Side-by-side comparison
| Parameter | DC Motor | DC Generator |
|---|---|---|
| Energy Conversion | Electrical energy → Mechanical energy | Mechanical energy → Electrical energy |
| Input | DC electrical supply (voltage and current) | Mechanical torque from prime mover (turbine, diesel engine) |
| Output | Shaft torque and rotation (rpm) | DC voltage and current at output terminals |
| Back EMF / Generated EMF | Back EMF E_b = V – I_a × R_a (opposes supply) | Generated EMF E_g = V + I_a × R_a (must exceed terminal voltage) |
| Armature Reaction | Distorts main field; causes commutation sparking | Same effect; more critical in generators under heavy load |
| Commutator Function | Converts AC induced EMF to DC mechanical output rotation | Converts AC generated EMF to DC output at brushes |
| Starting | Needs starter (3-point or 4-point) to limit high starting current | Field must be established before loading (build-up required) |
| Application | Cranes, traction (Delhi Metro), lathes, servo drives | Battery charging, electroplating, welding machines (MG set) |
Key differences
In a DC motor, the supply voltage V drives current through the armature against back EMF E_b = KΦN, so armature current I_a = (V – E_b)/R_a. A 220 V DC motor with R_a = 0.5 Ω at no-load runs at speed where E_b ≈ 218 V, giving I_a = 4 A; under full load E_b drops to 210 V and I_a = 20 A. In a DC generator, the generated EMF E_g must exceed terminal voltage: E_g = V + I_a × R_a. A separately excited generator producing 220 V at 50 A output with R_a = 0.4 Ω must generate E_g = 220 + 50×0.4 = 240 V internally. Same machine, opposite energy direction.
When to use DC Motor
Use a DC motor when you need controllable shaft speed and torque from a DC supply — for example, a separately excited DC motor driving a paper mill roll at precisely 500 rpm using a thyristor converter for speed control.
When to use DC Generator
Use a DC generator when you need a controlled DC voltage source from a mechanical input — for example, a DC generator coupled to a diesel engine in a remote mining site to provide 220 V DC for electroplating tanks.
Recommendation
For exam problems, identify energy flow first: electrical input means motor analysis (use E_b = V – I_a×R_a); mechanical input means generator analysis (use E_g = V + I_a×R_a). That direction-of-energy rule eliminates the most common sign error in DC machine problems.
Exam tip: Examiners test the voltage equation direction: for a motor, back EMF subtracts from supply; for a generator, internal EMF adds to drop across R_a to give terminal voltage — mixing these up loses all method marks.
Interview tip: Interviewers at traction or heavy engineering companies ask about regenerative braking — explain that the DC motor is reconnected as a separately excited generator with armature current reversed, returning energy to the DC bus or resistive braking grid.