Maintenance Features of Brush Assemblies in Printed-Armature Motors

This article covers the operation and maintenance of brush assemblies in DC motors with flat printed-disc armatures, including brush types, brush-holder designs, common collector contact-surface issues, and methods used to address them.

DC motors with flat armatures are widely used in applications where compact size, high power density, and precise control are important—from transportation to industrial automation.

Among the most common designs are brushed DC motors with so-called flat printed-disc armatures, such as PMG-132 / Printed Armature DC Motors or Flat Armature DC Motors. Flat-commutator brushed motors with copper Lynch-type segments (commutator bars) include models such as LEM-200, Agni 95R/119, ETEK, and Motenergy ME0708.

Maintenance requirements for brush assemblies in motors with printed armatures and segmented flat armatures differ significantly. Despite their visual similarity, the contact methods, collector designs, and brush requirements are substantially different.

Printed armatures, such as the PMG-132, require precise brush pressure, clean contact surfaces, minimal clearance (+0.025...+0.023 mm), and consistent spring force of approximately 20–35 N. Segmented disc commutators, by contrast, may allow the commutator surface to be machined.

A DC motor brush assembly, regardless of the type of armature—flat or cylindrical, segmented or disc-type—includes three main components:

• Armature
• Brush holder – a housing with a mechanism that feeds the brush toward the commutator, such as a spring or traverse system, and maintains stable brush pressure and reliable electrical contact.
• Carbon brush – a conductive component made from graphite-based material in various shapes and used to transfer electrical current in brush-commutator assemblies of motors and generators.

Common models include PMG-132, PLG-130, Printed Motor Works motors, and certain Emoteq and ThinGap motors.

Printed Armature

A printed armature is a type of armature (rotor) in which the winding is formed as a flat printed circuit, structurally similar to a printed circuit board.

The armature is typically shaped as a disc or a set of discs mounted on a shaft. Unlike a conventional cylindrical armature with wound copper conductors, a printed armature uses copper tracks formed on an insulating substrate.

The substrate is made from a heat-resistant dielectric material, such as:

• polyimide films, including Kapton;
• fiberglass laminate;
• ceramics.

The conductive layer is copper. It may be applied by foil lamination, electroplating, or vacuum deposition.

The winding pattern is formed by photolithography or chemical etching. The track configuration determines the number of poles and commutation sections.

After etching, the conductive tracks are protected with insulating and protective coatings such as varnish, polyimide, or fluoropolymer coatings.

Repairability of Printed Armatures

Repairing a printed armature outside the original manufacturing environment is extremely difficult and, in many cases, economically impractical. In most applications, the printed armature is effectively a non-repairable component. If it fails, the complete motor or rotor assembly usually has to be replaced.

The main reasons printed armatures are difficult to repair include:

• Thin and densely packed conductive tracks. Damage cannot normally be restored using conventional soldering methods.
• Heat-sensitive layers. Soldering or local replacement may overheat and damage adjacent layers.
• Photolithographically formed windings. Their geometry is difficult to reproduce under normal workshop conditions.
• Difficulty restoring rotor balance. Even minor changes can affect dynamic balance and motor performance.

Non-Repairable Flat Printed Armature

Dynamic Balancing of a Printed-Circuit Armature

Dynamic balancing is the uniform distribution of the mass of a rotating component around its axis of rotation so that vibration, runout, and uneven loads on bearings and the overall assembly do not occur during operation.

A printed-circuit armature is a thin, usually disc-shaped structure that rotates at high speed. Even small mass irregularities, such as a drop of solder, a soldering trace, or wear of a conductive track, can cause micro-vibrations, rotor runout, premature wear of brushes and bearings, and reduced accuracy and service life.

A printed-circuit armature is balanced during factory manufacturing by adding or removing microscopic amounts of balancing compound, by laser trimming, or by precision grinding. Any intervention in the original structure, for example repairing a conductive track by soldering or replacing insulation, can disturb the original balance. In practice, restoring the balance without specialized equipment is extremely difficult.

The long maintenance-free service life of a printed-circuit armature is achieved through its highly precise but relatively difficult-to-repair design. Under normal operating conditions, service life may reach several tens of thousands of operating hours. In miniature motors, it may reach approximately 10,000–20,000 hours.

Service life depends on:

Brushes. Brushes are one of the main sources of commutator wear. Brush wear produces graphite dust, while printed-circuit armatures are highly sensitive to contamination.

Temperature. Overheating can damage polyimide coatings and distort the geometry of the armature disc.

Mechanical loads. Vibration and shock can damage thin conductive tracks and cause microcracks.

Electrical load. Excessive current outside the specified operating range may cause overheating and burnout of conductive tracks.

Operating environment. Moisture, dust, and aggressive gases may cause corrosion and electrical breakdown.

Recommended Maintenance

• Inspect every 30–50 operating hours.

• Clean with a soft brush, isopropyl alcohol, or dry compressed air. Avoid oil contamination.

• Replace brushes when their length is less than 50% of the original length.

• Polish the contact surface without aggressive abrasives; manual finishing with P1000–P2000 fine abrasive paper may be used where permitted.

• Check the motor’s technical documentation for specified brush fit, brush-holder clearance, and spring pressure.

Repair and Upgrade of Brush Holders for Printed-Circuit Armatures

Brush holders used with printed-circuit armatures are designed to operate with flat printed contact tracks. They require very precise brush positioning and reliable protection against contamination. Major manufacturers include Helwig Carbon and Mersen.

Based on many years of maintenance and repair experience, we identified a weak point in the conventional design: the brass cover that secures the brush in the brush holder may fail after only the second bending cycle during brush replacement.

We redesigned the brush-holder assembly and modified the brush design accordingly, eliminating this vulnerable component.

We offer a fully compatible replacement solution that can substitute the original design without equipment readjustment and without modifications to the motor itself.

Benefits

• Extended service life of the brush assembly.

• No need to replace the complete brush holder.

Our company can perform an engineering assessment and propose modernization solutions for printed-circuit armature brush assemblies with a focus on reliability and service life.

Brush holders with non-removable springs must be replaced as complete assemblies.

Manufacturing Brushes for Flat Motors with Printed-Circuit Armatures

Brush assemblies of this type may use round, rectangular, or trapezoidal brushes.

Typical brush materials include:

• copper-graphite;

• electrographite;

• silver-graphite for miniature motors;

copper-graphite for miniature motors.

•  Round Copper-Graphite Brushes

• Round Electrographite Brushes

•  Ultra-Small Electric Brush 

• Trapezoidal Brush for a Printed-Circuit Armatur​​​​​​​

The operating characteristics of printed-circuit armatures require a specially adapted brush-holder design that keeps the brush aligned in a precisely defined direction with minimal play to prevent sparking. This places strict requirements on the dimensions and geometry of the brush during manufacturing.

Our company can manufacture brushes in the required size and shape for brush assemblies used in motors with printed-circuit armatures. We also manufacture extra-small copper-graphite brushes with tangential and axial dimensions starting from 1.5 mm, including versions with phosphor-bronze springs.

Source: Lyudmila A. Reznikova