DC Generators PDF 10/14/2020

10/14/2020 DC Generators Generator Theory, Construction, Maintenance And Voltage Regulation References  Aircraft Electricity & Electronics  Chapter 12, Pg. 190-244  Powerplant Textbook  Chapter 17, Pg.589-...

10/14/2020 DC Generators Generator Theory, Construction, Maintenance And Voltage Regulation References  Aircraft Electricity & Electronics  Chapter 12, Pg. 190-244  Powerplant Textbook  Chapter 17, Pg.589- end of chapter DC Generator  What does a Generator do?  Converts Mechanical energy into electrical energy https://www.youtube.com/watch?v=mq2zjm S8UMI 1  10/14/2020 DC Generator Theory  The AMOUNT of voltage generated depends on  1 The strength of the magnetic field,  2 The angle at which the conductor cuts the magnetic field,  3 The speed at which the conductor is moved, and  4 The length of the conductor within the magnetic field.  The POLARITY of the voltage depends on the direction of the magnetic lines of flux and the direction of movement of the conductor.  To determine the direction of current in a given situation, the LEFT-HAND RULE FOR GENERATORS is used. Left Hand Rule For Generators  This rule is explained in the following manner.  Extend the thumb, forefinger, and middle finger(NOT UP) of your left hand at right angles to one another.  Point your thumb in the direction the conductor is being moved.  Point your forefinger in the direction of magnetic flux (from north to south).  Your middle finger will then point in the direction of current flow in an external circuit to which the voltage is applied. Elementary Generator  The simplest elementary generator that can be built is an ac generator.  Basic generating principles are most easily explained through the use of the elementary ac generator.  An elementary generator consists of a wire loop placed so that it can be rotated in a stationary magnetic field.  This will produce an induced EMF in the loop. Sliding contacts (brushes) connect the loop to an external circuit load in order to pick up or use the induced EMF. 2  10/14/2020 Elementary Generator  https://www.youtube.com/watch?v=ATFqX2Cl3-w Elementary Generator  The pole pieces (marked N and S) provide the magnetic field.  The pole pieces are shaped and positioned as shown to concentrate the magnetic field as close as possible to the wire loop.  The loop of wire that rotates through the field is called the ARMATURE.  The ends of the armature loop are connected to rings called SLIP RINGS.  They rotate with the armature. The brushes, usually made of carbon, with wires attached to them, ride against the rings. The generated voltage appears across these brushes. Generator Action  The elementary generator produces a voltage in the following manner.  The armature loop is rotated in a clockwise direction. The initial or starting point is shown at position A. (This will be considered the zero-degree position.)  At 0º the armature loop is perpendicular to the magnetic field. The black and white conductors of the loop are moving parallel to the field.  The instant the conductors are moving parallel to the magnetic field, they do not cut any lines of flux. Therefore, no EMF is induced in the conductors, and the meter at position A indicates zero.  This position is called the NEUTRAL PLANE. 3  10/14/2020 Generator Action  As the armature loop rotates from position A (0º) to position B (90º), the conductors cut through more and more lines of flux, at a continually increasing angle.  At 90º they are cutting through a maximum number of lines of flux and at maximum angle.  The result is that between 0º and 90º, the induced EMF in the conductors builds up from zero to a maximum value. Generator Action A-B  From 0º to 90º , the black conductor cuts DOWN through the field. At the same time the white conductor cuts UP through the field.  The induced EMFS in the conductors are series-adding. This means the resultant voltage across the brushes (the terminal voltage) is the sum of the two induced voltages.  The meter at position B reads maximum value. Generator Action B-C  As the armature loop continues rotating from 90º (position B) to 180º (position C), the conductors which were cutting through a maximum number of lines of flux at position B now cut through fewer lines.  They are again moving parallel to the magnetic field at position C. They no longer cut through any lines of flux.  As the armature rotates from 90º to 180º , the induced voltage will decrease to zero in the same manner that it increased during the rotation from 0º to 90º.  The meter again reads zero. 4  10/14/2020 Generator Action C-D  From 0º to 180º the conductors of the armature loop have been moving in the same direction through the magnetic field. Therefore, the polarity of the induced voltage has remained the same.  This is shown by points A through C on the graph.  As the loop rotates beyond 180º (position C), through 270º (position D), and back to the initial or starting point (position A), the direction of the cutting action of the conductors through the magnetic field reverses.  Now the black conductor cuts UP through the field while the white conductor cuts DOWN through the field. Generator Action  As a result, the polarity of the induced voltage reverses.  Points C, D, and back to A, the voltage will be in the direction opposite to that shown from points A, B, and C.  The terminal voltage will be the same as it was from A to C except that the polarity is reversed (as shown by the meter deflection at position D).  The voltage output waveform for the complete revolution of the loop is shown on the graph. Generator Action 5  10/14/2020 What Do We Do To Make A DC Generator??  