chapter 4 dc machine [autosaved]

Introduction to DC Machine

CHAPTER 4CHAPTER 4

DKT 213DKT 213

Contents

– Overview of Direct Current Machines– Construction– Principle of Operation– Types of DC Machine– Power Flow Diagram– Speed Control

LEARNING OBJECTIVES

• Upon completion of the chapter the student should be able to: – State the principle by which machines convert

mechanical energy to electrical energy. – Discuss the operating differences between different

types of generators– Understand the principle of DC generator as it

represents a logical behavior of dc motors.

Overview of Direct Current Machines

• Direct-current (DC) machines are divided into dc generators and dc motors.

• Most DC machines are similar to AC machines: i.e. they have AC voltages and current within them.

• DC machines have DC outputs just because they have a mechanism converting AC voltages to DC voltages at their terminals.

• This mechanism is called a commutator; therefore, DC machines are also called commutating machines.

• DC generators are not as common as they used to be, because direct current, when required, is mainly produced by electronic rectifiers.

• While dc motors are widely used, such automobile, aircraft, and portable electronics, in speed control applications…

DC Generator

• A dc generator is a machine that converts mechanical energy into electrical energy (dc voltage and current) by using the principle of magnetic induction.

• In this example, the ends of the wire loop have been connected to two slip rings mounted on the shaft, while brushes are used to carry the current from the loop to the outside of the circuit.

Principle of magnetic induction in DC machine

DC Motor• DC motors are everywhere! In a house, almost every mechanical

movement that you see around you is caused by an DC (direct current) motor.

• An dc motor is a machine that converts electrical energy into mechanical energy by supplying a dc power (voltage and current).

• An advantage of DC motors is that it is easy to control their speed in a wide diapason.

Cutaway view of a dc motor Stator with poles visible.

Construction of DC machine

segments

brushes


Rotor of a dc motor.


Stator: non-moving coilRotor: rotating part

Armature coil

BrushesBrushes

Rotor is the rotating part - armatureStator is the stationary part - field

ARMATURE• More loops of wire = higher rectified voltage• In practical, loops are generally placed in slots of an iron core• The iron acts as a magnetic conductor by providing a low-reluctance path for magnetic lines of flux to

increase the inductance of the loops and provide a higher induced voltage. • The commutator is connected to the slotted iron core. • The entire assembly of iron core, commutator, and windings is called the armature. • The windings of armatures are connected in different ways depending on the requirements of the

machine.

Loops of wire are wound around slot in a metal core DC machine armature

ARMATURE WINDINGS• Lap Wound Armatures

– are used in machines designed for low voltage and high current – armatures are constructed with large wire because of high current – Eg: - are used is in the starter motor of almost all automobiles – The windings of a lap wound armature are connected in parallel.

This permits the current capacity of each winding to be added and provides a higher operating current

– No of current path, A=P

ARMATURE WINDINGS (Cont)• Wave Wound Armatures

– are used in machines designed for high voltage and low current – their windings connected in series– When the windings are connected in series, the voltage of each

winding adds, but the current capacity remains the same– are used is in the small generator in hand-cranked megohmmeters– No of current path, A=2

FIELD WINDINGS

• Most DC machines use wound electromagnets to provide the magnetic field.

• Two types of field windings are used :

– series field– shunt field

FIELD WINDINGS (Cont)• Series field windings

– are so named because they are connected in series with the armature – are made with relatively few windings turns of very large wire and have a very low

resistance– usually found in large horsepower machines wound with square or rectangular wire. – The use of square wire permits the windings to be laid closer together, which

increases the number of turns that can be wound in a particular space

FIELD WINDINGS (Cont)

Square wire permits more turns than round wire in the same area

Square wire contains more surface than round wire

– Square and rectangular wire can also be made physically smaller than round wire and still contain the same surface area

FIELD WINDINGS (Cont)

• Shunt field windings – is constructed with relatively many turns of small wire, thus, it

has a much higher resistance than the series field. – is intended to be connected in parallel with, or shunt, the

armature. – high resistance is used to limit current flow through the field.

FIELD WINDINGS (Cont)• When a DC machine uses both series and shunt fields, each pole

piece will contain both windings.

• The windings are wound on the pole pieces in such a manner that

when current flows through the winding it will produce alternate magnetic polarities.

