In one line
DC motors are classified by how the field winding is connected to the armature: separately excited and self-excited (shunt, series, compound), and each type suits different loads because of its speed-torque behaviour.
Explained simply
The field winding can be fed from its own supply, put side by side with the armature, put in line with it, or both. That small wiring choice changes the motor's personality: a shunt motor keeps a steady speed like a disciplined runner, while a series motor gives a huge push at start like a sprinter but speeds up dangerously if unloaded.
Technically, connection decides how flux depends on load current, and since \( T \propto \phi I_a \) and \( N \propto E_b/\phi \), it decides the torque and speed characteristics.
How it works
| Type | Field connection | Behaviour | Applications |
|---|---|---|---|
| Separately excited | field from separate DC source | wide, precise speed control | rolling mills, lab drives |
| Shunt | field in parallel with armature (many turns of thin wire) | flux almost constant, so nearly constant speed; moderate starting torque | lathes, drilling machines, fans, blowers, centrifugal pumps, conveyors |
| Series | field in series with armature (few turns of thick wire) | \( \phi \propto I_a \), so \( T \propto I_a^2 \) before saturation; very high starting torque; speed falls sharply with load and becomes dangerously high at no load | electric traction, cranes, hoists, starter motors, mixers |
| Compound (cumulative) | both shunt and series fields (long or short shunt) | high starting torque with safe no-load speed | presses, shears, punching machines, elevators, rolling mills |
- Compound motors may be cumulative (series field aids shunt field, the common type) or differential (opposes; rarely used because it can be unstable).
- A long-shunt compound has the shunt field across armature plus series field; short-shunt has it across the armature only.
- A series motor must never be started or run without load (or through a belt that may slip).
Key formula
Shunt: \( \phi \approx \text{constant} \Rightarrow T \propto I_a \). Series (unsaturated): \( \phi \propto I_a \Rightarrow T \propto I_a^2,\; N \propto \dfrac{E_b}{I_a} \)
\( T \) = torque (N·m), \( \phi \) = flux (Wb), \( I_a \) = armature current (A), \( N \) = speed (rpm), \( E_b \) = back EMF (V).
Quick example
Older electric locomotives and trams used DC series motors because a heavy train needs the very large starting torque that a series motor gives. A workshop lathe uses a shunt motor because the cutting speed must stay steady as the cut deepens.
Common mistakes
- Recommending a series motor for a belt-driven load (if the belt slips, the motor races).
- Saying shunt field windings have few turns of thick wire (that is the series field).
- Writing \( T \propto I_a^2 \) for a shunt motor.
How MU asks it
Numericals are not expected in this module (as per the MU syllabus), so questions are theory. Typical: "Classify DC motors and give applications of each" (5 marks) or "Why should a DC series motor never be started at no load?" (3 marks). For full marks draw connection diagrams of shunt, series, long- and short-shunt compound motors, sketch or describe their speed-torque behaviour, and give two applications for each with a reason.