Dc Motor and Genretor

8/8/2019 Dc Motor and Genretor

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DC MACHINES

SAJID MEHMOOD ARAIN

09 EL 12

QUEST NAWABSHAH

PAKISTAN


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DC Motor The direct current (dc) machine can be used as

a motor or as a generator.

DC Machine is most often used for a motor. The major advantages of dc machines are theeasy speed and torque regulation.

However, their application is limited to mills,mines and trains. As examples, trolleys and

underground subway cars may use dc motors. In the past, automobiles were equipped with dc

dynamos to charge their batteries.


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DC Motor Even today the starter is a series dc motor

However, the recent development of power

electronics has reduced the use of dc motorsand generators.

The electronically controlled ac drives aregradually replacing the dc motor drives infactories.

Nevertheless, a large number of dc motors arestill used by industry and several thousand aresold annually.


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Construction


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DC Machine Construction

General arrangement of a dc machine


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DC Machines The stator of the dc motor has

poles, which are excited by dccurrent to produce magneticfields.

In the neutral zone, in the middlebetween the poles, commutatingpoles are placed to reducesparking of the commutator.The commutating poles aresupplied by dc current.

Compensating windings aremounted on the main poles.These short-circuited windingsdamp rotor oscillations. .


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DC Machines The poles are mounted on an

iron core that provides aclosed magnetic circuit.

The motor housing supportsthe iron core, the brushes andthe bearings.

The rotor has a ring-shapedlaminated iron core with slots.

Coils with several turns are

placed in the slots. Thedistance between the two legsof the coil is about 180 electricdegrees.


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DC Machines The coils are connected in series

through the commutatorsegments.

The ends of each coil are

connected to a commutatorsegment.

The commutator consists ofinsulated copper segmentsmounted on an insulated tube.

Two brushes are pressed to thecommutator to permit current

flow. The brushes are placed in theneutral zone, where the magneticfield is close to zero, to reducearcing.


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DC Machines The rotor has a ring-shaped

laminated iron core with slots.

The commutator consists of

insulated copper segmentsmounted on an insulated tube.

Two brushes are pressed tothe commutator to permitcurrent flow.

The brushes are placed in the

neutral zone, where themagnetic field is close to zero,to reduce arcing.


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DC Machines The commutator switches the

current from one rotor coil tothe adjacent coil,

The switching requires theinterruption of the coilcurrent.

The sudden interruption of aninductive current generateshigh voltages .

The high voltage producesflashover and arcing betweenthe commutator segment andthe brush.


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DC Machine Construction

|

Shaft

Brush

Copper

segment

Insulation

Rotor

Winding

N S

Ir_dc

Ir_dc

/2

Rotation

Ir_dc

/2

Ir_dc

12

3

4

5

6

7

8

Pole

winding

Commutator with the rotor coils connections.


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DC Motor Operation


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DC Motor Operation In a dc motor, the stator

poles are supplied by dcexcitation current, which

produces a dc magneticfield.

The rotor is supplied bydc current through thebrushes, commutatorand coils.

The interaction of themagnetic field and rotorcurrent generates a forcethat drives the motor

ha t

rush

opper

segment

Insulation

otor

Winding

S

Ir_dc

Ir_dc

/2

Rotation

Ir_dc

/2

Ir_dc

12

3

4

5

6

7

8

ole

inding


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DC Motor Operation

Before reaching the neutral zone,the current enters in segment 1andexits from segment 2,

Therefore, current enters the coilend at slot a and exits from slotbduring this stage.

After passing the neutral zone, thecurrent enters segment 2 and exitsfrom segment 1,

This reverses the current directionthrough the rotor coil, when the coil

passes the neutral zone. The result of this current reversal is

the maintenance of the rotation.

(a) Rotor current flow from segment 1 to 2(slot a to b)

Vdc

30NS

Bv

v

a

b

1

2

Ir_dc

(b) Rotor current flow from segment 2 to 1(slot b to a)

30NS V

dc

a

b

1

2

B

v v

Ir_dc


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DC GeneratorOperation


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DC Generator Operation

The N-S poles produce adc magnetic field and therotor coil turns in thisfield.

A turbine or othermachine drives the rotor.

The conductors in theslots cut the magnetic fluxlines, which induce

voltage in the rotor coils. The coil has two sides:

one is placed in slot a, theother in slot b.

