6 DC MACHINES 6.1 Introduction DC machines can be mainly classified into two, viz; DC motor and DC generator. DC motor accepts electrical power as input and gives a corresponding mechanical power output. But in the case of DC generator, the input is mechanical power and output is electrical. electrical. Before the construction construction and operation operation of DC machines can be introduced, introduced, a few common terms must be understood.
6.2 DC MACHINE TERMINOLOGY (a) Tr!ina" #o"ta$
Terminal voltage, voltage, as applied to DC generators, is defined as the voltage that can be measured at the output of the generator or motor. (%) Countr&E"ctro!oti' orc (CEM)
In a generator using a rotating armature, the conductors cut the magnetic lines of force in the magnetic field. Voltage is induced in the armature conductors. his induced voltage opposes the applied voltage; it counteracts some of the applied voltage, which reduces the current flow through the armature. his induced voltage acts counter to applied voltage; therefore, it is called counter-electromotive counter-electromotive force !C"#$%. (c) A"id #o"ta$
Applied voltage is defined as the voltage that is delivered across the load. his voltage should be the same as terminal voltage; however, various circuit faults and losses may reduce the terminal voltage.
(d) Co!!utation
Commutation Commutation is the positioning of the DC generator brushes so that the Commutator segments change brushes at the same time the armature current changes direction. #ore simply stated, commutation is the mechanical conversion from &C to DC at the brushes of a DC machine, as shown in $igure '.(.
i$. 6.1 AC to DC Con'r*ion +it, a Co!!utator
In a DC generator, commutation provides for the conversion of &C to a DC output that is generated in the armature windings. OTOR AND GENERATOR 6.- GENERAL CONSTRCTION O DC MOTOR AND
Direct Direct curren currentt machin machines es !DC motor motor and DC generat generator% or% are energy energy trans transfer fer devices. hese machines can function as either a motor or a generator. DC motors and generators have the same basic construction, differing primarily in the energy conversion. In the case of motor, electrical energy is given at the terminals of the motor and motor shaft delivers corresponding mechanical power. )enerator does the reverse of it, that is,
mechanical power is inputted, and electrical power can be drawn from the generator terminals. 6.-.1 Ar!atur
he purpose of the armature is to provide the energy conversion in a DC machine !refer to $igure '.*%. In a DC generator, the armature is rotated by an e+ternal mechanical force, such as a steam turbine. his rotation induces a voltage and current flow in the armature. hus, the armature converts mechanical energy to electrical energy. he DC machine has twolayer winding of two types - lap winding and wave winding. In lapwinding, the ends of each armature coil are connected to adacent segments on the commutator so that the total number of parallel paths is e/ual to the total number of poles. In wave winding, the ends of each armature coil are connected to armature segment some distance apart, so that only two parallel paths are provided between the positive and negative brushes. In general, lapwinding is used in low voltage, high current machines and the wavewinding is used in highvoltage low current machines.
IG. 6.2 /a*ic DC !ac,in
In a DC motor, the armature receives voltage from an outside electrical source and converts electrical energy into mechanical energy in the form of tor/ue. 6.-.2 Rotor
he purpose of the rotor is to provide the rotating element in a DC machine !refer $igure '.*%. In a DC generator, the rotor is the component that is rotated by an e+ternal force. In a DC motor, the rotor is the component that turns a piece of e/uipment. In both types of DC machines, the rotor is the armature. 6.-.- Stator
he stator he stator is the part of a motor or generator that is stationary !refer to $igure '.*%. In DC machines, the purpose of the stator is to provide the magnetic magn etic field. he stator in $igure '.* is provided by a permanent magnet.
