1

Equivalent Circuit of a Separately-Excited

DC Motor

2

f

f f f f

aa a a a g

g v f

d t f a

d L

div R i L

dt

div R i L e

dt

e K i

T K i i

dT J B T

dt

3

0

f f f

g v f

a a a g

a a a v f

d t f a

d L

d d

Steady state

d

dt

V R I

E K I

V R I E

V R I K I

T K I I

T B T

P T

4

a a a v f

a a a a a a

v f f

v

f

V R I K I

V I R V I R

K I VK

R

Solve for the motor speed

5

Control of the motor speed

• Control armature

voltage, Va

– Voltage control

• Control the field

current, If

– Current control

• Control the armature

current, Ia

a a a

f

v

f

V I R

VK

R

6

Equivalent circuit of a DC Series Motor

7

( )

( )

( )

g v a

a a f a g

a a f a v f

d t a f

d L

a a f a

v f

E K I

V R R I E

V R R I K I

T K I I

T B T

V R R I

K I

8

Controlling the Motor Speed

• Control the armature

voltage, Va

• Control the armature

current, Ia

( )

( )

( )

g v a

a a f a g

a a f a v f

d t a f

d L

a a f a

v f

E K I

V R R I E

V R R I K I

T K I I

T B T

V R R I

K I

Single phase semi-controlled converter fed

dc separately excited motor

9

10

vOVm

0

iO

t

( )

t0

( )

11

12

13

Speed-Torque Characteristics

14

Single phase fully controlled converter fed

dc separately excited motor

15

16

17

18

19

Speed-Torque Characteristics

20

DC Series motor drive

21

22

Single phase semi-controlled

converter fed dc series motor

23

24

25

26

27

28

Speed-torque characteristics

29

Single phase Fully controlled

converter fed dc series motor

30

31

32

33

Speed-Torque Characteristics

34

Three phase Half controlled converter

connected to separately excited DC motor

35

36

ECE 442 Power Electronics 37

38

ECE 442 Power Electronics 39

40

Three phase fully controlled converter

connected to separately excited DC motor

41

ECE 442 Power Electronics 42

Where VL is rms value line voltage

43

Torque Speed Characteristic 44

DC series motor connected to 3 phase

half controlled bridge rectifier

45

46

ECE 442 Power Electronics 47

Equivalent Circuit

48

Speed torque characteristics

DC series motor connected to 3 phase

fully controlled bridge rectifier

49

ECE 442 Power Electronics 50

Equivalent circuit under steady state

51

Under steady state condition the following equations hold good

52

Torque speed curves for different firing angles

53