lecture dc machines compatibility mode

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DC Machines

DC Machines

A DC Machine

Armature along with

the commutator

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Significant Features of DC Machines

• Conventional DC generators are being replaced by the

solid state rectifiers where ac supply is available.

• The same is not true for dc motors because of

– Constant mechanical power output or constant torque

– Rapid acceleration or deceleration

– Responsiveness to feedback signals

• 1W to 10,000 hp

• Applications – in electric vehicles to extend their range

and reduce vehicle weight, in steel and aluminum rolling

mills, traction motors, electric trains, overhead cranes,

control devices, etc.

Introduction

Electromagnetic Energy Conversion:

1. When armature conductors move in a magnetic field produced

by the current in stator field winding, voltage is induced in the

armature conductors.

2. When current carrying armature conductors are placed in a

magnetic field produced by the current in stator field winding,

the armature conductors experience a mechanical force.

These two effects occur simultaneously in a DC machine

whenever energy conversion takes place from electrical to

mechanical or vice versa.

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Constructional Features of DC Machines

• Commutator along with the armature on the rotor

• Salient-pole on the stator and, except for a few smaller machines, commutating poles between the main poles.

• Field windings (as many as 4):

– Two fields that act in a corrective capacity to combact the detrimental effects of armature reaction, called the commutating (compole or interpole) and compensating windings, which are connected in series with the armature.

– Two normal exciting field windings, the shunt and series windings

Schematic Connection Diagram of a DC Machine

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Equivalent Circuit of a DC Machine

aaat

fff

RIEV

RIV

±±±±====

====

Ia_gen

If

Vf VtRf

+

- Ea

+

-

Ia_mot

Ra

+

Ia

If

VtRf

Ea

-

IL

Ra

+

+

-

Generated emf and Electromagnetic Torque

aaat

fff

RIEV

RIV

±±±±====

====

mdaa KE ωωωωφφφφ====

adae IKT φφφφ====

meaaem TIEP ωωωω========

Voltage generated in the armature circuit due the flux of the stator field current

Electromagnetic torque

Ka: design constant

Motor: Vt> EaGenerator: Vt > Ea

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Comparison between the Shunt and Series Connected DC Machines

Speed Control in DC Motors

Shunt motor:

Electromagnetic torque is Te=Ka φd Ia, and the conductor emf is Ea=Vt - RaIa.

For armature voltage control: Ra and If are constant

For field control: Ra and Vt are constant

For armature resistance control: Vt and If are constant

(((( ))))221 etm TKVK −−−−====ωωωω

(((( ))))(((( ))))3

2 e

ff

a

ff

tm T

IK

R

IK

V−−−−====ωωωω

(((( ))))(((( ))))1

2da

ae

da

tm

ada

etmda

K

RT

K

V

RK

TVK

φφφφ−−−−

φφφφ====ωωωω

φφφφ−−−−====ωωωωφφφφ

(((( ))))(((( ))))4

2 e

da

adja

da

tm T

K

RR

K

V

φφφφ

++++−−−−


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Speed Control in Shunt DC Motors

Armature Voltage Control:

Ra and If are kept constant and the armature

terminal voltage is varied to change the motor

speed.

For constant load torque, such as applied by an

elevator or hoist crane load, the speed will

change linearly with Vt. In an actual

application, when the speed is changed by

varying the terminal voltage, the armature

current is kept constant. This method can also

be applied to series motor.

(((( )))).constis;

KK;

KK

TKVK

d

dada

etm

φφφφφφφφ

====φφφφ

====

−−−−====ωωωω

221

21

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Field Control:

Ra and Vt are kept constant, field rheostat is varied to

change the field current.

For no-load condition, Te=0. So, no-load speed varies

inversely with the field current.

Speed control from zero to base speed is usually

obtained by armature voltage control. Speed control

beyond the base speed is obtained by decreasing the field

current. If armature current is not to exceed its rated

value (heating limit), speed control beyond the base

speed is restricted to constant power, known as constant

power application.


mm

aae

meaaat

.constIET

TIEconstIVP

ωωωω====

ωωωω====

ωωωω================

(((( )))) e

ff

a

ff

tm T

IK

R

IK

V2

−−−−====ωωωω

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Armature Resistance Control:

Vt and If are kept constant at their rated value,

armature resistance is varied.

