has been REVERSED.

If you hold the conductor MOTIONLESS, no deflection occurs.

A QUICK THRUST produces a LARGE DEFLECTION of the needle. And a slow movement of the conductor causes a small deflection.

Here is what you have observed-

PARTS OF ALTERNATING CURRENT GENERATOR

The Danish scientist, Oersted, discovered how to obtain an emf by ROTATING a LOOP of wire in a MAGNETIC FIELD. This device became known as the GENERATOR, a relatively simple machine with just four major parts-STATOR, ARMATURE, SLIP RINGS, AND BRUSHES (figure 59).

The ARMATURE is a loop of wire so placed in the magnetic field that during rotation the loop will cut across the flux lines.

The SLIP RINGS are two complete copper rings. In figure 59, the BLACK ring is attached to the BLACK leg of the loop, and the WHITE RING to the WHITE leg of the loop. While not shown in figure 59, the LOOP and both SLIP

The BRUSHES are pieces of carbon held stationary against the slip rings by the generator framework.

The GALVANOMETER is not a part of the generator, but is added to the drawing to indicate the direction of current flow.

OPERATION OF A GENERATOR

Here is how the generator works. In figure 60, the loop is rotating in a clockwise direction. At position A, the TOP leg is moving toward the north pole, and the LOWER leg toward the south pole. In position A, no flux lines are being cut since both legs are moving PARALLEL to the lines of flux. Since NO flux is cut, NO VOLTAGE is INDUCED, and the GALVANOMETER needle STANDS at ZERO.

In position B, the loop has rotated 1/4 of a turn (90°). The BLACK leg is moving DOWNWARD, and the WHITE LEG UP. In this position, BOTH legs are cutting across a MAXIMUM

At position C the loop has rotated 1/2 of a turn. The two legs are once more moving PARALLEL to the lines of flux, and the galvanometer stands at zero.

In the last drawing, D, the black leg is moving upward, and white leg downward. Both legs are again cutting a maximum number of lines of force, but in the direction OPPOSITE to that of position B. Since the legs are CUTTING the field in the OPPOSITE direction, the emf induced causes the CURRENT to FLOW in the OPPOSITE DIRECTION.

The next 1/4 turn brings the loop back to position A, and the cycle starts over again.

Now go back and see what happened during one rotation . The emf started at ZERO, increased to a MAXIMUM value in ONE DIRECTION, fell back to ZERO, increased to

Look at figure 61. The loop is shown in five positions. Below the loop diagrams is a graph of the induced emf.

In position 1, the loop is cutting no lines of force, so the induced emf is zero (point A on the graph).

One quarter turn later, the loop is in position 2. It is cutting a maximum number of lines of force, so the emf is maximum (point B).

At position 3, the loop has completed 1/2 of a turn, and no lines of flux are being cut, so the emf is back to zero at point C.

In position 4, the loop is cutting the field in the direction opposite to that of position 2. The voltage induced in the coil is maximum, but in the opposite direction (point D).

Position 5 is the same as 1, so the loop is ready to start over again.

WHAT IS AN ALTERNATING EMF?

The emf produced by the generator in figure 60 is an ALTERNATING emf. It starts at zero, rises to maximum in one direction (+), falls back to zero, rises to maximum in the opposite direction (-), and then comes back to zero.

Notice in particular that the graph of an alternating emf lies on BOTH sides of the ZERO line.

An alternating emf causes the current to flow first in one direction and then the other. Hence the name, ALTERNATING CURRENT, or just plain A.C.

If the emf is ALL On ONE SIDE of the ZERO LINE, whether all negative or all positive, it produces a DIRECT CURRENT. Hence, the emf of an ALTERNATING CURRENT is on both sides of the ZERO LINE (base line); but the emf of a DIRECT CURRENT remains on ONE SIDE.

THE SINE WAVE

A sine wave is actually a MATHEMATICAL expression showing the relationship between the ORDINATE and RADIUS values of a point as it rotates about the circumference

So you should be able to recognize the sine wave and know something about its structure.

Directly below point 2, and on the zero line, the number 90° is written. The portion of the base line between 0° and 90° includes angular ROTATION of the FIRST 1/4 rotation .

