---
title: "Speed control methods of shunt DC motor"
subject: "Electrical Machines"
chapter: 4
key: "semester_4/electrical_machines:dc-motor-speed-control-methods"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-motor-speed-control-methods
site: BCT Engineering
---

# Speed control methods of shunt DC motor

Electrical Machines · Chapter 4: DC Motor

- Asked in: 2071 Bhadra, 8 marks; 2069 Bhadra (asked 2 times)
- Video: [speed control of dc motor | speed control of dc shunt motor | dc motor speed control | motor control](https://www.youtube.com/watch?v=BPGt3zxUn_8) — Mind of Engineer 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-motor-speed-control-methods
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-4

## Question

Describe different methods of controlling the speed of shunt DC motor (armature control and field control methods).

## Short answer

Speed of a shunt motor can be controlled by armature rheostat control (adding series resistance reduces speed below base, suitable for constant torque but inefficient) or field control (reducing flux raises speed above base, efficient and suited for constant power). Armature voltage control (Ward-Leonard) gives smooth variation from zero to base speed.

## Answer

The speed of a shunt motor follows 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 armature 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 motors 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 (from 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. The speed of a shunt motor is proportional to:
   A. (V - IaRa)/Phi
   B. Phi / V
   C. IaRa / Phi
   D. V x Phi
   Answer: A. N is proportional to Eb/Phi. (Key point: Speed relation)

2. Armature (rheostatic) control gives speeds:
   A. Above base speed
   B. Below base speed
   C. Both above and below
   D. Only base speed
   Answer: B. Added series resistance reduces voltage across armature. (Key point: Armature control)

3. Increasing resistance in series with the shunt field:
   A. Raises flux and lowers speed
   B. Leaves speed unchanged
   C. Reduces flux and raises speed above base
   D. Reduces speed below base
   Answer: C. N is inversely proportional to Phi. (Key point: Field control)

4. Which is true for field control compared to armature control?
   A. It has higher loss
   B. It gives speeds below base only
   C. It suits constant torque loads
   D. It is more efficient with lower loss
   Answer: D. Field current is small, so loss is small; it suits constant power. (Key point: Comparison)

## Related in Electrical Machines

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## Related in other subjects

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- [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×