DC MOTOR
DIRECT CURRENT ELECTRICAL MACHINE // ENERGY CONVERSION SYSTEM
YY: 2026
A DC Motor is an electromechanical device that converts electrical energy (DC) into mechanical rotational energy. It operates on the principle of Lorentz Force — a current-carrying conductor placed in a magnetic field experiences a force. This phenomenon was first systematically demonstrated by Michael Faraday and later formalized by F = BIL sinθ.
V=supply, Ra=armature resistance
Self-regulating counter voltage
| TYPE | FIELD CONNECTION | SPEED CHARACTERISTIC | TORQUE | BEST USE |
|---|---|---|---|---|
| Series | Field in series with armature | VARIABLE / HIGH DROP | Very High (starting) | Traction, cranes |
| Shunt | Field parallel to armature | NEARLY CONSTANT | Moderate | Lathes, fans, pumps |
| Compound | Both series + shunt field | ADJUSTABLE | High & stable | Elevators, presses |
| PMDC | Permanent magnets | STABLE | Moderate | Robotics, toys, auto |
| BLDC | Electronic commutation | PRECISE | High & efficient | Drones, EVs, cooling |
Varying the DC supply voltage changes armature current and thus speed. Provides below-base speed control. Used in Ward-Leonard drives and modern PWM controllers.
Reducing field current weakens flux, increasing speed above base. Used for above-base speed. Suitable in shunt and compound motors with field rheostat.
Modern solid-state PWM (Pulse Width Modulation) choppers vary effective voltage efficiently. Very common in robotics and EVs — low losses, high responsiveness.
| PARAMETER | DC MOTOR | AC MOTOR (INDUCTION) |
|---|---|---|
| Power Supply | DC only | AC (Single/Three Phase) |
| Speed Control | EASY & PRECISE | COMPLEX |
| Starting Torque | HIGH | MODERATE |
| Maintenance | HIGHER (brushes) | LOW |
| Cost | HIGHER | LOWER |
| Construction | Complex (commutator) | SIMPLE & ROBUST |
| Applications | Precision / Variable speed | Constant speed / Industrial |
Fleming’s Left Hand Rule states: if the index finger points in the direction of magnetic flux (Field), the middle finger in the direction of current (I), then the thumb points in the direction of Motion (Force). This is the fundamental rule governing DC motor operation.
connected in series
to armature
field windings
PWM controls effective voltage by varying the duty cycle — the fraction of time the signal is HIGH. Higher duty cycle → higher average voltage → higher speed.
The commutator is a split-ring mechanism that reverses the current direction in each armature coil precisely as it passes through the magnetic neutral axis (MNA). This maintains unidirectional torque.
Without commutation, the force on each conductor would alternate direction, causing the motor to oscillate rather than rotate continuously.
Motor runs at low load. Iron and mechanical losses dominate as a percentage. Efficiency is low because fixed losses are a large fraction of input power.
Maximum efficiency is reached when variable losses (copper/I²R) equal fixed losses. This is the ideal operating point — typically at 70–80% full load.
Beyond rated load, copper losses grow quadratically (I²R). Temperature rises, insulation degrades. Sustained overload causes motor burnout.
| SYMPTOM | PROBABLE CAUSE | DIAGNOSIS | REMEDY |
|---|---|---|---|
| ● Motor won’t start | Open circuit / no supply | Check supply voltage, fuses | Restore power, replace fuse |
| ● Excessive sparking | Worn brushes / dirty commutator | Inspect brush contact surface | Replace brushes, polish commutator |
| ● Overheating | Overload / blocked ventilation | Check load, measure current | Reduce load, clear vents |
| ● Vibration / noise | Worn bearings / unbalanced rotor | Listen for grinding, check alignment | Replace bearings, balance rotor |
| ● Low torque | Weak field / high armature resistance | Measure field current and Ia | Check field circuit, measure Ra |
| ● Speed too high | Weak field (series motor no-load) | Never run series motor unloaded | Always load series motors |
K=motor const, Φ=flux, Ia=armature current