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REVIEW OF ELECTRICAL PRINCIPLES
 
A. MAGNETISM
 
1A1. Natural magnet. The power of a certain kind of iron ore to attract iron was first discovered thousands of years ago. The attracting power of such ore was named magnetism, and a piece of ore having this power was named a magnet.

1A2. Artificial magnet. An artificial magnet is made by stroking a piece of hard steel or soft iron with a natural magnet. These pieces can then be used to magnetize others. However, the properties of soft iron are such that, although easily magnetized, it loses its magnetism almost as soon as the means of magnetizing it have been removed. Hard steel, unlike soft iron, is more difficult to magnetize but retains its magnetism. Hence, soft iron when magnetized becomes a temporary magnet and hard steel a permanent magnet. The extent to which these metals retain their magnetism is an important factor when they are used in electrical equipment.

1A3. Polarity. If a bar magnet is dipped into a pile of iron filings, the greatest number of filings adheres to the ends of the bar. The ends, where the attraction is strongest, are known as the poles of the magnet, while the center of the magnet, where there is no apparent attraction, is known as the neutral line, or equator. When this magnet is swung on a thread secured around its equator, one pole points toward the north and the other toward the south. The end which seeks the north is called the north, or positive pole and the south-seeking pole is called the south, or negative pole.

1A4. Magnetic attraction and repulsion. If a bar magnet is suspended from its equator so that it swings freely and the north pole of another magnet is brought close to each of its poles in turn, the north pole of the suspended magnet is repelled and the south pole is attracted. If the ends of the suspended magnet are approached by the south pole of the other magnet, the north pole of the suspended magnet is attracted and the south pole repelled. This power of attraction and repulsion, which all magnets possess for other magnets and magnetic

  fields, is the basis upon which electric motors depend for their turning motion. It is expressed in an important law of magnetic attraction which states: Like poles repel each other and unlike poles attract each other.

1A5. Magnetic field. When an ordinary bar magnet is held under a piece of paper on which fine iron filings are sprinkled, the filings assume the shape of curved lines (Figure 1-1). Holding the magnet perpendicular to the plane of the paper causes the filings to form straight lines toward the ends of the magnet (Figure 1-2). The action of the filings indicates the presence of a force. The space surrounding the magnet in which this force is apparent is known as its magnetic field. The lines in which the filings arrange themselves are called lines of force. The number of magnetic lines in the field represents a certain amount of magnetism which is expressed as a unit of quantity called magnetic flux.

Figure 1-1. Lines of force surrounding a bar magnet.
Figure 1-1. Lines of force surrounding a bar magnet.

Figure 1-2. Lines of force surrounding the end of a
bar magnet.
Figure 1-2. Lines of force surrounding the end of a bar magnet.

 
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1A6. Magnetic circuits. A magnetic circuit is the path followed by the magnetic lines of force of a magnet. A closed magnetic circuit is one in which the lines of force produce flow around an unbroken metallic path or ring (Figure 1-3). Such a ring may be strongly magnetized and yet have no poles because its lines of force do not leave the metallic ring comprising the circuit. Ring magnetic circuits of this type are used where it is required that little or no external field be present; for example, in transformers and certain electrical instruments.

When this ring is cut and opened slightly, two poles are formed at the cut (Figure 1-4). This is known as an open magnetic circuit and is the type used for magnetic circuits of motors and generators. In the case of a motor or generator (Figure 1-5), the lines travel from the north pole piece, across the air gap to the armature, through the armature core until opposite the next pole piece, across the air gap, through the south pole piece through the outside frame or yoke of the machine, and then to the field core from which it is assumed they started.

  Figure 1-3. Closed magnetic circuit.
Figure 1-3. Closed magnetic circuit.

Figure 1-4. Open magnetic circuit.
Figure 1-4. Open magnetic circuit.

 
B. ELECTRIC CIRCUITS
 
1B1. Flow of electricity. An electric circuit is the closed path through which electricity moves. For explanatory purposes, the flow of electricity may be likened to the flow of water. In each case, the factors of current, quantity (the rate of flow), pressure (the factor which causes the flow), and resistance (the factor which tends to restrict the flow) must be considered.

