There are times when a DC motor or generator experiences a catastrophic failure and the customer wants to know why it happened. One type of failure that seems to stimulate lively conversation is when the failure involves dramatic damage to the brushholders and commutator. The term "flashover" describes the appearance of the failure; the very name conveys an accurate mental image of the failure. The questions that arise next are predictable: "What caused this?" and "What can be done to prevent a recurrence?" Or, if the motor was recently repaired: "What did you do to my motor to cause this?!" The purpose of this article is to help you answer those questions.

A convenient incoming inspection report for basic DC motor conditions and test values.

A special discounted collection of 6 webinar recordings focusing on DC motor electrical procedures.

Just $30 for EASA members!

The lowly brush is underrated and misunderstood. The brush grade, brush pressure and spring tension, as well as the effect of load and humidity are each important to brush performance in DC machines, wound rotor motors, and synchronous machines.

The brushes on a 4-pole, 700 hp DC motor were not wearing at the same rate. In this case, rapid brush wear occurred on two adjacent brush rows - one positive and one negative polarity. The other brushes had minimal wear. Electrical tests found no winding faults, and the air supply was clean. Most of us suspect low current-density when rapid brush wear occurs. A lightly loaded DC motor can "dust" a set of brushes in short order. Changing the brush grade (or removing some of the brushes) will usually solve the problem.

Ever run across brush arcing or vexing commutation issues? This paper, presented at the 2013 EASA Convention, provides a approach to analyzing brush and commutation problems and failures.

This webinar covers:

• How much voltage output is too much?
• What can cause higher than desired output voltage?
• Brush spacing, brush seating, field or interpole spacing & polarity
• Interpole circuits

This presentation demonstrates how to set brush neutral in DC machines, describing several methods along with the benefits and drawbacks of each.

This video shows how to adjust the brush neutral position of a DC machine to prevent sparking at the brushes at full load.

The worn carbon brush face indicates the operating conditions. Therefore, it can be utilized by brush experts as a highly effective diagnostic tool for troubleshooting and determination of root causes. If these warning signs shown at the brush face can be identified and proactively addressed in a timely manner, then major unexpected expensive catastrophic failures like flashover or repair of the contact surface can be avoided.