chapter2-2 types of dc motor

8/12/2019 Chapter2-2 Types of Dc Motor

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DC Motor - 2

Muhamad Zahim

Ext : 2312

BEE2123

ELECTRICAL MACHINES


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Contents

Overview of Direct Current Machines

Construction Principle of Operation

Types of DC Motor

Power Flow Diagram

Speed Control


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DC motor principles

DC motors consist of rotor-mountedwindings (armature) and stationarywindings (field poles). In all DC motors,

except permanent magnet motors,current must be conducted to thearmature windings by passing currentthrough carbon brushes that slide overa set of copper surfaces called acommutator, which is mounted on the

rotor. Parts of an electric motor

The commutator bars are soldered to armature coils. The brush/commutatorcombination makes a sliding switch that energizes particular portions of thearmature, based on the position of the rotor. This process creates north andsouth magnetic poles on the rotor that areattracted to or repelled by northand south poles on the stator, which are formed by passing direct current

through the field windings. It's this magnetic attraction and repulsion thatcauses the rotor to rotate.


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The Advantages

The greatest advantage of DC motors may be speed

control. Since speed is directly proportional to armature

voltage and inversely proportional to the magnetic flux

produced by the poles, adjusting the armature voltage

and/or the field current will change the rotor speed.

Today, adjustable frequency drives can provide precise

speed control for AC motors, but they do so at the

expense of power quality, as the solid-state switching

devices in the drives produce a rich harmonic spectrum.

The DC motor has no adverse effects on power quality.


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The drawbacks

Power supply, initial cost, and maintenancerequirements are the negatives associated with DC

motors Rectification must be provided for any DC motors

supplied from the grid. It can also cause power qualityproblems.

The construction of a DC motor is considerably morecomplicated and expensive than that of an AC motor,primarily due to the commutator, brushes, and armaturewindings. An induction motor requires no commutator orbrushes, and most use cast squirrel-cage rotor bars

instead of true windings two huge simplifications.


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Major types of dc motors

Self excited dc motor Series dc motor

Shunt dc motor

Compound dc motor

Separately excited dc motor

Permanent magnet dc motor


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Series motors

Series motors connect

the field windings in

series with thearmature.

Series motors lack good

speed regulation, but

are well-suited for high-

torque loads like powertools and automobile

starters because of their

high torque production

and compact size.

Ea

Rf

M VT (dc

supply)

Raia

)( faaaT RRiEV

La iinote :

aa IKKE 21


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Series Motor Power Flow Diagram

P

Pout

Pin= VTiL

Pca=ia2Ra

Pcf=ia2Rf

Pm

P is normally given

Pin= Pout+ total losses

Where,

Pca =armature copper loss

Pcf =field copper loss

P=stray, mech etc

in

out

mm

oo

P

PEf f iciency

N

Ptorquemechanicalfor

N

Ptorqueloadoutputfor

N

P

,

2

60,

2

60,/

2

60

Pm= Eaia


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Series Motor (cont)

Example 1:

A dc machine in Figure 1 is

consumed a 6.5kW when the12.5 A of armature current ispassing thru the armature andfield resistance of 3.3and 2.0respectively. Assume straylosses of 1.2kW. Calculate

a) terminal voltage, VTb) back emf, Ea

c) net torque if the speed is at3560rpm

d) efficiency of the machine

[520V, 453.75V, 12N-m, 68.8%]

Ea

Rf

M VT (dc

supply)

Raia

Figure 1


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Series Motor (cont)

Example 2:

A 600V 150-hp dc machine inFigure 2 operates at its full rated

load at 600rpm. The armature andfield resistance are 0.12and0.04respectively. The machinedraws 200A at full load. Assumestray losses 1700W. Determine

a) the armature back emf at full load,

Eab) developed/mechanical power anddeveloped/mechanical torque

c) assume that a change in loadresults in the line current droppingto 150A. Find the new speed inrpm and new developed torque.

{Hint: Ea=K1K2ia}

Ea

Rf

M VT (dc

supply)

Raia

Figure 2

[568V, 113.6kW, 1808Nm, 811.27rpm, 1017Nm]


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Shunt motors

Shunt motors use high-resistance field windingsconnected in parallel with the

armature. Varying the field resistance

changes the motor speed.