MACHINE WINDINGS OVERVIEW

Winding

LapA=P

WaveA=2

SeparatelyExcited

Frogleg

Self excited

armature field

series shunt compound

Principle operation of Generator

• Whenever a conductor is moved within a magnetic field in such a way that the conductor cuts across magnetic lines of flux, voltage is generated in the conductor.

• The AMOUNT of voltage generated depends on:i. the strength of the magnetic field,

ii. the angle at which the conductor cuts the magnetic field,

iii. the speed at which the conductor is moved, and

iv. the length of the conductor within the magnetic field

Principle of operation (Cont)

Fleming’s Right hand rule (Generator Rule)

• Use: To determine the direction of the induced emf/current of a conductor moving in a magnetic field.

• The POLARITY of the voltage depends on the direction of the magnetic lines of flux and the direction of movement of the conductor.

THE ELEMENTARY GENERATOR

• The simplest elementary generator that can be built is an ac generator.

• Basic generating principles are most easily easily explainedexplained through the use of the elementary ac generator.

• For this reason, the ac generator will be discussed first. The dc generator will be discussed later.

• An elementary generator consists of a wire loop consists of a wire loop mounted on the shaft, so that it can be rotated in mounted on the shaft, so that it can be rotated in a stationary magnetic fielda stationary magnetic field.

• This will produce an induced emf in the loopproduce an induced emf in the loop.

• Sliding contacts (brushes) connect the loop to an (brushes) connect the loop to an external circuit load in order to pick up or use the external circuit load in order to pick up or use the induced emf. induced emf.

Elementary Generator

• The pole pieces (marked N and S) provide the magnetic fieldprovide the magnetic field. The pole pieces are shaped and positioned as shown to concentrate the magnetic field as close as possible to the wire loop.

• The loop of wire that rotates through the field is called the The loop of wire that rotates through the field is called the ARMATUREARMATURE. The ends of the armature loop are connected to rings The ends of the armature loop are connected to rings called SLIP RINGScalled SLIP RINGS. They rotate with the armature.

• The brushes, usually made of carbon, with wires attached to them, ride against the rings. The generated voltage appears across these The generated voltage appears across these brushes. (brushes. (These brushes transfer power from the battery to the These brushes transfer power from the battery to the commutator as the motor spins – discussed later in dc elementary commutator as the motor spins – discussed later in dc elementary generatorgenerator).).

THE ELEMENTARY GENERATOR (Cont)

THE ELEMENTARY GENERATOR (A)

• An end view of the shaft and wire loop is shown.

• At this particular instant, the loop of wire (the black and white conductors of the loop) is parallel to the magnetic lines of flux, and no cutting action is taking place.

• Since the lines of flux are not being cut by the loop, no emf is induced in the conductors, and the meter at this position indicates zero.

• This position is called the NEUTRAL PLANE.

00 Position (Neutral Plane)

THE ELEMENTARY GENERATOR (B)

• The The shaft has been turned 90shaft has been turned 900 0 clockwise,clockwise, the the conductors cut through more and more lines of conductors cut through more and more lines of flux, and flux, and voltage is induced in the conductor.voltage is induced in the conductor.

• at a continually at a continually increasing angleincreasing angle , the induced , the induced emf in the conductors builds up from zero to a emf in the conductors builds up from zero to a maximum value or peak value. maximum value or peak value.

• Observe that from 0Observe that from 000 to 90 to 9000, the black conductor , the black conductor cuts DOWN through the field. cuts DOWN through the field.

• At the same time the white conductor cuts UP At the same time the white conductor cuts UP through the field. through the field.

• The induced emfs in the conductors are series-The induced emfs in the conductors are series-adding. adding.

• This means the resultant voltage across the This means the resultant voltage across the brushes (the terminal voltage) is the sum of the brushes (the terminal voltage) is the sum of the two induced voltages. two induced voltages.

• The meter at position B reads maximum value. The meter at position B reads maximum value. 900 Position

THE ELEMENTARY GENERATOR (C)

• After another 900 of rotation, the loop has completed 1800 of rotation and is again parallel to the lines of flux.

• As the loop was turned, the voltage decreased until it again reached zero.

• Note that : From 00 to 1800 the conductors of the armature loop have been moving in the same direction through the magnetic field.

• Therefore, the polarity of the induced voltage has remained the same

1800 Position

THE ELEMENTARY GENERATOR (D)• As the loop continues to turn, the

conductors again cut the lines of magnetic flux.

• This time, however, the conductor that previously cut through the flux lines of the south magnetic field is cutting the lines of the north magnetic field, and vice-versa.