6.-.0 i"d
he he purpo purpose se of the the field in a DC machine is to provide a magnetic field for producing either a voltage !generator% or a tor/ue !motor% !refer to $igure '.*%. he field in a DC machi machine ne is produ produce ced d by eith either er a perma permane nent nt magne magnett or an elect electro roma magn gnet et.. 0ormally, electromagnets are used because they have an increased magnetic strength, and the magnetic strength is more easily varied using e+ternal devices. In $igure '.*, the field is provided by the stator. In order for a DC generator g enerator to operate properly, the magnetic field must always be in the same direction. herefore, the current through the field winding must be direct current. his current is 1nown as the field the field excitation current and can be supplied to the field winding in one of two ways. It can come from a separate DC source e+ternal to the generator !e.g., a separately e+cited generator% or it can come directly from the output of the generator, in which case it is called a self-excited a self-excited generator . In a selfe+cited generator, the field winding is connected directly to the generator output. he field may be connected in series with the output, in parallel with the output, or a combination of the two. 2eparate e+citation re/uires an e+ternal source, such as a battery or another DC source. It is generally more e+pensive than a selfe+cited generator. 2eparately e+cited generators are, therefore, used only where selfe+citation is not satisfactory. hey would be used in cases where wh ere the generator must respond /uic1ly to an e+ternal control c ontrol source or wher wheree the the gene genera rate ted d volt voltag agee must must be vari varied ed over over a wide wide rang rangee duri during ng norm normal al operations.
6.-. Co!!utator
In a simp simple le one onelo loop op gener generat ator or,, the the comm commuta utato torr is made made up of two two semi semi cylindrical pieces of a smooth conducting material, usually copper, separated by an insulating material, as shown in $igure '.3. "ach half of the commutator segments is permanently attached to one end of the rotating loop, and the commutator rotates with the loop. he brushes, usually made of carbon, carbon, rest against the commutator commutator and slide along the commutator as it rotates. his is the means by which the brushes ma1e contact with each end of the loop.
i$. 6.- Co!!utator S$!nt* and /ru*,*
"ach brush slides along one half of the commutator and then along the other half. he brushes are positioned on opposite sides of the commutator; they will pass from one commutator half to the other at the instant the loop reaches the point of rotation, at which point the voltage that was induced reverses the polarity. "very time the ends of the loop reverse polarity, the brushes switch from one commutator segment to the ne+t. his means that one brush is always positive with respect to another. he voltage between the
brushes fluctuates in amplitude !size or magnitude% between zero and some ma+imum value, but is always of the same polarity !$igure .4%. In this manner, commutation is accomplished in a DC generator.
i$. 6.0 Co!!utation in a DC Gnrator
5ne important point to note is that, as the brushes pass from one segment to the other, there is an instant when the brushes contact both segments at the same time. he induced voltage at this point is zero. If the induced voltage at this point were not zero, e+tremely high currents would be produced due to the brushes shorting the ends of the loop together. he point, at which the brushes contact both commutator segments, when the induced voltage is zero, is called the 6neutral plane.6
6.0 DC GENERATOR T THEORY DC generators are widely used to produce a DC voltage. he amount of voltage produced depends on a variety of factors. here are three conditions necessary to induce a voltage into a conductor. & basic DC generator has four basic parts7 !(% a magnetic field; !*% a single conductor, or loop; !3% a commutator; and !4% brushes !$igure '.3%. he he magn magnet etic ic fiel field d may be supp suppli lied ed by eith either er a perm permane anent nt magn magnet et or an elect electro roma magn gnet et.. $or now, now, we will will use use a perma permane nent nt magne magnett to descr describ ibee a basic basic DC generator. & single conductor, shaped in the form of a loop, is positioned between the magnetic poles. &s long as the loop is stationary, the magnetic field has no effect !no relative motion%. If we rotate the loop, the loop cuts through the magnetic field, and an "#$ !e80.d9:dt% is induced into the loop.
i$ur 6. /a*ic Oration o a DC Gnrator
efer $ig.'.<. =hen we have relative motion between a magnetic field and a conductor in that magnetic field, and the direction of rotation is such that the conductor cuts the lines of flu+, an "#$ is induced into the conductor. he magnitude of the induced "#$ depends on the field strength and the rate at which the flu+ lines are cut. he stronger the field or the more flu+ lines cut for a given period of time, the larger the induced "#$. he commutator converts the &C voltage generated in the rotating loop into a DC voltage. It also serves as a means of connecting the brushes to the rotating loop. he purpose of the brushes is to connect the generated voltage to an e+ternal circuit. In order to do this, each brush must ma1e contact with one of the ends of the loop. 2ince the loop or armature rotates, a direct connection is impractical. Instead, the brushes are connected to the ends of the loop through the commutator.
