---
title: "DC series motor for starting heavy loads"
subject: "Electrical Machines I"
chapter: 4
key: "semester_4/electrical_machines_i:series-motor-heavy-load-starting"
source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/series-motor-heavy-load-starting
site: BCT Engineering
---

# DC series motor for starting heavy loads

Electrical Machines I · Chapter 4: DC Motor

- Asked in: 2069 Bhadra, 4 marks
- Video: [10 Characteristics of DC Series Motor](https://www.youtube.com/watch?v=DR5BGCm6Vto) — SP Engg. ACADEMY
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/series-motor-heavy-load-starting
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-4

## Question

What do you mean by dc series motor? Why is this motor used to start heavy loads?

## Short answer

A DC series motor has its field winding in series with the armature, so both carry the same current. Torque is proportional to the square of current before saturation, giving very high starting torque. It is used to start heavy loads because it produces large torque at low speeds and its speed falls as load increases, matching the demand.

## Answer

A **DC series motor** is a DC motor whose field winding is connected in series with the armature, so the same current flows through both. The series field has a few turns of thick wire, so its resistance is very low.

```text
   +o----[ Series field Rse ]----+
                                 |
                              ( A )  armature Ra
                                 |
   -o----------------------------+
        I = Ia = Ise
```

$$
V = E_b + I_a (R_a + R_{se})
$$

#### Why it is used for heavy starting loads

1. **Torque varies as square of current.** Since flux $\phi \propto I_a$ (before saturation), $T \propto \phi I_a \propto I_a^2$. If the current is doubled, the torque becomes about four times as large. A shunt motor would give only twice the torque for the same current.
2. **Very high starting torque.** At starting $E_b = 0$, so a large current flows (limited by the starter). This large current produces a strong field and a very large starting torque, which can accelerate heavy loads with high inertia.
3. **Speed adjusts automatically to load.** $N \propto E_b/\phi$; with heavy load the current and flux rise, so speed falls. Power ($T\omega$) stays roughly constant, so the motor is not overloaded much when the load torque rises.
4. **Less current from the supply for the same torque.** Because $T \propto I^2$, a given starting torque is obtained with less current than in a shunt motor.

**Applications:** electric traction (trains, trams), cranes, hoists, lifts, trolley buses, conveyors.

**Caution:** a series motor must never be started or run without load (or with a belt drive), because at light load the flux is very small and speed rises dangerously high ($N \propto 1/\phi$).

## Check yourself

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

1. What defines a DC series motor?
   A. Field winding in parallel with the armature
   B. Field winding in series with the armature, carrying the same current
   C. Two field windings with different currents
   D. Field winding supplied separately
   Answer: B. The series field has few thick turns and carries armature current. (Key point: Definition)

2. Why does doubling the starting current roughly quadruple the torque of a series motor?
   A. Speed doubles
   B. Flux is proportional to Ia, so T is proportional to Ia^2
   C. Back emf doubles
   D. Resistance halves
   Answer: B. T is proportional to phi x Ia and phi is proportional to Ia before saturation, so T is proportional to Ia^2. (Key point: Torque relation)

3. At starting, why is a large current and torque produced?
   A. Back emf is zero so the current is large (limited only by the starter), giving strong flux
   B. Speed is maximum
   C. The field is weak
   D. Supply voltage is doubled
   Answer: A. Eb = 0 at standstill, so Ia is large and with it the field flux and torque. (Key point: Starting condition)

4. Why must a series motor never run without load?
   A. Torque becomes zero
   B. At light load flux is very small, so speed (proportional to 1/flux) rises dangerously
   C. Armature overheats from large current
   D. Back emf collapses
   Answer: B. N is proportional to Eb/phi, so tiny flux at no load gives runaway speed. (Key point: Caution)

## Related in Electrical Machines I

- [Starting current and starting resistance of shunt motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-220v-starting-current-80pc) — Electrical Machines I, chapter 4
- [Need of starter and induction motor starting methods](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ainduction-motor-starter-methods) — Electrical Machines I, chapter 5
- [Starting, no-load and full-load current of slip ring motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aslip-ring-0p8-4ohm-400v-ratio4) — Electrical Machines I, chapter 5
- [Starting current and rotor resistance for 150 kW slip ring motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-150kw-3000v-starting-current-30a) — Electrical Machines I, chapter 5
- [Speed of 220 V series motor at reduced load current](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aseries-motor-220v-38a-speed) — Electrical Machines I, chapter 4
- [Series resistance to halve series motor speed](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aseries-motor-600rpm-250v-resistance-300rpm) — Electrical Machines I, chapter 4

## Related in other subjects

- [DC series motor should not start on no-load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-series-motor-no-load-start) — Electrical Machines, chapter 4
- [DFRT, starting methods and capacitor start-run motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Adfrt-starting-methods-csr) — Electric Machines II, chapter 3
- [Starting line current of capacitor start motor](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Acapacitor-start-line-current) — Electric Machines II, chapter 3
- [DC shunt motor speed at reduced voltage and starting resistance](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Adc-shunt-reduced-voltage-starting-resistance) — Utilization of Electrical Energy, chapter 3