Split the rings into 4 segments Elementary DC Generator  A single-loop generator with each terminal connected to a segment of a two-segment metal ring is shown in figure 1-4.  The two segments of the split metal ring are insulated from each other. This forms a simple COMMUTATOR.  The commutator in a DC generator replaces the slip rings of the AC generator.  This is the main difference in their construction.  The commutator mechanically reverses the armature loop connections to the external circuit.  This occurs at the same instant that the polarity of the voltage in the armature loop reverses.  Through this process the commutator changes the generated ac voltage to a pulsating dc voltage as shown in the graph.  This action is known as commutation. Elementary DC Generator Pulsing DC 6  10/14/2020  When the armature loop rotates clockwise from position A to position B, a voltage is induced in the armature loop which causes a current in a direction that deflects the meter to the right.  Current flows through loop, out of the negative brush, through the meter and the load, and back through the positive brush to the loop.  Voltage reaches its maximum value at point B on the graph.  The generated voltage and the current fall to zero at position C.  At this instant each brush makes contact with both segments of the commutator.  As the armature loop rotates to position D, a voltage is again induced in the loop. In this case, however, the voltage is of opposite polarity.  The voltages induced in the two sides of the coil at position D are in the reverse direction to that of the voltages shown at position B. Note that the current is flowing from the black side to the white side in position B and from the white side to the black side in position D. However, because the segments of the commutator have rotated with the loop and are contacted by opposite brushes, the direction of current flow through the brushes and the meter remains the same as at position B. 7  10/14/2020  The voltage developed across the brushes is pulsating and unidirectional (in one direction only).  It varies twice during each revolution between zero and maximum.  This variation is called RIPPLE. A pulsating voltage, such as that produced in the elementary generator, is unsuitable for most applications.  Therefore, in practical generators more armature loops (coils) and more commutator segments are used to produce an output voltage waveform with less ripple. Effects of Additional Coils Effects of Additional Coils  The effects of additional coils may be illustrated by the addition of a second coil to the armature.  The commutator must now be divided into four parts since there are four coil ends.  The coil is rotated in a clockwise direction from the position shown.  The voltage induced in the white coil, DECREASES FOR THE NEXT 90º of rotation (from maximum to zero).  The voltage induced in the black coil INCREASES from zero to maximum at the same time.  Since there are four segments in the commutator, a new segment passes each brush every 90º instead of every 180º.  This allows the brush to switch from the white coil to the black coil at the instant the voltages in the two coils are equal. 8  10/14/2020 Effects of Additional Coils  The brush remains in contact with the black coil as its induced voltage increases to maximum, level B in the graph.  It then decreases to level A, 90º later. At this point, the brush will contact the white coil again.  The graph in shows the ripple effect of the voltage when two armature coils are used. Effects of Additional Coils  Since there are now four commutator segments in the commutator and only two brushes, the voltage cannot fall any lower than at point A.  Therefore, the ripple is limited to the rise and fall between points A and B on the graph.  By adding more armature coils, the ripple effect can be further reduced.  Decreasing ripple in this way increases the effective voltage of the output. Summary NOTE: Effective voltage is the equivalent level of dc voltage, which will cause the same average current through a given resistance. By using additional armature coils, the voltage is not allowed to fall to as low a level between peaks. Practical generators use many armature coils. They also use more than one pair of magnetic poles. The additional magnetic poles have the same effect on ripple as did the additional armature coils. In addition, the increased number of poles provides a stronger magnetic field (greater number of flux lines). This increase output voltage because the coils cut more lines of flux per revolution. 9  10/14/2020 Electro Magnetic Poles  Nearly all practical generators use electromagnetic poles instead of the permanent magnets used in our elementary generator.  The electromagnetic field poles consist of coils of insulated copper wire wound on soft iron cores.  The main advantages of using electromagnetic poles are  1 increased field strength  2 a means of controlling the strength of the fields.  By varying the input voltage, the field strength is varied.  By varying the field strength, the output voltage of the generator can be controlled. Electromagnetic Poles Ground Field Terminal Commutation  Commutation is the process by which a dc voltage output is taken from an armature that has an ac voltage induced in it.  The commutator mechanically reverses the armature loop connections to the external circuit.  This occurs at the same instant that the voltage polarity in the armature loop reverses.  A dc voltage is applied to the load because the output connections are reversed as each commutator segment passes under a brush.  The segments are insulated from each other. 10  10/14/2020 Commutation  Commutation occurs simultaneously in the two coils that are briefly short-circuited by the brushes.  