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
DC DRIVES Vs AC DRIVES
DC drives:
Advantage in control unit
Disadvantage in motor
AC Drives:
Advantage in motor
Disadvantage in control unit
DC DRIVES
• 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
– DC strong presence – easy control – huge numbers
– AC will eventually replace DC – at a slow rate
DC Motors
• Advantage: simple torque and speed control
without sophisticated electronics
• Limitations:
• Regular Maintenance
• Expensive motor
• Heavy motor
• Sparking
General Torque Equation
Translational (linear) motion:
dv
F M
dt
F : Force (Nm)
M : Mass (Kg )
v : velocity (m/s)
Rotational motion:
d
TJ
dt
T : Torque (Nm)
J : Moment of Inertia (Kgm2 )
 : angular velocity ( rad/s )
Torque Equation: Motor drives
d
d
Te  TL  J
or Te  TL  J
dt
dt
Te : motor torque (Nm)
TL : Load torque (Nm)
Te  TL  0
Acceleration
Te  TL  0
Deceleration
Te  TL  0
Constant speed
…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.
Torque Equation: Graphical
Te
Speed
Forward Forward Reverse Reverse
running braking acc.
running
Reverse Forward
braking acc.
Load Torque
Load torque, TL, is complex, depending on applications.
In general:
TORQUE
TL = k
TL = k
TL = k2
SPEED
4Q OPERATION
F: FORWARD R: REVERSE M : MOTORING B: BRAKING
Te
SPEED

FB
Te
II

FM
I
TORQUE
RM
Te

III
IV
Te

RB
4Q OPERATION: LIFT SYSTEM
Positive speed
Negative torque
Motor
Counterweight
Cage
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
4Q OPERATION: LIFT SYSTEM
Speed
You are at 10th floor, calling
empty cage from gnd floor
FB
You are at 10th floor, calling
fully-loaded cage from gnd floor
FM
Torque
RM
You are at gnd floor, calling
empty cage from 10th floor
RB
You are at gnd floor, calling
Fully-loaded cage from 10th floor
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
Principle of Operation
DC Motors
Stator: field
windings
Rotor: armature
windings
Current in
Current out
Equivalent circuit of DC motor
Ra
+
Lf
La
ia
+
Rf
if
+
Vt
ea
Vf
_
_
_
di
v t  R a ia  L a  ea
dt
Te  ktia
ea  k E 
v f  R f if  L
Electromagnetic torque
Armature back e.m.f.
di f
dt
Torque-speed characteristics
Armature circuit:
dia
Va  Ra ia  L
 ea
dt
In steady state,
Va  Ra I a  Ea
Therefore speed is given by,
Ra
Va

T 
2 e
kT 
kT 
Three possible methods of speed control:
Armature resistance Ra
Field flux F
Armature voltage Va
Torque-speed characteristics of DC motor
Speed
No load speed
Full load speed
Maximum
load
Torque
Torque
Separately excited DC motors have good
speed regulation.
DC Motor Speed Control
Speed
By Changing Ra
Ra increasing
Maximum
Torque
Torque
• Power loss in Ra
• Does not maintain maximum torque capability
• Poor speed regulation
DC Motor Speed Control
Speed
By Decreasing Flux
Flux Decreasing
Maximum
Torque
Torque
Trated
• Slow transient response
• Does not maintain maximum torque capability
DC Motor Speed Control
Speed
By Changing Armature voltage
Va increasing
Maximum
Torque
Torque
Trated
• good speed regulation
• maintain maximum torque capability
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)
Armature voltage control
Field flux control
Torque
Line of
Maximum
Torque Limitation
base
speed
Methods of Armature Voltage Control
Phase-controlled rectifier (AC–DC)
ia

+
3-phase Or
1-phase
supply
Vt
Q2
Q1

Q3
Q4
T
Methods of Armature Voltage Control
1. Ward-Leonard Scheme
2. Phase-controlled rectifier (AC–DC)
3. Switch-Mode Converter (Chopper) (DC–DC)
Methods of Armature Voltage Control
Phase-controlled rectifier: 4Q Operation
1 or 3phase
supply
+
1 or 3phase
supply
Vt


Q2
Q1
Q3
Q4
T
AN ALTERNATIVE WAY
Phase-controlled rectifier : 4Q Operation
R1
F1
3-phase
supply
+
Va
F2
R2

Q2
Q1
Q3
Q4
-
T
Converters For DC motor Drives
Switch–mode converters: 1Q Converter

T1
+
Vt
-
Q2
Q1
Q3
Q4
T
Converters For DC motor Drives
Switch–mode converters: 2Q Converter

T1
D1
T2
+
Vt
D2 -
Q2
Q1
Q3
Q4
Q1  T1 and D2
Q2  D1 and T2
T
Converters For DC motor Drives
Switch–mode converters: 4Q Converter

T1
T4
D1
D3
+ Vt -
D4
D2
T3
T2
Q2
Q1
Q3
Q4
T
Advantages of Switch mode converters
Switch–mode converters
• Switching at high frequency
 Reduces current ripple
 Increases control bandwidth
• Suitable for high performance applications