The commutator in action
A purely mechanical inverter/rectifier — AC inside the coil, DC at the brushes. The single most elegant trick in machine design.
Step 1 — A single rotating coil between N–S poles
Reference notes
Use Next → on the narrator above to step through how the commutator turns an internally-alternating coil EMF into a DC terminal voltage.
The puzzle the commutator solves
A coil rotating in a fixed magnetic field naturally sees an alternating flux linkage. As the coil rotates through one full revolution, its sides pass under N then S then N again — by Faraday's law, the EMF induced in the coil is an alternating waveform, very nearly sinusoidal:
How, then, do you get a DC voltage at the terminals?
The commutator is a mechanical inverter / rectifier
Split a copper ring into two half-segments insulated from each other. Connect each coil end to one half-segment. Brushes rest on the segments and slide as the rotor turns. Crucially, the brushes are positioned so that as the coil rotates past the magnetic-neutral axis (where ecoil changes sign), the brushes switch from one segment to the other.
The result: the brushes always touch whichever coil end is at the positive polarity at that instant. The coil's AC EMF gets rectified at the brushes. The terminal voltage vbrush is the absolute value of the coil EMF — a pulsating DC waveform.
Generator mode vs motor mode
- Generator: shaft is mechanically driven. EMF is induced in the coil (Faraday). Commutator converts the internal AC into DC at the brushes. External circuit sees a DC source.
- Motor: external DC supply is applied at the brushes. Commutator converts that DC into the AC pattern the rotating coil needs (current must reverse twice per revolution). The coil sees AC; the supply sees DC.
The same machine, the same commutator, runs either direction.
Why the ripple, and why multiple coils smooth it
A single coil's rectified output is a half-sine pulsating between zero and Emax twice per revolution — usable, but rippley. Real DC machines have many coils distributed around the armature, each connected to its own commutator segment. At any moment, several coils are in the "high EMF" zone and several are switching — the brush sees the smoothed sum. A real machine's terminal voltage is essentially flat DC with a small high-frequency ripple from commutation events.
The deeper equivalent picture
From outside the machine, the rotor's armature MMF appears stationary in space, always pointing along the brush axis (the quadrature axis, 90° from the field axis) — regardless of how fast the rotor is spinning. The commutator keeps the armature MMF aligned this way by continuously switching which physical conductors carry which polarity. This is what makes DC-machine analysis so clean: the armature reaction sits in a fixed direction in space.
What you need from this lesson
- The commutator is purely mechanical — copper segments + sliding brushes.
- Inside the rotor, EMFs and currents are alternating.
- At the brush terminals, EMFs and currents are direct.
- The same hardware works as generator (AC→DC) or motor (DC→AC inside).
- The armature MMF stays put on the q-axis, regardless of rotation speed — that's why DC machines are easier to analyse than AC machines.
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