To Brush or Not to Brush?


A: Brushless alternators were developed a few years ago for commercial applications with one advantage over brush types—they didn’t have any brushes to wear out, so they lasted longer than the brush technology of the period. Bearings and regulators could still fail, but there were no brushes to wear out. These alternators use a dedicated stator and rotor to induce voltage across a secondary air gap to excite the rotor.

But this alternative design also generates a high cost, noticeable lack of efficiency, relatively low thermal resistance, and excessive size and weight. Compared to some brush types, the brushless types had inferior attributes, but were attractive to many because of their longer life. Some come with a 300,000-mile warranty.

So, the challenge for technology-driven alternator suppliers was to develop a reasonably priced, long-lasting brushed alternator. The industry needed a design that eliminated the only reason for using a costly brushless design, and enhanced the major advantages of the brush types.

Bosch was first to respond, with the brand new “Long Haul” alternator, which improves service life over the existing brush technology. This meant improving brush life. Two innovations resulted—smaller slip ring diameters to allow space for longer brushes, slower relative speed, and proprietary slip ring alloys to reduce wear. Bearings rated to 18,000 rpm and proprietary lubricants were also employed to contribute to longer life.

Service life was so extended that these designs now come with warranties up to 24 months or 250,000 miles—at nearly half the cost of a brushless.

Will both do the job for fleets, which put heavy demands on their alternators?

The design improvements in some brush type alternators have gone beyond longer life. Many of today’s commercial vehicles have higher electrical requirements and increased loads on the battery. The new alternator design provides higher output at all speeds. This means faster charging, more rapid battery recovery, and results in reduced battery cycling and increased battery life.

Twin internal fans improve cooling to maintain efficiency with the higher under hood temperatures developed by late model commercial vehicles. Further temperature defense comes from built in regulator self-protection. Instead of failing when exposed to extreme temperatures that would cause most alternators to expire, the regulator reduces output. This lowers internal heat to protect the unit from damage. The regulator returns to normal output as the temperature drops.

Brush designs have always been much more efficient than brushless. They take less energy to produce the same output as a brushless design. New designs such as the Long Haul further improve efficiency with a patented armature winding which incorporates up to 20 percent more copper.

Any impact on fuel usage? How do they stack up for maintenance?

A significant benefit of the brush type is the fuel savings realized by greater efficiency. A recent analysis made by a large fleet averaging between 150-190K miles per vehicle per year showed the same truck operating under the same conditions reported a significant increase in fuel economy with the Long Haul alternator over the previous brushless design. And, brush type units are engineered with fleet maintenance in mind.

Long Haul alternators, for instance, are available in the SAE J180 and pad mount. The higher output allows them to replace several lower output units. They can be operated as a one-wire design or connected to an indicator light and instruments. Multiple and universal connection terminals allow one unit to replace over 250 OE part numbers using original wiring.

Are brushless alternators really worth the money? In addition to greater operating efficiency and thermal resistance, fuel savings alone can pay for a brush type alternator long before it needs to be replaced. The longer life, and extended warranty combined with all of the other benefits make Long Haul brush designs a much more cost-effective solution for fleets.