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DC MOTORS
SEE 3433
ELECTRICAL MACHINES
DC MOTOR
- Shunt motors
- Separately excited
- Starter
DC MOTORS
DC MOTOR
+ -
+ -
DC motor
DC MOTOR
+
Va
-
Load torque opposing
the motor torque
+
Vf
+ -
+ Tm
Tload
DC motor
Mechanical Load: fans,
blowers, Compressors,
DC MOTOR
+
V
-
+
V
+ -
+ Tm
Tload
DC motor
F = mg
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 motor
If
Vt = IaRa + Ea
It
It = Ia + If
Rcf
+
Ia
Ra
Vt
Ea = k
Te = kIa
Rcw
Te

Tload
Mechanical
load

k = Vt - IaRa

Vt  IaR a
k

Vt
Ra

Te
2
k (k)
Three possible methods of speed control:
Field flux
Armature voltage Vt
Armature resistance Ra

Vt
Ra

Te
2
k (k)

Vt
kT
Vt
Ra

Te
2
k (k)
Varying Vt


TL
Vt ↓
Te
Requires variable DC supply

Vt
Ra

Te
2
k (k)
Varying Ra

Vt
kT

TL
Ra ↑
Te
Simple control
Losses in external resistor


Vt
kT

Vt
Ra

Te
2
k (k)
Varying 
TL
↓
Te
Not possible for PM motor
Maximum torque capability reduces
Method of speed control in DC motor drives
Armature voltage control : retain maximum torque capability
Field flux control (i.e. flux reduced) : reduce maximum torque capability
For wide range of speed control
0 to base  armature voltage, above base  field flux reduction
Armature voltage control
Field flux control
Te
Maximum
Torque capability
base

Te
Maximum
Torque capability
base

P Te
Constant torque
Constant power
Pmax

base
0 to base  armature voltage,
P = EaIa,max = kaIa,max
above base  field flux reduction
Pmax = EaIa,max = kabaseIa,max
   1/
0 to base  armature voltage,
above base  field flux reduction
0 to base  armature voltage,
If
Rcf
above base  field flux reduction
It
+
Ia
Ra
Rcw
Vt

BUT there are problems !
0 to base  armature voltage,
If
Rcf
above base  field flux reduction
It
+
Ia
Ra
Rcw
Vt

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 supply
for armature
What if we have an AC supply ?
DC supply
for field
0 to base  armature voltage,
above base  field flux reduction
3-phase AC
source
Separately Excited DC motor
AC to DC
converter
+
Vdc
+
Vdc
-
-
Armature voltage control
AC to DC
converter
Field voltage control
Starter in DC Motor
• At stand-still,  = 0  Ea = 0
 Ia
Ra
+
Ia 
Vt
Vt
Ra
eg, Vt = 100, Ra = 0.1  Ia = 1000 A !
–

Starter in DC Motor
• We can limit Ia at start-up by:
1) Controlling Vt using variable supply – e.g. using
power electronics converter
2) Adding external resistor  known as starter
 Ia
Ra
+
+
Ea
–
Vt
–
Ia 
Vt
R a  R st
When Ea = 0
Rst
• As speed builds up (so too Ea), Rst is gradually reduced
Starter in DC Motor
• As speed builds up (so too Ea), Rst is gradually reduced
Ia
Starter circuit
Imax 1
2
3
4
Imin
1
2
3
4
t (s)
speed
t (s)
Starter in DC Motor
Practical Starter circuit