Air Gap Measurement

What, why, and how


Hydro turbine-generators represent a mix of conventional radial vibration, thrust, and phase trigger measurements along with several measurements that are generally unique to hydro. In this article, we explore one such measurement – air gap – explaining what this measurement is, how it is made, and why it is important.


Hydropower is a significant part of the world’s energy supply and the renewable portfolio. Indeed, approximately 7% of the world’s electricity is produced by hydropower and nearly a dozen countries generate more than a quarter of their power from this renewable resource (Figure 1).

Figure 1: 2021 hydropower as a percent of total generating capacity by country. Countries generating more than 20% of their power via hydro are explicitly noted. Although some countries – such as the United States, China, and Russia – represent small percentages (2.6%, 7.8%, and 6.5% respectively), this still equates to a sizable number of machines and GW of production. For example, China represents more than 370GW of hydropower capacity – far exceeding countries with higher percentages yet smaller capacities.

While these percentages are important, they tell only part of the story: the percentage of a country’s energy portfolio coming from hydro. Table 1 conveys the rest of the story by showing the top 20 countries for hydropower based on installed generating capacity. As can be seen, even countries with a relatively small percentage of their power coming from hydro can still represent a very large installed base of generating capacity and thus numbers of machines.

We provide this preamble to underscore that there are many hydropower generators around the world and many of those are monitored by vibro-meter technology. While radial vibration, thrust, and phase trigger measurements are generally well-understood because they are present on most other machine types, air gap is less understood because it generally appears only on hydro units1.

Machine Size Implications

Although some hydropower is generated by smaller2, units (usually horizontal and may even use a gearbox3 between turbine and generator), as of 2022, a relatively small percentage4 (15%) of global hydropower comes from such machine trains. Also, they are not generally candidates for continuously monitored air gap due to the much smaller sizes of their generators. As such, those machines are not addressed in this article.

Instead, most hydro plants are large in scale – often marvels of civil engineering – featuring large dams and massive vertical turbines with salient pole generators and corresponding rotors that may exceed 10m in diameter and 450 tons (Figure 2)5. These machine trains are indeed among the largest on the planet in terms of both power output and physical size, some capable of 1GW from a single train – only exceeded in power output by the steam turbines in most nuclear plants and a small number of conventional thermal plants6.

Figure 2: A hydro generator rotor being lowered into place. Note the relative size of the ladder atop the rotor and of the workers along the railings. Such massive sizes are quite common in many hydro plants where vertically oriented turbine-generator units are used. Note also the single layer of stator-mounted air gap sensors (seven are visible) as indicated by the white arrows.

The size of these units means that the bearings carrying radial and axial loads are fluid-film type and thus use proximity probes for monitoring radial vibration and thrust position. Phase triggers are also used to provide a once-per-turn timing mark used in certain measurements (such as air gap profiles) that must be referenced to a precise location on the shaft. Although phase (degrees of rotation from the mark) can sometimes form part of a protective measurement, it is always valuable for condition monitoring and diagnostics and should thus be included on every machine regardless of size and regardless of whether air gap measurements are present.

The size of these units also means that the ramifications of mechanical failures can be very serious, as this series of before and after photos from the Sayano-Shushenskaya hydroelectric dam accident7 soberly convey.

Lastly, as we will show next, the size of these units also results in numerous sources of static and dynamic movement that must be understood and are often best measured by air gap and a suite of other measurements that help isolate and identify the underlying cause of changes in air gap. But first, we must define air gap.

What is Air Gap?

Air gap is simply the instantaneous gap between generator rotor and stator as shown in Figure 3. However, it will not necessarily be uniformly circular and must thus be measured using multiple, uniformly spaced sensors around the stator periphery so that a complete picture of rotor and stator shape and clearances (air gap) can be determined. Figure 4 is an example of one such plot (rotor profile) available from suitable air gap sensor data.

Figure 3: Air gap in a generator.

Figure 4: Air gap data is collected via multiple sensors placed at uniformly spaced intervals around the stator and allows the creation of plots such as this rotor profile. The data can be used to generate other important plots and information including rotor circularity, rotor eccentricity, min/max/average air gap, and more.

Why Measure Air Gap?

Air gap is at the heart of the turbine-generator because it is where the mechanical forces and electromagnetic forces intersect. Changes in air gap are indicative of many different problems and it is thus an important measurement that provides insight into more potential problems in a hydro unit than almost any other single measurement – more than even radial vibration.

