Basic Electrical and Electronics Engineering: Unit II: Electrical Machines
Characteristics of DC Motors
The characteristics curve of a motor are (i) Torque and armature current characteristic (Ta/Ia) (Electrical characteristic).(ii) Speed and armature current (N/Ia) characteristic.(iii) Speed and torque characteristic (N/Ta) (Mechanical characteristic).
CHARACTERISTICS
OF DC MOTORS
The
emf and torque equations are
Induced
emf, Ea = 
Ea
= Ka ϕN
Torque,
T =
T
= KTϕla
Speed,
N = K (V - IaRa) /ϕ
The
characteristics curve of a motor are
(i) Torque and armature current characteristic
(Ta/Ia) (Electrical characteristic).
(ii)
Speed and armature current (N/Ia) characteristic.
(iii)
Speed and torque characteristic (N/Ta) (Mechanical characteristic).
Τa
α ϕΙa, Ν α Eb/ϕ
Characteristics of series motors
(i) Ta/Ia
characteristic
In
series motor
ϕ
α Ιa
Τ
α ϕ Ιa
T α Ιa2
If
Ia increases Ta increases as the square of the current.
It is shown in fig 3.32.
(ii) N/Ia characteristics
Ν
α Eb/ϕ
If
Ia increases, ϕ also increases. Hence speed varies inversly as
armature current
as
shown in fig 3.33.
(iii)
N/Ta characteristic
If
speed is high, torque is low and vice-versa. This relation is shown in fig
3.34.
Characteristics of shunt motor
(i)
Ta/Ia characteristic
Assuming
ϕ is constant. Therefore Ta α Ia. Hence the electrical
characteristic is practically a straight line through the origin is shown in
fig 3.35.
(ii) N/Ia characteristic
If
o is assumed constant, then N α Eb. As Eb is practically
constant and also speed constant for more purposes.
But
increasing load, Eb and ϕ decreases so Eb decreases
slightly and also some decrease in speed, it is shown in fig 3.36 by the dotted
line.
(iii) N/Ta characteristic
It
is shown in fig 3.37
Characteristics of Compound Motor:
These
motors have both series and shunt windings. If series excitation helps the
shunt excitation (i.e) series flux is in the same direction then the motor is
said to be cumulatively compounded. If series field opposes the shunt field
then the motor is said to be differentially compounded. Both the motors shown
in fig 3.38.
The
characteristics of compound motor ie in between those of shunt and series
motors
as shown in fig 3.39.
Basic Electrical and Electronics Engineering: Unit II: Electrical Machines : Tag: : - Characteristics of DC Motors
Basic Electrical and Electronics Engineering: Unit II: Electrical Machines
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