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DC MOTORS
SEE 3433
ELECTRICAL MACHINES
DC MOTOR
- Shunt motors
- Separately excited
- Series (we’ll look briefly)
DC MOTORS
+ + --
DC motor
DC MOTOR
+ + --
DC motor
Mechanical Load: fans, blowers, Compressors,
+Va
-
+Vf
-
Tm Tload
Load torque opposing the motor torque
DC MOTOR
DC MOTOR
+ + --
DC motor
+V-
+V-
F = mg
Tm Tload
DC MOTOR
Hoist
DC MOTOR
- Some applications require the control the speed
- Some applications require the control the torque
- In order to control the torque or speed we need to know the T- characteristics of the motor and the mechanical load
Intersections between the two characteristics will determine the operating point
DC MOTOR
Shunt motorVt = IaRa + Ea
It = Ia + If
Ea = k
+
Vt
Ra
Rcf
Rcw
ItIf
Ia
Te = kIa
Te
Tload
Mechanicalload
k = Vt - IaRa
k
RIV aat
e2t T
)k(Ra
kV
Three possible methods of speed control:
Field flux
e2t T
)k(Ra
kV
Armature voltage Vt
Armature resistance Ra
2T
ea
T
t
k
TRkV
Te
TLT
t
kV
Vt ↓
Varying Vt
Requires variable DC supply
2T
ea
T
t
k
TRkV
Te
TLT
t
kV
Vt ↓
Varying Vt
Requires variable DC supply
Te
Varying Vt
Requires variable DC supply
TL
T
eaTt k
TR)k(V
Constant TL
Vt
aaTt RI)k(V
aaRI
rated,tV
base
Varying Vt
Constant TL
T
eaTt k
TR)k(V
Introduction
2T
ea
T
t
k
TRkV
Te
Ra ↑
TL
T
t
kV
Varying Ra
Simple controlLosses in external resistor
2T
ea
T
t
k
TRkV
Te
TL
T
t
kV
Varying
↓
Not possible for PM motorMaximum torque capability reduces
For wide range of speed control 0 to base armature voltage, above base field flux reduction
Armature voltage control : retain maximum torque capability
Field flux control (i.e. flux reduced) : reduce maximum torque capability
Te
MaximumTorque capability
Armature voltage controlField flux control
base
Method of speed control in DC motor drives
Te
MaximumTorque capability
base
Te
Constant powerConstant torque
base
0 to base armature voltage, above base field flux reduction
P = EaIa,max = kaIa,max
Pmax
Pmax = EaIa,max = kabaseIa,max
1/
P
0 to base armature voltage, above base field flux reduction
0 to base armature voltage, above base field flux reduction
+
Vt
Ra
Rcf
Rcw
ItIf
Ia
BUT there are problems !
0 to base armature voltage, above base field flux reduction
+
Vt
Ra
Rcf
Rcw
ItIf
Ia
Controlling Vt will also affect If
Controlling If via Rcf caused losses I2R
0 to base armature voltage, above base field flux reduction
Separately Excited DC motor
DC supplyfor armature
DC supplyfor field
What if we have an AC supply ?
Separately Excited DC motor
AC to DCconverter
AC to DCconverter
3-p
ha
se
AC
sou
rce
Armature voltage control Field voltage control
+Vdc
-
+Vdc
-
