Analysis of Sparking in Brushed DC Motors

Overview

In brushed DC motors, sparking between the brushes and commutator is a fairly common occurrence during operation. However, when spark intensity exceeds acceptable levels, it can degrade motor performance and even burn out the motor itself or connected equipment. The causes are varied — voltage fluctuations, brush wear, poor contact, and overloading among them. This article examines these causes in depth and proposes effective suppression measures.

The Physics Behind Motor Sparking

The fundamental cause of motor sparking is the forced interruption of a current-carrying inductive coil as the brush transitions between commutator segments. The magnetic energy stored in the coil releases instantaneously, ionizing the surrounding air and forming an arc.

In a brushed DC motor, the rotor consists of multiple coil windings, with each coil's two ends connected to a pair of commutator segments. Two stationary carbon brushes press against the surface of the rotating commutator, channeling current from the external power supply into the rotor coils. As the motor rotates, each brush slides from one commutator segment to the next — a process known as commutation.

During commutation, there is a brief moment when a brush simultaneously contacts two adjacent segments before lifting off the original one. The instant the brush leaves that segment, the current path through the associated coil is physically severed.

Coil is an inductive element, and inductance resists any sudden change in current. The moment current is interrupted, the coil generates a high reverse voltage — a back-EMF — in an attempt to sustain current flow. This self-induced voltage can easily reach tens of times the supply voltage.

Although the brush has physically separated from the commutator segment, the gap between them is extremely small — on the order of micrometers. The edges of commutator segments often carry microscopic burrs or wear-induced sharp points, where electric field intensity is concentrated. The high back-EMF is applied across this tiny air gap. When the voltage is sufficient, the air breaks down: molecules are ionized into electrons and positive ions, forming a conductive channel. Current then jumps across the gap rather than flowing through metal contact — and a spark is born.

From Spark to Arc

If the spark persists — due to poor brush contact, heavy loading causing commutation delay, or similar conditions — the ionized air gap drops to very low resistance, and current continues to flow through this high-temperature plasma channel, sustaining a full arc. Arc temperatures can reach 3,000°C to 8,000°C, hot enough to erode the copper surface of commutator segments, leaving pitting and oxidation. This simultaneously accelerates brush wear and, in hazardous environments such as coal mines or chemical plants, risks igniting flammable gases.

Spark Classification and Permissibility Standards

In theory, wherever inductance, current, and mechanical interruption coexist, sparking cannot be entirely eliminated. Minor sparking, however, is permitted by standard. To systematically assess the severity of commutation sparking, China's national standard GB755-87 defines five spark grades, evaluated by spark size, brightness, density, and the degree of damage to the commutator and brushes.

Based on engineering practice and national standards, these five grades fall into three permissibility categories:

Permitted for Continuous Long-Term Operation — Harmless Sparking

Grades 1, 1¼, and 1½ are all approved for sustained, continuous motor operation.

Grade 1: Theoretically spark-free — the ideal state, indicating excellent commutation performance.

Grade 1¼: Only isolated point sparks, causing no damage whatsoever to the commutator or brushes. Completely harmless.

Grade 1½: While slight surface marks may appear on the commutator and brushes, these do not progress further and pose no substantive threat to the motor. Long-term operation remains permissible.

Permitted Only Briefly Under Specific Conditions — Harmful Sparking

Grade 2 sparking is classified as harmful. Its arc energy is significant enough to cause burn damage to the commutator and brushes. It is not permitted under normal rated load, but may appear briefly in the following conditions:

During short-duration overcurrent or over-torque events — such as motor startup or sudden impact loading — sparking must not exceed Grade 2.

Under maximum working overload, commutation sparking must likewise remain below Grade 2.

Beyond these limits, irreversible damage to the commutator and brushes is likely.

Absolutely Prohibited During Normal Operation — Dangerous Sparking

Grade 3 sparking is classified as dangerous, characterized by:

Extremely high arc energy with severe destructive potential. In serious cases, it escalates into a ring fire event — the air around the entire brush track becomes intensely ionized, and the arc spreads across the full commutator surface, causing immediate and catastrophic motor damage or total failure.

Theoretically permissible only at the instant of direct-on-line starting or reversal, and only provided the commutator and brushes remain in a condition suitable for continued service.

Under all normal continuous operating conditions, Grade 3 sparking is strictly prohibited.

Measures to Prevent Harmful Sparking

Keep Inter-Segment Voltage Within Safe Limits

Maintain supply voltage within the motor's rated range. Pay attention to commutating pole adjustment — incorrect turn counts or reversed polarity can drive up inter-segment voltage to dangerous levels. Where possible, periodically inspect the rotor windings for short circuits or open circuits, using armature winding resistance tests and a short-circuit detector to verify that voltage distribution across commutator segments remains uniform.

Ensure Clean, Consistent Brush-to-Commutator Contact

Keep the commutator surface smooth and concentric. If the surface develops out-of-roundness, scoring, or an excessively thick oxide layer, polish it with fine-grit sandpaper — abrasive cloth must never be used. Verify that each brush fits its brush holder with the correct clearance. Monitor brush wear closely: as brushes wear down, their contact area with the commutator decreases, causing current density to rise and sparking to follow.

Avoid Sustained Overload Operation

Running the motor beyond its rated load for extended periods causes rotor speed to drop and current draw to climb — conditions that readily trigger sparking. Periodically check for mechanical binding or jam in the driven load, and ensure that motor ventilation and heat dissipation remain adequate.

Conclusion

Commutation sparking is unavoidable during the operation of brushed DC motors,but with proper design, quality components, and routine maintenance, harmful sparking is entirely preventable. Hongyang Motor's brushed DC motors are engineered with precision-wound armatures, high-grade carbon brushes, and tight commutator tolerances — all aimed at minimizing sparking and maximizing service life. Whether your application demands continuous high-load operation or precise speed control, our engineering team is ready to help you select the right motor for the job.