---
title: "Ward-Leonard system of speed control"
subject: "Utilization of Electrical Energy"
chapter: 3
key: "semester_7/utilization_of_electrical_energy:ward-leonard-speed-control"
source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/ward-leonard-speed-control
site: BCT Engineering
---

# Ward-Leonard system of speed control

Utilization of Electrical Energy · Chapter 3: Control of Electric Drive

- Asked in: 2080 Bhadra, 8 marks
- Video: [Ward Leonard Method of Speed Control of DC Motor | Notes4EE](https://www.youtube.com/watch?v=h25wpehqeB4) — Notes4EE - Electrical Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/ward-leonard-speed-control
- Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-3

## Question

Explain Ward-Leonard System of speed control with diagram. Also state the advantages and disadvantages of this method.

## Short answer

A motor-generator set supplies variable armature voltage to a DC motor, enabling smooth speed control from zero to base speed with inherent regenerative braking. Advantages: wide range, four-quadrant. Disadvantages: low efficiency, three machines of similar rating.

## Answer

The **Ward-Leonard system** controls the speed of a separately excited dc motor by varying its **armature voltage**, which is obtained from a separately excited dc generator driven at constant speed by an induction (or synchronous) motor. The motor field is kept constant.

#### Diagram
```text
 3-ph   +------+  shaft  +-----+      +-----+
 ac ----|  IM  |=========|  G  |------|  M  |===> load
        +------+         +-----+      +-----+
                           |            |
                        gen. field   motor field
                        (variable,   (constant,
                         reversible)  from exciter)
           exciter --[rheostat + reversing switch]
```
- **M–G set**: induction motor (IM) drives the dc generator G at constant speed.
- The generator armature is connected directly to the motor M armature.
- Generator field is fed from an exciter through a potential divider and reversing switch.

#### Working
1. Generator voltage $V_g \propto \phi_g$ (speed constant). Changing the generator field current changes $V_g$ smoothly from zero to rated value.
2. Motor speed $N \propto \dfrac{V_g - I_a R}{\phi_m}$, so speed rises from zero to base speed as $V_g$ is raised (constant torque region).
3. Reversing the generator field reverses $V_g$, so the motor reverses without switching the armature circuit.
4. Above base speed, the motor field can be weakened (constant power region).
5. **Regenerative braking**: when $V_g$ is reduced below the motor emf, current reverses; the motor acts as a generator, drives G as a motor, and G drives the IM above synchronous speed, returning energy to the supply. Hence four-quadrant operation is possible.

#### Advantages
- Very wide and smooth speed control (about 1 : 10 or more) in both directions.
- Speed from zero upward; good starting, no starting resistance.
- Inherent regenerative braking and four-quadrant operation.
- Speed regulation good; small control currents (field circuits only).
- With a synchronous motor or flywheel (Ilgner set), power factor and load peaks on supply are improved.

#### Disadvantages
- Three machines of nearly the same rating: high cost, large floor space and heavy foundation.
- Low overall efficiency (about 60–70 %), especially at light load.
- Noise, maintenance of commutators and brushes.
- Slower response than static drives.

It is used for rolling mills, elevators, mine winders, paper machines and cranes; today it is largely replaced by the **static Ward-Leonard** (thyristor converter) drive.

## Check yourself

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

1. Ward-Leonard controls motor speed by varying:
   A. The motor field only
   B. Armature voltage supplied by a variable-field generator
   C. Armature resistance
   D. Supply frequency
   Answer: B. Generator voltage is varied through its field current, giving smooth armature voltage control. (Key point: Principle)

2. How is the motor reversed?
   A. By reversing the generator field
   B. By reversing the IM
   C. By changing the motor armature connections
   D. By removing the exciter
   Answer: A. Reversing the generator field reverses Vg, so no switching in the armature circuit is needed. (Key point: Reversal)

3. How is speed obtained above base speed?
   A. Raise the generator voltage above rated
   B. Weaken the motor field (constant power region)
   C. Add armature resistance
   D. Increase IM supply
   Answer: B. Below base speed Vg varies at constant torque; above it the motor field is weakened. (Key point: Above base speed)

4. A main disadvantage is:
   A. Narrow speed range
   B. No regenerative braking
   C. Three machines of similar rating, high cost and low efficiency (60-70 %)
   D. Needs starting resistance
   Answer: C. The M-G set needs three machines, so it is costly, bulky and inefficient. (Key point: Disadvantage)

## Related in Utilization of Electrical Energy

- [Static Ward-Leonard four quadrant control](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Astatic-ward-leonard-four-quadrant) — Utilization of Electrical Energy, chapter 3
- [PID control of speed and torque](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Apid-control-speed-torque-motor) — Utilization of Electrical Energy, chapter 3, asked 2×

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