Electrically these factors are expressed in the following units:

a. Ampere. The quantity of water flowing through a pipe is measured by the amount of water that flows through that pipe in 1 second, as 1 gallon or 3 gallons per second. Similarly, flow, or current, of electricity is measured by the amount of electricity that flows through a conductor in one second, as 1 coulomb or 5 coulombs per second. The gallon and coulomb are units of quantity. The ampere is a rate of flow equal to 1 coulomb per second. Hence, 25 amperes means a current flowing at the rate of 25 coulombs per second. The term coulomb is

  rarely used because in most cases the quantity (coulomb) is of secondary importance to the rate of flow of electricity (amperes).

b. Volt. The quantity of water that flows through a pipe depends to a great extent upon the pressure under which it flows. Thus water pressure is measured in pounds per square inch. Similarly, the number of amperes, or coulombs per second of electricity, flowing in a conductor depends upon the pressure under which the electricity flows. The electrical unit of pressure is the volt.

The distinction between amperes and volts may be expressed as follows: Amperes represent the amount of current flowing through a circuit; volts represent the pressure which makes it flow. The voltage, or pressure difference between two points in an electrical circuit is sometimes referred to as the drop of potential or potential difference.

c. Ohm. The unit of electrical resistance is the ohm. A wire is said to have a 1-ohm resistance if a pressure of 1 volt forces a current

 
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Figure 1-5. Magnetic circuit of a simple dynamo.
Figure 1-5. Magnetic circuit of a simple dynamo.
of 1 ampere through it. If the resistance of a circuit is 2 ohms, the current will be only half as large and only half an ampere will flow. The relationship, in direct current, between pressure (volts), current (amperes), and resistance (ohms) is expressed as follows: The electric current in a conductor equals the voltage applied to the conductor divided by the resistance of the conductor. This is known is known as Ohm's Law and may be simply stated as follows:   Amperes = Volts / Ohms or

Ohms = Volts / Amperes or

Volts = Amperes X Ohms

This relationship always holds true when the quantities expressed are in the same system. Thus, if the law is applied to an entire circuit, the number of amperes in the entire

 
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circuit equals the number of volts in the entire circuit divided by the number of ohms of the entire circuit. If applied to a part of a circuit, the current in that part of the circuit equals the voltage across that part divided by its resistance.

It is possible to have a high pressure and no current. For example, when the path of a flow of water is blocked by a closed valve, there is no current, yet there may be a high pressure. Similarly, if the path of electricity is blocked by an open switch, there is no current (amperes) although the pressure (voltage) may be high. Thus, the amount of current depends upon the resistance that blocks the path; in this case, the closed valve or the open switch. The greater this resistance, the less the current which will flow under the same pressure.

1B2. Series circuit. A series circuit (Figure 1-6) is one in which all the component parts are so connected that there can be but one path through the entire circuit in which current can flow. The resistance of the circuit is the sum of the resistances of its component parts.

The voltage of a series circuit equals the algebraic sum of the voltages of its component parts. Thus, the amount of voltage that must be impressed on a series circuit to obtain a certain flow of current can be obtained by first ascertaining the number of volts required by each component and then adding these voltages to find the total voltage required. The current in a series circuit is the same at all parts of the circuit.

1B3. Parallel circuit. A parallel circuit (Figure 1-7), sometimes called a multiple or shunt

  Figure 1-6. Series circuit.
Figure 1-6. Series circuit.

circuit, is a circuit in which all components are arranged so that the current is divided among them. This type of circuit is generally used in connecting light and power loads. The principal distinction between the series and parallel circuits lies in the fact that in a series circuit the current value is maintained as a constant and the voltage is adjusted to the load requirements; whereas in a parallel circuit the voltage remains constant while the current value varies as more units, that is, more parallel paths, are cut in or out.

Figure 1-7. Simple parallel circuit.
Figure 1-7. Simple parallel circuit.

 
C. ELECTROMAGNETISM
 
1C1. Magnetic field around a wire. The relationship between electricity and magnetism, which is the basis of the operation of nearly all electrical machinery and measuring instruments, was discovered by a physicist named Oersted. He found that a wire carrying an electrical current exerts an effect on a magnetic needle held near the wire. This is an indication that a magnetic field exists around the wire. The existence of this field can be demonstrated by passing a wire vertically through a piece of paper on which fine iron filings are sprinkled. When current   flows through the wire, the filings arrange themselves in a concentric circular pattern around the wire (Figure 1-8). The needle of a compass placed on the paper points in the direction of the field (shown by the direction of the arrows in Figure 1-8). When the direction of current flowing through the wire is reversed, the shape of the field remains the same, but the direction of the compass needle is changed by 180 degrees.