• Since the conductors are cutting the flux lines of opposite magnetic polarity, the polarity of the induced voltage reverses.

• After 270' of rotation, the loop has rotated to the position shown, and the maximum terminal voltage will be the same as it was from A to C except that the polarity is reversed. 2700 Position

THE ELEMENTARY GENERATOR (A)

• After another 900 of rotation, the loop has completed one rotation of 3600 and returned to its starting position.

• The voltage decreased from its negative peak back to zero.

• Notice that the voltage produced in the armature is an alternating polarity. The voltage produced in all rotating armatures is alternating voltage.

3600 Position

Elementary Generator (Conclusion)• Observes

– The meter direction – The conductors of the armature loop

– Direction of the current flow

THE ELEMENTARY DC GENERATOR

• Since DC generators must produce DC current instead of AC current, a device must be used to change the AC voltage produced in the armature windings into DC voltage.

• This job is performed by the commutatorcommutator..

• The commutator is constructed from a copper ring split into segments with insulating material between the segments (See next page).

• Brushes riding against the commutator segments carry the power to the outside circuit.

• The commutator in a dc generator replaces the slip rings of the ac generator. This is the main difference in their construction.

• The commutator mechanically reverses the armature loop connections to the external circuit.

THE ELEMENTARY DC GENERATOR(Armature)

• The armature has an axle, and the commutator is attached to the axle.

• In the diagram to the right, you can see three different views of the same armature: front, front, side and end-on.side and end-on.

• In the end-on view, the winding is eliminated to make the commutator more obvious.

• We can see that the commutator is simply a simply a pair of plates attached to the axlepair of plates attached to the axle.

• These plates provide the two connections for These plates provide the two connections for the coil of the electromagnet. the coil of the electromagnet.

Armature with commutator view

THE ELEMENTARY DC GENERATOR(Commutator & Brushes work together)

• The diagram at the right shows how the commutator and how the commutator and brushes work together to let current flow to the brushes work together to let current flow to the electromagnetelectromagnet, and also to flip the direction that the electrons are flowing at just the right moment.

• The contacts of the commutator are attached to the axle The contacts of the commutator are attached to the axle of the electromagnet, so they spin with the magnet. of the electromagnet, so they spin with the magnet.

• The brushes are just two pieces of springy metal or The brushes are just two pieces of springy metal or carbon that make contact with the contacts of the carbon that make contact with the contacts of the commutator. commutator.

• Through this process the commutator changes the generated ac voltage to a pulsating dc voltage which also known as commutation process.

Brushes and commutator


• The loop is parallel to the magnetic lines of flux, and no voltage is induced in the loop

• Note that the brushes make contact with both of the commutator segments at this time. The position is called neutral plane.

00 Position (DC Neutral Plane)


• As the loop rotates, the conductors begin to cut through the magnetic lines of flux.

• The conductor cutting through the south magnetic field is connected to the positive brush, and the conductor cutting through the north magnetic field is connected to the negative brush.

• Since the loop is cutting lines of flux, a voltage is induced into the loop.

• After 900 of rotation, the voltage reaches its most positive point.

900 Position (DC)


• As the loop continues to rotate, the voltage decreases to zero.

• After 1800 of rotation, the conductors are again parallel to the lines of flux, and no voltage is induced in the loop.

• Note that the brushes again make contact with both segments of the commutator at the time when there is no induced voltage in the conductors

1800 Position (DC)


• During the next 900 of rotation, the conductors again cut through the magnetic lines of flux.

• This time, however, the conductor that previously cut through the south magnetic field is now cutting the flux lines of the north field, and vice-versa. .

• Since these conductors are cutting the lines of flux of opposite magnetic polarities, the polarity of induced voltage is different for each of the conductors. The commutator, however, maintains the correct polarity to each brush.

• The conductor cutting through the north magnetic field will always be connected to the negative brush, and the conductor cutting through the south field will always be connected to the positive brush.

• Since the polarity at the brushes has remained Since the polarity at the brushes has remained constant, the voltage will increase to its peak value in constant, the voltage will increase to its peak value in the same direction.the same direction.

2700 Position (DC)


• As the loop continues to rotate, the induced voltage again decreases to zero when the conductors become parallel to the magnetic lines of flux.

• Notice that during this 3600 rotation of the loop the polarity of voltage remained the same for both halves of the waveform. This is called rectified DC voltage.

