Lecture PPT

1. Direct Current Motor Drives:

2. DC DRIVES Vs AC DRIVES
DC drives:
Advantage in control unit
Disadvantage in motor
AC Drives:
Advantage in motor
Disadvantage in control unit

3. • DC DRIVES: Electric drives that use DC motors as the
prime movers
• Dominates variable speed applications before
PE converters were introduced
• DC motor: industry workhorse for decades
• Will AC drive replaces DC drive ?
– Predicted 30 years ago
– AC will eventually replace DC – at a slow rate
– DC strong presence – easy control – huge numbers
DC DRIVES

4. DC Motors
• Limitations:
• Advantage: simple torque and speed control
without sophisticated electronics
• Regular Maintenance • Expensive motor
• Heavy motor • Sparking

5. General Torque Equation
Translational (linear) motion:
dt
d
J
T


Rotational motion:
dt
dv
M
F 
F : Force (Nm)
M : Mass (Kg )
v : velocity (m/s)
T : Torque (Nm)
J : Moment of Inertia (Kgm2 )
 : angular velocity ( rad/s )

6. Torque Equation: Motor drives
dt
d
J
T
T
or
dt
d
J
T
T L
e
L
e






0

 L
e T
T Acceleration
0

 L
e T
T Deceleration
0

 L
e T
T Constant speed
Te : motor torque (Nm) TL : Load torque (Nm)

7. …continue
Drive accelerates or decelerates depending on
whether Te is greater or less than TL
During acceleration, motor must supply not only
the load torque but also dynamic torque, ( Jd/dt ).
During deceleration, the dynamic torque, ( Jd/dt ), has
a negative sign. Therefore, it assists the motor
torque, Te.

8. Torque Equation: Graphical
Te
Forward
running
Speed
Forward
braking
Reverse
acc.
Reverse
running
Reverse
braking
Forward
acc.

9. Load Torque
Load torque, TL, is complex, depending on applications.
SPEED
TORQUE
TL = k2
TL = k
TL = k
In general:

10. 4Q OPERATION
SPEED
TORQUE
I
III
II
IV

Te

Te

Te 
Te
FM
FB
RM
RB
F: FORWARD R: REVERSE M : MOTORING B: BRAKING

11. 4Q OPERATION: LIFT SYSTEM
Counterweight Cage
Motor
Positive speed
Negative torque

12. 4Q OPERATION: LIFT SYSTEM
Convention:
Upward motion of the cage: Positive speed
Weight of the empty cage < Counterweight
Weight of the full-loaded cage > Counterweight
Principle:
What causes the motion?
Motor : motoring P =T = +ve
Load (counterweight) : braking P =T = -ve

13. You are at 10th floor, calling
fully-loaded cage from gnd floor
You are at gnd floor, calling
empty cage from 10th floor
You are at 10th floor, calling
empty cage from gnd floor
You are at gnd floor, calling
Fully-loaded cage from 10th floor
Torque
Speed
FM
FB
RM RB

14. DC MOTOR DRIVES
Principle of operation
Torque-speed characteristic
Methods of speed control
Armature voltage control
Variable voltage source
Phase-controlled Rectifier
Switch-mode converter (Chopper)
1Q-Converter
2Q-Converter
4Q-Converter

15. Current in
Current out
Stator: field
windings
Rotor: armature
windings
Principle of Operation
DC Motors

16. Equivalent circuit of DC motor
a
t i
k
Te 
 Electromagnetic torque

 E
a k
e Armature back e.m.f.
Lf Rf
if
a
a
a
a
t e
dt
di
L
i
R
v 


+
ea
_
La
Ra
ia
+
Vt
_
+
Vf
_
dt
di
L
i
R
v f
f
f
f



17. Torque-speed characteristics
a
a
a
a E
I
R
V 

In steady state,
  


T
a
e
T
a
k
V
T
k
R


 2
Therefore speed is given by,
Three possible methods of speed control:
Armature resistance Ra
Field flux F
Armature voltage Va
a
a
a
a
a e
dt
di
L
i
R
V 


Armature circuit:

18. Torque-speed characteristics of DC motor
Torque
Speed
Maximum
load
Torque
No load speed
Separately excited DC motors have good
speed regulation.
Full load speed

19. DC Motor Speed Control
Torque
Speed
Maximum
Torque
By Changing Ra
Ra increasing
• Power loss in Ra
• Does not maintain maximum torque capability
• Poor speed regulation

20. DC Motor Speed Control
Torque
Speed
Maximum
Torque
Flux Decreasing
Trated
• Slow transient response
• Does not maintain maximum torque capability
By Decreasing Flux

21. DC Motor Speed Control
Torque
Speed
Maximum
Torque
By Changing Armature voltage
Trated
Va increasing
• good speed regulation
• maintain maximum torque capability

22. Speed control of DC Motors
Below base speed: Armature voltage control (retain maximum torque
capability)
Above base speed: Field weakening (i.e. flux reduced) (Trading-off torque
capability for speed)
Torque
speed
Line of
Maximum
Torque Limitation
Armature voltage control
Field flux control
base

23. Methods of Armature Voltage Control
Phase-controlled rectifier (AC–DC)
T
Q1
Q2
Q3 Q4

3-phase Or
1-phase
supply
+
Vt

ia

24. Methods of Armature Voltage Control
1. Ward-Leonard Scheme
2. Phase-controlled rectifier (AC–DC)
3. Switch-Mode Converter (Chopper) (DC–DC)

25. Phase-controlled rectifier: 4Q Operation
Q1
Q2
Q3 Q4

T
1 or 3-
phase
supply
1 or 3-
phase
supply
+
Vt

Methods of Armature Voltage Control

26. Phase-controlled rectifier : 4Q Operation
Q1
Q2
Q3 Q4

T
F1
F2
R1
R2
+ Va -
3-phase
supply
AN ALTERNATIVE WAY

27. Switch–mode converters: 1Q Converter
Q1
Q2
Q3 Q4

T
+
Vt
-
T1
Converters For DC motor Drives

28. Switch–mode converters: 2Q Converter
+
Vt
-
T1
D1
T2
D2
Q1
Q2
Q3 Q4

T
Q1  T1 and D2
Q2  D1 and T2
Converters For DC motor Drives

29. Switch–mode converters: 4Q Converter
Q1
Q2
Q3 Q4

T
+ Vt -
T1
D1
T2
D2
D3
D4
T3
T4
Converters For DC motor Drives

30. Switch–mode converters
• Switching at high frequency
 Reduces current ripple
 Increases control bandwidth
• Suitable for high performance applications
Advantages of Switch mode converters