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Torque Talk
PMDC Motor Brush Selection: Art? Science? Both?

Specifying the right brush for a PMDC motor is an inexact science at best. If a brush rates high in one property, it often sacrifices in another. But brush life and brush replaceability are virtually always key considerations.

To gauge brush life, use "Bullock's Rule":

Life is inversely proportional to the change in speed or the square of current through the brush.. For example, a gearmotor is capable of 3,000 hrs. brush life at 2,000 rpm input motor speed, drawing 2A brush current. At 4,000 rpm, expect brush life to be 1,500 hrs. at the same 2A current. At the same 2,000 rpm speed, but at lA current, expect four times the brush life, or 12,000 hrs.

What kind of brush materials does Bullock's Rule lead to?

• Metal graphite brushes, with low voltage loss and high wear rates, work well in low speed, low voltage motors.
• Carbon and carbon graphite brushes are best with low speed, high voltage, FHP applications.
• Electro-graphitic brushes are suited for high speed, high voltage, and higher power motors.
• Natural graphite brushes are softer, quiet, and have a slightly lower current capacity. They wear faster, but are best in high speed applications.

As an engineer, I've often faced tradeoffs in motor specification, so I know how important specialized technical support can be. Our application engineers can provide expert advice on the best Bison PM motor choice for your project.

Call 1-800-AT-BlSON for a free catalog and details on our technical support.