6. EM E3uation o DC Gnrator
>et Φ 8 flu+:pole in weber ? 8 total number of armature conductors. con ductors. @ 8 number of generator poles. & 8 number of parallel paths in armature 0 8 armature rotation in revolutions per minute. " 8 "#$ induced in any parallel paths in armature. &verage emf generated:conductor 8 d9:dt Volts $lu+ cut: conductor in one revolution, dΦ 8 Φ@ weber ime for one revolution 8 'A:0 second
&verage "#$ induced in one conductor per revolution 8 flu+ cut by one conductor in one revolu revolutio tion n : time time ta1en ta1en for one revolution. 8 @9:!'A:0% 8 !@90% : 'A Volts Volts otal otal "#$ induced in ? conductors 8 !0o. of conductors% + !&vg. !&vg. "#$ per conductor% ie, " 8 !?90% : 'A Volts "#$ across generator terminals 8 total "#$ induced in ? conductors : 0o. of parallel paths. herefore, " 8 !?90 : 'A% !@:&% Volts ie; " 8 !?90@ : 'A&% Volts Volts =hich is called "#$ e/uation of DC generator.
6.6 Dirction o Inducd Currnt "o+ in a Gnrator
he direction of the induced current flow can be determined using the 6lefthand rule6 for generators. his rule states that if you point the inde+ finger of your left hand in the direction of the magnetic field !from 0orth to 2outh% and point the thumb in the direction of motion of the conductor, the middle finger will point in the direction of current flow !$igure '.'%. In the generator shown in $igure '.', for e+ample, the conductor closest to the 0 pole is traveling upward across the field; therefore, the current flow is to the right, lower corner. &pplying the lefthand rule to both sides of the loop will show that current flows in a countercloc1wise direction in the loop.
i$ur 6.6 Lt&Hand Ru" or Gnrator*
6.4 T5* o DC Gnrator* & DC generator may be constructed in a variety of ways depending upon the relationship and location of each of the fields. Based on the field e+citation, we can classify DC generators mainly into three. a.% 2hunt=ound DC )enerators b.% 2eries DC generator c.% Compound DC generator
6.4.1 S,unt&ound DC Gnrator* =hen the field winding of a generator is connected in parallel with the generator armature, the generator is called a shuntwound generator !$igure '.%. he e+citation current in a shuntwound generator is dependent upon the output voltage and the field
resistance. 0ormally, field e+citation is maintained between A.< and < percent of the total current output of the generator.
i$ur 6.4 S,unt&ound DC Gnrator
he shunt shuntwou wound nd generat generator or,, runnin running g at a constan constantt speed speed under under varyin varying g load load conditions, has a much more stable voltage output than does a serieswound generator. 2ome change in output voltage does ta1e place. his change is caused by the fact that, as the load current increases, the voltage drop !Iaa% across the armature coil increases, causing output voltage to decrease. &s a result, the current through the field decreases, reducing the magnetic field and causing voltage to decrease even more. If load current is much higher than the design of the generator, the drop in output voltage is severe. $or load current within the design range of the generator, the drop in output voltage is minimal !$igure '.E%.
i$ur 6.7 Outut #o"ta$&'*&Load Currnt or S,unt&ound DC Gnrator
6.4.2 Sri*&ound DC Gnrator* =hen the field winding of a DC generator is connected in series with the armature, the generator is called a serieswound generator !$igure '.F%. he e+citation current in a serieswound generator is the same as the current the generator delivers to the load. If the load has a high resistance and only draws a small amount of current, the e+citation current is also small. herefore, the magnetic field of the series field winding is wea1, ma1ing the generated voltage low.
