---
title: "Reason: DC series motor on ac supply"
subject: "Electrical Machine"
chapter: 4
key: "semester_4/electrical_machine:series-motor-ac-supply-reason"
source: https://www.bctengineer.com/q/semester_4/electrical_machine/series-motor-ac-supply-reason
site: BCT Engineering
---

# Reason: DC series motor on ac supply

Electrical Machine · Chapter 4: DC Motor

- Asked in: 2071 Bhadra, 4 marks
- Video: [DC motor on AC supply |  What happen if DC motor gets AC supply ? | Shunt motor vs Series motor ||](https://www.youtube.com/watch?v=BCYa4T0n-H4) — Ed Prince Tutorial
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machine/series-motor-ac-supply-reason
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-4

## Question

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

## Answer

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.

```text
 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.

## 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. Torque direction is unchanged because flux and armature current reverse together
   B. Because the armature does not carry current
   C. Because the field is not needed on AC
   D. Because AC has a higher voltage
   Answer: A. Both Φ and I_a reverse together, so T ∝ ΦI_a keeps its sign. (Key point: Reason for AC operation)

2. In a series motor, how is torque related to current (unsaturated)?
   A. T is independent of I_a
   B. T ∝ I_a^2
   C. T ∝ I_a
   D. T ∝ 1/I_a
   Answer: B. T = KΦI_a with Φ ∝ I_a gives T ∝ I_a^2, which is always positive. (Key point: Torque dependence)

3. What is the nature of the torque on AC supply?
   A. Pulsating at supply frequency with zero average
   B. Constant, no pulsation
   C. Pulsating at twice supply frequency with positive average
   D. Alternating with zero average
   Answer: C. I_a^2 pulsates at 2f but never goes negative, so the average is positive. (Key point: Torque pulsation)

4. Why are the yoke and poles laminated in an AC series motor?
   A. To increase the number of poles
   B. To reverse the direction of rotation
   C. To increase armature resistance
   D. To reduce eddy-current loss
   Answer: D. Alternating flux causes eddy currents in solid iron; lamination reduces them. (Key point: Modifications)

## Related in Electrical Machine

- [DC series motor operated from ac supply](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-series-motor-ac-supply) — Electrical Machines, chapter 4