dc motors Flashcards
why do you need a load on a generator
create counter emf and flow
when a conductor cuts through a field an emf is generated in the conductor.
a)what determines the magnitude of the emf?
b) what determines the polairity of the emf?
a) amount of flux being cut/second
b)direction of rotation relative to the magetic field
a) what would the output emf be if the fields were arranged to provide the same flux polarity on both sides of the loop in a simple generator
b) what would happen to the output polairity if the conductor was moved in the opposite direction
a)no voltage. One voltage would cancel out the other
b)polarity would flip
A generator converts _________ energy into __________ energy.
mechanical into electrical
what ways can you change the output voltage of a generator
speed, flux, # of turns, lenth of condictor that is in the magnetic field.
what does a commutater do in a generator
converts AC to a DC waveform. (switched positive to negative)
how often do split rings change directions in a generator
180 degrees
what is EMF
rate of change in flux, amount of lines cutting per second
inducted voltage in a generator will be ________ of the voltage that created it
opposite
For faradays right hand rule what does the thumb,pointer finger and middle fingers mean
thumb-thrust
pointer- polarity (north and south poles)
middle -directon of current flow
what does a frame in a genrator do
supports the field poles and provide part of the low reluctance magnetic cct.
frame is thicker then required because a low reluctance path for the field is important.
what do field poles in a generator do
iron pole pieces, laminated with one or more windings. they are permanant megnats for a generator for the south and north poles
For DC machines how many poles do they need to have
2-24
what is the armature and what does it consist of. why is it mounted on bearings
the rotating portion, consists of the drive shaft, a commutator a slotted iron core and the armature winding.
so it can rotate within the magnetic fiels provided by the field poles
what is a commutator made from
wedge shaped copper bars, seperated by sheets of mica insulation
what does a commutator do
serves as a connection point for the armature coils within the brushes. also provides sliding contact that allows the armature to turn. acts as a rotating switch that changes the armature connections as the armature coils pass from pole to pole
what does commutate mean
reverse the direction of an electrical current
what are armatire windings in a genrator made from and how are they arranged
made from a series of coils. connected to the commutator and arranged so they pas under one pole as they loop out of the commutator as the next returnd
where are the brushes in a generator and what are they made from
fit into box shaped brush holders. mounters above the surface of the commutator. made from carbon composite
what are commitating poles in a generator
(interpoles) they control armature reaction. their windings are connected in the armature cct so their influence on the fiels varies with the load
what are interpoles
commutating poles mounted between the main field poles
what is armature reaction
effect that the mmf generated by armature current has on the magnectic field.
in a generator what do the armature conductos do
are driven through the firld and drag the field flux in the opposite direction.
in a motor what does the armature reaction and what does it do
armature reaction is opposite to the direction of rotation. armature mmf pushes against the field flux to create the force that drives the load.
armature reaction only occurs _______________
under load
what are compensating windings
they do the same as interpoles but on bigger machines. the windings are busbars. they eliminate armature reaction and cancel most of the armature reactance
what are end covers on a generator
enclose the ends of the machine and support bushes and bearings. important to put on the correct way. they protect the armature, windings and brush assembly and spports the bearings
what functions does the frame provide other then mechanical support
low reluctance path for the magnetic cct and completes the magnetic cct.
in what 2 ways can the main field of a DC machine may be provided?
wound poles or permanant magnents
what are the main components of the armature
drive shaft
commutator
slotted iron core
armature windings
list the functions of the commutator and brushes.
commutator-switch AC to DC
brushes-slide connection coils
whats the cause of armature reacrion and its effect on the main field
fies being dragged, distorted fields. caised by mmf generated by armature current
what do commutating poles and compensating windings do
eliminate armature reaction
when a DC machine is used as a motor armature current is driven by the supply voltage. what drives the armature current when the same machine is used as a genreator?
load-closes the cct loop
no load=no current
load=more voltage=more current
current goes up. so does the voltage.
the more voltage the faster it spins and the more lines of flux cut
on a diagram how many humps does a shunt fiels and series have in a generator
shunt field- 4 (f1,f2)
series-3(s1,s2)
what is a shunt field winding designed for
parallel connection to a DC source. wound with lots of turns of small conductor.has max V and I rating.current is limited by the resistance od the field and can be controlled by changing the field supply voltage or installing a shunt firls rheostat in series with the field winding. high resistance= very magnetic
what is the saftey warning about shunt fields
very inductive. field discharge on shutdown can cause serious arching at the switch and make high voltages. dont open at an energized cct exept at the normal diconecct point.
what are the windings like on a series field
few turns of large conductor.series winding is intended to be conncected in series with the armature of the machine and is rated for the FLC of the current machine. resistance of the winding is very low. current is limited by the armatuer cct.
