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CHAPTER 11
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Introduction, Although DC is not widely used by consumers DC machines have played a major role in industry. over the years ,FE elevators use DC machines ,DC MACHINE . DC machine operates as the result of the interaction of rotating coils with a stationary magnetic. field , The rotor contains the armature winding a number of coils mounted in lots on the rotor . The ends of the coils are connected to the commutator at the end of the rotor . Carbon brushes ride on the surface of the commutator to make electrical contact from outside. the motor to the armature coils , The stationary magnetic field is provided either by DC coils on the stator of the machine or by. permanent magnets mounted inside the stator , 2, The Stator.
DC machine stator , a Cross section , b Side view . It is simpler and less expensive to make a stator using assembled stack of stamped laminations . The ear shaped projections in the center are the pole faces which actually become a north and. south pole during operation of the machine , The field coils are mounted behind the ear shaped projections and provide the magnetomotive. force MMF required to establish the magnetic field . MMF is the product of current and the number of turns in the coil . Thus MMF can be provided with high current and few turns or with low current and 3. many turns , In the DC machine there are two types of field windings that are commonly used . 1 Shunt field Field windings are connected in parallel with the armature to the. source voltage or, Field windings are connected to a separate voltage source . To keep the losses of the machine low and to reduce the power requirements for. the source shunt field coils are constructed of a large number of turns of smaller . gauge wire , 2 Series field Field winding are connected in series with the armature .
Series field windings are constructed of a few turns of heavy gauge wire and. placed in series with the armature , The types of field coils and the manner in which they are connected has a. significant impact on the performance of the machine . 4, The Rotor, The rotor contains the armature winding which is the. high power winding of the DC machine , The slots in the lamination provide space for the. armature coil , When operating as a generator the armature. winding is rotated through the field generating, a DC voltage that is delivered to the load .
For motor operation the armature winding, draws current from the source and the current. in the armature coils interacts with the field to. create an electromagnetic torque that drives a, mechanical load . Details of a DC machine armature The commutator consists of a number of. a Side view of rotor copper segments that are electrically insulated. b Cross section of commutator from each other and from the rotor shaft . c Cross section of lamination stack , The brushes are held by springs against the. rotating of the commutator so as to make, The armature windings carry AC current which electrical contact and to provide a connection. is rectified by the switching action of the from the rotating coils to the outside world . commutator , 5, Types of DC Machines, One big advantage of DC machines is the variety of ways that the field excitation.
can be provided a wide range of operating characteristics. There are two types of field windings series and shunt . A DC machine may have either or both therefore there are several possible types. of DC machines , Notation , A1 A2 the armature terminals. F1 F2 the shunt field, S1 S2 the series field, Shunt field windings. Separately excited Self excited, use separate DC operating machine. power supply provides its own, excitation , 6, Separately excited DC machine . Notation , A1 A2 the armature terminals, F1 F2 the shunt field.
S1 S2 the series field, The separately excited field is provided with a separate DC source . Because it requires a separate DC power supply the separately. excited machine is usually used only when a wide range of. voltage control generator or speed control motor is required . The field current is typically only a few percent of the rated output. current so a small field current controls a much larger armature. current , The separately excited DC generator effectively operates as a power. amplifier , 7, Shunt motor or self excited generator. Notation , A1 A2 the armature terminals, F1 F2 the shunt field. S1 S2 the series field, Self excited means they rely on the generator s own voltage.
to provide the excitation note residual magnetism to. provide initial flux is required to build up the initial excitation . The shunt connection is in which the field is connected in parallel or shunt with the. armature , The voltage from the armature is applied directly to the field in the case of a generator . while for motor operation the source voltage is applied to the field and armature . The rheostat is used for changing the field current without changing the voltage and. therefore provides a measure of voltage control for the generator and speed control. for the motor , 8, The series machine, Notation , A1 A2 the armature terminals. F1 F2 the shunt field, S1 S2 the series field, In the series connected generator the field flux is obtained by passing the armature. current through the series field winding , Because the excitation is provided by the armature current it varies widely with load . especially for a generator and therefore suffers from very poor voltage regulation . When the series machine is connected as a motor the field terminals S1 and S2. have to be reversed because the current would be going into the machine instead of. out of it , 9, Compound machines, When both series and shunt fields are used on a DC machine .
it is called a compound machine , The long shunt connection the shunt winding is located. directly across the terminals of the machine and the series. winding is connected between the shunt and the armature . The short shunt connection the shunt winding is directly in. parallel with the armature and the series winding is between. the shunt winding and the terminals of the machine . Notation ,A1 A2 the armature terminals,F1 F2 the shunt field. S1 S2 the series field 10, Compound machines, The short or the long shunt can be connected so that the two. fields reinforce each other cumulative compounding or so that. they are in opposition to each other differential compounding . For the cumulatively compounded machine the series winding. provides additional flux as the load current increases . For the generator operation the current would be from the. armature to the terminals so the windings produce flux in the. same direction when current enters , For the motor operation they would be differentially compounded. because the current would be into the motor and the two fields. would be in opposition , Notation , A1 A2 the armature terminals.
F1 F2 the shunt field, S1 S2 the series field 11, DC Generators. When operated as a generator the armature of the DC machine is. driven by a prime mover , When the armature coils move through the flux created by the. stator field a voltage is induced in them , The voltage in a coil is a function of the velocity of the coil through. the flux the length of the coil and the, flux density . The armature circuit resistance includes the armature winding. resistance plus the resistance of the interpoles and. compensating windings ,Complete equivalent circuit of a separately.
excited DC generator , The interpole is a small narrow winding mounted on the back. iron of the stator in the neutral zone between the poles to speed. up commutation in the generator , The compensating winding mounted on the back iron of the. stator and consist of a single heavy turn carrying the armature. current in the opposite direction that it is carried in the armature. coils in order to cancel the armature reaction Compensating. windings are generally very expensive and are used only in large. machines , 12, Generated Voltage Equation for a DC Generator . To understand the operation of the DC generator we need to obtain the relationship. for the generated voltage in terms of flux the physical design of the machine and its speed . Faraday s law tell us,where N is the of series conductors . During each revolution a conductor cuts the flux of P poles where P is the number of poles. in the machine ,The total amount of flux cut in one revolution .
the flux per, pole, If the speed of the machine in RPM is represented by n then the time to complete one. revolution is , 13, Number of series We need to find the number of series. conductors The total number of armature conductors. conductors in the armature to calculate, the generated voltage. The number of parallel paths, Depends on the armature. winding structure, expression for the generated voltage of a DC generator .
Kg is called the generator constant for a DC machine and is a function of the design of the. machine specifically the number of poles and the type of winding . 14, Armature Windings, Lap Winding , The ends of each coil are connected to adjacent. commutator segments , For a lap wound armature the number of parallel paths. through the armature is equal to the number of poles . Wave Winding , The armature coils are connected to commutator. segments of commutator on opposite sides of the, armature . The wave winding has only 2 parallel paths between. brushes regardless of the number of poles in the machine . Lap winding provides more parallel paths which means higher current and lower voltage . The wave winding has more series coils yielding higher voltage and lower current .


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