Coil B is short-circuited by the negative brush.  Coil Y, the opposite coil, is short- circuited by the positive brush. The brushes are positioned on the commutator so that each coil is short-circuited as it moves through its own electrical neutral plane. Commutation  There is no voltage generated in the coil at that time. Therefore, no sparking can occur between the commutator and the brush.  Sparking between the brushes and the commutator is an indication of improper commutation.  Improper brush placement is the main cause of improper commutation. Armature Reaction  All current-carrying conductors produce magnetic fields.  The magnetic field produced by current in the armature of a dc generator affects the flux pattern and distorts the main field.  This distortion causes a shift in the neutral plane, which affects commutation.  This change in the neutral plane and the reaction of the magnetic field is called ARMATURE REACTION.  For proper commutation, the coil short-circuited by the brushes must be in the neutral plane. 11  10/14/2020 Armature Reaction  Consider the operation of a simple two-pole dc generator.  View A of the figure shows the field poles and the main magnetic field.  The armature is shown in a simplified view in views B and C with the cross section of its coil represented as little circles. Armature Reaction  The symbols within the circles represent arrows. The dot represents the point of the arrow coming toward you, and the cross represents the tail, or feathered end, going away from you.  When the armature rotates clockwise, the sides of the coil to the left will have current flowing toward you.  The side of the coil to the right will have current flowing away from you. Armature Reaction  The field generated around each side of the coil is shown in view B.  This field increases in strength for each wire in the armature coil, and sets up a magnetic field almost perpendicular to the main field. 12  10/14/2020 Armature Reaction  Now you have two fields — the main field, view A, and the field around the armature coil, view B.  View C shows how the armature field distorts the main field and how the neutral plane is shifted in the direction of rotation.  If the brushes remain in the old neutral plane, they will be short- circuiting coils that have voltage induced in them.  Consequently, there will be arcing between the brushes and commutator.  To prevent arcing, the brushes must be shifted to the new neutral plane. Break Part two next week Review  How do we generate AC?  1  2  3  How did we convert it to DC?  How did we make it usable power? 13  10/14/2020 Review  What happens when a current is flowing in a wire ?  What shifts _____________ when a load is placed on a generator?  How do we fix this problem? Compensating Windings  Shifting the brushes to the advanced position (the new neutral plane) does not completely solve the problems of armature reaction.  The effect of armature reaction varies with the load current. Therefore, each time the load current varies, the neutral plane shifts.  This means the brush position must be changed each time the load current varies.  In small generators, the effects of armature reaction are reduced by actually mechanically shifting the position of the brushes.  The practice of shifting the brush position for each current variation is not practiced except in small generators.  In larger generators, other means are taken to eliminate armature reaction. Compensating Windings  COMPENSATING WINDINGS or INTERPOLES are used for this purpose.  The compensating windings consist of a series of coils embedded in slots in the pole faces.  These coils are connected in series with the armature.  The series-connected compensating windings produce a magnetic field, which varies directly with armature current.  Because the compensating windings are wound to produce a field that opposes the magnetic field of the armature, they tend to cancel the effects of the armature magnetic field.  The neutral plane will remain stationary and in its original position for all values of armature current.  Because of this, once the brushes have been set correctly, they do not have to be moved again. 14  10/14/2020 Compensating Windings and Interpoles Interpoles  Another way to reduce the effects of armature reaction is to place small auxiliary poles called "interpoles" between the main field poles.  The Interpoles have a few turns of large wire and are connected in series with the armature.  Interpoles are wound and placed so that each interpole has the same magnetic polarity as the main pole ahead of it, in the direction of rotation.  The field generated by the interpoles produces the same effect as the compensating winding.  This field, in effect, cancels the armature reaction for all values of load current by causing a shift in the neutral plane opposite to the shift caused by armature reaction.  The amount of shift caused by the interpoles will equal the shift caused by armature reaction since both shifts are a result of armature current. Motor Reaction in a Generator  When a generator delivers current to a load, the armature current creates a magnetic force that opposes the rotation of the armature. This is called MOTOR REACTION.  A single armature conductor is represented in view A.  When the conductor is stationary, no voltage is generated and no current flows. Therefore, no force acts on the conductor. 15  10/14/2020 Motor Reaction in a Generator  When the conductor is moved downward (view B) and the circuit is completed through an external load, current flows through the conductor in the direction indicated.  