Shunt motors are prone toarmature reaction, a distortionand weakening of the fluxgenerated by the poles that

results in commutationproblems evidenced by sparkingat the brushes.

Installing additional poles, calledinterpoles, on the statorbetween the main poles wired inseries with the armature

reduces armature reaction.

Ea VT (dc

supply)

Raia

if

RfM

iL

)( aaaT RiEV

ffT

faL

RiV

iiinote

:


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Shunt Motor (power flow diagram)

PPout

Pin=VTiL

Pca=ia2Ra

Pcf=if2Rf

Pm

P is normally given


Where,



P=stray, mech etc

in

out

mm

oo

P

PEf f iciency

N


NPtorqueloadoutputfor

N

P

,

2

60,

260,/

2

60

Pm= Eaia


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Shunt Motor

Example :

A voltage of 230V is applied to armature of amachines results in a full load armature currents

of 205A. Assume that armature resistance is

0.2. Find the back emf, net power and torque by

assuming the rotational losses are 1445W at full

load speed of 1750rpm.

[189V, 37.3kW, 203.5Nm]


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Compound motors

the concept of the

series and shunt

designs arecombined.

Ea VT (dc

supply)

Raia

if

Rf1M

iLRf2

)(2faaaT

RRiEV

1

:

ffT

faL

RiV

iiinote


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Compound motor (power flow diagram)

P is normally given


Where,



P=stray, mech etc

in

out

mm

oo

P

PEf f iciency

N


NPtorqueloadoutputfor

N

P

,

2

60,

260,/

2

60

Pm= Eaia

PPoutPin=VTiL

Pca=ia2Ra

Pcf1=if2Rf1

Pm

Pcf2=ia2Rf2


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Separately Excited Motor

There is no direct connection between the

armature and field winding resistance

DC field current is supplied by an

independent source

(such as battery or another generator or prime

movercalled an exciter)


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Separately Excited Motor (Cont)

nKniKC

pnZ

E fffa 60

2

fff RiV

aaaT RiEV

Where p= no of pole pair

n= speed (rpm)

Z=no of conductor

=Flux per pole (Wb)C= no of current/parallel path

=2p (lap winding)

=2 (wave winding)

KVL:

Circuit analysis:

La iinote :

Ea

Ra Laia

M

Rf

VTVfLf

If


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Permanent Magnet motors

PMDC is a dc motor whose poles are made of

permanent magnets.

Do not require external field circuit, no copper losses

No field winding, size smaller than other types dc

motors

Disadvantage: cannot produce high flux density,

lower induce voltage


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Speed Control for shunt motor and

separately excited dc motor

Torquespeed characteristic for shunt and separately

excited dc motor

a

ff

a

ff

a

a

a

aa

aa

R

nIK

R

IVK

excitedseparatelyassame

nE

REV

n

IE

IEtorqueDeveloped

22

,

2

2

,

22

a

ff

R

IVKc

2

Starting

torque

a

ff

R

nIK

slope 2

22

nNLnn

m

=0

n=0


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By referring to the Torquespeed characteristic for shunt andseparately excited dc motor

note that, there are three variables that can influence the speed of

the motor, VIf

Ra Thus, there are three methods of controlling the speed of the

shunt and separately excited dc motor,

i. Armature terminalvoltage speed control

ii. Field speed control

iii. Armature resistance speed control

a

ff

a

ff

RnIK

RIVK

22

22

Variables


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i. Armature resistance speed control

- Speed may be controlled by changing Ra

- The total resistance of armature may be varied by means of a

rheostat in series with the armature

- The armature speed control rheostat also serves as a starting

resistor.

- From -n characteristic,

a

ff

a

ff

start

R

nIKslope

R

IVKc

2

2

22 Will be changed


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Torquespeed characteristic

Ra1

nNL nn1

m

Ra2

Ra3

Ra1 < Ra2< Ra3

n2n3


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Advantages armature resistance speed control:

i. Starting and speed control functions may be combined in one

rheostat

ii. The speed range begins at zero speed

iii. The cost is much less than other system that permit control

down to zero speed

iv. Simple method

Disadvantages armature resistance speed control :

i. Introduce more power loss in rheostat

ii. Speed regulation is poor (S.R difference nLoaded& nno loaded)

iii. Low efficiency due to rheostat


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ii. Field Speed Control

- Rheostat in series with field winding (shunt or separately ect.)

