3.1. Series Motors
DC shunt or series motors rotate in the same direction regardless of the polarity of the supply. Thus
the motor can operate on AC supply. But it has been found that:
(i). Shunt motors develop little torque when connected to an AC supply. The reason of it
is that the field winding, owing to its high inductance, causes the field current to lag the armature
current by such a large angle that a very low net torque results.
(ii). In series motors, the field and armature currents are in phase, theoretically same torque is
developed with a given AC as with a like amount of DC in a series motor.
When ordinary DC series motor is connected to an AC supply, it would operate, but not very
satisfactorily. The reasons being:
(i) Since the field and armature currents both reverse every half cycle, the torque would be
exerted at double frequency in one direction.
(ii) The alternation in the field flux would cause excessive eddy-current losses in the field core
and yoke.
(iii) The induce voltage and currents in the armature coils, short circuited by the brushes when
undergoing commutation would result in heavy sparking.
(iv) The inductance of the field winding would result in abnormal voltage drop and low power
factor which results in poor performance.
Hence some modification must be made in order for a DC series motor to operate satisfactory.
3.1.1. Improvements
(i) To reduce the reactance of the series field, ac series motors are built with as few turns as
possible.
(ii) Reduction in the number of turns on the field winding would reduce the flux

for a
given current hence the speed will increase and torque reduced. To develop the required load
torque the number of armature conductors would have to be increased proportionally.
(iii) The increase in armature conductors would increase the inductive reactance of the
armature so that the net inductive reactance of the total machine circuit would not be
improved. In order to overcome this difficulty, the machine is provided with a compensating
winding.
(iv) In order to lower the transformer emf in the short-circuited element of the armature
winding, the motor is provided with a large number of poles with lesser flux per pole.
(v) The armature coils are single turn coils and brush of less width are employed in order
not to short circuit more than two coils at a time.
(vi) To obtain sufficient flux with few ampere-turns per pole, the reluctance of the magnetic
circuit must be reduced to minimum. This is accomplished by operating the iron at low flux
density and therefore at high permeabilities. Also a short air gap is used. Because of the small
number of field ampere-turns and the very low flux density a short pole is required.
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(vii) All parts of magnetic circuit of an ac series motor are laminated in order to reduce the eddy
current losses.
(viii) The operating voltage is kept low in order to reduce the inductance. However, high
voltage motor with proportionally low current would require a large number of turns to produce
the given flux.
(ix) Since inductive reactance is directly proportional to frequency so ac series motor
operating characteristics are better at low frequencies. However fractional horse power motors
may be designed to operate satisfactory at commercial frequency of 50Hz