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Reason: DC series motor on ac supply

Short answer

In a series motor, field and armature currents reverse simultaneously with AC so torque remains unidirectional (T ∝ I_a²). To operate on AC, yoke and poles are laminated, field turns reduced, and a compensating winding added, making it a universal motor.

Give reason: DC series motor can also be operated from ac supply.

A DC series motor can run on AC because the direction of torque does not depend on the direction of the supply current.

Reason

  • In a series motor the same current flows through the armature and the field winding.
  • Torque T = KΦ I_a, and Φ ∝ I_a, so T ∝ I_a².
  • When the AC supply reverses, both the field flux Φ and the armature current I_a reverse at the same instant. The product Φ I_a keeps its sign.
  • Therefore the torque is always in the same direction (it is pulsating at twice the supply frequency, but with a positive average value), and the motor rotates in the same direction. In a shunt motor the field is highly inductive, and the flux lags the armature current by almost 90°, so the average torque is poor.
 supply +ve half: Ia (+), Phi (+) -> T (+)
 supply -ve half: Ia (-), Phi (-) -> T (+)

Modifications required for AC operation

  1. The yoke and field poles are laminated to reduce eddy-current loss.
  2. The field has fewer turns, to reduce reactance and improve power factor.
  3. A compensating winding is provided to neutralise armature reactance and give better commutation.
  4. Higher armature turns are used for the torque.

Such a motor is called an AC series motor or universal motor, used in drills, mixers, vacuum cleaners and sewing machines.

✓ Answer checked by Claude Opus 5.5 (high effort)

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