Page 225 - Electrician - TT (Volume 1)

Eb
Ta
2π
P
ΦZN
Eb
60
ΦZN
P
60

Ta

2π
N
a
60

or
K.Φ
Ia

Ta
Φ
α

Ia
Ta
ΦZN
P
Eb
a
60
ΦN
Eb
K

Eb

N
Φ
Eb
k
Eb1
eq.(1)

N1
Φ1
k
Eb2
N2

Φ2
Φ
Eb
N1
2
1

Eb
N2 α N Φ K Ia N a 60 or Ia eq.(2)
Φ
2
1
N1 Eb 1 Φ 2
ELECTRICIAN - CITS

N2 Eb 2 Φ 1
Ta 1 Ia 1 Φ 1
For Shunt motor, Ta α Ia
Ta 2 Ia 2 Φ 2
In this characteristic torque is directlyproportional to the armature current.
T α Φ Ia, α N E.b
Φ
where, Φ is constant, so,
T α Ia α T Φ Ia , so,
1
whichgivesstraight line in practically.
α N
T
Speed / armature current characteristics
E.b
α N
Φ
For shunt motor, Φ is alomst constant
V
Eb
Ia
“Eb” is approximately constant
Ra
Constant speed motor
when loaded;
Practically “Eb” and “Φ” decrease,
“Eb” decreases a little more
Hence speed decreases with increase in load
Speed / torque characteristics
In a DC shunt motor, when load is increased, speed reduces slightly. Eb reduces due to that.
SoI is increased. Torque increases within capacity of motor.
a
Characteristics of dccompound motors
Compound motor have combine characteristics of both series and shunt motors. It can be started with load. Load
can be thrown suddenly. Effectively, is used in intermittent loads
Cumulatively compounded
Generally compounded motors are connected cumulatively
Application
Rolling mills, heavy tool machines, shears, conveyors
Differential compound
If heavy load is suddenly given; Series filed current increases and its flux. Total flux reduces, Attains a dangerously
high speed
212
CITS : Power - Electrician & Wireman - Lesson 30-37