The field intensity in both cases depends upon the strength of the current and the distance of the compass from the conductor.

 
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Figure 1-8. Magnetic field around a conductor.
Figure 1-8. Magnetic field around a conductor.

1C2. Solenoid type of electromagnetic field. When a wire is formed into a single loop and a current is passed through it, a field exists around the loop. The intensity of the field varies with the strength of the current. The field has a north and south pole and acts in exactly the same manner as that of a bar magnet. The circular lines of force around the conductor curl around it in the same direction, entering at one face of the loop and leaving at the other (Figure 1-9).

If several turns or loops of wire are wound to form a loose coil (Figure 1-10), most of the flux lines produced by each of the turns will encircle the entire coil instead of encircling only the turn that generates them. This results in a field shaped similarly to that around a bar magnet. A temporary magnet therefore can be produced by passing an electric current through a coil of wire. This is known as a solenoid. The direction of the magnetic flux inside a solenoid can be found by grasping the solenoid in the right hand with the fingers pointing in the direction of the current flow. The thumb will then

Figure 1-9. Magnetic field around a single loop of wire.
Figure 1-9. Magnetic field around a single loop of wire.

  Figure 1-10. Magnetic field around a coil of wire.
Figure 1-10. Magnetic field around a coil of wire.

point in the direction of the magnetic field inside the solenoid.

1C3. Electromagnets. A very powerful magnet can be made by inserting a piece of soft iron through which a current is flowing into the air space of a solenoid (Figure 1-11). Such a magnet is called an electromagnet. The direction of the lines of force in an electromagnet is the same as through the solenoid alone but the number of lines is increased tremendously by the ability of the soft iron to carry magnetism. The number of lines produced depends upon the current passing through the solenoid and the number of turns or loops in it.

Figure 1-11. Magnetic field around an electromagnet.
Figure 1-11. Magnetic field around an electromagnet.

In practically all electrical apparatus in which motion occurs, the motion is produced by magnetism. The chief advantages of an electromagnet are: 1) it can be turned on or off; 2 ) the strength can be varied; and 3) the movements can be controlled by controlling the current.

 
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D. ELECTROMAGNETIC INDUCTION
 
1D1. General. The operation of motors or generators is dependent on the principle of electromagnetic induction discovered by Faraday. This principle is based on the induction of an electromotive force (emf) in a wire. An electromotive force is the force that establishes the electrical pressure or voltage that will cause current to flow if the circuit is complete. An electromotive force can be induced in one of three ways: 1) by pushing or withdrawing a magnet through a coil of wire; 2) by winding a coil around an iron rod, and magnetizing and demagnetizing the rod by another coil from a separate current source; or 3) by passing a conductor through a magnetic field in such a direction as to cut the lines of magnetic flux.

In the first method, the emf developed is induced by a change in the number of magnetic lines threading through the coil. In the second case, when the separate circuit is closed, a momentary current is produced which in turn sets up lines of force to oppose the producing field. The third case is that of a generator, which is described in detail below. The emf and current so produced are called the induced emf and current.

1D2. Principle of the simple generator. If a conductor is moved downward (Figure 1-12) so as to cut the lines of force between unlike poles of magnets, an electrical current-detecting instrument connected to the ends of the conductor will indicate that an emf sufficient to produce a measurable current has been set up in the circuit.

Figure 1-12. A conductor cutting lines of force.
Figure 1-12. A conductor cutting lines of force.

  When the conductor is moved upward, cutting the lines of force in the opposite direction, the detector shows a deflection in the opposite direction, proving that the emf produced is acting in the opposite direction to the previously induced emf. The amount of deflection, or the value of the emf produced, varies with the rate at which the conductor cuts the lines.

When the conductor is moved horizontally from pole to pole, no lines are cut, since the direction of motion is parallel to the lines, and no deflection is produced. Thus, it is evident that the direction of the emf produced depends upon the direction of motion of the conductor. The value of the emf induced is proportional to the speed at which the conductor cuts the lines. The reason for the direction of the motion of the emf is stated in Lenz's Law as follows: Electromagnetically induced currents always have such a direction that the action of the magnetic fields set up by them tends to oppose the motion which produced them. This law will become more meaningful after a study of motor action (Section 1F1).

