Loading Characteristics of Shunt and Compound DC Generators

8/11/2019 Loading Characteristics of Shunt and Compound DC Generators

1/12

1

Loading Characteristics of Shunt and Compound DC Generators

Christopher Reynolds

Electrical Machines

Professor Paula Bourne

October th! "#$


2/12

2

Loading Characteristics of Shunt and Compound DC Generators

%ntroduction

Li&e DC generators DC motors 'or& off of the same principles but in a re(erse) %nstead of the

armature deli(ering the po'er to the shunt and the terminal! (oltage is fed into the motor!

supplying the shunt field and the armature 'ith po'er and allo'ing them to both create an EM*)

+hese lines of flu, react 'ith each other- the armature.s field distorts that of the shunt.s and

creates tor/ue) 0Ryff! 11$2 +his happens as the armatures conductors are pulled from a region of

high flu, density into one 'ith lo'er density) 3ith a multiple conductors this reaction is made

continuous and allo's us to ha(e a de(ice that 'ill! for the most part! efficiently con(ert

electrical energy into mechanical)

Safety Precautions

During this e,periment there is the use of high (oltages and the testing of li(e 'ires) %n

addition this (oltage 'ill be produced in DC form and 'ill ha(e an increased le(el of danger

then that of 4C) 5ands must be dry during any e,periments and there is absolutely no food ordrin&s allo'ed in the lab room) +his is to a(oid accidently damaging any e/uipment or creating

the potential for electrocution) Ensure that po'er supplies are turned off before ma&ing or

changing connections! and 'hile doing these connections ma&ing sure to only use one hand) By

only using one hand you a(oid accidently grounding a li(e connection and causing in6ury) +he

presence of rotating e/uipment also possesses a potential ha7ard to loose clothing! articles or

hair) +hese items can get caught in the e/uipment and cause serious in6ury) Long hair should be

tied bac& and loose clothing or articles should be secured or remo(ed) 8o open toed foot'ear is

allo'ed in the class and the use of electrical insulated 'or& boots or steal toes is re/uired) +his is

to pre(ent in6ury caused by the potential of falling e/uipment)

Ob6ecti(es

Lab 5:

+he ob6ecti(e of lab fi(e 'ill be to become familiar 'ith the 'iring of a DC shunt motor and

a manual motor starter) 3e 'ill be obser(ing the effects on current during motor starting and

ho' incorporating resistance can alter these effects) %n addition 'e 'ill be collecting data from

our circuit as the motor.s armature is physically loaded 'ith the use of a dynamometer) Each step

or motor is tor/ued do'n 'e 'ill be noting the tor/ue! speed! line (oltage! field current! and

armature current) *rom that data 'e 'ill then calculate the line current! po'er in! po'er out!horsepo'er! and the motor.s efficiency)

Lab 6:

3ith lab si, 'e 'ill be in(estigating the characteristics of a DC compound motor) 3e 'ill be

doing the same loading process as in lab fi(e but instead 'e 'ill be using a DC compound

motor) +his configuration of the motor includes the use of a series field to help aid the motor


3/12

3

through the loading process) %n addition 'e 'ill be manipulating our series field through the use

of a field di(erter) 3e 'ill be collecting four sets of data! one 'ith not field di(erter and three

'ith) +he three 'ith the field di(erter 'ill be done 'ith one at high! one at medium! and one at

lo' di(erter resistance) +he same data and calculations as in lab fi(e 'ill be done 'ith these four

setups! 'ith the addition of po'er losses for full and no load scenarios)

Procedure

Lab 5 Loading of a DC Shunt Motor:

Part9

3e began this lab by first chec&ing and implementing all safety precautions) Once 'e 'ere

sure that the lab could be performed safely 'e began to 'ire the lab board as per the dra'ing

0see appendi, 42! 'hile at the same time noting all of the e/uipment.s nameplate information)

+he prime mo(er 'as also set up to perform as a dynamometer and the manual motor starter set

to operate 'ith $ points) Once 'ired and chec&ed by the professor 'e began the testing

procedure)

*irst part of the lab 'as to e,periment 'ith inrush currents) +he motor 'as started 'ith the

manual motor starter ma&ing sure not to pass the first :; resistor mar&ing) 3hile this 'as done

the other lab partner made note of the inrush current through the armature! as 'ell as the steady

state current and the motor.s speed) +his 'as done three more times! each time going to the ne,t

resistor possession on the starter) 4ll data 'as collected and entered into table )

Part "9

3ith the manual motor starter fully in gauged and loc&ed in 'e set the motor to ) 3e then used the prime mo(er 0no' set to be a dynamometer2 to apply

tor/ue to our motor.s armature) 3e increased the tor/ue in lb)in increments each time

recording the motor.s speed! line (oltage! field current! and armature current) +his process

continued until the armature current reached ?"=@ of the motor.s rated ma, current) 4dditional

data 'as then calculated! and the test board 'as po'ered do'n and disassembled)

