Why Are Commutators Segmented Instead of Solid Rings?

If you work with DC motors, universal motors, or generators long enough, this question eventually appears on a drawing review call:

“Why don’t we just use a solid copper ring here? Wouldn’t that be simpler?”

Short answer: a solid ring stops being a commutator. It becomes a slip ring. And your machine stops behaving like the DC machine you specced.

The longer answer is where things get interesting for engineers, and for purchasing.

1. What a “solid commutator” would actually do

Imagine you replace the segmented commutator with one continuous copper ring, still pressing carbon brushes on it.

Now you have:

• One ring
• One continuous contact surface
• No discrete bars, no mica insulation between them

Electrically, that’s a slip ring arrangement. Power transfer is still possible, sure, but you’ve removed the core function of a DC commutator: switching current between individual armature coils so the torque stays roughly unidirectional.

So with a solid ring:

• No segment-to-segment switching
• No controlled commutation zone
• No way to map individual coils to specific angular positions

You can still build a machine around that, of course. It just isn’t a DC commutated machine anymore.

For a buyer: a “solid commutator” on a DC motor drawing is really a red flag. Either the term is being misused, or the design isn’t what the spec claims.

2. Electrical reasons the commutator must be segmented

2.1 Current reversal has to be local, not global

In a practical DC machine, each armature coil terminates on a dedicated pair of commutator segments. As the rotor turns, the brush spans two adjacent segments for a brief angle. During that short time, the coil is effectively shorted and its current reverses.

That only works because:

• Each segment is electrically isolated from the next by mica or another insulation
• The brush only connects a few segments at once

With a solid ring, there is nothing to switch. Every coil end tied to the same copper body would just sit at the same potential. No commutation. No way to reverse individual coil currents.

So segmentation is not a feature. It is the whole method.

2.2 Torque ripple and “too few segments” reality

Why not just use a tiny number of big segments? Textbooks show the “two segment” picture. Real machines almost never do that.

More segments mean:

• At any rotor angle, more coils are near their ideal torque position
• The torque ripple is reduced
• The machine is easier to start and runs more smoothly

Engineers on design forums often summarise it very bluntly: adding more segments and coils keeps at least one coil in a good torque position at all times.

But there’s a trade:

• More segments → more bars to manufacture, more joints, more inspection points
• Fewer segments → cheaper part, but higher ripple torque, rougher running, higher brush stress

For an industrial buyer, this is where cheap commutators start to get expensive again in the field.

2.3 Managing inductive stress and sparking

Each coil carries current and has inductance. During commutation you are forcing that coil’s current to change sign in a very short time window. That means high di/dt, which wants to create voltage spikes and brush sparking.

Segmented design helps you manage this:

• Each commutating coil is tied to a small part of the circumference
• The brush shorts only a couple of segments at once
• The voltage per segment is limited, so the arc energy is limited

Educational material often points out that segmented commutators allow more precise control over commutation and reduce the risk of sparking when designed correctly.

With a solid ring, there is no per-segment voltage, because there are no segments. Every coil end is effectively on the same electrode. You lose the whole control mechanism.

3. Mechanical and thermal reasons for segmentation

Even if you ignored the physics and tried to force a “solid commutator”, the mechanics would fight you.

3.1 Copper bars plus mica are a stress relief system

Practical commutators are built as:

• Many wedge-shaped copper bars
• Radially locked in a hub
• Each bar separated by segment mica or a similar insulator

This construction:

• Electrically isolates each bar
• Allows the copper to expand and contract with temperature
• Lets the stack behave like a flexible ring instead of a rigid band

A solid thick copper ring, shrunk onto a steel hub, would see:

• Differential expansion between copper and steel
• High circumferential stress
• Out-of-round issues as it heats and cools

Which you will eventually meet as:

• Uneven brush loading
• Localised wear
• Noise, vibration, brush chatter

The segmented construction is partly about electricity, partly about letting the mechanics breathe.

3.2 Slots, contamination, and wear control

Real commutators live in dust, oil, and carbon brush debris. Segment slots, undercut depth, and slot shape all affect how that contamination behaves.

Design references talk about V-slots versus U-slots:

• V-slots: collect less dirt, better in slow or dirty environments
• U-slots: allow the bar to wear deeper before undercutting is needed

You cannot tune that behaviour with a featureless solid ring. Segments and their slots are part of the tribology package: current density, brush grade, surface speed, environment.

3.3 Repairability and “refilling”

On larger industrial machines, commutators are built to be repairable:

• Individual segments can be removed
• Insulation can be replaced
• The stack can be “refilled” instead of scrapping the whole rotor

This matters for:

• Mining drives
• Steel mill motors
• Large DC generators

If you commit to a solid ring concept, you also commit to full-rotor replacement on any serious damage.