The principle of a moving conductor cutting a magnetic field is applied in the operation of direct current generators and motors, the conductors being positioned in slots around the armature which is rotated between the poles of electromagnets.

1D3. Generation of an alternating electromotive force. An alternating emf is produced by continuously moving the conductor up and down, cutting the lines of force (see Figure 1-12). A detector in the circuit would indicate that the emf thus induced tends to cause the current to flow first in one direction and then in the other.

Figure 1-13 (reading down) illustrates the production of a simple alternating emf as a coil or loop of wire is revolved in a field between two magnetic poles. The loop consists of two conductors joined at one end and connected to two slip rings which are insulated from each other and from the spindle on which they are mounted. The circuit is completed by a resistance known as the external circuit which is

 
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connected by sliding connections, called brushes, to the two slip rings.

If this loop is turned on its spindle so that the conductor A cuts the lines of force in a downward direction, and conductor B cuts them in an upward direction, the emf produced in the two arms of the loop would be in opposite directions, but since the two arms are connected in series, the resulting current flows around the completed circuit.

In position 1 (Figure 1-13), no emf is produced, since no lines are being cut, but as the plane of the loop becomes more horizontal, the number of lines cut per second increases until

Figure 1-13. The simple alternator in four positions.
Figure 1-13. The simple alternator in four positions.

  the maximum emf (position 2) is produced. As position 3 is reached, the number of lines cut decreases until the emf produced is again zero. As position 4 is reached, the emf again increases to maximum, but acts in the opposite direction in the conductors to that shown in position 2 because the conductors are cutting the lines in the opposite direction. Finally, in position 1, the emf produced is zero again and the cycle is back at the starting point.

The current maintained by such an emf is known as an alternating current and the arrangement producing it is called an alternator.

1D4. Generation of a steady electromotive force. An alternating emf is not suitable for all forms of electrical work. It is necessary therefore to produce an emf that has the same direction constantly. This is accomplished by the use of a commutator which serves to interchange the connections between the conductors and the outside circuit each time the direction of the emf induced in the conductors reverses. The commutator is arranged so that the brushes pass from one commutator segment to the next only at the points where zero emf is being generated. A simple two-segment commutator is shown in Figure 1-14. (A detailed description of a commutator is given in Section 1E8.)

Figure 1-14. Sectional view of a two-segment
commutator.
Figure 1-14. Sectional view of a two-segment commutator.

1D5. Multipolar field. Up to this point it has been assumed that the conductor is in a magnetic field in which the lines of force are practically parallel, such as would be found between a single pair of magnetic poles.

Instead of rotating in such a field, the conductors usually rotate in a field created by

 
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several pairs of poles spaced evenly around the circumference of a circle. Such a field, produced by more than two poles, is known as a multipolar field.

In the four-pole field (Figure 1-15), each conductor goes through a full cycle in half a revolution instead of in a full revolution as previously described. As in the case of a conductor rotating in a two-pole field, when the conductor reaches a point midway between two adjacent poles, it is moving parallel to the lines of force and hence no emf is being generated.

In Figure 1-15 it will be noted that the direction of current in the field coils is represented by the symbol (+) on one side and ( . ) on the other side. These may be thought of as the feathered tail of an arrow (+) disappearing into the page, and the point of the arrow ( . ) appearing through the page.

  Figure 1-15. Multipolar field.
Figure 1-15. Multipolar field.
 
E. DIRECT CURRENT GENERATORS
 
1E1. Definition. A generator is a machine used to change mechanical energy into electrical energy by utilizing the principle of electromagnetic induction (Section 1D1). The principal parts of a direct current generator and their functions are described below.

1E2. Field structure. The field structure (Figure 1-16) consists of the field frame, or yoke; the field poles, or pole pieces; and the field coils. The assembly produces the magnetic field necessary in every generator.

The frame is usually a large ring of formed, or cast, steel or iron which supports the field poles and coils in its inner diameter and has feet on its outer surface to support the machine on its foundation.

The field poles or pole pieces are constructed of laminated steel sheets and are bolted around the field frame. The arrangement of the poles around the frame is always such that they alternate in polarity. The ends of the poles may flare out to increase the surface that faces the armature, thereby providing better distribution of the flux. This flared portion also serves to hold the field coils in place and is sometimes referred to as the pole shoe.