decribe and compare the physical characteristics od shunt and series field windings
field= lots of turns on the shunt field of thin wire, high resistance
series= low turns, thick wire, low resistance
why do the individual field pole windings are connected with alernating electrical polarity
creates opposite magnetic polarity
what are the standard terminal markings for the shunt and series field
series= s1,s2
shunt fiels=f1,f2
why are shunt fields so inductive
lots of loops/turns
what would happen if you disconnect one lead from the shunt field of a large DC machine, when the field is energized
arc because magnetic field collapses
why are commutating poles and compensating windings not usually considered as field windings
in series with armature not field cct
if the shunt field of a generator is connected with F1 positive, F2 negative and the machine is driven in clockwise roatation. (viewed from opposite the driven end) what is the polariry of the output?
output will be positive at A2 and negative at A1
what are the 2 common armature winding configuration
lap
wave
characteristics of lapwound on the brushes and poles
of brushes= # of poles
wave wound characteristics
have at least 4 poles. dont need brushes for each pole bc there are only 2 parallel paths in the armature cct. starts on one end and termininates on the other. will alwasy have 2 brushes and 2 parallel paths
machines with less bruh rows then poles are ___________
wave wound
if one side of an armature coil is directly under a south field pole, where will the opposite side of the coil be
north
if the lead end of coil 1 of a simplex lap winding terminates at commutator segment 1 and the finish end at segment 2, where will the start and finish ends of coil 2 terminate
start at 2 end at 3
when a simplex armature winding is completed but the brushes havent been installed, are the coils connected in series or parallel
series
how many parallel paths are there in the armature cct of
a) a simplex lap wound 6 pole DC machine
b)a simplex wavewound 6 pole machine
a) 6 paths, 6 brushes
b)2 paths, 2 brushes
what is the minimum number of poles for a machine with a:
a)wave wound ammature
b)lap wound armature
a)4 poles (will only have 2 brushes)
b) 2 poles (will have 2 brushes)
besides the armature windinfs what parts of a large 8 pole generatir are connected between A1 and A2
commuatating poles, interpoles,compensation windings (busbars)
draw tge different coil symbols for shunt field, series field and commutating windings
pg 23
F1 in a generator must always be ___________
positive
to reverse polarity or roation of a generator what do you reverse
only the armature (a1,a2)
what is the standard rotation for a DC generator
clockwise (f1 and A2 are positive)
what is standard rotation for DC motors
counter clockwise (F1 and A1 are positive)
what determines the polarity of a voltage induced into a conductor passing through a field
polarity of field poles, direction of armature conductors
what determines the direction od the force applied to a conductor current carrying current through a magnetic field
connections at field terminals
what happens to the mag polarity of the field poles, if the connections to the field windings are reversed
reverses the polariry
is it best the reverse the field or armature connections if the outpur of a generator, or rotation of a motor has to be reversed
armature
direction of roation depoends on the location of the viewer. relative to the machine. what is the standard viewpoint
opposite of drivepoint
the rotation always goes towards ________________
A1
in permant magnets of DC machines when is the field polarity set
during construction and cant be changed
the output polarity of a permant magnent generator is determined by….
direction of rotation and will reverse if the rotation is reversed.
what determines the output voltage of a permant magnent generator
speed
self exited generators depend on __________________
residual flux
what is resitidual flux
magnetism retained in the iron poles after the machine is shut down to provide initial exitation
does self exited or seperatly exited have better voltage regulation
seperatly exited
witg a standard connection what are the polarities of A1 and A2
A1-, A2+
what happens when the polarity if the shunt is reversed on
a) seperatly exited generator
b)self-exited generator
a) reverses polarity of the load
b)will fail to build a mag field, output voltage will go low and may cause a flash
what is the purpose of a shunt field rheostat
current control through field windings and controls magnetism
if the self-exited generator fails to build up and the roation and connections are correct
a)whats the problem
b)what should be checked
c)how can the problem be corrected
d)what procedures should be followed
a)residual magnetism could be gone
b)shunt field rheostat may be set too high of a resistance
c)decreases resistance to allow current to pass, flash the field
d)check rheostat before flashing the field and leave connected for awhile so the magnetism can build up
what makes series generator/motor
when a wound field machine is connected with the series field alone
why arnt series generators used often
output voltage varies with load current. no load voltage is low bc there is no field current. full load voltage is too high bc the field is at a maximum
how to change roation in a series generator
flip A1 and A2
self exited current will be _____________ as the load current
the same
seperatly exited motor has __________ voltage regulation
better
the lower the self regulation percentage the __________ the regulation
better
in seperatly exited motors why is the field current unaffected by the load
fed from a seperate source
the ___________ is the source for a generatior
armature
what is a cumatively compounded generator connection
series winding has same polarity and they add together
what is differentially compounded generator connection
series winding has opposite polarity and work against eachother (subtract)
what does the degree of compounding depend on
mmf generated by the series field relative to the mmf of the shunt field. also determines machine characteristics
undercompounded machines have the __________ series field
weakest
over-compounded machines have the ____________ series field
strongest
shunt generators characteristics
near perfect
good voltage regulation
(voltage drops only a small amt from no load to full load)
characteristics of under-compounded machines
have better voltage regulations then shunt
voltage still drops slightly as the machine is loaded
characteristics of flat compounded machines
-near perfect voltage regulation
-voltage is the same at full load as it is at no load. may be slightly higher at partial load
characteristics of over-compounded machines
-better for long distance
–negative voltage regulation
-voltage rises slightly with increaing load and is the highest at full load.
