Shunt, Series and Compound
Three machines differing in one connection, and behaving nothing like each other. Includes the one that runs away.
Skip to the animationShunt, series and compound machines differ only in where the field winding is connected, and because that decides whether the flux is constant or follows the load, it determines the whole speed-torque characteristic through one equation.
One equation, three behaviours
N ∝ (V − I_aR_a)/Φ applies to all three. The only question is what the connection does to Φ as the load changes — and the characteristic follows in one step.
| Type | Field connection | Flux behaviour | Result |
|---|---|---|---|
| Shunt | Across the armature; many thin turns | Essentially constant | Nearly constant speed |
| Series | In series; few thick turns | Rises with load current | Speed collapses under load |
| Compound | One of each on the same poles | Partly load-dependent | Series torque, bounded speed |
Shunt
The field is across the supply, so its current and Φ are fixed. Speed falls only by the small I_aR_a term — about 5% from no load to full — which is why it is called a constant-speed motor. Lathes, fans and pumps use it.
Series
The field carries the armature current, so Φ ∝ I_a and therefore `T ∝ I_a²`. Torque grows as the square of current, giving huge starting torque, while speed collapses as load rises — exactly what traction needs.
And a real hazard: at no load I_a is small, so Φ is small, and N ∝ 1/Φ sends the speed toward destruction. A series motor runs away unloaded, which is why traction motors are geared or chain-coupled and never belt-driven — a broken belt would unload the motor instantly.
Compound, in both flavours
Cumulative compound has both windings aiding: series-like starting torque with the shunt winding setting a finite no-load speed. Used for presses, hoists and rolling mills — heavy intermittent loads that must not run away.
Differential compound has the series field opposing the shunt, so rising load weakens Φ and raises speed — positive feedback. On heavy overload the net flux can reverse and so can the rotation. It survives in syllabuses as a demonstration that the speed equation permits things practice will not.
The numbers you will be asked for
- Speed
N ∝ (V − I_a·R_a) / Φ
The one equation behind all three.
- Series torque
T ∝ I_a²
Because Φ ∝ I_a before saturation.
- Shunt speed regulation
(N_nl − N_fl)/N_fl ≈ 5%
Small, from the I_aR_a drop.
- Series runaway
as I_a → 0, Φ → 0, N → ∞
Never run one unloaded.
Advantages and disadvantages
Advantages
- Three distinct characteristics from one winding change.
- Series gives outstanding starting torque for traction.
- Shunt holds speed nearly constant without any controller.
- Compound combines the useful halves of both.
Disadvantages
- Series motors run away unloaded, which has caused real accidents.
- Shunt motors run away if the field circuit opens.
- Differential compound is unstable and effectively unusable.
- All of them keep the commutator's maintenance burden.
Watch it work
Check yourself
question 1 / 4
One question at a time. Pick an answer to see why it is right or wrong, then move on — there is no score to keep and nothing is saved.