SOLID STATE DRIVES
UNIT-II
CONVERTER / CHOPPER FED DC MOTOR
1
Contents
 Review of dc motor equivalent circuit

DC motor speed control

Converters used in dc motor drives
2
3
4
5
SCR “phase-angle controlled” DC drives
•
•
•
•
By changing the firing angle, variable DC output voltage can
be obtained.
Single phase (low power) and three phase (high and very
high power) supply can be used
The line current is unidirectional, but the output voltage can
reverse polarity. Hence 2- quadrant operation is inherently
possible.
4-quadrant is also possible using “two sets” of controlled
rectifiers.
6
Single phase Full converter fed Dc drive
(a)
(b)
(c )
Fig. (a) 1-PHASE THYRISTOR BRIDGE WITH R-L-E LOAD
(b) CONTINUOUS CONDUCTION RECTIFICATION (Mode-A)
(c ) DISCONTINUOUS CONDUCTION RECTIFICATION (Mode-B)
7
(a)
(b)
Fig. (a) CONTINUOUS CONDUCTION INVERSION MODE (Mode-C)
(b) DISCONTINUOUS CONDUCTION INVERSION MODE (Mode-D)
8
Vd 
2

Vm cos   Vc  I d R.....(1), Vc  K m  K ' N ...(2), Te  KI d ...(3)
TYPICAL TORQUE-SPEED CURVES OF DC MOTOR WITH 1PHASE BRIDGE CONVERTER
9
Three phase converter fed DC drive
Fig. THREE-PHASE BRIDGE CONVERTER WITH DC MOTOR LOAD
10
v
va

vb
Q1
vc
Q3
Q5
vd
0
t

Q6
Q2
(a)
Q4
60°
vd
0
iQ
iQ
iQ
3
1
t
(b)
5
iQ
Id
0
iQ
iQ
6
iQ
2
t
(c)
4
iQ
Id
0
ia
0
t
120°
(d)
30 + 
120°
60°
Id
t
(e)
ib
0
iL
0
t
(f)
Id
Id
t
60°
(g)
60°
60°
Fig. 3-PHASE THYRISTOR BRIDGE WAVEFORMS IN
RECTIFICATION MODE ( = 40) (Mode-A)
11

Q4
Q6
va
v
Q2
vb
vc
vd
0
t
Q3
Q5
Q1
Q3

vd

3
0
vba
t
vca
vcb
vab
iQ
iQ
vac
vbc
iQ
iQ
5
3
1
0
Id
iQ
iQ
4
iQ
t
iQ
6
2
0
t
va
i1
ia
t

Fig. PHASE THYRISTOR BRIDGE WAVEFORMS IN
INVERTING MODE ( = 150) (Mode – C)
12
Vab
Vac
Vbc
Vba
Vca
Vd
Vc
(a)
id
 
0


Q1Q6

t
2
t
Q1Q2

Q 1Q 6
Q 1Q 2
id
0

 /3
(b)
Vc
vbc
vba
vca
vcb
vab
vac
Fig. WAVEFORMS OF THREE-PHASE THYRISTOR BRIDGE CONVERTER AT DISCONTINUOUS
CONDUCTION
(UPPER) RECTIFICATION MODE (Mode-B), (LOWER) INVERSION MODE(Mode-D)
13
Fig. TYPICAL TORQUE-SPEED CURVES OF DC MOTOR WITH
THREE-PHASE THYRISTOR BRIDGE CONVERTER
14
Chopper fed DC drives
Supply is DC (maybe from rectified-filtered AC, or some other
DC sources).
 DC-DC converters (coppers) are used.
 suitable for applications requiring position control or fast
response, for example in servo applications, robotics, etc.
 Normally operate at high frequency
– the average output voltage response is significantly faster
– the armature current ripple is relatively less than the controlled
rectifier
 In terms of quadrant of operations, 3 possible configurations
are possible:
– single quadrant,
– two–quadrant and four–quadrant

15
16
17
18
References
G.K. Dubey, ‘Power semi-conductor controlled drives’,
prentice hall of india,1989.
19