DC machine review sheets Flashcards by Paige Petracek

Motors covert ________ energy into _________

-electrical into mechanical

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starting torque of series DC motors is commonly _________ of the full load torque

450%

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Starting torque of shunt DC motors is commonly ____________ to _____________ % of the full load torque

125%-200%

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Generator effect of a DC motor produces a _____________

counter emf

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Counter EMF limits armature current as acts as a _______________ for DC motors

-speed governor

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As load is added to the motor, the armature _________________________

current increases

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Torque is _________________ to armature current

proportional

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Torque in DC motors is produced from interaction between _______________ in the poles and the _______________

-magnetic fields
-armature

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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

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____________ the field of a DC motor will cause the RPM to ___________ above base speed

-weakening
-increase

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A DC shunt motor that is not connected to a load will _______________ if the field is opened.

-overspeed

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Voltage reduction to the armature of a DC shunt motor will cause the RPM to __________ below base speed

-decrease

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The standard direction of rotation for DC motors is _______________ as viewed from opposite the drive end.

opposite

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to change the direction of a DC motor, change the _______________

armature leads only

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Speed of a series motor is controlled by the load, at no load a series motor will ______________

overspeed

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Generators convert ___________ energy into ________________ energy

mechanical into electrical

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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

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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

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As load is added to a generator, an _________ in torque from the prime mover is required

increase

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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

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Voltage drop in the armature of a generator is caused by ___________________ through the resistance of the armature windings (E=IxR)

load current flowing

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When the load is added to a shunt generator, the output terminal voltage _____________ due to the voltage drop in the armature

drops

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on a self-exited generator, the voltage will not build up if the direction of rotation is __________________________ unless the armature leads are ___________________

-reversed
-reversed

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the output voltage characteristics of generators are classified as:

over-compound
2.flat-compounded
3.under-compounded

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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

1. 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?

1. polarity of flux 2. direction of the cutting

What do generators convert into electrical energy

mechanical energy

Why must a frame have a low reluctance path?

1. So the frame is thicker then needed 2. completes the magnetic path/cct

Describe field poles

1. iron pole peices that are often laminated to reduce eddy currents 2. can have 2-24 poles 3. 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?

1. 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.