---
title: "DC series motor operated from ac supply"
subject: "Electrical Machines"
chapter: 4
key: "semester_4/electrical_machines:dc-series-motor-ac-supply"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-series-motor-ac-supply
site: BCT Engineering
---

# DC series motor operated from ac supply

Electrical Machines · Chapter 4: DC Motor

- Asked in: 2071 Bhadra, 4 marks
- Video: [What Happens When DC Series Motor Connected to AC Supply l AC Supply to DC Motor l What If Series](https://www.youtube.com/watch?v=FPbv8BD7sYE) — Electrical Orbit
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-series-motor-ac-supply
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-4

## Question

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

## Short answer

Because field and armature currents are the same and reverse together, torque direction remains unchanged (T ∝ I²). For AC operation, the stator and field cores are laminated, field turns reduced to lower reactance, and a compensating winding is used. This motor is called a universal motor.

## Answer

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

- In a series motor the same current flows through the field winding and the armature, so flux $\Phi\propto I$ and the torque is
$$
T\propto\Phi I_a\propto I^2
$$
- When the AC supply reverses, the current in the armature and the field both reverse together. The product $\Phi I_a$ keeps the same sign, so the torque is always in one direction (pulsating at twice the supply frequency, smoothed by the rotor inertia).

Such a motor is called a **universal motor** (works on both DC and AC). For good performance on AC the motor needs these changes:
1. The field core and yoke are **laminated** to reduce eddy-current and hysteresis loss.
2. The field winding has **fewer turns** and the armature has more, to reduce the reactance voltage drop and improve power factor.
3. A compensating winding is used to neutralise armature reaction and reduce commutation sparking.

It is used in vacuum cleaners, mixers, drills and sewing machines.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why can a DC series motor run on AC?
   A. Its field is separately excited
   B. Field and armature currents reverse together, so torque (proportional to I^2) keeps one direction
   C. The commutator converts AC to DC
   D. AC has no reactance
   Answer: B. Phi and Ia are the same current, so their product never changes sign. (Key point: Torque direction independent of polarity)

2. On AC, how does the torque behave?
   A. Constant with no ripple
   B. Reverses every cycle
   C. Pulsates at twice the supply frequency, smoothed by inertia
   D. Is zero
   Answer: C. I^2 is always positive but varies at 2f. (Key point: Torque pulsation)

3. Which change is needed for good AC performance?
   A. Solid field core
   B. Laminated field core and yoke
   C. More field turns than armature turns
   D. Removing the compensating winding
   Answer: B. Lamination reduces eddy-current and hysteresis loss under alternating flux. (Key point: Design changes)

4. Why is the field winding given fewer turns for AC use?
   A. To reduce reactance voltage drop and improve power factor
   B. To increase flux
   C. To reduce the speed
   D. To raise the starting current
   Answer: A. Fewer field turns lower the reactance, improving power factor. (Key point: Fewer field turns)

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## Related in other subjects

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