-can be used to compensate for line loss to provide a near constant voltage at the end of a long supply limne
characteristics of differentially-compounded machines
-very poor voltage regulation
-used where their natural current limiting characteristics are more important than a constant output voltage
In generators, differentially compounding is used to provide ____________. ________ in the load and ____ series field mmf. this _____ the main field and _____ output voltafe. the ____ the load are connected in ___________
-used to provide current limiting.
-increase in the load means higher armature current and more series fiels mmf
-this weakens the main field and reduced output voltage
-armature and load are in //
shunt field current can be adjusted with a series connected _______________
shunt field rheostat
series field current can be adjusted with a ___________
// connected diverter resistor
shunt field rheostats are an _________
operating control
a motor rheostat is connected with ___________________ resistance fully ___________ which __________motor speed
-max resistance fully clockwise which increases motor speed
series field diverter allows some of the _________________ to _________________
-armature current to bypass the series field windings
-wired in // with s1 and s2
high current= low resistance
reducing the resistance of a diverter________________________
increases its share of the armature current which reduces the current through the series field
-are not an operating control
series field diverters are specialized devices with _______________resistance and __________ current ratings
-very low resistance
-high current
what is the difference between a long-shunt and short-shunt connection?
-long- has series windings and connected across both armature and series field
-short-shunt field connected only across armature
what is meant by the term “cumulativeky compounded”
series windings same polarity and add together
compare no- load and full-load voltages od under-,flat-, and over-compounded generators
flat= near perfect, voltage reg is the same at full load and no load (FL=NL)
over=- voltage reg full load is higher then no load
under= better voltage reg, slight volt drop when machine is loaded. no load is higher then full load
what is the usual reason for using differential compounding in a generator
-current limiting characteristic
-when load is higher voltage is lower
how is a series field diverter connected and when is it usually set
- in // with s1 and s2
-set when machine is commistioned and not changed
how would a shunt field rheostat have to be connected to provide an increase in generator voltage with a clockwise roatation of the control?
-connected to provide max resistance with the control turned fully counter clockwise
for DC machines what is the ambient temperature if its not specified
40 degrees celcius
how to calculate max rated operating temperature
40 degrees+ temperature it says
a generator is rated at 10kW, 240V. will it be operating within its ratings if it is supplying 10kW at 200V?
-no.
-power ratings based on operation at full voltage
-current will be too high
what effect does operating speed have on genreated voltage
-speed the armature conductors cut through the field
under what conditions should generator output voltage match the nameplate rating
if operating at ratyed speed witgh rated shunt field and full load
why does generator voltage slightly decline as the machine heats up
resistamce of windings increase=reduces shunt field current=weakens the field=increases internal voltage drops and loses=decreases output
what is an appropreate ohmmeter range for measuring armature resistance
lowest range avaliable.
can armature resistance be measured accurately at the ammuture terminals (a1,A2)
-no
-bc measurements will include the resistance of commutating windings and brushes
what would happen if the series field were accidentally connected as the shunt field
short out supply
what happens to the output voltage of an operational self-exited shunt generator, if the shunt field connections are reversed
fail to build up voltage the next time its brought up to speed.
why do you connect a load to obtain a useful voltage from a series genertator
-no load=no current
-field flux comes from residual magnetism
how to you test for grounds
maggometer
if a machine is in good working order, the insulation is clean and there are no connections to the terminals, what should the resistance readings be from the terminals to ground?
infinate
*what can cause leakage to ground in a DC machine?
-moisture
-airborne contaminants such as carbon dust from brushes
*why would a ground on the armature winding read a low resistance on both A1 and A2?
-resistance of armature is very low
why would a ground on the shunt field give a much higher resistance reading to ground on one terminal, compared to the other?
