Tools | Measuring hydro air gaps Accurate measurement of rotor–stator air gaps is essential for generator performance
and reliability. A new static measurement tool has been developed to offer faster, repeatable data collection while reducing technician hours compared with traditional parallels
KAMAN’S STATIC AIR GAP TOOL (AGT) is a portable measurement device that helps reduce downtime by providing a fast, repeatable and reliable means of measuring one of the most critical aspects of hydro-generator operation – the installed static air gap between the generator rotor and stator. Operating the AGT is designed to be simple: insert the device into the air gap to display and record measurement data, ensuring alignment of the generator rotor and stator. The Kaman AGT significantly reduces the number of overhaul technician hours required to inspect and analyse this critical operating parameter. Recent field
tests have proven that a reduction in hours of 90% is achievable.
Generator Air Gap 101 An air gap is the distance between the rotor and stator
of a large-scale hydroelectric generator. This gap is a very small distance in relation to the size of the rotor. Its measurement to tight tolerances is critical to prevent damage to and maintain efficiency of the generator. Static measurement of the air gap is required during initial installation, periodic maintenance checks and again at reassembly of the generator following major maintenance operations. Figure 1 is a cut-away view of a representative generator model. The arrow points to the air gap between the rectangular stator winding on the left, and the rotor on the right. This gap is typically measured at individual poles from above and again from the underside. Figure 2 shows the upper side of a typical 100
Right: Figure 1. Air gaps are a small part of the overall system and must be set to a tight tolerance Source: Kaman Measuring
KVA generator with the cover plate removed. Air gap measurements are taken between the black rotor poles on the left and red stator windings on the right.
Taking a closer look at Kaman’s AGT There are many sensors available for dynamic
Right: Figure 2. Generator air gaps leave virtually no room for error or lack of precision Source: Kaman Measuring
monitoring of the air gap when the generator is rotating. However, typical tools for static measurement of the air gap prior to generator rotation called “parallels” take the measurement mechanically, which is a time-consuming process. Kaman’s new AGT has been developed to make this static measurement electronically simply by sliding the spring end of the robust measurement paddle into the gap between the rotor and stator. Compression of the springs to match the gap is measured in real time by an eddy current sensor coil located between the springs. The compression springs are wide enough to account for variability in different rotor and stator windings. This permits the tool to be used with different winding sizes and patterns without adjustment. Signal conditioning electronics at the handle end of the paddle accurately translate the inductance change in the sensor coil to distance – available in imperial or metric units – and wirelessly transmit to the operator’s handheld device. The AGT is user friendly and accurate. Like the parallels, the AGT can be used in any orientation, horizontally or vertically depending on the design of the generator. The AGT is powered by a long-life rechargeable battery permitting up to eight hours of operation between charges. The compression springs are located at the
bottom and signal conditioning electronics at the top of the device, shown in figure 3. It is equipped with adjustable wedges in the middle to provide a
28 | October 2025 |
www.waterpowermagazine.com
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