Lab 6 Loading of a DC Compound Motor:

Part 9

3e began this lab by first chec&ing and implementing all safety precautions) Once 'e 'ere

sure that the lab could be performed safely 'e began to 'ire the lab board as per the dra'ing

0see appendi, B e,cluding the field di(erter2! 'hile at the same time noting all of the

e/uipment.s nameplate information) +he prime mo(er 'as also set up to perform as a

dynamometer and the manual motor starter set to operate 'ith $ points) Once 'ired and chec&ed

by the professor 'e began the testing procedure)


4/12

4

%n this lab 'e 6umped to the same process as in part t'o of lab fi(e) +his time 'e incorporated

the use of a series field that 'as connected in series 'ith our armature and an initial line (oltage

of "#>DC) +he motor 'as loaded in steps of "lb)in till the armature current 'as ?"=@ of the

motor.s rated ma, current) *or each "lb)in increment 'e recorded the same sets of data as in part

t'o of the last lab) Once this 'as complete the board 'as po'ered do'n and 'e proceeded to

part " of the lab)

Part "9

*or the second part of the e,periment a field di(erter 'as incorporated across the series field)

Once 'ired in and chec&ed by the professor 'e repeated the same loading procedure as in part

one of this lab) +his 'as done three times o(er 'ith the field di(erter set at minimum! medium!

and ma,imum resistance) +his ga(e us three different sets of data! one for each le(el of

resistance) Once complete 'e po'ered do'n the board! disconnected the 'iring and made the

necessary calculations to complete the tables)

E/uipment

Lab 5&6:

AE/uipment cabinet 0LabA>olt olt olt 1#:"A#2

A DC Manual Starter 0LabA>olt


5/12

5

Results and 4nalysis

F*ormulas used for calculations can be found in appendi, C

Lab 5:


6/12

6

Part 9

%n the first part of this lab 'e e,amined the concept of inrush current) +his high le(el of

current is due to the lo' resistance of the armature as 'ell as the little to no counter EM* that is

present during initial motor start up) +his counter EM* 0'hen at full strength2 opposes the line

(oltage and limits the amount of current that flo's through the armature) 0Ryff! 11$2 +his is

also called the armature reaction and basically means the motor also partially acts as a generator

'hile in full operation) 3hen there is little to no counter EM* being created (oltage has a free!

lo' resistance path through the armature 'indings and 'e get our spi&e in current) +his current

can be damaging to both our motor.s 'indings as 'ell as our po'er supplies) %n order to help

'ith this one of t'o things must be done to help limit the current) Ether the (oltage must be

lo'ered and gradually increased so that the armature is able to slo'ly build a field and opposing

(oltage to limit the current) Or 'e must incorporate an in line resistance to limit the amount of

current)

Referring to the data in table one you can see that 'ith 'ithout the incorporation of resistance

our inrush current much higher than the current readings ta&en 'ith the incorporation of

resistance) +his is because 'ith the resistors 'e get a (oltage drop before our armature) +his

drops our inrush current and allo's for safer starting of the motor) 4s you can see one do'nfall

to this method is that 'e get a decrease in our machine.s speed) +his is due to the additional

(oltage drop caused by our added resistance) Steady state current through the armature remains

the same because the resistors are in series 'ith the armature) %t is a good idea to note ho'e(er!

that this motor starter is designed so that 'hen in the full on position the resistors are no longer

part of the circuit and the armature is able to recei(e full (oltage)

Part "9

%n part t'o of the lab 'e e,amined the characteristics of a basic DC shunt motor) +he data in

table t'o as 'ell as the graphs illustrated abo(e sho' a comparison of the data that 'e collected!

as 'ell as calculated) 4s you can see as 'e increased the tor/ue on the armature the speed of the

motor dropped) +his is because the EM* being de(eloped by the shunt field and the counter

EM* from the armature 'as not strong enough to maintain a constant speed) 4s the rotation

slo'ed the current began to increase) +his increase in current 'as because the armature

conductors 'ere cutting the shunts field lines at a slo'er rate! there for reducing the (oltage

created by the armature reaction) +his decrease in armature reaction meant that there 'as less

opposing (oltage being created in the armature 'indings! allo'ing more line current to flo'

through the armature) +his increase in current creates additional heat 'ithin the motor and is 'hy

'hen a motor e,periences loc&ed rotor it can /uic&ly o(erheat and cause damage)

3hen 'e apply a restricting force to the spinning armature tor/ue is created) +he creation of

tor/ue can be e,pressed as the product of an amount of force offer a distance) 4s 'e add

resistance to our motor tor/ue is generated because the mo(ement of the armature can be related


7/12

7

to a liner distance) +he motor.s speed decreases as force is added so it ta&es a balance bet'een

the rotational speed and the force being added in order to achie(e a gi(en amount of tor/ue)

E,amining the data collected it.s clear that this &ind of DC motor is not the greatest 'hen it

comes to handling large amounts of physical load) %t is ho'e(er good at maintaining speed 'ith

minimal load)

