---
title: "Series motor load start; shunt motor speed control"
subject: "Electrical Machines"
chapter: 4
key: "semester_4/electrical_machines:dc-series-load-shunt-speed-control"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-series-load-shunt-speed-control
site: BCT Engineering
---

# Series motor load start; shunt motor speed control

Electrical Machines · Chapter 4: DC Motor

- Asked in: 2065 Chaitra (old course), 2+3+3 marks
- Video: [14 Speed Control of DC Series Motor](https://www.youtube.com/watch?v=8tdI-XOQkzA) — 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/dc-series-load-shunt-speed-control
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-4

## Question

Explain why the dc series motor can not be started without some mechanical load. Also discuss the armature control and field control method for speed control of dc shunt motor.

## Short answer

A series motor cannot start without load because low armature current yields minimal flux, leading to dangerously high speed. Shunt motor speed control methods: armature rheostat control reduces speed below base (high loss, constant torque); field control increases speed above base (efficient, constant power); armature voltage control provides 0 to base speed.

## Answer

#### Why a series motor cannot be started without load

In a series motor the field winding carries the armature current, so the flux depends on the load: $\Phi\propto I_a$ (below saturation). The speed is

$$
N \propto \frac{E_b}{\Phi} \approx \frac{V - I_a(R_a + R_{se})}{\Phi}
$$

- At no load $I_a$ becomes very small, so the flux $\Phi$ becomes very small (only the residual value).
- The numerator $E_b\approx V$ stays nearly constant, so the speed $N\propto 1/\Phi$ rises to a **dangerously high value** (several times rated speed).
- This is called **racing**. The centrifugal forces can burst the armature, throw the winding out of the slots and damage the commutator and bearings.
- The torque $T\propto\Phi I_a\propto I_a^2$ is small at no load, so the motor has no load to hold the speed down.

Hence a series motor must always be started and run with a load connected (preferably directly coupled or geared, never by a belt that may slip or break).

#### Speed control of a DC shunt motor

From the motor equations:

$$
N = \frac{V - I_aR_a}{K\Phi} \propto \frac{E_b}{\Phi}
$$

So the speed of a shunt motor can be changed by varying the flux $\Phi$, the armature circuit resistance, or the applied voltage.

#### 1. Armature (rheostatic) control

A variable resistance $R_x$ is connected in series with the armature (field kept constant at full value):

$$
N = \frac{V - I_a(R_a + R_x)}{K\Phi}
$$

- The speed is **reduced below the normal (base) speed**; the greater $R_x$, the lower the speed.
- Torque for a given $I_a$ is unchanged ($T\propto\Phi I_a$), so it suits constant-torque loads.
- **Disadvantages:** large $I_a^2R_x$ loss, poor efficiency, and poor speed regulation (the speed changes with load); it is used for small and short-time duty.

#### 2. Field (flux) control

A rheostat $R_f$ is connected in series with the shunt field:

$$
I_f = \frac{V}{R_{sh} + R_f}, \qquad N \propto \frac{1}{\Phi}
$$

- Increasing $R_f$ lowers $\Phi$, so the speed rises **above the base speed** (typically up to 2:1 or 3:1).
- It is simple, cheap and efficient (field loss is small, since $I_f$ is small).
- Limits: commutation worsens and the machine can become unstable (armature reaction) at very weak fields; the torque for a given $I_a$ falls as $\Phi$ falls, so it suits constant-power loads.

```text
   Speed
     ^       field control (N up)
     |      /
  base N ---+--- 
     |      \  armature control (N down)
     +--------------------> Rheostat setting
```

#### 3. Armature voltage control (Ward-Leonard)

Varying the voltage applied to the armature (with a separate source) changes the speed smoothly from zero up to the base speed.

#### Comparison

| Point | Armature control | Field control |
|---|---|---|
| Speed range | below base speed | above base speed |
| Loss | high ($I_a^2R_x$) | low |
| Efficiency | poor | good |
| Load suited | constant torque | constant power |
| Speed regulation | poor | good |

## Check yourself

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

1. Why can a series motor not be started without mechanical load?
   A. Starting current is too high
   B. The armature reaction is excessive
   C. Flux is tiny at light load, so speed rises dangerously
   D. The commutator sparks
   Answer: C. Flux depends on Ia, so at no load N proportional to 1/Phi becomes extremely high. (Key point: Series motor needs load)

2. In armature rheostatic control of a shunt motor, what is the effect on speed?
   A. Speed rises above base speed
   B. Speed falls below base speed; larger Rx gives lower speed
   C. Speed stays the same but torque rises
   D. Speed becomes independent of load
   Answer: B. Extra series resistance increases the drop Ia(Ra+Rx), reducing Eb and N. (Key point: Armature control)

3. Increasing the shunt field rheostat Rf does what?
   A. Raises flux and lowers speed
   B. Lowers flux and raises speed above base speed
   C. Reduces armature current only
   D. Has no effect on speed
   Answer: B. N is proportional to 1/Phi, so weaker flux gives higher speed. (Key point: Field control)

4. Which is correct comparing armature and field control?
   A. Armature control: efficient; field control: high loss
   B. Both give speeds above base
   C. Field control: constant-power loads, low loss; armature control: high I^2Rx loss
   D. Armature control gives good speed regulation
   Answer: C. Field current is small so field control loss is low; armature control wastes Ia^2Rx and regulates poorly. (Key point: Comparison)

## Related in Electrical Machines

- [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
- [Shunt motor speed with series armature resistance](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adcm-shunt-half-full-load-torque) — Electrical Machines, chapter 4
- [Shunt motor speed with 0.1 Ω armature resistance, less load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-240v-5a-armature-0p1) — Electrical Machines I, chapter 4, asked 3×
- [DC series motor started at no load: true/false](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-series-no-load-true-false) — Electrical Machines, chapter 4
- [Speed control methods of shunt DC motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-motor-speed-control-methods) — Electrical Machines, chapter 4, asked 2×
- [Shunt motor speed with 1 Ω armature resistance added](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-240v-27a-1ohm-torque) — Electrical Machines I, chapter 4, asked 5×

## Related in other subjects

- [Speed control methods of DC shunt motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Ashunt-motor-speed-control-methods) — Electrical Machine, chapter 4, asked 2×
- [Shunt motor speed with field resistance added (250 V, 1400 rpm)](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-250v-0p2ohm-1400rpm-50ohm) — Electrical Machines I, chapter 4, asked 5×
- [Shunt motor speed with 50 Ω field resistance and lower torque](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-220v-25a-50ohm-torque) — Electrical Machines I, chapter 4, asked 4×
- [Shunt motor speed at 30 A armature current](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Ashunt-motor-60a-230v-speed) — Electrical Machine, chapter 4