Wedge Tightness Testing
Importance of Wedge Tightness Testing
All generators have some type of wedge design that helps prevent the stator bars in the stator core slots from vibrating. The design also helps in preventing a short circuit that would arise from the bars lifting out of the core slots. Historically, generator manufacturers have experimented with many different wedge designs and materials. In the very beginning of generator manufacturing, the choice of wedge material was very limited. Most of the wedges were made out of wood (usually hard maple). These wooden wedges were prone to shrinkage in the dovetail making them become loose, thereby allowing the bars to vibrate in the slot. As a result, they had to be replaced quite often. However, on the mica asphalt bar insulation, it was not too big of a problem as the insulation in the bar swelled during operation due to heat that was created. This made the bars tighten up within the slot and it limited the bar vibration. The wedges were really there to prevent the bars from popping out of the slot which would cause a short circuit.
Due to girt cracking of asphalt winding insulation, manufacturers had to devise new insulation systems. Today, most of them are using some kind of Mica epoxy mixture system. Some of these systems are more rigid and some are more flexible, depending on which one is used. The bars using these new insulation systems are more prone to vibration and movement in the slot, which has to be stopped. To prevent the bar movement in the slot, they had to add conductive side packing or conductive side ripple springs. In some cases, they had to add ripple springs under the wedge.
Today, the wedges are no longer made out of wood. The wood material was replaced by many different fiber materials. Some are better than others, but none are perfect.
I have seen a wedge system done in the factory 30 years ago and after 30 years of use, the wedges were still tight. It all amounts to who does it, what material they use for it and the procedure they are using to install them.
So, then, why is it so important for the wedges to be tight?
Loose wedges will let the bars vibrate causing them to move radially. Now, if the side packing or the side ripple springs are also loose, it will also worsen the abrasion of the bar in the slot as the bar is now moving both radially and circumferentially. The bar armor is coated with a conductive material to help control the current from discharging from the bar to the ground (stator core), thus, in a way, acting as a current limiting resistor. Over time, the conductive material wears off due to abrasion. This will cause the current discharge in the slot to increase, burning and degrading the bar insulation. If left uncorrected, it will destroy the bar ground insulation and the generator will fail to ground. So, early detection of loose wedges is very important. If the conductive material on the bars wears off, it is too late to correct. At this point, rewinding the generator is the only option to correct the problem.
History of Checking Wedge Tightness
There have been all kinds of methods used to determine the wedge tightness on generators. Each of these methods depended on the experience of the craftsmen and the tools that they used. Because of this, there was major inconsistency in the interpretation of what constituted a really loose wedge from a tight wedge.
I started in generator work 50 years ago and I learned from these, what I called at that time, experts. To test the tightness, they used anything up to a 1.5-inch open ended wrench and from a 12-16oz hammer to tap the wedges. While tapping on the wedges, they listened to the sound each tap made and depending on the sound, determined if the wedge was tight or loose. Each craftsman had their own interpretation for the sound that came from tapping the wedge. This is not a very scientific or accurate process, especially if there was a generator next to the one you were testing that was making a lot of noise.
As time went on, I believe that maintenance and factory service departments began offering electronic devices for determining the tightness of a wedge. Some of them went out of business, but there are still some around using Equotip hardness testers that have been adapted to their methods of interpretation. I worked with the Equotip 2 for 20 years, but was never satisfied with the results. There was nothing wrong with the test equipment, but the accuracy of the results always depended on the operator. The Equotip 2 is now obsolete (as is the Equotip 3, which was the hardness tester we used as our experimental instrument to develop and refine our testing method).
Currently, we are supplying the Equotip 550 Leeb with our product, which is the latest in technology. We mount the Equotip hammer on the trolley (a trolley designed and made by us that will accommodate different generators) which will always keep the hammer at the same angle to the wedge and at the same initial pressure on the wedge between taps.
We believe this is the most reliable instrument on the market today for testing the tightness of wedges.