Figure 5 shows the key components in the generator8 rotor/stator assembly while Figure 6 superimposes onto the same illustration the numerous areas in which relative movement between the two can occur, resulting in a non-uniform air gap.

Figure 5: Cross-sectional diagram of a vertical hydro-generator rotor, stator, and supports. The inset photo shows a comparatively small unit where workers are installing rotor pole pieces with the aid of a crane. The spider (gray) is clearly visible as is the shaft, the rim (black), and the pole pieces (red).

Figure 6: Multiple sources of static and dynamic movement exist in large hydro-generator assemblies resulting in non-uniform air gap. Continuous measurement of air gap around the entire periphery of the rotor allows problems to be detected and isolated.

Other sources of non-uniform air gap include constantly varying centrifugal, thermal, and magnetic forces that are capable of distorting the stator and rotor of the generator. For this reason, air gap cannot simply be measured on the unit at rest because there are dynamic contributions – not just static. Lastly there is the issue of manufacturing and assembly tolerances that cannot be precise enough to result in rotors and stators of such large geometries that are perfect in concentricity, circularity, alignment, and all other aspects.

As the size of a hydro-generator increases, all of these contributing factors become more pronounced and the need for monitoring air gap becomes more acute. This explains why air gap measurements are generally warranted on hydro units above 50 MW in size9.

Air gap is thus a combination of contributing factors that may act in isolation or interact in complex fashion. Root cause could be anything from a loose pole to a shifting foundation to a thermal hot spot, or a combination of factors.

While non-uniform air gap reduces operational efficiency, this is not the only reason (or even primary reason) it is monitored. Instead, non-uniform air gap can be indicative of more serious problems that lead to potentially catastrophic breakdowns if the causes are not understood and corrected. Consequently, the incentives to monitor air gap entail far more than simply optimizing maintenance intervals – they include the avoidance of catastrophic failures and the associated safety and economic consequences.

How to Measure Air Gap?

Sensing Chain

Vibro-meter’s air gap sensing chain consist of two components: a sensor with 5m or 10m of integral cables and a companion signal conditioner as shown in Figure 7. The two required cables (transmit and receive) are provided with separate connectors but within a single jacket. The 5m and 10m length options allow sufficient length to connect the stator-mounted sensor to its associated signal conditioner, located in a nearby junction box. The 10m length is used on stators where the additional length is needed to exit the machine, or where the junction box is not immediately adjacent.

While the sensor is designed to withstand the large magnetic fields10 inside a generator, it is important to note that it is intended only for air-cooled generators – not the hydrogen-cooled generators typically associated with large steam turbine-generator trains.

Two sizes are offered. The LS120 is designed to measure air gaps between 5mm and 30mm while the LS121 is designed to measure air gaps between 20 and 60mm as encountered on larger generator geometries11. Each size has its own corresponding signal conditioner – the ILS730 and the ILS731.

Figure 7: The vibro-meter air gap sensing chains.


Air gap sensors are affixed to the stator and the measurement is thus sometimes referred to as “stator-mounted air gap” to distinguish it from rotor-mounted12 air gap. The sensors are placed uniformly around the stator periphery as shown in Figure 8. The recommended number of sensors is generally a function of rotor outer diameter and can vary from as few as 4 to as many as 12 (Table 2).

The number of sensors can also vary based on the height of the rotor pole pieces (H in Figure 9). However, the need for multiple layers tends to be more subjective based on customer preferences and practices, but is done to account for the tilt that can occur due to vertical misalignment and the corresponding differences in air gap between the top of the rotor versus the middle and bottom of the rotor. The sensors can thus be arranged in layers, allowing multiple profiles – one for each layer. A very large machine with a rotor diameter of 13m and a height of 2m might thus have 3 layers of sensors with 12 sensors per layer for a total of 36 air gap sensors. Profiles would then be available for three vertically spaced layers: top, middle, and bottom. Other machines might require only a single layer and still others only two layers. The uppermost layer is the most important in protecting against rotor-to-stator rubs.