-ground is near one end of the winding
-winding has significant resistnace
A DC generator has a full load output voltage of 248V, and no load output of 265V. what is the percent voltage?
use formula= 6.85%
draw a connection diagram for a self-exited shunt generator, with a rheostat for voltage contril. generator is to be driven clockwise.
pg 54
what connection should be changed if the roatation of a generator has to be reversed
armature connections
the exitation provided by a series winding ___________ as the load is added
increases
A series generator has ______________ voltage regulation
poor
A compunded generator has a no load output of 125V and a full load output of 145V
a)what is the percent voltage regulation
b)is the generator under, over, or flat compounded
a) -13.8%
b)over
draw a connection diagram for a cumulatively compounded generator with counter clockwise roatation
pg 58
what device could be connected to convert an over compounded generator to flat compounding
-series field diverter
-connected in // with the series field
if the output of a flat-compounded generator is 120V at no load, what is it at full load?
120V
what is the percent voltage regulation of a flat-compounded genrator?
0%
right hand rule is for _____
generators
left hand rule is for __________
motors
what was the __________ in a generator is now the ______ for a motor
-load in generator
-source for motor
how is mmf created in a motor
-through current conductors
-interacts with the main field
-creating a force that can be used to move a mechanical load
in a motor the __________ creates the force that becomes the motors mechanical output
-mmf
in a motor the direction of motion and the polarity of the field are ______________ so the polarity of the emf is also ________
-the same
-the same
in a motor it becomes a ____________ that opposes the ___________ and acts to _____motor ______
-counter emf
-applied voltage
-regulate motor current
generator action occurs when a _______________ is used to drive the _________________ through a field
-mechanical force
-conductors
motor action occurs when _____________ from an electrical supply is driven through a conductor in the field
-current
motors demand ____________ to turn it into __________
-electrical
-mechanical
when a current passes through a conductor in a field a mechanical force is generated that attempts to push the conductor at right angles to the current and the field.motor
a) what determines the magnetiude of th eforce?
b)what determines the direction of the force?
a)strenth of the 2 magnetic fields (1 in armature and one in field)
- density of the field
b)polarity of the field and current reralative to eachother
a)what would output force be if the fields were arranged to provide the same flux polarity on both sides of the loop in a simple motor
b)what determines the direction of force?
c) what would happen if the polarity of both fields were reversed?
a)will sit and buzz bc both forces are going in opposite directions whichb results in no motor action
b) roatation would flip/reverse
c)would still go the same direction bc it would double flip
reversing the polarity of the field OR the armature connections in a motor _______________
reverses the direction of rotaton in a motor
standard rotation for a motor is ________________
counter clockwise (f1 and A1 are positive)
-viewed from end opposite the drive
what determines the direction of the force applied to a conductor current carrying current through a magnetic field?
-polarity of the 2 fields
what happens to the magnetic polarity of the field poles, if the connections to the field winding are reversed?
-flips
is it best to reverse the field armature connections if the roatation of a motor has to be reversed?
armature
direction of rotation depends on the location of the viewer, relative to the machine. what is the “standard viewpoint”?
opposite end of the drive
if a motor is connected with F1 and A2 positive. which way will it rotate in a motor
clockwise
the direction of the rotation for permanant magnet motors can be reversed by _________________________
changing the polarity of the supply connections
what connections must be changed to reverse the rotations od a permanant magnent motor
A1/A2 or supply
in what way is a permanat magnent motor unique amoung DC motors
-only motor that can change roatation by changing polarity of the supply (armature) connections
for more resistance in a motor __________ magnetism=___________ torque=____________ speed
less=less=more
for less resistance in a motor _________ magnetism=___________ torque=__________speed
more=more=less
in a motor what will happen if you dont have a high enough starting resistance
destroy the armature bc they have low resistance and need to be high resistance
where can you find the base speed on a motor
name plate
can DC motors be started across the line
no
shunt motors should alwasys be started with ____________ and maximum starting torque to provide and ensure a ________of __________
-full rated field
-rapid build up of current limiting cemf
(start at max resistance with a armature rherostat)
the base speed of a motor is ___________and listed on the _________
motor speed at full load
listed on the name plate
adjusting the _______provides speed control below base speed with ____________ from 0rpm
-armature voltage
-below base speed
-full torque
adjusting field current provides __________ speeds at ___________
-above base speed
-reduced torque
a constant motor maintains a ______________speed under a changing load
constant speed
under changing load
the speed of a varying-speed motor changes with _________ in the load
changes in the load
an adjustable speed motor is a __________ that can be speed ___________
-constant speed
-speed contrilled over a defined range (you control)
an adjustable varying speed motor can be speed _________ but also __________________
-controlled
-changes speed with changes in load
a DC shunt motor is a _____________ that can be operated as an ________________
-constant speed motor
-adjustable speed motor
weakening the field of a motor _________ motor speed but ____________ torque
increases
lowers
what will happen if the motor cant handle the load if the torque has been lowered
motor will stall
if a series diverter is added to a cumulativley compounded motor should the speed regulation improve(lower %) or get worse (higher %)
imorove