The value of Radj can be adjusted to obtain

various speed such that the armature current Ia(hence torque, Te=KaφdIa) remains constant.

Armature resistance control is simple to

implement. However, this method is less

efficient because of loss in Radj. This resistance

should also been designed to carry armature

current. It is therefore more expensive than the

rheostat used in the field control method.


(((( )))) ee

da

adja

da

tm TKKT

K

RR

K

V652

−−−−====φφφφ

++++−−−−


Armature Voltage Control:

A variable dc voltage can be applied to a series motor to

control its speed. A variable dc voltage can be obtained

from a power electronic converter.

Torque in a series motor can be expressed as

Speed Control in Series DC Motors

(((( ))))[[[[ ]]]]

sae

t

sa

sa

sae

tm

samsa

tsa

asaadae

KKT

V

KK

RR

KKT

V,or

RRKK

VKK

IKKIKT

≈≈≈≈++++

−−−−====ωωωω

++++++++ωωωω====

====φφφφ====

2

2

2

(((( ))))(((( ))))

(((( )))) (((( ))))

samsa

ta

saamasa

saamda

saaat

asd

RRKK

VI

RRIIKK

RRIK

RRIEV

IK

++++++++ωωωω====

++++++++ωωωω====

++++++++ωωωωφφφφ====

++++++++====

====φφφφ

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Field Control:

Control of field flux in a sries motor is achieved by

using a diverter resistance.

The developed torque can be expressed as.


ds

dsa

aads

dsaadae

RR

RandKKK,where

IKIRR

RKKIKT

++++====ρρρρ====

ρρρρ====

++++====φφφφ==== 22

(((( )))) (((( ))))(((( ))))

asm

ta

aasm

aasmasa

aasamda

aaads

dsat

RRK

VI,or

IRRK

IRRIKK

RIRIK

RIIRR

RREV

++++ρρρρ++++ρωρωρωρω====

++++ρρρρ++++ρωρωρωρω====

++++ρρρρ++++ωωωωρρρρ====

++++ρρρρ++++ωωωωφφφφ====

++++

++++++++====


2

++++ρρρρ++++ρωρωρωρωρρρρ====

asm

te

RRK

VKT

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Armature Resistance Control:

Torque in this case can be expressed as

Rae is an external resistance connected in series with

the armature.

For a given supply voltage and a constant developed

torque, the term (Ra+Rae+Rs+Kωm) should remain

constant. Therefore, an increase in Rae must be

accompanied by a corresponding decrease in ωm.


(((( ))))

K

RRR

KT

V,or

VT

KKRRR,or

T

KVKRRR

sadja

e

tm

te

msadja

e

tmsadja

++++++++−−−−====ωωωω

====ωωωω++++++++++++

====ωωωω++++++++++++2

2

(((( ))))22

msadja

te

KRRR

KVT

ωωωω++++++++++++====

Power Division in DC Machines

Input from

prime-mover

Elec-magnetic

Power =EaIa

Arm. terminal

power = Vta Ia

Output power

= Vt IL

No-load rotational loss (friction

+windage+core)+stray load loss

Arm. copper loss

Ia2Ra+brush contact loss

Series field loss IL2Rs

+shunt field loss If2Rf

Input power from

mains =Vt IL

Elec-magnetic

Power =EaIa

Arm. terminal

power = Vta Ia

Output available

at the shaft

No-load rotational loss (friction

+windage+core)+stray load loss

Arm. copper loss

Ia2Ra+brush contact loss

Series field loss IL2Rs

+shunt field loss If2Rf

DC Motor

DC Generator

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Efficiency

InputPower

Losses

InputPower

LossesInputPower

InputPower

OutputPower

−−−−====

−−−−====

====ηηηη

1

The losses are made up of rotational losses (3-15%), armature

circuit copper losses (3-6%), and shunt field copper loss (1-5%).

The voltage drop between the brush and commutator is 2V and

the brush contact loss is therefore calculated as 2Ia.

DC Machines Formulas

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Problem 9-1 to 9-7 (Page 621)

Solution to Problem 9-1 (Page 621)

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Problem 9-13 (Page 623)


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lecture dc machines compatibility mode

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