30NS

Vdc

Bv

v

a

b

1

2

Ir_dc


30 NS Vdc

a

b

1

2

vv

B

Ir_dc



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In Figure 8.11A, theconductors in slot a arecutting the field linesentering into the rotorfrom the north pole,

The conductors in slot bare cutting the field linesexiting from the rotor tothe south pole.

The cutting of the field

lines generates voltage inthe conductors.

The voltages generated inthe two sides of the coilare added.

30NS

Vdc

Bv

v

a

b

1

2

Ir_dc


30 NS Vdc

a

b

1

2

vv

B

Ir_dc



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The induced voltage isconnected to the generatorterminals through thecommutator and brushes.

In Figure 8.11A, the induced

voltage in b is positive, and ina is negative.

The positive terminal isconnected to commutatorsegment 2 and to theconductors in slot b.

The negative terminal isconnected to segment 1 andto the conductors in slot a.

30NS

Vdc

Bv

v

a

b

1

2

Ir_dc


30 NS Vdc

a

b

1

2

vv

B

Ir_dc



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When the coil passes theneutral zone: Conductors in slot a are

then moving toward thesouth pole and cut flux linesexiting from the rotor

Conductors in slot b cut theflux lines entering the inslot b.

This changes the polarityof the induced voltage inthe coil.

The voltage induced in ais now positive, and in b isnegative.

30NS

Vdc

Bv

v

a

b

1

2

Ir_dc


30 NS Vdc

a

b

1

2

vv

B

Ir_dc



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The simultaneously thecommutator reverses itsterminals, which assuresthat the output voltage(V

dc

) polarity isunchanged.

In Figure 8.11B

the positive terminal isconnected to commutatorsegment 1 and to the

conductors in slot a. The negative terminal is

connected to segment 2 andto the conductors in slot b.

30NS

Vdc

Bv

v

a

b

1

2

Ir_dc


30 NS Vdc

a

b

1

2

vv

B

Ir_dc



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Generator


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DC Generator Equivalent circuit The magnetic field produced by the stator poles induces a

voltage in the rotor (or armature) coils when the

generator is rotated.

This induced voltage is represented by a voltage source.

The stator coil has resistance, which is connected in

series.

The pole flux is produced by the DC excitation/field

current, which is magnetically coupled to the rotor

The field circuit has resistance and a source

The voltage drop on the brushes represented by a battery


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DC Generator Equivalent circuit

Equivalent circuit of a separately excited dc generator.

Rf

Ra

Vbrush

VdcE

agV

f

*max

If

Iag

Load

Mechanical

power in

Electrical

power out


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DC Generator Equivalent circuit The magnetic field produced by the stator poles

induces a voltage in the rotor (or armature) coils

when the generator is rotated. The dc field current of the poles generates a

magnetic flux

The flux is proportional with the field current if

the iron core is not saturated:

fag IK1!*


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DC Generator Equivalent circuit The rotor conductors cut the field lines

that generate voltage in the coils.

The motor speed and flux equations are :

vBNE grag N2!

2

gDv [! ggag DBN!*


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DC Generator Equivalent circuit The combination of the three equation

results the induced voltage equation:

The equation is simplified.

[[[agrggr

g

grgrag NDBND

BNvBNE *!!

!! NNN

222

[[[ fmfragrag IKIKNNE !!*! 1


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DC Generator Equivalent circuit When the generator is loaded, the load current produces

a voltage drop on the rotor winding resistance.

In addition, there is a more or less constant 13 V voltagedrop on the brushes.

These two voltage drops reduce the terminal voltage ofthe generator. The terminal voltage is;

brushaagdcag VRIVE !


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Motor


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DC Motor Equivalent circuit

Figure 8.13 Equivalent circuit of a separately excited dc motor

Equivalent circuit is similar to the generator only the currentdirections are different

Rf

Ra

Vbrush

VdcE

am

Vf

*max

If

Iam

Mechanical

power out

Electrical

power in

DC Power

supply


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DC Motor Equivalent circuit The operation equations are:

Armature voltage equation

brushaamamdc IEV !

The induced voltage and motor speed vs angular

frequency

[fmam IKE !mnT[ 2!


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DC Motor Equivalent circuit The operation equations are:

The combination of the equations results in

The current is calculated from this equation.The output

power and torque are:

mamdcamfm RIVEI !![

amamout IEP ! fammout II

PT !!

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Dc Motor and Genretor