Dc motor characteristics are useful for
deciding which dc motor to be used for which application.
By looking at various dc motor characteristics, you
can get rough idea about the choice of the particular type of dc motor for a given
application. Though, exact shape of the characteristics will depend on motor
construction.
DC Series Motor Characteristics
The following expressions are used in determining dc
series motor characteristics.
Motor equation,
Eb= vt -
Ia(Ra+Rf) ...(1)
DC
Series Motor Speed-Current (N-Ia)
Characteristics
Putting (3) in (2)
and then (2) in (1) , we get
The characteristics can be drawn from the equation where speed
is inversely proportional to current giving a rectangular hyperbola curve.
As you can see from above plot, at no-load, armature current is very small, hence speed rises to
a dangerously high value which may destroy machine. That is why
a dc series motor should never be started without a mechanical load attached to
it.
DC
Series Motor Torque-Current (Ta-Ia)
Characteristics
From (4)
Ta= K1 K2 Ia2
Upto magnetic saturation of the field poles, Torque-Current Characteristic is a parabola. But for larger
armature currents magnetic saturation sets in and net flux tends to remain
constant and therefore , Ta
∝ Ia , the
curve becomes linear.
As prior to magnetic saturation, torque increases as the square
of armature current, therefore, dc series motors are used where high
starting torque is required e.g electric traction, cranes etc.
DC
Series Motor Speed-Torque (N-Ta)
Characteristics
From (4)
So, the speed torque characteristics of dc series motor is a rectangular hyperbola.
As clear from the characteristic, it is a variable speed motor
i.e speed is low at high torque and vice-versa ∴ dc series motor is used where the load is
subjected to heavy fluctuations and the speed is automatically required to
reduce at high torques and vice-versa.
