DC Drives WITH DC DC (1).ppt

DC – DC Converter Fed Drives
 To obtain variable DC voltage from fixed DC source
 Self-commutated devices preferred (MOSFETs, IGBTs,
GTOs) over thyristors
 Commutated by lower power control signal
 Commutation circuit not needed
 Can be switched at higher frequency for same rating
 Improved motor performance (less ripple, no discontinuous
currents, increased control bandwidth)
 Suitable for high performance applications
 Regenerative braking possible up to very low speeds even
when fed from fixed DC voltage source
1

Dr. Ungku Anisa, July 2008 EEEB283 - Electrical Machines & Drives 2

- Step Down Class A Chopper
Motoring
3
T
Q1
Q2
Q3 Q4


S is ON (0  t  ton)
4
Motoring
V
E
dt
di
L
i
R a
a
a
a 


Duty
Interval
- ia 

S if OFF (ton  t  T)
5
Motoring
0


 E
dt
di
L
i
R a
a
a
a
Freewheeling
Interval
- ia 

DC – DC Converter Fed
Motoring
 Duty cycle
 Under steady-state conditions
 Motor side:
 Chopper side:
 Hence,
6
period
chopper
where 
 T
T
t
k on
E
I
R
V
kV a
a
a 


a
a
R
E
kV
I


kT
Freewheeling
Interval
- ia 
Duty
Interval
- ia 
E
I
R
V a
a
a 

kV
Va 
average Va
average Ia

- Step Up Class B Chopper
Regenerative Braking
7
T
Q1
Q2
Q3 Q4

•Possible for speed
above rated speed
and down to nearly
zero speed
•Application:
• Battery operated
vehicles
• Regenerated
power stored in
battery

S is ON (0  t  ton)
8
Regenerative
Braking
Energy Storage
Interval
- ia 
 Va = 0
 ia increases due to E
 Mechanical energy
converted to
electrical (i.e.
generator)
 Energy stored in La
E
dt
di
L
i
R a
a
a
a 


S if OFF (ton  t  T)
9
Regenerative
Braking
Duty
Interval
- ia 
 ia flows through diode
D and source V
 Energy stored in La &
energy supplied by
machine are fed to
the source E
V
dt
di
L
i
R a
a
a
a 



Regenerative Braking
 Duty cycle
 Under steady-state conditions
 Generator side:
 Chopper side:
 Hence,
period
chopper
where 
 T
T
t
k on
  a
a
a I
R
E
V
V
k 



1
 
a
a
R
V
k
E
I



1
 T
Duty
Interval
- ia 
Energy Storage
Interval
- ia 
a
a
a I
R
E
V 

 V
k
Va 
 1
average Va
average Ia

- Two-quadrant Control
 Forward motoring Q1 - T1 and D2
 Forward braking Q2 – T2 and D1
11
D2
+
Va
-
T1
D1
T2
D2
+
V
-
T
Q1
Q2
Q3 Q4

No Speed
Reversal

 Forward motoring Q1
 T1 conducting: Va = V
 D2 conducting: Va = 0
T
Q1
Q2
Q3 Q4

T1
T2
D1
+
Va
-
D2
ia
+
V

T1
T2
D1
+
Va
-
D2
ia
+
V

•Average Va positive
•Average Va made larger
than back emf Eb
•Ia positive
Va Eb

 Forward braking Q2
 D1 conducting: Va = V
 T2 conducting: Va = 0
T
Q1
Q2
Q3 Q4

T1
T2
D1
+
Va
-
D2
ia
+
V
 T1
T2
D1
+
Va
-
D2
ia
+
Vdc

Va
Eb
•Average Va positive
•Average Va made
smaller than back emf Eb
•Ia negative
13

- Four-quadrant Control
 Operation in all quadrants
 Speed can be reversed
+ Va -
T1
D1
T2
D2
D3
D4
T3
T4
T
Q1
Q2
Q3 Q4

14

 Forward Motoring Q1
 T1 and T2 on
 Va = V
 Ia increases
 Reverse Braking Q4
(Regeneration)
 T1 off but T2 still on
 Va = 0
 Ia decays thru T2 and D4
 T1 and T2 off
 Va = -V
 Ia decays thru D3 and D4
 Energy returned to supply
+ Va -
T1
D1
T2
D2
D3
D4
T3
T4
T
Q1
Q2
Q3 Q4

+
V
-
T3 and T4 off
15

 Reverse Motoring Q3
 T3 and T4 on
 Va = -V
 Ia increases in reverse direction
 Forward Braking Q2
(Regeneration)
 T3 off but T4 still on
 Va = 0
 Ia decays thru T4 and D2
 T3 and T4 off
 Va = V
 Ia decays thru D1 and D2
 Energy returned to supply
+ Va -
T1
D1
T2
D2
D3
D4
T3
T4
T
Q1
Q2
Q3 Q4

+
V
-
T1 and T2 off
16

Closed-loop Control
 Feedback loops may be provided to satisfy one or more of the
following:
 Protection
 Enhancement of speed response
 Improve steady-state accuracy
 Variables to be controlled in drives:
 Torque – achieved by controlling current
 Speed
 Position
 Controllers are designed based on a linear averaged model
17

Closed-loop Control
 Torque – achieved by controlling current
 Commonly employed current sensor:
 Current shunt – no electrical isolation, cheap
 Hall effect sensor – provides electrical isolation
 Speed is governed by torque:
dt
d
J
T
T L
e



firing
circuit
current
controller
controlled
rectifier

+
Va
–
vc
iref
+
-
e.g. With phase-controlled rectifier

Closed-loop Control
 Speed – with or without current loop
 Commonly employed speed/position sensor:
 Tachogenerator – analog based
 Digital encoder – digital based, converts speed to pulses
 Torque is governed by speed demand:
 Without current loop: no limit on current – can be too high
 With current loop: current can be limited
19

Closed-loop Control
 Speed control without current loop:
 Simple implementation
 Current can be too high  may damage converter
20
Speed
controller
Power
Electronic
Converters
* +
-
+
va


vc
Tacho

Closed-loop Control
 Speed control with current loop:
 Two controllers required: speed and current
 Current limited by limiting ia*
21
Speed
controller
Power
Electronic
Converters
* +
-
+
va


vc
Tacho
Current
controller
ia*
ia
+
-

References
 Rashid, M.H, Power Electronics: Circuit, Devices and
Applictions, 3rd ed., Pearson, New-Jersey, 2004.
 Dubey, G.K., Fundamentals of Electric Drives, 2nd ed., Alpha
Science Int. Ltd., UK, 2001.
 Krishnan, R., Electric Motor Drives: Modeling, Analysis and
Control, Prentice-Hall, New Jersey, 2001.
 Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
 Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
22

DC Drives WITH DC DC (1).ppt

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