- If field current, If is varied, hence flux is also varied

- Not suitable for series field

- Refer to -n characteristic,

- Slope and nNLwill be changed


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If1 < If2< If3

1< 2< 3

nNL3 nn1

m

n2 n3 nNL2

nNL1Base speed


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Advantages field speed control:

i. Allows for controlling at or above the base speed

ii. The cost of the rheostat is cheaper because Ifis small value

Disadvantages field speed control :

i. Speed regulation is poor (S.R difference nLoaded& nno loaded)

ii. At high speed, flux is small, thus causes the speed of themachines becomes unstable

iii. At high speed also, the machines is unstable mechanically,

thus there is an upper speed limit


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iii. Armature terminal voltage speed control

- Use power electronics controller

- AC supplyrectifier

- DC supplychopper

- Supply voltage to the armature is controlled

- Constant speed regulation

- From -n characteristic,- C and nNLwill be change

- Slope constant


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nNL1nn1

m

V3< V2< V1

n2n3 nNL2

nNL3


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Advantages armature terminal voltage speed control:

i. Does not change the speed regulation

ii. Speed is easily controlled from zero to maximum safe speed

Disadvantages armature terminal voltage speed

control :

i. Cost is higher because of using power electronic controller


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FACTORS AFFECTING THE

PERFORMANCE OF DC MACHINE

There are two factors affecting the

performance of dc machine

1. Armature reaction

2. Armature inductance


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Armature Reaction

Definition of armature reaction:1. It is the term used to describe the effects of the armature

mmf on the operation of a dc machineas a "generator" nomatter whether it is a generator or motor.

2. It effects both the flux distribution and the flux magnitude inthe machine.

3. The distortion of the flux in a machineis called armaturereaction

Two effects of armature reaction:1. Neutral Plane Shift

2. Flux Weakening


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Armature Reaction

Effect on flux distribution:Neutral plane shift

When currentis flowing inthe field winding, hence aflux is producedacross themachine which flows from

the North pole to the Southpole.

Initially the pole fluxisuniformly distributed andthe magnetic neutral planeis vertical


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Armature Reaction

Effect on flux distribution:

Neutral plane shift

effect by the air gapon the

flux fieldcauses the

distribution of flux is no longer

uniform across the rotor.

There are two points on the

periphery of the rotor where

B= 0.


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Armature Reaction

Effect on flux distribution:Neutralplane shift

when a load connected to themachines a resulting magneticfield produced in the armature

If the armatureis rotated at aspeed by an external torqueeach armature coil experiences

a change in flux t as itrotates.

A voltage is generatedacrossthe terminals of each windingaccording to the equation e =t


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Armature Reaction

Effect on flux distribution:Neutral plane shift

Both rotor and pole fluxes (fluxproduced by the field windingand the flux produced by thearmature winding) are addedand subtracted togetheraccordingly

The fields interact to produce adifferent flux distribution in therotor.

Thus, the flux on the middleline, between the two field

poles, is no longer zero.


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Armature Reaction

Effect on flux distribution:Neutral planeshift

The combined flux in the machinehasthe effect of strengthening or weakeningthe flux in the pole.Neutral axis istherefore shifted in the direction ofmotion.

The result is current flow circulatingbetween the shorted segments and largesparks at the brushes. The ending resultis arcing and sparking at the brushes.

Solution to this problem:

placing an additional poles on theneutral axis or mid-point that willproduce flux density component,which counter-acts that produced bythe armature.


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Armature Reaction

Effect on flux magnitude:FluxWeakening

Most machine operate at saturationpoint

When the armature reaction happen,at location pole surface:

The add of rotor mmf to pole mmfonly make a small increase in flux

The subtract of rotor mmf frompole mmf make a large decreasein flux.

The result is the total average fluxunder entire pole face isdecreased.

This is called Flux Weakening

dflux decrease under subtracting section of poles


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Armature Inductance

When rotor turns, thus we have inductance value, e1= L(di/dt). Let say current ia1.

That means, we have ability to store energy If the machine is turn off, thus, e1will decreased.

This will affect the current as well. Say ia2. When the machine is turn on again, it will produce

e2while e1is still inside. The current now is reverseddirection from previous (decreasing) current.

Thus, it will cause sparking resulting the sameaching problem caused by neutral plane shift.

chapter2-2 types of dc motor

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