The field coils are the insulated wire or

  strap coils wound around the field poles through which current is forced to produce the magnetic field. Two distinct types of field windings known as shunt and series are used.

1E3. Shunt generators. In a shunt generator the field coils are connected in series with each other and the complete shunt field circuit is connected in shunt or parallel with the armature circuit (Figure 1-17). The coils are composed of many turns of fine wire. The resistance of the coils is comparatively high, to prevent the field from taking too much current from the

Figure 1-16. Field frame of a generator.
Figure 1-16. Field frame of a generator.

 
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armature circuit. Many turns of wire must be used in order to obtain the necessary ampereturns which determine the strength of the magnetic field produced. The voltage produced by a shunt generator is practically independent of the current taken by the external circuit.

Figure 1-17. Diagram of shunt generator connections.
Figure 1-17. Diagram of shunt generator connections.

1E4. Series generators. In a series generator the field is connected in series with the armature and the external circuit (Figure 1-18). The coils consist of a few turns of heavy wire having a low resistance in order to carry the whole current from the armature to the external circuit. In a generator of this type, the voltage increases as the load increases, for when more current is taken from the machine, more goes through the field coils, thus causing a stronger magnetic field.

Figure 1-18. Series generator connections.
Figure 1-18. Series generator connections.

1E5. Compound generators. A compound generator has both shunt and series fields wound on the same poles (Figure 1-19). When wound in such a direction that it helps the shunt field, the series may be designed to have just enough

  strength to overcome the slight decrease in voltage with increased load of a shunt machine. When wound in the opposite direction, it may be designed to give a definite voltage drop with increased load. This feature is desirable in certain applications, notably submarine auxiliary generators.

Figure 1-19. Compound generator connections.
Figure 1-19. Compound generator connections.

1E6. Methods of excitation. Generators are termed self-excited when the field coils are energized by current from the generator itself, or separately excited when the field coils are energized by a source outside the generator. Main propulsion generators on submarines are shunt wound and separately excited, the current to the fields being supplied by the battery. The voltage is controlled through a variable resistance in series with the shunt field. The voltage, being dependent on the strength of the field, can thus be regulated by weakening or strengthening the field by means of this resistance which is known as the shunt field rheostat.

1E7. Armature. The armature (Figure 1-20) of a generator is composed of the winding in which the emf is induced and the structure that supports this winding. This structure is made up of a number of slotted steel punchings assembled in the form of a cylinder and mounted on a spider. The spider is then attached to the armature shaft. On small machines the armature laminations may be mounted directly on the shaft. The windings are shaped to fit in the slots and are held there by means of wedges and steel banding wire.

 
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Figure 1-20. Generator armature.
Figure 1-20. Generator armature.
1E8. Commutator. The commutator (see Figure 1-20) is a cylindrical form mounted on one end of the armature shaft. It performs the function of changing an alternating emf to a direct emf. It is built up of a number of longitudinal segments of copper which are insulated from each other and from the armature shaft that supports them. The number of segments is proportional to the number of coils in the armature, each of which is connected to the segments in a sequence determined by the particular type of armature winding.

1E9. Brushes and brush rigging. The brushes bear upon the commutator, collect the current from the armature winding, and lead it to the external circuit. The brushes are supported on holders which in turn are bracket-mounted around the inner diameter of the brush yoke. The complete assembly is known as the brush rigging (Figure 1-21).

The brushes are secured in the rigging in definite positions around the commutator to

  insure sparkless commutation over the range of loading. Adjacent groups of brushes in large machines usually are staggered axially so that the commutator will wear evenly. Provision is always made to permit rotating the brush rigging with respect to the commutator in order to pick up the best plane of commutation. This provision also permits rotation of the rigging so that brush holders may be brought to an accessible spot for maintenance or renewal of the brushes.

1E10. Armature reaction. The current flowing in the conductors of the armature sets up a magnetic field which tends both to weaken and to distort the main field set up by the shunt field windings. This effect is illustrated in Figure 1-22 which shows progressively: A, the main field; B, the field resulting from current in the armature conductors; and C, the field resulting from the combination of these two fields. Since the strength of the field is in part due to the armature current or load current, the resultant field C will vary both in strength and position as the

 
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Figure 1-21. Generator brush rigging.
Figure 1-21. Generator brush rigging.

load current flowing through the armature changes.