Purchasing feels that as:

• Higher spares cost
• Longer outages (rotor replacement vs commutator rebuild)
• More logistics complexity on large equipment

Segmentation is one of the reasons big DC machines are still serviceable after decades.

4. Segmented commutator vs solid ring vs slip ring

To line things up for both engineers and buyers, here is a compact comparison. The “solid commutator” column is really a thought experiment — in practice it behaves like a slip ring for most of these points.

If you are reading this as a buyer: the column that actually exists in your RFQs is the first one.

5. Design choices that make segmentation work (or not)

Once you accept that “segmented” is not optional, the real engineering decisions begin.

5.1 Number of segments

More segments:

• Lower voltage per bar
• Shorter commutation per bar
• Smoother torque and lower ripple
• Smaller current step per segment

But also:

• More complex manufacturing
• More joints to inspect
• Tighter tolerance on bar width and gap

Too few segments, on the other hand:

• Increase bar voltage
• Increase brush sparking risk
• Can limit maximum speed under load

A design team typically balances segment count against rated voltage, speed, allowable ripple, and cost. Reference material on DC machines makes the same point: extra coils and segments are added specifically to smooth the pulsating output.

5.2 Segment geometry and connection style

Not all segmented commutators look the same inside.

Common options:

• Conventional (slotted) commutators: each segment has slits to hold the magnet wire, then the wire is fused; these are very robust and popular for high-vibration or high-RPM uses.
• Tang / hook type: wires are bent around tangs or hooks on the riser, with different assembly advantages.
• Groove or shell designs: for larger machines, focused on mechanical strength and cooling.

Geometry choices affect:

• Vibration resistance
• Assembly cost
• Ease of rework
• How compact the motor can be for a given rating

Again, something a “solid ring” simply cannot do: there is nowhere to land and secure the winding ends.

5.3 Insulation system

The unsung hero here is mica and related materials.

Industrial commutator guides note that:

• Copper bars are separated from each other and from the hub by mica
• Mica stays stable under the pressure and temperature of operation
• The combination of copper + mica segments gives both insulation and mechanical stability

For purchasing, “mica grade” is often just a line item. For the engineer, it has consequences:

• Temperature rating
• Moisture resistance
• Machinability and undercut behaviour
• Long-term dimensional stability

If the insulation creeps or cracks, your segment stack loses alignment or tracking. At that point, solid ring or segmented, everyone is unhappy.

5.4 Brush system pairing

Segmentation and brush system are a matched set.

Brush width, brush material, and spring pressure are chosen so that:

• The brush spans around 2–3 segments
• The commutating coil is properly shorted in the commutation zone
• Current density stays within allowable limits at rated load

A change in segment width or gap that looks minor on a drawing can move you into a region where:

• The brush intermittently bridges too many segments
• Commutation is extended in angle
• Sparking increases

So when a supplier proposes a “simplified” commutator design, make sure brush geometry is being rechecked alongside.

6. What this means for sourcing and specification

Engineers usually ask, “How many segments do we really need?”

Purchasing usually asks, “What levers do I have on cost without risking field failures?”

Segmentation is one of those areas where a bit of structure in the specification can prevent bad surprises.

6.1 Practical checklist for buyers

When you send a commutator RFQ, consider making these points explicit:

• Type of machine and duty
  • DC motor, universal motor, generator
  • Duty cycle, start/stop frequency, reversal or not
• Electrical data
  • Rated voltage and speed
  • Target ripple torque range (if known)
  • Expected overloads, short-term and continuous
• Mechanical constraints
  • Maximum outer diameter and stack length
  • Allowable runout and out-of-round
  • Vibration environment
• Segmentation details
  • Target number of segments (or at least a range)
  • Preferred bar material
  • Slot style (U/V etc.) if your application is sensitive to contamination
• Service expectations
  • Whether segment replacement / refilling must be possible
  • Minimum expected resurfacing allowance

Suppliers that specialise in commutators often publish their own design guidance and standard series. They will happily tune segment count, bar shape, and mica system to your duty, as long as they have enough detail.

7. FAQs: segmented commutators vs solid rings

8. Key takeaways for engineers and buyers

A commutator is segmented because it has to be:

• Electrically, to give you controlled current reversal, manageable voltage per segment, and acceptable torque ripple.
• Mechanically, to survive thermal expansion, contamination, and decades of brush contact without turning into an oval.
• Commercially, so that large machines can be serviced by replacing segments instead of entire rotors, keeping lifecycle cost under control.

So when someone asks, “Why can’t we use a solid ring?”, the real answer is:

Because then it isn’t a commutator anymore. It’s just copper. And all the useful behaviour lives in the segments.