Figure 8: Air gap sensors are mounted on the stator at locations between its windings (see also inset photo showing three sensors in place on the stator). The number will depend on the rotor diameter and may be as few as 4 for small machines to as many as 12 for large machines. Although there is a difference between the measured air gap and the true air gap due to the thickness of the air gap sensor and the layer of adhesive affixing it to the stator, this difference is compensated for in VibroSight software, ensuring that plots and data always reflect the true air gap.

Figure 9: Depending on height (H), multiple layers (horizontal planes) of air gap sensors may be required.

Signal Formats

The sensing chain’s principle of operation is that as the distance (air gap) between the rotor and stator changes, the capacitive coupling between the transmitter and receiver elements of the sensor will change. This results in a modulated signal at the receiver, reflecting the instantaneously changing air gap – known as the pole profile – in the form of a time-varying waveform, similar to the output from any other dynamic sensor.

The signal conditioner (Figure 10) also processes this waveform into two additional measurements: minimum gap and rotor profile. Minimum gap is exactly as would be expected: the smallest air gap regardless of where it occurs. It is typically used as the input to a protection system such as our VM600 or VibroSmart where it can be displayed, alarmed, and perhaps trended.

Figure 10: Air gap signal conditioner showing available signals (min gap, rotor profile, and pole profile).

In contrast, the rotor profile signal is used primarily during system commissioning and verification. It is not further detailed here but is discussed at greater length in the product manual.

Lastly, the pole profile (waveform) signal is used in companion condition monitoring software such as our VibroSight suite where numerous plot types and measurement extractions are available along with supplementary software alarming. In those instances where continuous condition monitoring is not installed, the pole profile signal is available at the monitor’s buffered output connector for access by portable instrumentation. It is worth noting that the minimum gap can be provided as a proportional 4-20mA signal13 for connection to a PLC or other platform when a conventional vibration monitoring system is not used.

Figure 11: The signal conditioner takes the instantaneous raw sensor signal (pole profile) and generates two additional measurement signals useful for monitoring and alarming: rotor profile and minimum gap.


A significant part of vibro-meter’s heritage is an emphasis on quality. Although it is embodied in all our products, nowhere is it more important than in our sensors given the harsh locations in which they must often survive. Unlike monitoring systems and software that can be addressed even when a machine is running, this is not the case for sensors such as air gap and robustness becomes even more important. Replacing an air gap sensor means not just stopping the unit, but pulling the generator rotor and thus entailing a major, planned outage.

Another area where our air gap sensing chain differentiates itself is in the signal quality. As was noted above, our air gap sensors output minimum air gap and rotor profile, not just the raw signal (pole profile). This means that they do not necessarily require an accompanying monitoring system to extract measurements from the raw pole profile signal and can thus be applied to smaller machines where monitoring might be done in a PLC using a 4-20mA signal. Also, our signal conditioner does not require special linearization14 and ships from the factory already calibrated for use with the sensing element. In contrast, some competitors require field linearization and thus increased commissioning costs.


As was previously noted, the minimum gap is available directly from the signal conditioner as a proportional voltage and/or current and is frequently brought into a permanent protection system such as our VM600 or VibroSmart where it can be displayed and alarmed using adjustable ALARM and TRIP setpoints. The pole profile, on the other hand, is a dynamic waveform and is used in our VibroSight condition monitoring system to generate the various measurements and plot types useful for condition monitoring.


As has been conveyed, the air gap measurement is useful precisely because it reflects many different sources of static, quasi-static, and dynamic movements within the hydro-generator and its support structure. In this respect, it is like an audio waveform that may reflect the contributions of a 20-member band. To use this analogy further, the job of the analyst becomes that of extracting the guitar, the bass, the keyboards, the cymbals, the kick drums, etc. When air gap non-uniformities occur, it thus becomes necessary to determine what is contributing to the change and how to remedy the situation. In some cases, examination of the various air gap plots alone can isolate the issue. In other cases, supplemental measurements are needed. For example, if the stator has shifted, a soleplate displacement measurement can be helpful in isolating this. Or, if a preload exists on the shaft, X-Y radial vibration probes would allow shaft eccentricity to be correlated with generator rotor eccentricity as revealed by air gap sensors and profiles.

A discussion of the various data types, presentation formats (see Figure 4, for example), and interpretation thereof is beyond the scope of this article, but may be the topic of a separate future article. Suffice to say here that our VibroSight software possesses the capabilities (Table 3) to collect, process, extract, and display the air gap data necessary for proper analytics and can even be used to generate automated analytics using the Rule Box component of the software suite.

