Welcome to series motor runaway.
If the load being driven by a series DC motor becomes disconnected, a runaway condition occurs.
In this situation, the motor speed may accelerate until the motor physically breaks apart.
Under normal operating conditions, the current flowing through the armature and the series
field coil develops the amount of flux that produces just enough torque to turn the load.
At the moment the load is removed the following conditions develop. The amount of current that
flows through the coils is still at the level that was required to drive the load.
Now that there is no load, this large amount of current is higher than what
is needed to turn the armature so the motor speed increases. As the motor speed increases
the counter electromotive force generated by the armature also increases. Because the CEMF
increases, the current through the armature and field decreases.
Let's review. When the load is removed: the motor speed increases, the CEMF increases,
and the current through the armature and field coils decreases.
Since current through the armature and field coils decreases,
the strength of the flux lines around them weakens. If the strength of the flux lines
around the coils were reduced at the same rate as the current flowing through them,
both would decrease at the same rate that the speed increases. Therefore, the CEMF
would stop increasing, the current would become constant, and the armature speed would stabilize.
However, because the series field coil has only a few turns of heavy wire,
its flux strength decreases at a faster rate than the armature current decreases.
This condition keeps the CEMF from building at a rate as high as the rate at which the motor speed
increases. Since the CEMF is unable to reduce the current fast enough to stop the motor speed
from increasing, the motor breaks apart. Due to their runaway characteristics, it is recommended
that belts or chain drives that can break should not be coupled to the shaft of the series motor.
indicate what happens to the following conditions when the load is removed from the series motor.
The counter electromotive force or CEMF:
the CEMF increases. Field strength: the field strength decreases.
Motor current: the motor current decreases. And armature speed: the armature speed increases.
How would you complete this sentence? The strength of the flux lines decreases at _____ rate that
the armature current decreases due to the few turns of the series motor field coil.
A.the strength of the flux lines decreases at a faster rate than the rate that the
armature current decreases due to the few turns of the series motor field coil.
And one final question. An unloaded series motor will break apart
because the _____ is unable to reduce the current at a rate that is as fast as the rate at which
the armature speed increases. The answer is B. the counter electromotive force or CEMF.
Congratulations! You have completed this learning object Series motor runaway.