This sets up lines of flux around the conductor in a clockwise direction. Motor Reaction in a Generator  The interaction between the conductor field and the main field of the generator weakens the field above the conductor and strengthens the field below the conductor.  The main field consists of lines that now act like stretched rubber bands. Thus, an upward reaction force is produced that acts in opposition to the downward driving force applied to the armature conductor. Motor Reaction in a Generator  If the current in the conductor increases, the reaction force increases. Therefore, more force must be applied to the conductor to keep it moving.  With no armature current, there is no magnetic (motor) reaction. Therefore, the force required to turn the armature is low.  As the armature current increases, the reaction of each armature conductor against rotation increases. 16  10/14/2020 Motor Reaction in a Generator  The actual force in a generator is multiplied by the number of conductors in the armature.  The driving force required to maintain the generator armature speed must be increased to overcome the motor reaction.  The force applied to turn the armature must overcome the motor reaction force in all dc generators.  The device that provides the turning force applied to the armature is called the PRIME MOVER. The prime mover may be an electric motor, a gasoline engine, DIESEL ENGINE(VW TDI), a steam turbine, or any other mechanical device that provides turning force. Practical DC Generators  The actual construction and operation of a practical dc generator differs somewhat from our elementary generators.  The differences are in the construction of the armature, the manner in which the armature is wound, and the method of developing the main field.  A generator that has only one or two armature loops has high ripple voltage. This results in too little current to be of any practical use.  To increase the amount of current output, a number of loops of wire are used. These additional loops do away with most of the ripple. Practical DC Generator  The loops of wire, called windings, are evenly spaced around the armature so that the distance between each winding is the same.  The commutator in a practical generator is also different. It has several segments instead of two or four, as in our elementary generators.  The number of segments must equal the number of armature coils. 17  10/14/2020 Gramme-Ring Armature  A GRAMME-RING armature is shown in the figure view A.  Each coil is connected to two commutator segments as shown.  One end of coil 1 goes to segment A, and the other end of coil 1 goes to segment B.  One end of coil 2 goes to segment C, and the other end of coil 2 goes to segment B.  The rest of the coils are connected in a like manner, in series, around the armature.  To complete the series arrangement, coil 8 connects to segment A. Therefore, each coil is in series with every other coil. Gramme-Ring Armature Gramme-Ring Armature  View B shows a composite view of a Gramme-ring armature.  It illustrates more graphically the physical relationship of the coils and commutator locations.  The windings of a Gramme-ring armature are placed on an iron ring.  A disadvantage of this arrangement is that the windings located on the inner side of the iron ring cut few lines of flux. Therefore, they have little, if any, voltage induced in them.  For this reason, the Gramme-ring armature is not widely used. 18  10/14/2020 Drum Type Armature  The armature windings are placed in slots cut in a drum-shaped iron core.  Each winding completely surrounds the core so that the entire length of the conductor cuts the main magnetic field.  Therefore, the total voltage induced in the armature is greater than in the Gramme-ring.  The drum-type armature is much more efficient than the Gramme- ring.  This accounts for the almost universal use of the drum-type armature in modem DC generators. Drum Type Armature Lap and Wave Windings  Drum-type armatures are wound with either of two types of windings — the LAP WINDING or the WAVE WINDING.  The lap winding is illustrated in view A  This type of winding is used in DC generators designed for high- current applications.  The windings are connected to provide several parallel paths for current in the armature.  For this reason, lap-wound armatures used in DC generators require several pairs of poles and brushes. 19  10/14/2020 Lap and Wave Windings High Current High Voltage Lap and Wave Windings  View B, shows a wave winding on a drum-type armature.  This type of winding is used in dc generators employed in high- voltage applications.  Notice that the two ends of each coil are connected to commutator segments separated by the distance between poles.  This configuration allows the series addition of the voltages in all the windings between brushes.  This type of winding only requires one pair of brushes. In practice, a practical generator may have several pairs to improve commutation. Field Excitation  When a dc voltage is applied to the field windings of a dc generator, current flows through the windings and sets up a steady magnetic field.  This is called FIELD EXCITATION.  This excitation voltage can be produced by the generator itself or it can be supplied by an outside source, such as a battery.  A generator that supplies its own field excitation is called a SELF- EXCITED GENERATOR.(which most of you are)  Self-excitation is possible only if the field pole pieces have retained a slight amount of permanent magnetism, called RESIDUAL MAGNETISM. 20  10/14/2020 Field Excitation  When the generator starts rotating, the weak residual magnetism causes a small voltage to be generated in the armature.  This small voltage applied to the field coils causes a small field current.  