• The voltage is pulsating. It does turn on and off, but it never reverses polarity. Since the polarity for each brush remains constant, the output voltage is DC.

00 Position (DC Neutral Plane)


• Observes– The meter direction– The conductors of the armature loop– Direction of the current flow

Effects of additional turns

• To increase the amount of output voltage, it is common practice to increase the number of turns of wire for each loop.

• If a loop contains 20 turns of wire, the induced voltage will be 20 times greater than that for a single-loop conductor.

• The reason for this is that each loop is connected in series with the other loops. Since the loops form a series path, the voltage induced in the loops will add.

• In this example, if each loop has an induced voltage of 2V, the total voltage for this winding would be 40V

(2V x 20 loops = 40 V).

Effects of additional turns

Effects of additional coils• When more than one loop is used, the average

output voltage is higher and there is less pulsation of the rectified voltage.

• Since there are four segments in the commutator, a new segment passes each brush every 900 instead of every 1800.

• Since there are now four commutator segments in the commutator and only two brushes, the voltage cannot fall any lower than at point A.

• Therefore, the ripple is limited to the rise and fall between points A and B on the graph. By adding more armature coils, the ripple effect can be further reduced. Decreasing ripple in this way increases the effective voltage of the output.

Effects of additional coils

The Practical DC Generator• The actual construction and operation of a practical

dc generator differs somewhat from our elementary generators

• Nearly all practical generators use electromagnetic poles instead of the permanent magnets used in our elementary generator

• The main advantages of using electromagnetic poles are:

(1) increased field strength and

(2) possible to control the strength of the fields. By varying the input voltage, the field strength is varied. By varying the

field strength, the output voltage of the generator can be controlled.

Four-pole generator (without armature)

DC Motor Operation

• In a dc motor, the stator poles are supplied by dc excitation current, which produces a dc magnetic field.

• The rotor is supplied by dc current through the brushes, commutator and coils.

• The interaction of the magnetic field and rotor current generates a force that drives the motor

DC Motor Operation

• The magnetic field lines enter into the rotor from the north pole (N) and exit toward the south pole (S).

• The poles generate a magnetic field that is perpendicular to the current carrying conductors.

• The interaction between the field and the current produces a Lorentz force,

• The force is perpendicular to both the magnetic field and conductor

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

Vdc30

NS

Bv

v

a

b

1

2

Ir_dc

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

30NS Vdc

a

b

B

v v

Ir_dc

DC Motor Operation

• The generated force turns the rotor until the coil reaches the neutral point between the poles.

• At this point, the magnetic field becomes practically zero together with the force.

• However, inertia drives the motor beyond the neutral zone where the direction of the magnetic field reverses.

• To avoid the reversal of the force direction, the commutator changes the current direction, which maintains the counterclockwise rotation.


Vdc30

NS

Bv

v

a

b

1

2

Ir_dc


30NS Vdc

a

b

B

v v

Ir_dc

DC Motor Operation

• Before reaching the neutral zone, the current enters in segment 1 and exits from segment 2,

• Therefore, current enters the coil end at slot a and exits from slot b during this stage.

• After passing the neutral zone, the current enters segment 2 and exits from segment 1,

• This reverses the current direction through the rotor coil, when the coil passes the neutral zone.

• The result of this current reversal is the maintenance of the rotation.


Vdc30

NS

Bv

v

a

b

1

2

Ir_dc


30NS Vdc

a

b

B

v v

Ir_dc

DC Machine 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


1. Permanent magnet2. Separately excited3. Self-excited


1. Permanent magnet• The poles are made of permanent magnets. • No field winding required.• Small size.• Disadvantage is low flux density, so low torque.


2. Separately excited The field flux is derived from a separate power source

independent of the generator itself.

B

Field winding

Armature winding


3. Self-excited • Shunt machine

The field flux is derived by connecting the field directly across the terminals of the generator.

B


3. Self-excited

Series machine• field are connected in

series with armature

B


3. Self-excited • Cumulatively compounded

• Differentially compounded

BB

BB


3. Self-excited

Compounded dc generator• both a shunt and a series field are present


Compounded dc motor• both a shunt and a series

field are present

3. Self-excited

Equivalent circuit of a DC motor

The armature circuit (the entire rotor structure) is represented by an ideal voltage source EA and a resistor RA. A battery Vbrush in the opposite to a current flow in the machine direction indicates brush voltage drop.The field coils producing the magnetic flux are represented by inductor LF and resistor RF. The resistor Radj represents an external variable resistor (sometimes lumped together with the field coil resistance) used to control the amount of current in the field circuit.

