Oh, the laws of physics!

{Author’s Note. To avoid using the term “centrifugal force,” the author visited “Wikipedia, the free encyclopedia.” The first sentence in the article cautions, “Not to be confused with Centripetal force.” Who, on the other side of the Internet, knew we might be blurring those two concepts?}

When a mass (like the rotor of a motor or generator) is spinning, the force that causes the circular motion is aimed at the center, or axis, of the motion, and is called centripetal force. Mass wants to travel in a straight line, which would resemble a tangent to the rotor. However, Sir Isaac Newton’s Third Law of Motion tells us that the rotating mass exerts an equal force but in the opposite direction to the centripetal force. The term for this force is “reactive centrifugal force.”

The peripheral rotor speed is generally limited to the speed of sound, which is 1100 feet per second in dry air. That’s a lot of mass spinning pretty fast. It is the reactive centrifugal force that can manifest itself as rotor damage in wound rotor wind generators. You can see the devastating effects of this force in several videos we have recently posted on our YouTube Channel.

See the two videos on our YouTube Channel at  http://www.youtube.com/user/BrithineeElectric. This article is demonstrated in “Examples of Generator Failure-Brithinee Electric and Examples of Generator Failure II.”  You can subscribe to our channel to be notified of new motor and control panel videos on this site.

The first video is an illustration of metal fatigue or welding failures that caused pieces of metal (about four inches across) to flair out from the reactive centrifugal force. This is an Elin one megawatt, Austrian-built, water-cooled wind generator The flared metal fragments become like knives that do amazing damage. The rotor of the generator cut holes into the windings of the stator causing massive shorts in the stator winding.

The second example is a Canterey two megawatt wound-rotor induction generator, made in Spain. The rotor banding has not held the winding on one end, and the resultant flared rotor coils has wiped out the stator winding. The wind powering the rotor blades may have gone over speed, causing the winding to flare outside its normal position, but in any case, the fiberglass banding tape, which opposes the reactive centrifugal force, failed. The rotor coils lifted up, creating a rotating saw which destroyed the stator windings.

Brithinee Electric makes an effort to establish the root cause of failure of each motor or generator we service. Only in this way can measures be devised that prevent a recurrence. Brithinee Innovation is our way of making significant improvements to the manufacturer’s original design, and a major reason we have a near-zero failure rate when rewinding wind generators.

Donald P. Brithinee