Electric Traction Motor: AC Series Motor

Many single phase AC motors have been developed for traction purposes but only compensated series type commutator motor is found to be best suited for traction services. Practically, the AC series motor is best suited for traction purpose because of high-starting torque.

When DC series motor is fed from AC supply, it works but not satisfactorily due to the following reasons −

  • If a DC series motor is fed from AC supply both the field and the armature currents reverse for every half cycle. Thus, unidirectional torque is developed in the motor at double frequency.

  • When AC supply is given to the DC series motor, the series field winding inductance will result abnormal voltage drop and low power factor that leads to the poor performance of the motor.

  • The induced EMF and the current flowing through the armature coils undergoing commutation will cause sparking at the brushes and the commutator segments.

  • Alternating flux produced by the series field winding causes excessive eddy current loss, which increases the temperature of the motor. Therefore, the operating efficiency of the motor will decrease.

Therefore, some modifications are required for the satisfactory operation of the DC series motor on the AC supply, (i.e. AC series motor) and they are as follows −

  • The magnetic circuit of the AC series motor must be laminated to reduce the eddy current losses.

  • In order to reduce the inductive reactance of the series field winding, the series field winding of the AC series motor must be designed for few turns.

  • The decrease in the number of turns in the series field winding reduces the load torque, i.e., when series field winding turns decrease, its MMF decreases and hence the flux, which will increase the speed of the motor and hence the torque developed will decrease. But, to maintain constant load torque, it is necessary to increase the armature turns proportionally. Thus, in AC series motor, large number of armature conductors are used.

  • When the number of armature turns increase, the inductive reactance of the armature would increase. Thus, in case of AC series motor, compensating windings are provided to neutralize the effect of increased armature inductance.

  • An AC series motor should be operating at low voltage because the high voltage and low current supply would require large number of turns to produce desired magnetic flux.

  • An AC series motor should be operating at low frequency, because the inductive reactance of the field winding is directly proportional to the frequency. Therefore, at low frequency, the inductive reactance of the field winding decreases.

A schematic diagram of a single-phase AC series motor with inter-pole and compensating windings is shown in the figure below.

The average torque available on the motor shaft is given by,

$$\mathrm{\mathit{T\mathrm{\: =\: }\frac{\mathrm{1}}{\mathrm{2\sqrt{2}}\pi }\left ( \frac{ZP\phi _{m}}{A} \right )I\, \mathrm{cos}\, \theta }}$$


  • 𝑰 is the effective value of current

  • 𝝓𝒎 is the maximum value of flux per pole

  • 𝜽 is the phase angle between the phasors φ and I.

At the time of starting of an AC series motor, the power factor is low, hence from the above expression, for a given current, the torque developed by an AC series motor is less compared to the DC series motor. Therefore, AC series motors are not suitable for suburban traction services with frequent stops and preferred for main line service where high acceleration is not required.

Updated on: 19-May-2022

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