dc motor drivesrhabash/elg4139dcdrives.pdf · dc motor drives methods of speed control armature...
TRANSCRIPT
ELG4139: DC Drives Types of DC Motors: Shunt; Series; Compound;
Separately; excited; Switched reluctance motors.
Motor Selection: Cost; Thermal capacity; Efficiency;
Torque-speed profile; Acceleration; Power density,
volume of the motor; Ripple, cogging torque; Peak
torque capability.
Motor Drives
Speed is determined by Armature Voltage
Torque is determined by Armature Current
Output - AC
- DC
Input
Source - AC
- DC
- unregulated
Reference
sensors
DC Motor Example
• V = 12 volts
• Max current = 4 A
• Max power out = 25 W
• Max efficiency = 74%
• Max speed = 3500 rpm
• Max torque = 1.4 N-m
• Weight = 1.4 lbs
• Forward or reverse (brushed)
Torque Equation: Motor Drives
dt
dJTTor
dt
dJTT LeLe
0 Le TT Acceleration
0 Le TT Deceleration
0 Le TT Constant speed
Te : motor torque (Nm) TL : Load torque (Nm)
Equivalent Circuit of DC Motor
at ikTe Electromagnetic torque
Ea ke Armature back e.m.f.
Lf Rf
if
aa
aat edt
diLiRv
+
ea
_
La Ra
ia +
Vt
_
+
Vf
_
dt
di L i R v f
f f f
Torque-Speed Characteristics
aaaa EIRV In steady state,
T
ae
T
a
k
VT
k
R
2
Therefore speed is given by,
Three possible methods of speed control:
Armature resistance Ra
Field flux F
Armature voltage Va
aa
aaa edt
diLiRV
Armature circuit:
DC Motor Drives
Methods of speed control
Armature voltage control
Variable voltage source
Phase-controlled Rectifier
Switch-mode converter (Chopper)
1Q-Converter
2Q-Converter
4Q-Converter
• DC Drives: Electric drives that use DC motors as the prime movers.
• DC motor: industry workhorse for decades.
• Will AC drive replaces DC drive ?
– Predicted many years ago
– AC will eventually replace DC; at a slow rate.
– DC strong presence; easy control; huge numbers
DC Drives
• Advantage: simple torque and speed control without sophisticated electronics.
• Limitations: Maintenance; expensive and heavy;
the problem of sparking!
Power Electronic Converters in Electrical Drive Systems
DC Drives AC Drives
DC Source AC Source
AC-DC-DC AC-DC
AC Source
Const.
DC
Variable
DC
AC-DC-AC AC-AC
NCC FCC
DC Source
DC-AC DC-DC-AC
DC-DC DC-AC-DC
AC-DC Drive
Shunt
Field
M
Field Supply
3 Phase AC
Power
SCR
Gate
DC Armature
T1 T3 T5
T2 T4 T6
L1 L2 L3
+
-
Firing Circuit
Regulato
r
Operator
Control Station
Current
Feedback
Voltage
Feedback
AC-DC-DC Drives
control
AC
DC-DC
Q1 Q2
Q3 Q4
T
+
Vt
-
T1
Switch Mode Converter: 1Q Converter or Class A
• When chopper is ON, supply voltage V is connected across the load. • When chopper is OFF, vO = 0 and the load current continues to flow in the same
direction through the freewheeling diode. • The average values of output voltage and current are always positive.
• It is used to control the speed of dc motor.
Vs
Is Ia
Va
Va= D Is Po=Va Ia Is = D Ia Ri = Vs / Is
+
Vt
-
T1
D1
T2
D2
Q1 Q2
Q3 Q4
T
Q1 T1 and D2
Q2 D1 and T2
Switch Mode Converter: 2Q Converter
Output current
Thyristorgate pulse
Output voltage
ig
i0
v0
t
t
ttON
T
CH ON
FWD Conducts
T1 conducts va = Vdc
Q1 Q2
Va
Ia
T1
T2
D1
+
Va
-
D2
ia
+
Vdc
DC-DC: Two-quadrant Converter
Q1 Q2
Va
Ia
T1
T2
D1
+
Va
-
D2
ia
+
Vs
D2 conducts va = 0
Va Eb
T1 conducts va = Vdc
Quadrant 1: The average voltage is made larger than the back EMF
DC-DC: Two-quadrant Converter
Is Ia
Q1 Q2
Va
Ia
T1
T2
D1
+
Va
-
D2
ia
+
Vdc
D1 conducts va = Vdc
DC-DC: Two-quadrant Converter
Q1 Q2
Va
Ia
T1
T2
D1
+
Va
-
D2
ia
+
Vdc
T2 conducts va = 0
Va Eb
D1 conducts va = Vdc
Quadrant 2: The average voltage is made smaller than the back EMF,
forcing the current to flow in the reverse direction.
DC-DC: Two-quadrant Converter
Q1 Q2
Q3 Q4
T
+ Vt - T1
D1
T2 D2
D3
D4
T3
T4
Switch-Mode Converter: 4Q Converter