DC MOTOR HAND NOTES by ELECTRICAL ZINDAGI | VAIBHAV GUPTA
DC motors are among the most fundamental electromechanical devices you'll meet in power and control systems. This note will give you the practical ideas you need — clear definitions, exam-friendly tricks, quick examples, and maintenance pointers — all in one place. Read on and use the download button at the end to grab printable digital notes.
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At its heart, a DC motor converts electrical energy (direct current) into mechanical rotation. The basic operation relies on the interaction between magnetic fields: a current-carrying conductor placed in a magnetic field experiences a force (Lorentz force), producing torque on the rotor.
The main parts are: stator (field poles), armature (rotor), commutator, and brushes. The commutator reverses current through the rotor windings so torque stays unidirectional. Small trick: think of the commutator as a mechanical H-bridge reversing current at just the right moment.
When current flows in the rotor conductors inside the stator field, each conductor sees a magnetic force. Combined forces create torque. A simple mnemonic: F = BIL (force = magnetic flux density × current × conductor length). For quick exam recall: torque ∝ flux × armature current.
Common DC motors: separately excited, shunt, series, and compound. A one-line memory aid:
- Series: high starting torque — good for starters & traction.
- Shunt: nearly constant speed — good for fans and lathes.
- Compound: combination advantages — used where both torque and speed regulation matter.
Remember three lines:
- Series: torque falls rapidly with speed (steep curve).
- Shunt: torque roughly proportional to armature current; speed is nearly constant under load.
- Compound: in between, can be short- or long-shunt depending on winding.
Example 1 (Starter motor): Use series DC motor where starting torque must be high (starter motor in older vehicles concept).
Exam trick: If a question asks what happens when field is reduced (shunt motor), answer: speed increases (weak field → less back-emf for same voltage → higher speed). Short explanation wins marks.
Brush wear is the most common maintenance item. Regularly check commutator for pitting and clean with appropriate tools; avoid chemical solvents that leave residues. When replacing brushes, match hardness & grade. For thermal protection, ensure the motor has proper ventilation and fitted thermal cutoffs for continuous-duty motors.
Key formulas to memorize:
- Torque ∝ Φ × I_a
- Back emf: E_b = kΦω
- Electrical equation: V = E_b + I_a R_a
If you want printable notes, click the download button below — it opens the digital notes in Google Drive.
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