Failure modes · Electromechanical

Brushed DC motor — Failure Modes & Failure Rate

DC motor using carbon brushes and a commutator to deliver current to the rotating armature. Cheap, easy to control, ubiquitous in automotive (mirror, window, wiper), industrial actuators, and small pumps. End-of-life is dominated by brush wear-out — a wear-out failure, not random.

λ typical

5.0×10-6 / h

Range

1.0×10-6 – 2.0×10-5

Source

NPRD-2016

Failure modes

Brush wear-out

Root causes: Carbon erosion at the brush-commutator interface, accelerated by current density, sparking, and dust/contamination. Predictable and progressive — typical brush life 1,000 to 10,000 hours depending on duty.
Detection: Increasing brush spark; decreasing torque output for the same current; visible brush length below replacement threshold; brushless current ripple shifts.
Mitigation: Specify brush length and grade for the duty cycle; periodic inspection in service-accessible installations; switch to brushless DC (BLDC) for long-life applications.

Commutator damage

Root causes: Pitting and grooving from sustained sparking; mica undercutting too shallow leaves the segments uneven; tar build-up between segments shorts adjacent commutator bars.
Detection: Audible / visual sparking; sudden current spikes during rotation; intermittent torque drops.
Mitigation: Match brush grade to commutator material; control duty cycle to avoid sustained over-current; clean commutator periodically in dirty environments.

Bearing failure

Root causes: Sleeve-bearing wear from inadequate lubrication; ball-bearing fatigue; axial load from misaligned coupling.
Detection: Increased noise/vibration; rotor wobble measurable with a dial indicator; current fluctuation as load varies.
Mitigation: Specify bearings rated for actual radial and axial load; align coupling carefully; sealed bearings in dusty environments. See <a href="/failure-modes/ball-bearing">ball-bearing failure modes</a>.

Winding insulation failure

Root causes: Thermal stress from over-current or stalled rotor; voltage transient breakdown; moisture ingress on non-sealed motors.
Detection: Insulation-resistance (megger) test below threshold; turn-to-turn short detected as low impedance or back-EMF anomaly; visible discolouration or burn smell.
Mitigation: Thermal protection (PTC or bimetal) embedded in windings; class-H insulation for high-temperature service; surge suppression on the supply rail.

Typical applications

Automotive auxiliary actuators (windows, wipers, mirrors, seat positioning); small industrial pumps and fans; cordless tools; medical-device linear actuators; toys and consumer electronics.

How to model in a fault tree

For ASIL functions like electric park-brake actuators or steering assist, the brush-wear-out mode invalidates the constant-λ assumption — fault-tree quantification should use a Weibull or piecewise-constant rate that captures end-of-life rise. For shorter-mission applications (e.g. window closure, ASIL B), constant-λ over the warranty period is usually accepted with a documented mission-time bound. See PMHF for the ISO 26262 metric.