EE2352 SOLID STATE DRIVES

UNIT-II

CONVERTER / CHOPPER FED DC MOTOR

Mr. R. Essaki raj

Senior Lecturer

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 Contents

 Review of dc motor equivalent circuit

 DC motor speed control

 Converters used in dc motor drives

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 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.

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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)

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 (a)

(b)

Fig. (a) CONTINUOUS CONDUCTION INVERSION MODE (Mode-C)

(b) DISCONTINUOUS CONDUCTION INVERSION MODE (Mode-D)

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 2
Vd  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 1-
PHASE BRIDGE CONVERTER

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 Three phase converter fed DC drive

Fig. THREE-PHASE BRIDGE CONVERTER WITH DC MOTOR LOAD

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 v
 va vb vc
Q1 Q3 Q5

vd
0 (a)
t

Q6  Q2 Q4

vd 60°

0 (b)
t
iQ 1
iQ
3
iQ
5

iQ
Id
0 (c)
t
iQ 6
iQ 2
iQ 4

iQ
Id
0 (d)
120° t
30 + 
ia

0 (e)
120° 60° Id t

ib

0 (f)
t

iL Id
Id
0 (g)
t
60° 60°
60°

Fig. 3-PHASE THYRISTOR BRIDGE WAVEFORMS IN

RECTIFICATION MODE ( = 40 ) (Mode-A)

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 
Q4 va Q6 vb Q2 vc
v
vd
0
t

Q3 Q5 Q1 Q3


vd

0  t
3

vba vca vcb vab vac vbc

iQ iQ iQ
iQ 5 1 3

0
Id t

iQ iQ iQ
iQ 4 6 2

0
t
va
i1
ia

t

Fig. PHASE THYRISTOR BRIDGE WAVEFORMS IN

INVERTING MODE ( = 150 ) (Mode – C)

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 Vab Vac Vbc Vba Vca
Vd

(a) Vc

id

  t
0  
Q1Q6
 Q1Q2

 Q 1Q 6 Q 1Q 2
id

0 t
2

 /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)

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Fig. TYPICAL TORQUE-SPEED CURVES OF DC MOTOR WITH
THREE-PHASE THYRISTOR BRIDGE CONVERTER

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 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

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References

G.K. Dubey, ‘Power semi-conductor controlled drives’,

prentice hall of india,1989.

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