Now if you want to know the emf generated after 45° of rotation, draw a VERTICAL line upward from the 45° point on the base line (half way between 0° and 90°) until it cuts the curve at point A. Draw a horizontal line from point A over to the scale to the left, and you

After the first 90°, the emf begins to fall. It continues dropping until it is back to "zero," after 180° of rotation (point 3). The loop has now completed 1/2 of a rotation and is ready to start upward.

One quarter of a turn later, 270° after the start of the rotation, the voltage is again maximum, but this time in a NEGATIVE direction (point 4).

Beyond point 4, the voltage again falls to zero as the loop returns to its starting point.

The next time around, the loop will generate the same alternating emf, and so on for each rotation.

A complete rotation is called a CYCLE, and each rotation thereafter is another cycle.

Thus, if 10 complete rotations are made in one second, the FREQUENCY of rotation is 10 CYCLES PER SECOND SIXTY ROTATIONS per second is a FREQUENCY of 60 cycles per second-the frequency of the ALTERNATING CURRENT used with commercial electricity power systems. You have heard about that before.

MORE ABOUT FREQUENCIES AND CYCLES

You will hear the word FREQUENCY used thousands of times in radio work. You will use it most commonly when referring to the tuning of your receiver or transmitter.

As an example, you may say your transmitter is tuned to a frequency of 4,200 kilocycles. What does it mean?

First of all, the word KILOCYCLE means 1,000 cycles per second; therefore, 4,200 kilocycles is the same as 4,200,000 CYCLES per SECOND. Thus your transmitter is generating an ALTERNATING CURRENT with a frequency of 4,200,000 cycles per second.

In radio, you will use three expressions of frequency-

cycles
kilocycles
megacycles

699198°-46-6

Hence, a frequency of 31 megacycles is-

  31,000 kilocycles, or 31,000,000 cycles

The megacycle is usually abbreviated Mc. or mc., the kilocycle Kc. or kc., and the cycle (˜) just one cycle of a sine wave.

Frequencies used in radio are divided into two basic classes-

AUDIO FREQUENCIES, from about 20 to 20,000 CYCLES per second, are those your ear can HEAR.

RADIO FREQUENCIES are all GREATER than 20,000 cycles and extend well above 30,000,000,000 cycles (30,000 Mc.) .

It is the usual practice to refer to all frequencies BELOW 20,000 cycles as A.F. or a.f., and those ABOVE 20,000 cycles as R.F. or r.f.

ELECTROMAGNETIC STATOR

In order to obtain a larger emf, a stronger ELECTROMAGNET is used for a stator instead of a permanent

ARMATURE

The armatures used with actual generators have many turns of wire instead of a single loop. The wire is wound on a core made of SHEETS of SOFT IRON, tightly clamped together. The iron core is attached to shaft, which is turned by a pulley and drive belt.

PART OF A DIRECT CURRENT GENERATOR

If you make a slight change in the slip rings of an a.c. generator, you can obtain direct current instead of alternating current.

The two BRUSHES are on opposite SIDES of the SPLIT RING, mounted in such a manner that each brush is in contact with only one segment at a time.

HOW A DIRECT CURRENT GENERATOR WORKS

The generation of the emf by the loop cutting across the magnetic field is the same in a d.c. as it is in an a.c. generator. The change to d.c. takes place at the COMMUTATOR.

In position B, the flux is being cut at a maximum rate. The BLACK BRUSH is contacting the BLACK SEGMENT and the WHITE BRUSH the WHITE SEGMENT. And the galvanometer needle is deflected to the RIGHT.

At position C, the loop has completed 180° of rotation. No flux is being cut, so the emf is zero. The important thing to observe in position C is the action of the segments and brushes. The BLACK BRUSH is SLIPPING off the black segment and ON TO the WHITE. At the same

The SWITCHING of commutator segments also switches legs of the loop. In this way the BLACK BRUSH is ALWAYS in contact with the leg moving DOWNWARD, and the WHITE brush in contact with the leg moving UPWARD. While the current is actually reversed in the loop it is ALWAYS FLOWING in the same direction through the galvanometer.

A graph for one cycle of a d.c. generator is given in figure 66. The generation of the emf for positions A, B,

The d.c. furnished by a single loop armature is very bumpy. It starts at zero, rises to maximum, and falls back to zero TWICE for each rotation of the loop. To produce a smoother d.c., more loops of wire are added to the armature.