Considering first the shunt wound motor. If one were to measure the resistance of
the motor and using this and ohm's law to calculate the current taken, it would be
found that the result was very much greater than anticipated for the motors rating.
This is because another factor, back emf, comes into play to control the eventual
The initial current will though conform to the calculated value but as the motor
commences to rotate it also acts as a generator, generating a voltage (back emf)
whose polarity opposes that of the supplied voltage thereby limiting the current.
As the motor speed, and back emf, continue to increase, the current will fall until
the current is just sufficient to provide the torque the load demands. Incidentally,
with a constant field strength, torque developed is proportional to current. At this
point the back emf is only just short of the applied voltage, typically say for a
12V motor a back emf in the region of 11V. As it is this difference in voltage, 1V
in the example, that drives the current, any change in load and therefore current
demanded, will require a change in back emf to make the change possible.
Let us consider that the torque demanded by the load doubles requiring twice the
current. In this case the 1V difference will require to increase to 2V which can
only be achieved by the back emf reducing to 10V. As back emf is proportional to
speed, the speed of the motor will have to reduce by the same proportion. Actually,
in an attempt to keep my figures simple the speed variation suggested, around 10%,
is rather greater than is likely to occur in practice, especially at higher power
A series motor is though much more difficult to describe and understand in detail,
though the basic principles of back emf, etc., are still applicable. Whilst with
the shunt motor the field strength is constant, in the case of the series motor it
will vary with torque demanded. Therefor, twice the motor current will produce four
times the torque as both the field, field-strength and armature, field-strength will
increase by a factor of two. This produces the main advantage of the series motor,
that is, it capable of a greater torque output for short periods, either whilst running
or during start up. On the down side, speed variation due to load is greater than
with a shunt wound motor.
I mentioned above that the initial current at start up is limited only by the resistance
of the armature. As this becomes proportionately less as the motor size increases
some form of limiter to give it a soft start becomes essential, In the early days
this would have been a resistor in series that would be switched out of circuit after
a few seconds or in some cases, a tapped resistor which would be switched out gradually
At what power rating this becomes necessary I am not sure as my experience has been
with much larger motors.
AC/DC commutator motors
Alternating current commutator motors work in exactly the same way as DC motors but
when used on AC they benefit from a minor internal modification to their construction.
For this reason, motors labelled for use on DC should not be used on AC, though if
labelled for use on AC they should work reasonably well on DC. Some motors are though
labelled as being suitable for both.