applications of DC machines

applications of DC machines

1. Why DC Machines Still Used (Despite AC Dominance)

DC machines offer excellent speed control (wide range, smooth) and high starting torque (series) via simple voltage control, and generator can provide variable DC. Though AC induction + VFD now dominates, DC still used where precise control needed.

2. Generator Applications (historical, now mostly rectified AC or electronic DC, but exam still asks)

Type	Voltage Characteristic	Application
Shunt generator	Voltage drops slightly with load (due to $I_aR_a$ + armature reaction)	Battery charging, exciter for AC alternator, lighting where load varies little
Series generator	Voltage rises with load (field ∝ $I_a$)	Booster in traction to compensate line drop, arc welding (rising) — rarely used now
Compound (cumulative)	Fairly constant/flat voltage (series aids shunt)	Power supply where constant $V$ needed despite load, electroplating

3. Motor Applications (dominant)

Motor Type	Speed-Torque Characteristic	Typical Applications
Shunt motor	Nearly constant speed ($N$ drops ~5% from no-load to full, due to $E_b$), medium starting torque	Lathe, drill, milling, centrifugal pump, blower, conveyor where constant speed needed
Series motor	High starting torque ($T∝I_a²$), speed falls sharply with load, dangerous no-load (speed → ∞)	Traction (electric train, tram, crane, hoist, forklift), electric vehicle where high torque at start
Compound (cumulative)	Moderate starting torque, fairly constant speed but droops less than shunt	Rolling mills, presses, elevators, heavy loads with flywheel

4. Worked Selection

Need to drive lathe at constant 1000 rpm under varying cut load: shunt motor (speed regulation ~5%)

Need to lift 10-ton load from rest: series motor (starting torque ~3× full load) or compound

Need to charge 12V battery from varying mechanical speed? Shunt generator with regulator.

5. Exam Points

• List generator applications (shunt — charging, series — booster, compound — constant) — 5 marks, frequent.
• List motor applications: shunt (constant speed, lathe/pump), series (high torque, traction/crane), compound (rolling mill) — 7 marks, very frequent (the most common applications question).
• Mention why DC motor speed control is easy ($N∝(V-I_aR_a)/Φ$) — 3 marks.
• Show speed-torque curves difference — 5 marks, frequent.