How Does a Split Ring Commutator Reverse Current?

A split ring commutator reverses current by acting as a timed rotary switch: every half-turn (180°), each end of the armature coil swaps which brush it touches, so the supply polarity seen by the coil flips, while the polarity seen by the external circuit stays consistent. That’s the whole trick.

The core idea: a mechanical polarity flip

Forget the exam line for a moment and see it as a very simple device: two copper half-rings on the shaft, separated by an insulating gap, with stationary brushes pressing on them. The half-rings rotate with the coil. The brushes do not.

Each half-ring is permanently wired to one end of the coil. The brushes are permanently wired to the DC supply. As the rotor turns, the brushes slide from one half-ring to the other. At a certain angle, each brush crosses the gap and changes which half-ring (and therefore which coil end) it is connected to. That swap is the current reversal in the coil. Textbooks compress this into a single sentence: “The split ring commutator reverses the current every half-turn to keep torque in the same direction.”

In other words, the commutator doesn’t generate anything clever. It just keeps re-wiring the same two coil ends to the same two brushes at very specific angles.

Two points of view: brushes vs coil

The confusing part is that “direction of current” depends on who you are pretending to be.

From the point of view of the brushes and the DC supply, nothing special happens. One brush is “always” positive, the other “always” negative. They sit still. Current leaves the positive terminal, passes through the coil in some route, and returns to the negative terminal. No alternation here; the external circuit always sees a fixed polarity.

From the point of view of the coil conductors (AB and CD in standard diagrams), life is different. As the commutator rotates, the conductor that used to be on the “positive” side of the coil becomes connected to the negative brush, and the one that used to be on the “negative” side becomes connected to the positive brush. So, inside the rotating frame, the current through AB and CD actually flips direction every half-turn.

Same hardware. Two descriptions. That mismatch in description is where a lot of exam confusion comes from.

Timing: why the flip happens at 90° or 180° in diagrams

In the standard school diagram, the coil is horizontal when torque is maximum and vertical when torque would naturally change sign. Left alone, the magnetic force on each side of the coil would reverse as it passes through this vertical position, and the motor would hesitate or even stop.

The commutator is built so that the brushes cross the gaps right around this “dangerous” position. During that brief moment, contact is poor or nearly zero; the coil is effectively open-circuited. As the coil moves a touch past vertical, each brush lands on the opposite half-ring. Now the current through the coil has reversed, so the force on each side of the coil also reverses. Instead of dragging the coil back, it pushes it on in the same rotational direction.

So the sequence over one half-turn is roughly:

The coil approaches vertical, current still flowing in its original direction, torque falling. Near vertical, contact almost breaks; torque is minimal anyway, so that brief loss doesn’t matter. Just past vertical, current re-appears, reversed in the coil. Torque rises again, now pushing the coil in the original rotational sense.

Strictly speaking, real motors often have many segments and coils, so the transition is smoother than this cartoon. But the principle is the same.

A compact map of what’s flipping

It helps to pin down what changes sign and what does not. Treat AB and CD as the two active sides of the coil, and assume a simple two-pole field.

The direction labels (A → B, etc.) are just a bookkeeping choice. The key is that:

The brush that is “positive” in the circuit keeps its sign. The coil side that is “going up” always carries current in the direction needed to create an upward force, even though the current in that particular piece of copper has reversed compared with half a turn before.

That’s the whole game: reassign which physical conductor is attached to each brush, so forces stay consistent even though the rotor has spun half a turn.

Why this keeps torque one-sided

If you write the magnetic force on a straight conductor as F = I L × B, the sign of the torque depends on the sign of I in each active side. Without a commutator, after half a revolution the geometry flips, so the same current would now produce torque in the opposite direction.

By reversing the current in each conductor exactly when the geometry flips, the product “geometry sign × current sign” stays the same. In basic teaching notes this is usually stated as “The split ring commutator reverses the current every half-turn so that the coil continues to rotate in the same direction.”

So the commutator doesn’t increase torque. It just stops the torque changing sign. Very ordinary, very useful.

Connection to DC generators: the same mechanism, reversed

In a DC motor, you feed in DC and get mechanical rotation; the commutator flips the internal current to keep torque direction fixed.

In a DC generator, the physics flips around. The rotating coil in the field creates an induced emf that naturally alternates. Left alone, with slip rings, you would get an AC output. Replace the slip rings with a split ring commutator and now, just as the induced emf in the coil changes sign, the connections to the external brushes also swap, so the external circuit still sees a one-directional output.

Inside the coil, the induced emf is AC. At the brushes, after the mechanical switching, the output is a rectified, always-same-sign waveform. So in a generator, the commutator is not “reversing the current in the coil” for a torque reason; it is “reversing which coil end is connected to which brush” to tidy up the output.

Same hardware, different role. That is usually not emphasised, but it helps the idea feel less arbitrary.

Slip ring vs split ring: why bother splitting at all?

A continuous slip ring is a simple rotating contact used where you do not want the polarity to change as seen by the rotating part: transferring power or signals into a rotating antenna, an AC rotor, or a turntable, for example. It is just a ring.

A split ring is that ring cut into two insulated halves, each half connected to a different end of the armature. Because it is cut, each brush can slide from one half to the other as the shaft rotates. That sliding creates the periodic swapping of connections and therefore the effective current reversal in the armature, which is what DC motors and DC generators need.

So the “split” is not aesthetic. It embeds timing into solid metal.

Common points of confusion

Many learners mix up four slightly different statements. Treat them as separate:

“Current in the external circuit is DC.” True for a DC motor or DC generator with a split ring; the brushes see fixed polarity, though the waveform may ripple.

“Current in each armature conductor reverses every half-turn.” True as a description in the rotor frame; which conductor is attached to which brush changes.

“The commutator reverses direction of current in the coil every half revolution.” A compact teaching slogan, usually meant in the rotor sense.

“The commutator reverses direction of torque every half revolution.” Not correct; it prevents the torque from reversing by flipping coil polarity at the right time.

Mixing these up leads to vague phrases like “it keeps the motor going” with no clear mechanism behind them. Once you separate the views, the logic becomes less mysterious.

Real-world quirks that the simple diagram hides

Real motors rarely use a single loop and two commutator segments. They use many coils spaced around the armature, with many commutator bars. That spreads the switching over angle, making the current and torque more uniform. At any instant, some coils are just entering the active region, some are at peak torque, and some are being switched. The same basic switch-every-180° rule is applied per coil, but the effect is averaged out.

Brushes are usually carbon, and there is contact resistance, arcing, and wear. The neat textbook picture where contact is zero exactly at vertical is really a gradual transition over a small angle. Engineers worry about this; exam questions usually do not.

Designers can also shift the brush position slightly away from the ideal geometrical neutral plane to compensate for armature reaction in larger machines. That changes the exact angle at which reversal occurs but keeps the same underlying rule: switch when the conductors are in a region where the induced emf is small, so the commutation is cleaner.

A short wrap-up

A split ring commutator is just a slip ring with a carefully placed cut and two brushes in contact. As the rotor turns, that cut causes each brush to swap which coil end it touches every half-turn. Seen from outside, polarity at the terminals stays fixed. Seen from inside the rotor, current in the conductors keeps flipping to match their changing position in the field, so the torque on the shaft keeps pushing the same way.