In series motors stator windings
and field windings are connected in series with each other. As a result the
field current and armature current are equal. Heavy currents flow directly from
the supply to the field windings. To carry this huge load, field windings are
very thick and have few turns. Usually copper bars form stator windings. These
thick copper bars dissipate heat generated by the heavy flow of current very
effectively. Note that the stator field windings S1-S2 are in series with the
rotating armature A1-A2.In a series motor electric power is supplied between
one end of the series field windings and one end of the armature. When voltage
is applied, current flows from power supply terminals through the series
winding and armature winding. The large conductors present in the armature and
field windings provide the only resistance to the flow of this current. Since
these conductors are so large, their resistance is very low. This causes the
motor to draw a large amount of current from the power supply. When the large
current begins to flow through the field and armature windings, the coils reach
saturation that results in the production of strongest magnetic field possible.
The strength of these magnetic
fields provides the armature shafts with the greatest amount of torque
possible. The large torque causes the armature to begin to spin with the
maximum amount of power and the armature starts to rotate.
In series
motors, a linear relationship exist between the current flowing through the
field windings and the amount of torque produced. As heavy currents flow
through the very thick series field windings, large torques are produced in
series motors. This feature makes series motors to be used as starter motors
for industrial applications. Series motors can move comparatively heavier shaft
loads. A series motor can start an automobile’s engine by drawing a heavy
current of 500A. In a factory series motors can help operate huge cranes by
carrying several thousands of amperes. Series motors generally operate for a
very less duration, about only a few seconds, just for the starting purpose.
Motor speed control is achieved by controlling the voltage applied
to the motor. This essentially controls the torque developed by the motor. To
increase the speed of a series DC motor a low resistance is placed in parallel
with the series field. This shunt resistance lowers the field current, which
produces a drop in magnetic flux and an increase in speed. To lower the speed
an external resistance is connected in series with the field and the armature.
This results in armature voltage reduction and a fall in speed. When the
armature speed increases the field current reduces, reducing the induced back
emf. This results in further increase in speed (as current is reduced because
it is directly proportional to overall voltage) and virtually there is no upper
speed limit. So running a series motor with no load is very risky, as it can
accelerate to destruction.