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DC excitation: shunt vs series vs compound

Three field arrangements, three personalities — constant-speed shunt, high-torque series, balanced compound.

Freshman ~9 min

Step 1 — Three classical ways to feed the field winding

Animation speed 0.55×

type — N/N_NL — T —

Reference notes

Use Next → on the narrator above to walk through the three classical DC machine excitation schemes and their characteristics.

The three classical excitation arrangements

Separately excited — field from an external DC source, independent of the armature. Used for tests and precise drives.
Shunt — field winding placed across the armature terminals. Many turns, high resistance, small field current. Flux nearly constant.
Series — field winding in series with the armature, carrying full armature current. Few turns, low resistance. Flux scales with load.
Compound — both shunt and series fields on the same poles. Cumulative (fields add) or differential (fields oppose; rare and unstable).

Shunt motor characteristics

Because Φ ≈ constant, the speed equation N = (V_t − I_a·R_a) / (K_a·Φ) shows N falls only slightly as load grows (R_a·I_a drop is small). Typical full-load speed regulation is 2–5 %.

Speed-load curve: nearly flat, slight downward droop.
Torque-current curve: T = K_a·Φ·I_a — linear in I_a (Φ ≈ constant).
Best for: applications needing approximately constant speed regardless of load — lathes, fans, blowers, machine tools.

Series motor characteristics

Because Φ ∝ I_a (the same current that drives torque drives flux), T ∝ I_a² (before saturation) and N ∝ 1/I_a (approximately).

Speed-load curve: starts very high, drops steeply as load grows. Danger: if unloaded, the motor accelerates without bound (no-load speed → ∞). Series motors must always run with a coupled load.
Torque-current curve: parabolic up to saturation, then linear. Very high starting torque.
Best for: traction (trains, trams, electric vehicles), cranes, hoists, starter motors in cars — anywhere you need huge starting torque and don't mind speed varying with load.

Compound motor characteristics

Cumulative compound: shunt provides baseline flux; series field adds proportional to load. Combines the high starting torque of a series motor with the safe no-load speed of a shunt motor.

Speed-load curve: between shunt and series — droops more than shunt but stays bounded at no-load.
Torque-current curve: between linear and parabolic.
Best for: rolling mills, shears, presses, elevators — high starting torque needed but safe to run unloaded.

Side-by-side comparison

Type	Flux behaviour	Speed-load	Starting torque	No-load behaviour
Shunt	~constant	nearly flat (2–5% droop)	moderate	safe (≈ rated speed)
Series	∝ I_a	drops sharply with load	very high (T ∝ I_a²)	RUNAWAY — must have load
Compound (cumulative)	grows with load (≪ series)	droops more than shunt	high	safe (bounded by shunt)

Take-away. The choice of excitation determines the entire personality of a DC machine. Same hardware, different field arrangement → wildly different speed-torque behaviour. Pick the field topology to match the load's demands — and the speed curve will follow automatically.

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