dc motors see 3433 electrical machines. dc motor - shunt motors - separately excited - series...
TRANSCRIPT
+ + --
DC motor
Mechanical Load: fans, blowers, Compressors,
+Va
-
+Vf
-
Tm Tload
Load torque opposing the motor torque
DC MOTOR
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
Introduction
2T
ea
T
t
k
TRkV
Te
Ra ↑
TL
T
t
kV
Varying Ra
Simple controlLosses in external resistor
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
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
+
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 ?