A requirement of good commutation is that the brushes short circuit the commutator segments at a time when there is no induced current flowing in the conductors to which they are connected, or, in other words, that the brushes pass from one commutator segment to the next when the conductors of the armature to which they are connected are moving parallel to the field responsible for inducing the current. This position is called the plane of commutation, or the neutral plane. Obviously, this neutral plane shifts in position with change of load current. If the machine were to operate at constant speed and load, and always in the same direction, the brushes could be shifted to the neutral plane position and left there with good commutation thus effected. Such a machine is rarely encountered and in any case would not meet the requirements for submarine propulsion.

Since it is impractical to shift the brushes with each change of load, direction, or speed, recourse is made to auxiliary fields called the commutating fields and the compensating windings. The effect of these fields counteracts the effects of armature reaction and maintains the

  neutral plane in a fixed position throughout the range of load and speed of the machine, and, in the case of motors required to run in reverse, in both directions of rotation.

Figure 1-22. Effect of armature reaction on field
of generator.
Figure 1-22. Effect of armature reaction on field of generator.

1E11. Commutating field windings.The commutating fields, or interpoles, as they are sometimes called because of their position relative to the main poles, consist of a series of small poles similar to the main field poles in construction and method of fastening, but

 
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having a winding that consists of a few turns of heavy copper bus bar of high current capacity and low resistance (Figure 1-23).

The commutating pole windings are all connected in series with each other and with the armature circuit. A resistor connected in parallel with the commutating pole windings is adjusted and permanently set at the factory to give the commutating pole strength that results in the best commutation. Most of the armature current goes through the commutating pole windings; only a small amount goes through the shunting resistor. Since the armature reaction increases when the armature load current increases, and the effect of the commutating poles also increases, the result is that the neutral or commutating plane is maintained in a fixed position throughout the load range.

With this method of correction, some distortion of the field still remains because the commutating fields, being small, are not completely effective in correcting the distortion in the vicinity of the main pole tips. This latter condition is especially true of the high-power, compact machines used for submarine propulsion.

Figure 1-23. Effect of commutating field windings.
Figure 1-23. Effect of commutating field windings.

1E12. Compensating windings. To neutralize completely the effects of armature reaction, a second set of auxiliary field windings, known

  as the compensating windings, is used in high-power d.c. machines. These windings consist of a few turns of low-resistance copper bar laid in slots in the faces of the main shunt field pole pieces and so connected that the windings carry current in the reverse direction to that of the immediately adjacent armature conductors. The compensating windings are connected in series with each other and with the armature winding in a manner similar to the commutating windings so that they also oppose the field set up by armature reaction. The current in them is then equal to that in the armature (Figure 1-25).

The field resulting from the compensating windings is wide in comparison with the commutating fields but weaker since the flux is less concentrated. The effect of the two windings acting in conjunction is to neutralize completely the effects of armature reaction in respect to the shifting of the neutral plane, and to eliminate almost completely the distorting effects. Thus it is insured that the neutral plane will remain in fixed position throughout the entire range of load and speed of the machine, and, in the ease of a motor, in both directions of rotation. Good commutation is thus effected with the brushes located in a fixed position. Figure 1-24 shows the construction of these fields and windings.

Figure 1-24. Construction of compensating windings.
Figure 1-24. Construction of compensating windings.

1E13. Voltage control. The voltage produced by a generator is proportional to the strength of the magnetic field times the speed of rotation of the armature. The voltage of a shunt wound generator can be increased in any of the following ways:

 
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Figure 1-25. Currents in armature conductors, compensating windings, and commutating pole windings.
Figure 1-25. Currents in armature conductors, compensating windings, and commutating pole windings.
1. Keep the speed constant and increase the current through the field coils. This increases the magnetic field and the voltage.

2. Keep the field current and magnetic field constant and increase the speed of the engine that drives the generator.

  3. Change both the field current and engine speed in such a way that the product of the magnetic field times the speed is increased.

Changing the engine speed and field current in the opposite direction causes the generator voltage to decrease.

