Chapter 2 - DC Motor - Slides

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Chapter 2

TET411 – Power Electronics & Drive

Semester 1 – 2011/2012

E.A. Rafiqi, PPKSE 2011



TET411 Semester 1 - 2011/20122

2.1 Introduction of DC Motor

The DC motors are popular because the staring torque of DC

machine is large, some of them can be used with either AC orDC supply & they are cheap, simpleto operate & control.

The main components of theDC machine are: field circuit,armature circuit, commutator& brushes.

The field is normally anelectric magnet fed by a DC power source. In small machines,the field is energized by permanent magnet.

N

S

+

Rotor

Brush

Armaturewindings

V DC

Stator



TET411 Semester 1 - 2011/20123

2.1 Introduction of DC Motor (cont’d)

The armature circuit is composed of the windings, commutator

& brushes. The windings & the commutator are mounted onthe rotor shaft which will rotate together with the shaft.

The brushes are mounted on the stator & are stationary but

they are in contact with therotating commutator segments.

The brushes allow thecommutator segments to be

connected to an externalDC source.

+

Rotor

Brush

Armaturewindings

V DC



TET411 Semester 1 - 2011/20124


The rotor windings are composed of several coils; each has two

(2) terminals connected to the commutator segments on theopposite sides.

The commutator segments are electrically isolated from one

another. The segments areexposed & the brushes touchtwo (2) opposing segments.

+

Rotor

Brush

Armaturewindings

V DC



N

S

+

Rotor

Brush

Armaturewindings

φ

V DC

Stator

TET411 Semester 1 - 2011/20125


The stator field produces flux, φ from the north, N pole to the

south, S pole. The brushes touch the terminals of the rotor coilunder the pole.

When the brushes are connected to an

external DC source of potential V ,a current, I enters the terminalof the rotor coil under the N pole& exits from the terminal underthe S pole.



TET411 Semester 1 - 2011/20126


The presence of the stator flux & rotor current produces a

force, F on the coil known as the Lorentz force.

This force produces torque that rotatesthe armature counterclockwise.

The coil that carries the currentmoves away from the brush &is disconnected from theexternal source.

The next coil moves under thebrush & this produces a continuousforce & continuous rotation.

N

S

+

ω

Rotor

Brush

Armaturewindings

φ

F

F

V DC

Stator


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The DC motors can be classified into four (4) groups which

are:

1. Seperately-excited – the field winding is composed of alarge number of turns with small cross-section wire. This

type of field winding is designed to withstand the rated voltage of the motor. The field & armature circuits areexcited by separate sources.

2. Shunt – the field circuit is the same as that for seperately-

excited machines but the field winding is connected inparallel with the armature circuit. A common source is usedfor the field & armature windings.




3. Series – the field winding is composed of a small numberof turns with a large cross-section wire. This type isdesigned to carry large currents & is connected in series

with the armature winding.

4. Compound – this type uses the shunt & series windings.



2.2 Separately-excited Motor

The equivalent circuit of a separately-excited motor which

consists of field & armature circuit is shown below.

The field circuit is mounted on the stator & is energized by aseparate DC source, V f . The field current is:

In small motor, the field circuitis a permanent magnet. Thusthe flux of the field is constant& cannot be adjusted.

E a

RaI f

I a

R f V f

V t

f

f

f R

V I



2.2 Separately-excited Motor (cont’d)

The armature circuit is mounted on the rotor & is composed of

a rotor winding & commutator segments.

The external voltage source, V t is connected to the armature todrive the load via the commutator segments & brushes.

Relative to the field circuit, the armature carriesa much higher current. Thus, the armatureresistance, Ra is much smaller thanthe field resistance, R f .

The field voltage is usually inthe same order of magnitude asthe armature voltage.

E a

RaI f

I a

R f V f

V t




The armature current, I a is:

The developed power, P d is:

The back-emf, E a is:

The developed torque, T d is:

By substituting I a & E a into T d equation, we get:

a

a t a

R

EV I

d a a d T I E P

K E a

a d I K T

a

t

d R

K V K T




Thus, the speed equation, ω is:

At no-load operation, assuming therotational losses can be ignored, thedeveloped torque, T d & I a is 0, whichmakes the no-load speed is:

d

a t T K

R

K

V 2

K V t

0

Operating point

ω0

TorqueT m

Speed

Load

ω




At starting, assuming the motor is initially at rest, the motor

speed is 0. Thus, the starting torque, T st & starting current, I st are:

From the T st equation, T st is proportional to the source voltage.If V t is at rated value, T st is very large. Hence, this feature ishighly desirable when starting under heavy loading conditions.

However, I st is also proportional to the source voltage & whenV t is at rated value during starting, I st will be large & mightdamage the motor windings.

a

t st

R

V K T

a

t st

R

V I



Example 1:

A DC , separately-excited motor has the following data:

K φ = 3 Vsec V t = 600 V Ra = 2 Ω

I a = 5 A at full load

Calculate the rated torque, T d , starting torque, T st & startingcurrent , I st at full voltage.

Sketch the speed vs. current characteristic of the motor.




Example 2:

Using the same separately-excited motor from the previousexample, if the starting current must be limited to six (6) timesthe rated value, calculate the new source voltage, V t. Sketch thespeed-current characteristic of the motor.



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Example 3:

Using the same separately-excited motor from the 1st example,if the starting current must be limited to six (6) times the rated

value, calculate the value of added resistance, Radd . Sketch thespeed-current characteristic of the motor.


Chapter 2 - DC Motor - Slides

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