classification

classification

1. What Classification Refers To (DC Machines)

Classification of DC machines = categorization by how field winding is connected to armature (self-excited vs separately excited) and by application/behaviour. It determines characteristics ($E$ vs $I$, $T$ vs speed).

2. Based on Excitation (how $Φ$ field produced)

Type	Connection	Field Current $I_f$ vs Armature $I_a$	Characteristics
Separately excited	Field winding separate DC source, not connected to armature	$I_f$ independent	Constant $Φ$ (if $I_f$ constant) ⇒ $E∝N$, $T∝I_a$
Self-excited — Shunt	Field parallel with armature across supply	$I_f = V/R_f$ (small, parallel)	$Φ$ ≈ constant (since $V$ nearly constant) ⇒ shunt generator $V$ slightly droops, shunt motor nearly constant speed
Self-excited — Series	Field in series with armature (same current)	$I_f = I_a$	$Φ∝I_a$ ⇒ generator voltage rises with load, motor high starting torque ($T∝I_a²$), speed falls with load
Self-excited — Compound	Both shunt and series windings (cumulative or differential)	Mix	Cumulative compound (series aids shunt) ⇒ fairly constant $V$ (generator) or stable speed (motor); Differential subtracts ⇒ rarely used

3. Based on Function

• DC Generator: converts mechanical → DC (prime mover drives).
• DC Motor: DC → mechanical (load driven). Same machine reversed.

4. Worked Selection

Need constant speed drive (lathe): shunt motor (speed drop ~5% from no load to full load) ✓

Need high starting torque (crane, traction): series motor (T∝I²) ✓

Need stable voltage (generator for battery charging): compound generator (shunt alone droops, series alone rises, compound flat) ✓

5. Exam Points

• List classification: separately excited, shunt, series, compound (with diagram connections) — 5-7 marks, very frequent (almost certain to ask types).
• Show $I_f$ relation and $Φ$ behaviour per type — 5 marks, frequent.
• Mention generator vs motor use — 3 marks.
• Applications: shunt (constant speed), series (high torque), compound (stable) — 5 marks, very frequent.