 
F. DIRECT CURRENT MOTORS
 
1F1. Principles of operation. An electric motor is a machine for transforming electrical energy into mechanical energy. In this respect it is the reverse of a generator although it is based fundamentally upon the same general principles.   In construction, a direct current motor is the same as a direct current generator. When a motor is connected to a source of emf as, for example, a generator, the emf developed by the generator impels a current through the motor armature and field windings. Electromagnetic
 
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reactions between the fields of the armature and the main field then cause the motor armature to rotate and pull its load.

The operation of a motor is based on the fact that a conductor carrying a current, when placed in a magnetic field, tends to move at a right angle to the field. Figure 1-26 (A) illustrates a magnetic field in which a conductor carrying no current is placed. In Figure 1-26 (B) the magnetic field has been removed and the conductor is shown carrying a current in a direction leading away from the reader. The current in the conductor has created a cylindrical magnetic field around it. The direction of this magnetic field may be determined by the right-hand rule: Grasp the wire in the right hand with the thumb pointing in the direction of the current. The fingers will then point in the direction of the magnetic field around the wire.

WARNING. Never grasp a real wire when it is hot. Put your fingers around an imaginary wire carrying current in the same direction.

  magnetic field above the conductor but opposes it below the conductor. This action creates a crowding of the flux in the region above the conductor, and a reduction of the flux density in the region below the conductor. The crowding effect of the flux lines creates a force, comparable to elastic bands under tension and endeavoring to straighten out. This force exerts a downward pressure on the conductor; it is represented in the illustration by the arrow pointing downward.

When the current in the conductor flows in the opposite direction to that shown in Figure 1-26 (D), the crowding of the flux lines occurs below the conductor and tries therefore to force the conductor upward. This force is represented by the arrow pointing upward.

1F2. Counter electromotive force. As the motor armature rotates, an emf is induced in the armature exactly as in a generator. The emf induced in the armature is in a direction opposing, but never as great as, the emf impressed on

Figure 1-26. Force acting on a conductor carrying
current in a magnetic field.
Figure 1-26. Force acting on a conductor carrying current in a magnetic field.
Figure 1-26 (C) shows the magnetic field obtained by combining the main magnetic field and the magnetic field created by the current carrying conductor. The field created by the conductor acts in conjunction with the main   the armature, causing it to rotate. Since this emf tends to cause a current in a direction opposite to that of the current causing the armature to turn, it is known as the counter, or back electromotive force. This counter emf is the
 
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difference between the impressed voltage and the product of the armature current times the armature resistance.

1F3. Starting resistance. The effect of counter emf is to limit the current in a motor armature. The armature of a motor, as in a generator, is of very low resistance in order to reduce as much as possible current losses in the machine. When a motor at rest is suddenly connected to a source of current supply, an abnormally high current flows in the armature circuit because the counter emf is not present to oppose the applied voltage. For example, the armature resistance of a submarine main motor is only a few thousandths of an ohm. If a starting voltage of 250 volts were applied to the terminals, the current flowing the first instant would be enormous, resulting in serious damage to the motor and seriously overloading the generator supplying the current and the cables and contactors connecting them.

As soon as the motor starts to rotate, however, it generates a counter emf which increases as the motor gathers speed, thereby constantly reducing the armature current. To avoid this initial high inrush of current, a resistance is placed in series with the armature. This resistance is of such value that when the armature circuit is first closed, a current value about 1.5 times normal full load current flows. As the motor gathers speed, a portion of the resistance

  is cut out, allowing an increased current to flow again, thus supplying more torque, or turning tendency, which in turn speeds up the motor still more. This process continues until the motor terminals are connected directly across the supply line, the current by that time having been limited to a safe value by the counter emf. A motor should always be started with a strong field so that the counter emf may build up as rapidly as possible and also to provide the necessary torque.

1F4. Speed control. The most common method of controlling the speed of a motor is through variation of the shunt field strength. This method is based upon the fact that as the value of the flux is reduced, the motor speed is increased. The value of the flux is varied by placing a resistance (rheostat) in series with the shunt field circuit. Increasing the value of the resistance in series with the shunt field decreases the amount of current flowing through the field, and hence decreases the strength of the field. Any decrease in the strength of the field decreases the counter emf in the armature coils since the counter emf is dependent upon the number of lines of force cut by the coils on the armature. It is evident that with a weakened field, the lines of force cut are fewer and the counter emf produced is lower. This allows a greater current to flow from the external voltage, which in turn causes an increase in the motor speed.

 
15

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