How does varying load current affect the Torque, flux, and speed of a DC Series Motor?
The armature current also happens
to be the excitation current of the series field and hence the flux variation resembles the
magnetization curve of the machine. At the large value of the armature currents, the useful flux
would be less than the no-load magnetization curve for the machine. Similarly for small
values of the load currents, the torque varies as a square of the armature currents as the flux
is proportional to the armature current in this region. As the magnetic circuit becomes more
and more saturated the torque becomes proportional to Ia as flux variation becomes small. The figure shown below shows the variation of E1, flux, torque, and speed following the above procedure
from which the torque-speed characteristics of the series motor for a given applied voltage
V can be plotted. The initial portion of this torque-speed curve is
seen to be a rectangular hyperbola and the final portion is nearly a straight line.
Why DC series motor should not be started at no-load?
The speed
under light load conditions is many times more than the rated speed of the motor. Such
high speeds are unsafe, as the centrifugal forces acting on the armature and commutator
can destroy them giving rise to a catastrophic breakdown. Hence series motors are not
recommended for use where there is a possibility of the load becoming zero. In order to safeguard the motor and personnel, in modern machines, a ‘weak’ shunt field is provided
on series motors to ensure a definite, though small, value of flux even when the armature
current is nearly zero. This way the no-load speed is limited to a safe maximum speed. It is
needless to say, this field should be connected so as to aid the series field.
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| Load Characteristics |
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| Torque-Speed Characteristics |