DC machine review sheets Flashcards
Motors covert ________ energy into _________
-electrical into mechanical
starting torque of series DC motors is commonly _________ of the full load torque
450%
Starting torque of shunt DC motors is commonly ____________ to _____________ % of the full load torque
125%-200%
Generator effect of a DC motor produces a _____________
counter emf
Counter EMF limits armature current as acts as a _______________ for DC motors
-speed governor
As load is added to the motor, the armature _________________________
current increases
Torque is _________________ to armature current
proportional
Torque in DC motors is produced from interaction between _______________ in the poles and the _______________
-magnetic fields
-armature
base speed is when _____________ is applied to both the __________ winding and the _____________ of a DC motor under full load
-full load voltage
-shunt field winding
-armature
____________ the field of a DC motor will cause the RPM to ___________ above base speed
-weakening
-increase
A DC shunt motor that is not connected to a load will _______________ if the field is opened.
-overspeed
Voltage reduction to the armature of a DC shunt motor will cause the RPM to __________ below base speed
-decrease
The standard direction of rotation for DC motors is _______________ as viewed from opposite the drive end.
opposite
to change the direction of a DC motor, change the _______________
armature leads only
Speed of a series motor is controlled by the load, at no load a series motor will ______________
overspeed
Generators convert ___________ energy into ________________ energy
mechanical into electrical
The strength of a magnetic field depends on what 3 things
1.amount of current
2. number of conductor loops through which the current flows
3. type of core material
The amount of induced voltage depends on what 3 things.
1.strength of the magnetic field
2.speed of cutting action
3. number of conductor loops cutting the field
As load is added to a generator, an _________ in torque from the prime mover is required
increase
the voltage build up on a series generator is directly proportional to the ____________. The greater the __________, the greater the ____________ resulting in _____________ voltage output
1.connected load
2.load
3.field strength
4.higher
Voltage drop in the armature of a generator is caused by ___________________ through the resistance of the armature windings (E=IxR)
load current flowing
When the load is added to a shunt generator, the output terminal voltage _____________ due to the voltage drop in the armature
drops
on a self-exited generator, the voltage will not build up if the direction of rotation is __________________________ unless the armature leads are ___________________
-reversed
-reversed
the output voltage characteristics of generators are classified as:
- over-compound
2.flat-compounded
3.under-compounded
over-compounding of a generator means that the full-load voltage is ____________________ than the no load voltage
higher
over-compound generators can ve used to _______________ for line volt drop
compensate
Generators can be ________________ or ___________
seperatly exited or self exited
self-exited generators can be connected as: (4)
1.series
2.shunt
3.cummulative or differential long shunt
4.cummulative or differential short shunt
cumualitive compounding is accomplished by connecting the _____________ and _______________ windings so that the current flows in the same direction through both windings
1.series
2.shunt windings
3.same
to prevent sparking, wasted energy and damage to the generator brushes must be located over the ______________
neutral plane
all rotating machines generate ______________________
alernating current
in a DC genreator, the generated alternating current is converted to direct _____________ by a __________ called a commutator
-current
-rotating switch
A commutatir is made up of segments of ____________ which are seperated by _____________
copper
mica insulation
In a DC shunt generators they have drooping output voltage curve (positive % voltage regulation) which means they are well suited to be connected for ____________ operation
parallel
in a DC shunt generator they will naturally share shanges in the load levels if the machines have close or equal _________________
voltage regulation
DC shunt generators must be rated for the ___________ output voltage
same
DC shunt generators outmuts must be connected with ______________ polarity
the same
In DC cummlative compound generators, flat or overcompounded machines require an ____________- connected between the two machines to __________________ connect the series windings
-equalizer bus
-to parallel
In a DC cummulative compounded generator rhe equalizer bus forces the machines to ______________________ share any _______________ in the load
-proportionaly
-changes
In a DC cumulative compounded generator the resistance of the equalizer bus must not be more then ________________ of the resistance of the series winding of the ____________ generator
- 20%
smallest
When 2 generators are operating in parallel, increasing ___________ on one generator causes it to ‘pick up’ load, and the bus voltage to __________
-exitation current
-increase
to properly shift load from one generator to another when 2 DC generators are connected in parallel _____________ exitation current on one to allow it ‘pick up’ load, and _____________ exitation on the other to give up some load.
increase
decrease
Generator A and Generator B are operating in parallel and sharing a load. Power is loast or fuel is depleated to the prime mover of generator A, Generator A will run in the ___________ direction, at a __________ speed. Generator B will pick up the _____ load and _____________ to the motoring generator
1.same
2.reduced speed
3.entire load
4. supply power
What determines the polarity of an induced EMF?
- polarity of flux
- direction of the cutting
What do generators convert into electrical energy
mechanical energy
Why must a frame have a low reluctance path?
- So the frame is thicker then needed
- completes the magnetic path/cct
Describe field poles
- iron pole peices that are often laminated to reduce eddy currents
- can have 2-24 poles
- wound with 1 or more field windings
What is the roatating portion of a DC machine called
the armature
What is used to change AC voltage into DC voltage.
the commutator
Why is most iron laminated in a DC machine?
to reduce eddy currents
what do end covers enclose on a DC machine
-armature
-windings
-brush assembly
-support bearings
describe a shunt winding and how its connected
-designed for // connection
-wound with many turns of small guage wire
-has specific voltage rating
- the current is limited by the resistance of the field
-can be controlled by changing the supply voltage or installing a shunt rheostat in series with the windings.