LAB 6:


8/12

8

Part 9

3ith it this lab 'e e,perimented 'ith a DC compound motor) +he difference 'ith this motor

is that it incorporates the use of a series field) +his field is 'ired in series 'ith the armature so

our current through the armature 'ould also be the same through our series field) 4naly7ing the

data directly abo(e you can see that there is an e,tremely large drop in speed bet'een our no

load and our full load speeds) %f this motor 'as to ha(e the same characteristics as our shunt

motor 'e 'ould see a massi(e amount of armature current! but instead 'e don.t) +he biggest

difference bet'een this motor and the one found in lab fi(e is that this motor is able create large

amounts of tor/ue 'ithout creating too much current through the armature) +his is because of the

series field that has no' been incorporated) 4s force is applied to the armature the motor.s speed

begins to reduce) %n a shunt motor this 'ould cause the armature to create less and less counter

EM* to oppose the line (oltage! causing the armature current to increase) %n this instant ho'e(er!

the addition of the series field is able to counter act this by incorporating an additional field into

the e/uation) Because the series field does create resistance in our line the speed does drop

/uic&er! but 'e are able to support a much higher amount of tor/ue) +his motor is able to

produce more tor/ue 'ith less current dra' and is more efficient than that of the motor in lab =)

Part "9


9/12

9

+he last part of this lab 'e added a field di(erter across the series 'inding) +his ga(e us the

ability to manipulate the amount of current through the series field) By doing this 'e 'ere able

to ad6ust the motor from being able to handle high tor/ue to being able to maintain high speed)

4s you can see in the graphs and data abo(e there is a definite difference bet'een the three

le(els of current di(ersion) One thing that.s made more noticeable in this part of the e,periment

is the field current for the three different tests) +he reason each test has % different le(el of shunt

field current is because each test re/uired a different amount to get the motor up to speed for the

set (oltage) 3hen the series field 'as being used 'e needed less shunt current because 'e had


10/12

10

the series field helping it) Minimum di(erter resistance let most of the current around the series

field so 'e re/uired a larger shunt current to get the motor up to speed)

%n the tor/ue (s) speed graph 'e can see the difference bet'een full! partial! and no use of the

series field) Li&e in lab fi(e 'hen the motor has full current through the armature and none

through a series field 'e are able to maintain a higher le(el of speed but aren.t able to support

large amounts of tor/ue) On the opposite end of things the motor 'ith full use of a series field isable to support more tor/ue but is not able to maintain speed (ery 'ell)

Conclusion

%n conclusion to our e,perimentation of series and compound generators 'e 'ere able to

establish a list of &ey characteristics that each motor has and ho' these are created) Shunt motors

are able to maintain speed more efficiently than that of compound but are not (ery good at

dealing 'ith large amounts of tor/ue) +his is due to the fact that as speed of the armature

decreases the counter EM* in the armature begins to deplete lessoning its ability to oppose the

line (oltage) +his allo's current through the armature to rise and the motor to reach its limits

more /uic&ly) +he compound motor on the other hand incorporated the use of a series field to

help e,ploit this increase in current) 4s tor/ue increased on the armature the speed began to drop!

and current began to increase) 3ith the addition of a series field this current 'as put to use and

'as con(erted into a usable EM* that allo'ed our motor to continue producing tor/ue 'ithin

rated operating conditions e(en though the speed 'as (ery slo') Compound motors ho'e(er are

not great at maintaining speed because it adds an additional (oltage drop in series 'ith the

armature pre(enting the motor from maintaining speed)

Lab uestions 4ssignments

Lab 5

2 See appendi, 4"2 See lab = analysis

2 See lab = analysis

$2 See lab = analysis


11/12

11

=2 8reg@H 08 no load A 8I rated load2 8I rated loadF##@

H 0


12/12

12

through the 'indings and the resistance of those 'indings) 4s our counter EM*

dissipates! current increases! and 'e get a larger loss across the armature and series fields)

+his ho'e(er doesn.t ma&e up all of our losses) +hings li&e core losses and friction also

play a part in our determining our total losses)

12 +he series field di(erter can be useful because it gi(es you the ability to control ho'

the motor reacts to load) By ad6usting the di(erter you can allo' the motor to be a littlebetter at maintaining consistent speed! or a little better at handling high tor/ue) +his could

help the shunt field rheostat by allo'ing the motor to achie(e a gi(en speed 'ith less

current needing to be passed through the shunt field) By increasing the resistance of the

di(erter you could increase the EM* of the series field and allo' for less current to be

needed through the shunt) +his may be used for cases 'here there isn.t enough (oltage

a(ailable to get the motor up to speed or maintain a speed)

References

Ryff! P) *) 011$2)Electric machinery) Prentice 5all! pgA$)

Loading Characteristics of Shunt and Compound DC Generators

Documents

Transcript of Loading Characteristics of Shunt and Compound DC Generators