2.
T
K
R
K
V a
t
2
• Resistance in armature circuit: When a resistance is
inserted in the armature circuit, the speed drop (D)
increases and the motor speed decreases.
• Terminal voltage (armature voltage): Reducing the
armature voltage of the motor reduces the motor speed.
• Field flux (or field voltage): Reducing the field voltage
reduces the flux. The motor speed then increases.
D
o
Speed Control Variables
3. Attributes of good speed controller
• Soft transition
• Sufficient speed damping
• Over-voltage must not exceed the tolerable limit
of the system components.
• The magnitude of the inrush current should be
kept under control
• Natural electromechanical oscillations should be
avoided.
4. Concept of Speed Control by Adding
Resistance
V
f
If
Ia
Radd
Rf
Ra
Ea
Vt
6. Example 6.1
A 150 V, dc shunt motor drives a constant-torque
load at a speed of 1200 rev/min. The armature and
field resistances are 1 W and 150 W respectively.
The motor draws a line current of 10 A at the given
load.
a. Calculate the resistance that should be added to the
armature circuit to reduce the speed by 50%.
b. Calculate the resistance that must be added to the
armature circuit to operate the motor at holding
condition.
7. Solution: Part a
A
I
I
I f
a 9
150
150
10
1
1
)
R
R
(
I
V
K
E
R
I
V
K
E
1
add
a
2
a
2
2
a
a
1
a
1
1
a
K
T
I d
a 2
a
1
a I
I
)
R
R
(
I
V
R
I
V
n
n
E
E
1
add
a
a
a
a
2
1
2
1
2
a
1
a
)
R
1
(
9
150
1
9
150
1200
5
.
0
1200
1
add
W
83
.
7
1
add
R
Speed
Torque
Ra
Ra + Radd1
Ra + Radd2
Ra + Radd3
Radd1<Radd2<Radd3
1
2
3
4
Ra
I
If Rf Vt
Ea
Ia
8. Solution: Part b
0
)
R
R
(
I
V
K add
a
a
W
67
.
15
1
9
150
a
a
add R
I
V
R
Speed
Torque
Ra
Ra + Radd1
Ra + Radd2
Ra + Radd3
Radd1<Radd2<Radd3
1
2
3
4
9. Concept of Speed Control by Adjusting
Armature Voltage
1f
1a
Vf Rf
Ea
Ra
Vt
13. Example 6.3
• A 150 V, dc shunt motor drives a constant-
torque load at a speed of 1200 rev/min. The
armature and field resistances are 2 W and
150 W respectively. The motor draws a line
current of 10 A. Assume that a resistance is
added in the field circuit to reduce the field
current by 20%. Calculate the armature
current, motor speed, the value of the added
resistance, and the extra field losses.
15. Solution
2
a
2
1
a
1
d I
K
I
K
T
1
a
2
1
2
a I
I
A
25
.
11
9
8
.
0
1
I
I
I
I 1
a
2
f
1
f
2
a
a
2
a
2
2
2
a
a
1
a
1
1
1
a
R
I
V
K
E
R
I
V
K
E
a
2
a
a
1
a
2
1
2
1
R
I
V
R
I
V
n
n
2
25
.
11
150
2
9
150
n
1200
8
.
0
1
2
rpm
n 86
.
1448
2
o1
o2 Vf2
Vf1
Td
Torque
Vf1>Vf2
1
2
22. Example 6.3
• A 1-hp dc shunt motor is loaded by a
constant torque of 10 NM. The
armature resistance of the motor is 5 W,
and the field constant K = 2.5 V sec.
The motor is driven by a half wave SCR
converter. The power source is 120 V,
60 Hz. The triggering angle of the
converter is 60o, and the conduction
period is 150o. Calculate the motor
speed and the developed power.
23. Solution
A
K
T
Iave 4
5
.
2
10
4
5
5
.
2
360
150
)
150
60
cos(
)
60
cos(
2
120
2
sec
/
22
.
16 rad
rpm
n 88
.
154
W
I
k
I
E
P ave
ave
a
d 162
4
22
.
16
5
.
2
ave
a
max I
R
K
2
]
cos
[cos
2
V
29. Example 6.4
• For the motor in Example 6.3, assume
that the converter is a full-wave type.
The triggering angle of the converter is
60o, and the conduction period is 150o.
Calculate the motor speed and the
developed power.
31. Example 6.5
• A dc separately excited motor has a
constant torque load of 60 NM. The
motor is driven by a full-wave converter
through a 120 V ac supply. The field
constant of the motor K = 2.5 and the
armature resistance is 2 W. Calculate
the triggering angle for the motor to
operate at 200 rev/min. The motor
current is continuous.