-high resistance
-very magnetic
Describe a series winding and how its connected
-few turns of large conductor
- connecrted in series with the armature, (part of of the armature)
-rated to carry the full load of machine
-low resistance
-current limited by armature cct
how are commutating poles and compensation windings connected?
internally connected in series with the armature
describe the layout of a lap winding?
-a continuous series loop that closes in on itself after one loop around the armature.
-start and end leads terminate at adjacient segments and the start lead of the following coil terminates on the same segment as the end lead of the preceding coil.
- the end lead of the last coul connects to the start lead of the first coil
what is meant by the term “duplex coil”
-2 coils wound into each slot
what are the standard rotations for DC machines
Generator-clockwise (F1, A2 positive)
motor-counter clockwise (F1, A2 negative)
how will the output polarity of a permanant magnent generator be determined?
-direction of rotation
what is the difference in a self exited generator and a sepreatly exited generator?
self- depends on residual flux for inital exitation
seperatly- depends on another source for initial exitation
what device is used to control thr field strength in a shunt generator?
shunt field rheostat
what is residual magnetism
magnetism retained in the iron poles after the machine is shut down
what is flashing the field
temperarly connecting another power supply to change the magnetic polarity of the field poles and the residual flux.
why do self-exited generators have a max field cct resistance?
-if the rheostat is set too high there will be too much resistance in series with the field and the generator will fail to build up.
describe the connections in a series generator
-output voltage varies with the load current
-no load voltage is low because there is no field current
-full load voltage is high because the field current is at its max
why are series generators almost unusable?
-self exited
-wont build up voltage without a load
-wont build up voltage at all if the initial field current weakens the residual flux
what is the difference between cumulativley compounded generators and differentially compounded generators?
cumulativley- connected with same polarity and are additive
differentially- connected with opposite polarity and are subtractive
Describe the 3 different types of cummulativley compounded generators
under- weakest series field, voltage drops as the machine is loaded.
flat- near perfect voltage regulation, voltage is the same at no load as it is at full load.
over- negative voltage regulation, voltage increases as load increases and is its highest at full load, can be used to compensate for line loss at the load end to supply constant voltage at the end of a long supply.
how is a series field diverter installed
in parallel with shunt field current
how does a series field diverter operate
-allows some of armature current to bypass the series field windings
-reduces resisitance of diverter= increases share of armature current= reduces current through series windings
what are listed on the nameplate of a DC generator
1.full load power output in kW
2.output voltage
3.full load output current
4.shunt field voltage
5.shunt field current
how could you identify the field windings and the series windings?
- look at number of turns
2.size of conductor
3.how its connected
what would cause leakage to ground in a dc machine
-airborne contaminants
-moisture
how is the voltage regulated in a typical shunt generator
-adjusting field winding resistance
-armature resistance
-armature speed
what determines the current that flows through the winding in a series generator
the load
How can a cumulatively compounded generator be turned into a differential generator?
reverse series coil
when a motor is turned and the armature is rotating in a magnetic field, what is the term used for the voltage that is generated
mmf
what does a motor convert electrical energy into?
mechanical energy
what is the only motor that can be reversed by reversing the supply leads?
permenant magnent motors
what settings should the 2 rheostats be at when starting a shunt motor?
shunt-minimum resistance
armature-max resistance
when the motor is running at base speed, what can be done to speed the motor up?
adjust armature voltage
what is an adjustable speed motor?
constant speed motor that can be speed controlled over a defined range (you control)
what would cause a shunt motor to over speed
-if field is weaked and the motor isnt loaded or its under loaded.
what happens if under a heavy load the field current is lost
wont restart
list applications of a series motor
-hoisting
-traction duty
-trains
Why must a series motor not be belt driven?
-spin faster then designef for at max speed
-cause mechanical damage
what is the difference in a long-shjunt and short-shunt motor
-long in series with armature and series field
short- only in // with armature
what is the difference in a shunt field rheostat that is used for a motor or generator
series field connections are reversed
what is required for a DC motor with a power supply code “a”
pure dc (generator DC output or battery)
how could you reduce the base speed of a shunt motor?
increase field current
Can a series motor have a field winding installed in the motor?
yes in series with the armature
How can the voltage regulation of a compound motor be changed?
shunt field rheostat
what would be some advantages of parallelling DC generators
-share the load
-more efficient
-operatre at less than full rated value most of the time
-allow for backup
When 2 DC generators are paralleled, how can we move the load from one machine to the other?
-increase setting voltage on incoming generator so it picks up some load
-decreased voltage regular setting of generator so it will gives up some load
what is the voltage that is measured on the output of both paralled generators called?
bus voltage
what is the bus voltage is one machine is slowed down and the other remins at the same speed?
bus voltage would drop
what are the conditions required to parallel 2 DC generations
-series fields must be //
-same rated voltage
- polarities must be + to +, - to -
-equlaizer connnected to armature
when is an equalizer bus required?
-over, flat, under compounded generators.
