Lecture DC Machines

dcmotor 1

Basics of a Electric Motor

DC Machines

http://www.wisc-online.com/objects/ViewObject.aspx?ID=IAU9508

http://www.youtube.com/watch?v=RAc1RYilugI&feature=player_embedded

dcmotor 5

A Two Pole DC Motor

dcmotor 6

A Four Pole DC Motor

dcmotor 7

Armature of a DC Motor

DC Machines

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.

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

Armature Reaction

If a load is connected to the terminals of the dc

machine, a current will flow in its armature windings.

This current flow will produce a magnetic field of its

own, which will distort the original magnetic field from

the machines field poles. This distortion of the magnetic

flux in a machine as the load is increased is called the

armature reaction.

16

Armature Reaction(AR)

AR is the magnetic field produced by the

armature current

AR aids the main flux in one half of the

pole and opposes the main flux in the

other half of the pole

The net effect is the reduction of the field current

17

Minimizing Armature Reaction

Since AR reduces main flux, voltage in

generators and torque in motors reduces

with it. This is particularly objectionable

in steel rolling mills that require sudden

torque increase.

Compensating windings put on pole

faces can effectively negate the effect

of AR. These windings are connected

in series with armature winding.

18

Minimizing commutation problemsSmooth transfer of current during

commutation is hampered by

a) coil inductance and

b) voltage due to AR flux in the interpolar

axis. This voltage is called reactance

voltage.

Can be minimized using interpoles. They

produce an opposing field that cancels

out the AR in the interpolar region. Thus

this winding is also connected in series

with the armature winding.

Note: The UVic lab motors have

interpoles in them. This should be

connected in series with the armature

winding for experiments.

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

adae IKT

meaaem TIEP

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

Electromagnetic torque

Ka: design constant

Motor: Vt > EaGenerator: Vt > Ea

21

Magnetization Curve

maa KE

Flux is a non-linear

function of field current and

hence Ea is a non-linear

function of field current

For a given value of flux Eais directly proportional to

m

Types of DC Machines

Both the armature and field circuits carry direct current in the case

of a DC machine.

Types:

Self-excited DC machine: when a machine supplies its own

excitation of the field windings. In this machine, residual

magnetism must be present in the ferromagnetic circuit of the

machine in order to start the self-excitation process.

Separately-excited DC machine: The field windings may be

separately excited from an eternal DC source.

Shunt Machine: armature and field circuits are connected in

parallel. Shunt generator can be separately-excited or self-excited.

Series Machine: armature and field circuits are connected in series.

Separately-Excited and Self-Excited DC Generators

IfIL

If

DC Supply VtRf

+

- Ea

+

-

Ra

+

Ia

VtRf

Ea

-

IL

Ra

+

Separately-Excited Self-Excited

24

Shunt Field Coil Armature

RA

Shunt Excited DC Machine

25

Series Field Coil

Armature

RA

Series Excited DC Machine

26


RA

Compound Excited DC Machine

Series Field Coil

If the shunt and series field aid each other it is called a cumulatively

excited machine

If the shunt and series field oppose each other it is called a differentially

excited machine

27

Field CoilArmature

R

a

Vf

Separately Excited DC Generator

+

-

Rf

Vt

+

-

Field equation: Vf=RfIf

If

Ia

+

-

Ea

Armature equation: Vt=Ea-IaRa

Vt=IaRL, Ea=Kam

RL

28

Shunt Generators

Field equation: Vt=Rf If

Rf=Rfw+Rfc

Armature equation: Vt=Ea-Ia Ra

Vt=(Ia If) RL, Ea=Kam


R

a

RL

If Ia Ia If

Vt

+

-

Rfc

Ea

+

-Field coil has Rfw :

Implicit field resistance

Example 1

A 100-kW, 250-V DC shunt generator has an

armature resistance of 0.05 W and field circuit

resistance of 60 W. With the generator operating at

rated voltage, determine the induced voltage at (a) full

load, and (b) half-full load.

Solution to Example 1(a) At full load,

Vt=Ea-IaRaIf=250/60=4.17 A

IL_FL=100,000/250=400 A

Ia=IL_FL+If=400+4.17=404.17 A

Ea=Vt+IaRa=250+404.17*0.05=270

.2 V

(b) At half load,

If=250/60=4.17 A

IL_HL=50,000/250=200

A

Ia=IL_HL+If=200+4.17=2

04.17 A

Ea=Vt+IaRa=250+204.17

*0.05=260.2 V

Armature

R

a

RL

If Ia Ia If

+

-

Rfc

Ea

+

-

Example 2

Example 2-solution

Field CoilArmature

R

a

Vf+

-

Rf

Vt

+

-

If

Ia

+

-

Ea

RL

DC Generator Characteristics

In general, three characteristics specify the steady-state

performance of a DC generators:

1. Open-circuit characteristics: generated voltage versus field

current at constant speed.

2. External characteristic: terminal voltage versus load current

at constant speed.

3. Load characteristic: terminal voltage versus field current at

constant armature current and speed.


Open-circuit and load characteristics

The terminal voltage of a dc

generator is given by

aa

mf

aaat

RI

dropreactionArmatureIf

RIEV

,


It can be seen from the external

characteristics that the terminal

voltage falls slightly as the load

current increases. Voltage regulation

is defined as the percentage change

in terminal voltage when full load is

removed, so that from the external

characteristics,

External characteristics100

V

VEregulationVoltage

t

ta

Self-Excited DC Shunt Generator

Schematic diagram of connection

Open-circuit characteristic

Maximum permissible value of the field

resistance if the terminal voltage has to build up.

Comparison between the Shunt and Series Connected DC Motor

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

Efficiency

InputPower

Losses

InputPower

LossesInputPower

InputPower

OutputPower

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

Example 3

Example 3-solution

Problem 9-1 to 9-5

Solution to Problem 9-1 (Page 621)

Problem 9-13 (Page 623)

The End

Lecture DC Machines

Documents

Transcript of Lecture DC Machines