i$ur 6.8 Sri* DC Gnrator
Conve Convers rsely ely,, if the the load load draw drawss a larg largee curr current ent,, the the e+ci e+cita tati tion on curr current ent is also also high high.. herefore, the magnetic field of the series field winding is very strong, and the generated voltage is high.
i$ur 6.19 Outut #o"ta$&'*&Load Currnt or Sri*&ound DC Gnrator
&s we see in $igure '.(A, in a series generator, changes in load current drastically affect the generator output voltage. & series generator has poor voltage regulation, and, as a result, series generators are not used for fluctuating loads. &s is the case for the shunt wound generator, a serieswound generator also e+hibits some losses due to the resistance of the windings and armature reaction. hese losses cause a lower terminal voltage than that for an ideal magnetization curve.
6.4.- Co!ound Gnrator* 2erieswound and shuntwound generators have a disadvantage in that changes in load current cause changes in generator output voltage. #any applications in which generators are used re/uire a more stable output voltage voltage than can be supplied by a serieswo serieswound und or shuntwound generator. 5ne means of supplying a stable output voltage is by using a compound generator. he compound generator has a field winding in parallel with the generator generator armature !the same as a shuntwound shuntwound generator% generator% and a field winding winding in series with the generator armature !the same as a serieswound generator% !efer $igure '.((%.
i$ur 6.11 Co!ound DC Gnrator
he two windings of the compounded generator are made such that their magnetic fields will either aid or oppose one another. If the two fields are wound so that their flu+ fields oppose one another, the generator is said to be differentially-compounded . If the two fields of a compound generator are wound so that their magnetic fields aid one another, the generator is said to be cumulatively-compounded . &s the load current increases, the current through the series field winding increases, increasing the overall magnetic field strength and causing an increase in the output voltage of the generator. =ith proper design, the increase in the magnetic field strength of the series winding will compensate for the decrease in shunt field strength. herefore, the overall strength of the combined combined magnetic magnetic fields remains almost unchanged, so the output voltage voltage will remain constant. In reality, the two fields cannot be made so that their magnetic field strengths compensate for each other completely. here will be some change in output voltage from
the noload to fullload conditions. In practical compounded generators, the change in output voltage from noload to fullload is less than < percent. & generator with this characteristic is said to be flat-compounded be flat-compounded !$igure '.(*%.
i$ur 6.12 #o"ta$&#*&Currnt or a Co!oundd DC Gnrator
$or some applications, the series winding is wound so that it overcompensates for a change in the shunt field. he output gradually rises with increasing load current over the normal normal operati operating ng range range of the machine machine.. his his type type of generator generator is called called an overcompounded generator. he series winding can also be wound so that it under compensates for the change in shunt field strength. he output voltage decreases gradually with an increase in load current. his type of generator is called an under-compounded generator.
6.7 DC MOTOR 6.7.1 :rinci" o oration
DC voltage is applied across the field and armature terminals and a stationary magnetic field is setup. he current entering in the armature winding via brushes and commutator gets disturbed such that all the conductors under one magnetic polarity carry current in one particular direction and the conductors undo other magnetic polarity carry current in opposite direction. "ach conductor in the magnetic field develops a force $ 8 BI>. he forces developed by the conductor acting in the shaft causes net tor/ue d to develop. he direction of force and tor/ue can be found by $lemings >eft Gand ule. If the tor/ue developed is greater than that of the frictional tor/ue and load tor/ue, the armature starts rotating in the direction of force or tor/ue developed. &s the armature starts rotating, the flu+ lin1ed with the armature conductors changes and an "#$ is induced in it as per $aradays $irst >aw. his induced "#$ is called bac1 "#$. !" b% 6.7.2 Tor3u Tor3u E3uation
If we neglect the losses and assume that tha t the input electrical power is completely converted into mechanical power, then7 @m 8 @e =here @e is the electrical power inputted and @m is the output mechanical power.