The armature is represented by an ideal voltage source EA and a resistor RA.

The brush voltage drop is represented by a small battery Vbrush opposing the direction of the current flow in the machine.

The field coils, which produce the magnetic flux, are represented by inductor LF and RF.

The separate resistor Radj represents an external variable resistor used to control the amount of current in the field circuit.

DC Motor Equivalent Circuit.

Equivalent Circuit of a DC Motor.

The brush drop voltage is often only a very tiny fraction of the generated voltage in the motor.

Therefore, in cases where it is not critical, the brush drop voltage may be left out or approximately included in the value of RA.

Also, the internal resistance of the filed coils is sometimes lumped together with the variable resistor, and the total is called RF , Figure below.

A Simplified Equivalent Circuit eliminating the Brush Voltage Drop and Combining Radj with the Field Resistance .

The Equivalent Circuit of Separately Excited dc Motor.

From the above figure,

Motor types: Separately Excited DC motors.

F

FF R

VI

AAAT RIEV

AL II

Separately excited DC motor:a field circuit is supplied from a separate constant voltage power source.

The Equivalent Circuit of a Shunt dc Motor.

From the above figure,

F

FF R

VI

AAAT RIEV

FAL III

Motor types: Shunt DC motors.

Shunt DC motor:a field circuit gets its power from the armature terminals of the motor.

Motor types: The permanent-magnet DC motor

A permanent magnet DC (PMDC) motor is a motor whose poles are made out of permanent magnets.

Advantages:1. Since no external field circuit is needed, there are no field circuit copper

losses;2. Since no field windings are needed, these motors can be considerable

smaller.

Disadvantages:

1. Since permanent magnets produces weaker flux densities then externally supported shunt fields, such motors have lower induced torque.

2. There is always a risk of demagnetization from extensive heating or from armature reaction effects (via armature mmf).

Motor types: The series DC motor

A series DC motor is a DC motor whose field windings consists of a relatively few turns connected in series with armature circuit. Therefore:

SAAAT RRIEV

Motor types: Compounded DC motor

A compounded DC motor is a motor with both a shunt and a series field.

Long-shunt connection

Short-shunt connection

Current flowing into a dotted end of a coil (shunt or series) produces a positive mmf.

If current flows into the dotted ends of both coils, the resulting mmfs add to produce a larger total mmf – cumulative compounding.

If current flows into the dotted end of one coil and out of the dotted end of another coil, the resulting mmfs subtract – differential compounding.

Motor types: Compounded DC motor

The Kirchhoff’s voltage law equation for a compounded DC motor is

The currents in a compounded DC motor are

The mmf of a compounded DC motor: Cumulatively compounded

Differentially compoundedThe effective shunt field current in a compounded DC motor:

Number of turns

(5.85.1)

(5.85.2)

(5.85.3)

(5.85.4)

(5.85.5)

SAAAT RRIEV

FLA III F

TF R

VI

ARSEFnet FFFF

F

ARA

F

SEFF N

FI

N

NII *

Torque Equation

T = torque of armature (N-m)

kA = geometry constant

= flux/pole (Wb)

IA = armature current (A)

AA IkT

Geometry Constant

p = number of field poles

N = number of active conductors on armature

M = number of parallel paths in armature winding (=p for lap winding, =2 for wave winding)

)(60

),/(2

' rpmM

pNksrad

M

pNk AA

Power Equation

P=power (W) – not counting lossesE = EMF induced in armature (back EMF)IA = armature current (A)T = torque of armature (N-m) = speed of rotation (rad/s)

Note that Pin = VLIL which will be higher than P because of loss in the field and armature windings as well as rotational (friction) losses.

TEIP A

EMF Equation

E = EMF induced in armature (V)

kA = geometry constant

= flux/pole (Wb)

= speed of rotation (rad/s)

n = speed of rotation of armature (rpm)

nkkE AA '

2

60n

Terminal Voltage Equation

VT = voltage at motor terminals

E = EMF induced in armature (V)

IA = armature current (A)

RA = armature resistance

AAT RIEV E VT

+

-

+

-

RA

Speed Equation

(applies to shunt connected motor only)

Note that can also be written as kfIf where kf is

/If (normally a constant ratio)

'A

AAT

k

RIVn

Ratio Equation

1

2

1

2

E

E

n

n

Speed-Torque

Torque

Speed

Series

Shunt

Cumulative Compound

Differential Compound

Power flow and losses in DC machines

Unfortunately, not all electrical power is converted to mechanical power by a motor and not all mechanical power is converted to electrical power by a generator…

The efficiency of a DC machine is:

or

%100xP

PP

in

lossin

%100xP

P

in

out

The losses in DC machines

There are five categories of losses occurring in DC machines.

