Why do brushed motors spark?

What Is Commutation, and Why Does It Cause Sparking?

A brushed DC motor turns because the direction of current in the rotating armature windings must be reversed at exactly the right moment. This job is done by two parts working together: the commutator and the brushes.

Commutator (not a slip ring)

The commutator is a copper ring on the rotor shaft that is split into insulated segments. Each pair of opposing segments connects to a specific winding. A common mistake — including in many older articles — is to call this part a “slip ring.” That is incorrect: a slip ring is a continuous, unsegmented ring used in AC machines and wound-rotor motors to pass a steady current to a rotating part. A commutator is segmented precisely so it can reverse (commutate) the winding current. Sparking is a direct consequence of that segmented switching action.

Brushes

Brushes are stationary blocks of conductive material — usually a graphite–copper composite (about 50% copper in small motors, per maxon’s DC motor commutation documentation) or a plated precious-metal contact for micro-motors — spring-loaded against the spinning commutator. As the rotor turns, each brush slides across the segment gaps, momentarily short-circuiting and then re-routing current.

Reactance voltage and inductive kick

The core electrical cause of sparking is reactance voltage (also called the inductive kick). An armature winding stores energy in its magnetic field. When the brush leaves a segment, the circuit opens abruptly and the collapsing field induces a voltage spike e = −L·(di/dt). If this spike exceeds the dielectric strength of the opening air gap (roughly 3 kV/mm at the microscopic scale involved), the air ionizes and current jumps the gap as a spark. This is why sparking is worse at high speed, high load, and with high-inductance windings.

How a Spark Forms: Step-by-Step

1. Current flows through the winding. A brush feeds DC current into an armature coil via one commutator segment, energizing the coil and building a magnetic field.
2. The brush reaches a segment gap. As the rotor turns, the brush begins to bridge two segments — the coil is momentarily short-circuited so its current can start reversing (this interval is the “commutation period”).
3. The circuit opens. The brush lifts off the departing segment. The coil current has not fully reversed, so a “residual current” remains that must be interrupted.
4. Inductive kick fires. The winding inductance resists the change, inducing a high reactance voltage across the shrinking air gap. Armature reaction shifts the magnetic neutral, worsening the imbalance.
5. The air ionizes → spark. The voltage breaks down the gap and an arc discharges. Research on high-speed DC motors measured a typical arc duration of about 42 µs and residual current near 0.55 A across 1,000–5,000 rpm (Sawa et al., 2017 IEEE Holm Conference).

This sequence repeats for every segment on every rotation — thousands of times per minute — which is why a brushed motor produces a near-continuous shower of tiny sparks inside its housing.

Normal Sparking vs. Abnormal Sparking

Not all sparking is a fault. The key skill is telling harmless micro-sparks from destructive arcing. NEMA MG 1 and DC machine practice grade commutation on a spark scale from ½ (pinpoint) to 3 (heavy, destructive).

Brushed vs. Brushless: Why One Sparks and the Other Doesn’t

The single biggest reason engineers migrate from brushed DC motors to brushless DC (BLDC) motors is the elimination of the sparking commutation contact. Learn more in our guide to the difference between AC and DC motors.

For a deeper look at BLDC trade-offs, see the disadvantages of brushless DC motors and why brushless DC motors have 3 wires.

Engineering Data: Formulas, Temperatures & Brush Grades

Key formulas that govern sparking

Brush systems compared (maxon commutation data)

maxon reduces brush fire further by using an odd number of commutator segments (e.g., 7 bars on the DC-max 16), so less energy is switched per commutation event, cutting both sparking and EMI.

Insulation class & temperature limits (IEC 60034-1)

Sparking generates localized heat, so insulation-class limits matter. Greensky’s standard round DC brush motors use Class B insulation (130 °C, ≤80 K rise).

Spark-suppression techniques

• RC snubber / CLL capacitor — absorbs the inductive-kick energy across the brush gap (used in maxon precious-metal systems).
• Interpoles (commutating poles) — in larger DC machines, auxiliary poles generate a counter-voltage that cancels reactance voltage. A Hitachi study verified near sparkless commutation on a 2,000 kW DC motor using tuned interpole flux (Koharagi et al., 1993).
• Brush-grade matching — hardness and conductivity matched to voltage/current.
• Correct brush pressure — uniform across all brushes, within ±10% of spec.
• Neutral-plane adjustment — align brushes with the magnetic neutral axis under load.

Where Brushed Motors Are Fine — and Where Sparking Is a Deal-Breaker

Selection Guide: Minimizing Sparking in Your Design

1. Define duty and environment. Intermittent + sealed favors brushed; continuous, EMI-sensitive, or hazardous favors BLDC.
2. Estimate operating current. Use T = K_t·I to size current; keep it below the brush’s rated current density to limit Q = I²Rt heating.
3. Choose the brush grade. Graphite–copper for higher power; precious metal for clean low-current signals.
4. Cap the speed. Brush wear rises sharply above ~3,000 rpm (Sawa 2017); add a gearbox instead of over-speeding.
5. Verify thermal margin. Confirm winding stays within the insulation-class limit (Class B/F) under worst-case load.
6. Add suppression. Specify RC snubber/CLL and EMI filtering where required by the application.

Common Engineering Mistakes

Troubleshooting: Problem → Cause → Solution

Frequently Asked Questions

Is it normal for a brushed motor to spark?

Yes. A small, blue-white, discontinuous spark at the trailing edge of the brush is a normal by-product of mechanical commutation and has little effect on motor life. Large, continuous, yellow-red arcing or ring fire is abnormal and signals a fault.

Why exactly does the spark happen?

Because the armature windings are inductors. When a brush breaks contact with a segment, the winding’s collapsing magnetic field induces a high reactance voltage (e = −L·di/dt) that ionizes the opening air gap, producing an arc.

Do brushless motors spark?

No. BLDC motors replace the mechanical brush/commutator with electronic commutation, so there is no sliding contact to arc. This is the main reason they are chosen for EMI-sensitive and explosive environments.

Does sparking reduce motor efficiency?

Yes. Every spark is energy lost as heat, light, and radio-frequency noise instead of torque. Brushed motors typically peak at 65–80% efficiency versus 85–92%+ for comparable BLDC motors.

How do I reduce sparking in a brushed motor?

Match the brush grade to the current, set uniform brush pressure, keep the commutator clean and true, respect the rated speed, align the brushes to the neutral plane, and add an RC snubber or CLL capacitor for suppression.

Can excessive sparking damage the motor?

Yes. Heavy arcing rapidly erodes both the carbon brushes and the copper commutator, creates conductive carbon dust, and can escalate to a destructive “ring fire” that burns out the motor.