If H is the angular velocity and d is the tor/ue developed; @m 8 H d @m 8 !*0:'A% d
!'(%
@e 8 " b Ia
!'*% φ Z N P
Bac1 "#$, " b 8
'A A
Ia !'3%
=here 9 is the flu+ developed, ? is the number of armatures per slot and @ is the number of poles. "/uating !'(% and !'3%; (
d 8 *π 9? !@:&% Ia his is the tor/ue e/uation for DC motor. motor. $rom the e/uation, we can see that the developed tor/ue is directly proportional to the armature current and flu+. 6.8 T5* o DC Motor*
Depends on the field connection, con nection, DC motors are mainly classified into7 !a% DC 2huntwound 2huntwound motor motor or 2hunt 2hunt motor. motor. !b% DC 2eries motor. !c% DC Compound motor. !d% 2eparately e+cited DC motor.
&mong these, we will discuss shunt motor, series motor and compound motor.
6.8.1 S,unt&ound Motor
i$ur 6.1- DC S,unt !otor
$igure '.(3 shows a shunt DC motor. he motor is called a 6shunt6 motor because the field is in parallel, or 6shunts6 the armature.
Tor3u&'*&Sd c,aractri*tic* o a S,unt&ound DC Motor i$ur 6.1010 Tor3u&'*&Sd
he speedtor/ue characteristics of a typical shuntwound motor are shown in $igure '.(4. & shuntwound shuntwound DC motor has a decreasing tor/ue when speed increases. he decreas decreasing ing tor/ue tor/ueVs Vss spee peed d is caused caused by the armatu armature re resist resistance ance voltag voltagee drop drop and armature reaction. &t a value of speed near *.< times the rated speed, armature reaction becomes e+cessive, causing a rapid decrease in field flu+ and a rapid decline in tor/ue until a stall condition is reached.
6.8.2 DC Sri* Motor
i$ur 6.1 DC *ri* !otor
$igure '.(< shows a series DC motor. he motor field windings for a series motor are in series with the armature. 2ince the armature and field in a serieswound motor are connected in series, the armature and field currents become identical, and the tor/ue can be e+pressed as7 or/ue, 8 JIa*
i$ur 6.16 Tor3u&#*&Sd Tor3u&#*&Sd or a Sri*&ound Sri*&ound Motor
he tor/ueVsspeed characteristics of a serieswound motor with a constant voltage source are shown in $igure '.('. &s the speed decreases, the tor/ue for a series wound motor increases sharply. &s load is removed from a series motor, the speed will increase sharply. $or these reasons, serieswound motors must have a load connected to prevent damage from high speed conditions.
6.8.- Co!oundd Motor
i$ur 6.14 Co!oundd DC !otor*
$igures '.( !a% and '.(!b% show different types of compounded DC motor. & compounded DC motor is constructed so that it contains both a shunt and a series field. $igure '.(!a% is called a 6cumulativel 6cumulativelycom ycompounded pounded66 DC motor because the shunt and series fields are aiding one on e another.
i$ur 6.17 Cu!u"ati' co!ound !otor c,aractri*tic*
he speedtor/ue characteristics of a cumulative compound motor is shown in $igure '.(E. In cumulative compound motor, a better constancy of speed is obtained than that of DC shunt motor. $igure '.(!b% is called a 6differentiallycompounded6 DC motor because the shunt and series field oppose one another. he direction of the arrows indicates the direction of the magnetic fields.
i$ur 6.18 Dirntia"&co!ound !otor c,aractri*tic*
In differentiallycompounded motor, a series type speedtor/ue characteristic is desired without the problem of dangerous noload speed. If differential compounded is used, the noload speed corresponds to the shunt field as shown in the $igure '.(F. he compou compounded nded motor motor is desira desirable ble for a variet variety y of applica applicati tions ons because because it combines the characteristics of a serieswound motor and a shuntwound motor. he compounded motor has a greater tor/ue than a shunt motor due to the series field; however, it has a fairly constant speed due to the shunt field winding. >oads such as presses, shears, and reciprocating machines are often driven by compounded motors.