1. Electrical or copper losses – the resistive losses in the armature and field windings of the machine.

Armature loss:

Field loss:

Where IA and IF are armature and field currents and RA and RF are armature and field (winding) resistances usually measured at normal operating temperature.

AAA RIP 2

FFF RIP 2


2. Brush (drop) losses – the power lost across the contact potential at the brushes of the machine.

Where IA is the armature current and VBD is the brush voltage drop. The voltage drop across the set of brushes is approximately constant over a large range of armature currents and it is usually assumed to be about 2 V.

Other losses are exactly the same as in AC machines…

ABDBD IVP


4. Mechanical losses – losses associated with mechanical effects: friction (friction of the bearings) and windage (friction between the moving parts of the machine and the air inside the casing). These losses vary as the cube of rotation speed n3.

3. Core losses – hysteresis losses and eddy current losses. They vary as B2 (square of flux density) and as n1.5 (speed of rotation of the magnetic field).

5. Stray (Miscellaneous) losses – losses that cannot be classified in any of the previous categories. They are usually due to inaccuracies in modeling. For many machines, stray losses are assumed as 1% of full load.

The power-flow diagram

On of the most convenient technique to account for power losses in a machine is the power-flow diagram.

For a DC motor:

Electrical power is input to the machine, and the electrical and brush losses must be subtracted. The remaining power is ideally converted from electrical to mechanical form at the point labeled as Pconv.

The power-flow diagram

The electrical power that is converted is

And the resulting mechanical power is

After the power is converted to mechanical form, the stray losses, mechanical losses, and core losses are subtracted, and the remaining mechanical power is output to the load.

AAconv IEP

mindconvP

Example 1

A 6 pole, 3.0 hp 120V DC lap-wound shunt motor has 960 conductors in the armature. It takes 25.0 A from the supply at full load. Armature resistance is 0.75, flux/pole=10.0 mWb, field winding current is 1.20A. Find the speed and torque.

AAAIII FLA 8.232.125

VAVRIVE AAT 10275.08.23120

153

62

9606

2

MpN

K A

sradx

V

K

E

KE

A

A

/9.661010153

1023

rpmn 6382

60

kWhp

WhpP 24.2

7463

mNsrad

kWPT 5.33

/9.66

24.2

Example 2

A 10hp, 115V Dc series motor takes 40A at its full load speed of 1800rpm. What is the torque at 30A?

srad

n/188

60

18002

60

2

kWhp

WhpP 46.7

74610

mNsrad

kWPT

TP

6.39/188

46.7

AFFAAA IIKKIKT

AF II 2

AFA IKKT

025.0

40

6.3922

A

mN

I

TKK

A

FA

mNAIKKTnewAFAnew 2.2230025.0 22

Example 3 (a)

A 220V DC shunt motor draws 10A at 1800rpm. The armature resistance is 0.2 and field winding resistance is 440.

(a) What is the torque?

AV

R

VI

F

TF 5.0

440

220

AAAIII FLA 5.95.010

VAVRIVE AAT 2182.05.9220

kWAVEIP A 07.25.9218

srad

n/188

60

18002

60

2

mNsrad

kWPT 0.11

/188

07.2

Example 3 (b)

A 220V DC shunt motor draws 10A at 1800rpm. The armature resistance is 0.2 and field winding resistance is 440.

(b) What will be the speed and line current at a torque of 20 N-m (if field current is constant)?

16.1/188

218

srad

VEK

KE

A

A

AmN

K

TI

IKT

AA

AA

3.1716.1

20

AAAIII FAL 8.175.03.17

VVRIVE AAT 2172.03.17220

rpmxn

sradV

K

E

A

31079.12

60

/18716.1

217

(shunt is constant speed)

chapter 4 dc machine [autosaved]

Education

dc motor dc motors

dc generators

dc voltages

dc machinesare

dc outputs

dc power voltage

principle of dc generator

dc motor stator