Although small, this field current strengthens the magnetic field and allows the armature to generate a higher voltage.  The higher voltage increases the field strength, and so on.  This process continues until the output voltage reaches the rated output of the generator. Generator Classification  Self-excited generators are classed according to the type of field connection they use.  There are three general types of field connections — SERIES-WOUND, SHUNT-WOUND (parallel), and COMPOUND-WOUND. Series-Wound Generators  Uses very low resistance field coils, which consist of a few turns of large diameter wire.  The voltage output increases as the load circuit starts drawing more current.  Under low-load current conditions, the current that flows in the load and through the generator is small. Since small current means that a small magnetic field is set up by the field poles, only a small voltage is induced in the armature.  If the resistance of the load decreases, the load current increases.  Under this condition, more current flows through the field. This increases the magnetic field and increases the output voltage.  A series-wound dc generator has the characteristic that the output voltage varies with load current.  This is undesirable in most applications. For this reason, this type of generator is rarely used in everyday practice. 21  10/14/2020 Series-Wound Generator Shunt Wound Generator  The field coils consist of many turns of small wire.  They are connected in parallel with the load.  They are connected across the output voltage of the armature.  Field winding current is independent of the load current (currents in parallel branches are independent of each other).  Since field current, and therefore field strength, is not affected by load current, the output voltage remains more constant than the output series- wound generator.  In actual use, the output voltage in a dc shunt-wound generator varies inversely as load current varies.  The output voltage decreases as load current increases because the voltage drop across the armature resistance increases (E = IR). Shunt Wound Generators 22  10/14/2020 Compound-Wound Generators  Series Wound : output voltage varies directly with load current.  Shunt-wound: output voltage varies inversely with load current. A combination of the 2 can overcome the disadvantages of both called the compound-wound DC generator.  Compound-wound generators have both series-field and shunt-field windings.  The shunt and series windings are wound on the same pole pieces.  When load current increases, the armature voltage decreases just as in the shunt-wound generator.  This causes the voltage applied to the shunt-field winding to decrease, which results in a decrease in the magnetic field.  This same increase in load current, since it flows through the series winding, causes an increase in the magnetic field produced by that winding. Compound-Wound Generator Compound Wound Generators  By proportioning the two fields so that the decrease in the shunt field is just compensated by the increase in the series field, the output voltage remains constant.  By proportioning the effects of the two fields (series and shunt), a compound-wound generator provides a constant output voltage under varying load conditions. 23  10/14/2020 Generator Construction  The Following figure illustrates the component parts of a DC Generator. Brushes 24  10/14/2020 DC Generator Cut-Away Question  How can I control a generator ?  length of wire  speed  Magnetic strength  or _______________ DC Voltage Regulation  Voltage control is either  1 manual  2 automatic.  In most cases the process involves changing the resistance of the field circuit. This controls the field current.  Controlling the field current permits control of the output voltage.  The major difference between the various voltage control systems is merely the method by which the field circuit resistance and the current are controlled.  VOLTAGE REGULATION should not be confused with VOLTAGE CONTROL.  Voltage regulation is an internal action occurring within the generator whenever the load changes.  Voltage control is an imposed action, usually through an external adjustment, for the purpose of increasing or decreasing terminal voltage. 25  10/14/2020 Manual Voltage Control  The hand-operated field rheostat, is a typical example of manual voltage control.  The field rheostat is connected in series with the shunt field circuit.  This provides the simplest method of controlling the terminal voltage of a dc generator. Manual Voltage Control  This type of field rheostat contains tapped resistors with leads to a multi- terminal switch.  The arm of the switch may be rotated to make contact with the various resistor taps.  This varies the amount of resistance in the field circuit.  Rotating the arm counterclockwise increases the resistance and lowers the output voltage.  Rotating the arm clockwise decreases the resistance and increases the output voltage.  Most field rheostats for generators use resistors of alloy wire. They have a high specific resistance and a low temperature coefficient. These alloys include copper, nickel, manganese, and chromium. Automatic Voltage Control  Automatic voltage control may be used where load current variations exceed the built-in ability of the generator to regulate itself.  An automatic voltage control device "senses" changes in output voltage and causes a change in field resistance to keep output voltage constant.  Whichever control method is used, the range over which voltage can be changed is a design characteristic of the generator.  The voltage can be controlled only within the design limits. 26 10/14/2020 27