Series Motors:

        A series dc motor, as the name implies, is one in which the field
coil and the armature coil are in series. A basic series diagram is the
same as a series generator diagram. Since the starting current through
the armature also flows through the field coils, the series motor develops
a high starting torque, but has poor speed regulation. As the speed 
decreases, the amount of torque delivered to the load increases. The
reason for this is that as the armature slows down, the CEMF
developed in the armature decreases, the current through the armature
and field increases and the torque increases. Remember that increasing
the current through the field coils and the armature increases their 
fields which causes the increased torque. This means that a series motor
runs fast with a light load, and runs slower as the load is increased.
In fact, if a series motor is allowed to run without a load, it may run
so fast that the armature will fly apart. It is for this reason that
series motors should not be used with belt coupling to the load. If
the belt were to break, the motor would be without a load, and could
destroy itself.
Top of Page