1. GENERAL ELECTRICAL DRIVES
What is electrical drives?
Components of electrical drives
Advantages of electrical drives
DC drives Vs AC drives
Torque equations
Load torque profiles
Four quadrant operation

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
DC motor: industry workhorse for decades
Dominates variable speed applications before PE converters were introduced
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

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

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

6. Torque Equation: Motor drives
Te : motor torque (Nm)
TL : Load torque (Nm)
Acceleration
Deceleration
Constant speed

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, ( Jdw/dt ).
During deceleration, the dynamic torque, ( Jdw/dt ), has
a negative sign. Therefore, it assists the motor
torque, Te.

8. Torque Equation: GraphicalTe
Speed
Forward
running
Forward
braking
Reverse
acc.
Reverse
running
Reverse
braking
Forward
acc.

9. Load Torque Load torque, TL, is complex, depending on applications.
In general:
TORQUE
TL = k
TL = kw
TL = kw2
SPEED

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

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

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 =Tw = +ve
Load (counterweight) : braking P =Tw = -ve

13. 4Q OPERATION: LIFT SYSTEM
Speed
You are at 10th floor, calling
empty cage from gnd floor
You are at 10th floor, calling
fully-loaded cage from gnd floor FB FM
Torque RM RB
You are at gnd floor, calling
empty cage from 10th floor
You are at gnd floor, calling
Fully-loaded cage from 10th floor

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

16. Equivalent circuit of DC motorLf Rf if + ea _ La Ra ia Vt Vf dt di L i R v f =
Electromagnetic torque
Armature back e.m.f.

17. Torque-speed characteristics
Armature circuit:
In steady state,
Therefore speed is given by,
Three possible methods of speed control:
Armature resistance Ra
Field flux F
Armature voltage Va

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

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

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

21. DC Motor Speed Control By Changing Armature voltage Va increasing
Maximum
Torque
Torque
Trated
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
Armature voltage control
Field flux control
Line of
Maximum
Torque Limitation
speed
base

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

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

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

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

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

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

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

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