CEATI Conformity

Vibro-meter is active in numerous industry groups as it allows us to better understand the needs of our customers, industry trends, applicable standards, and evolving best practices. One such industry group is CEATI (Centre for Energy Advancement through Technological Innovation). CEATI has developed several thousand technical reports and guides over the years, including one dealing with alignment and circularity measurements in hydro generators15(Figure 12). We have produced a separate application note16 on this topic which you are encouraged to download and read. It details the importance of such measurements and why adherence to this CEATI guideline is likewise important. The guideline reflects the combined experience and ensuing best practices compiled from dozens of hydroelectric operators, OEMs, and consulting engineers.

Figure 12: The guidelines in this CEATI document15 are essential for properly computing hydro generator rotor circularity. As such, vibro-meter uses them in both our monitoring hardware and in our VibroSight® software. We have also published a companion application note16 with additional information on the topic of rotor circularity calculations.


Air gap is perhaps the single most useful measurement for assessing the overall health of large-diameter hydro turbine-generator units. That said, it is best used in conjunction with a suite of other measurements to provide total protection and condition monitoring capabilities.

Vibro-meter’s air gap monitoring capabilities extend from the sensing chain to the monitor to the condition monitoring environment and are thus comprehensive.

This article has briefly explored what air gap is, how it is monitored, and perhaps most importantly, why it is monitored. You can learn more about not just air gap, but all the various hydro-related machinery measurements by contacting your local vibro-meter sales professional to learn about our portfolio of products and services. You can also visit our hydro application pages.


1 Continuous air gap monitoring is also used in other niche applications, such as the gearless drives for mills used in the mining industry and occasionally very large horizontally mounted generators driven by steam and gas turbines.

2 The definition of so-called “small hydro” varies by country but is generally deemed to be installations smaller than 30MW, whether from a single machine or multiple machines. Small hydro is considered to have a smaller environmental impact because it diverts only part of a stream or river rather than the entire flow and may even use existing irrigation canals or reservoirs.

3 Gearboxes are used to increase the speed from the turbine to a speed that allows a generator with fewer poles to be used. For example, if the turbine output speed is increased to 1500 rpm via a gearbox, a 4-pole generator can be used for 50Hz electricity. Gearboxes can be used on turbines up to about 10MW in size. Above that, the generator is directly driven by the turbine.

4 Source: GlobalData (, power industry database

5 Refer to this informative Quora post for a succinct description of why salient pole generators must be so large.

6 The largest machines by power output to-date are the 1770MW steam turbines provided to the Hinkley Point Nuclear Station in the UK. A small number of conventional thermal plants (such as Belews Creek in North Carolina) have steam turbines of 1GW or larger, but this is rare.

7 The exact root cause has been postulated but not conclusively isolated. What is known is that excessive vibration had been present for nearly a decade prior to the accident and that it had increased hours prior to the accident.

8 Because air gap measurements pertain to the generator (not the turbine), we confine ourselves to discussion of the generator in this article.

9 The importance of air gap measurement is actually more closely associated with generator rotor diameter than with power output. However, a general rule of thumb is that air gap is usually measured on units with power outputs of 50MW or greater because these units generally have generator rotor diameters large enough to warrant measurement. Regardless, there are notable exceptions such as the Bieudron Hydroelectric Power Station in Switzerland which has units that are each 423MW, but no corresponding need for air gap measurements because of the unique generator designs resulting in relatively small rotor diameters.

10 Up to 1.5 Tesla

11 Vibro-meter air gap sensing chains can also be used to measure extended ranges 25% larger than those shown here, which are the ranges for ±1.5% linearity. Consult the product datasheets for the transfer characteristics and linearity over these extended ranges.

12 Rotor-mounted air gap has been attempted over the years by others but is no longer commercially available. Rotor-mounted sensors are generally discouraged in machines due to complications in the supply of necessary power to so-called “flying electronics” along with the centrifugal forces generated by the spinning rotor that can make it challenging to the securely retain such sensors on rotating parts – particularly at the outmost diameter of a generator rotor where centrifugal forces are greatest.

13 A factory-changeable option also exists to use the 4-20mA output for the rotor profile or the pole profile; minimum gap, however, is the default and typical use of this output.x

14 The underlying operating prnciples of vibro-meter air gap sensors result in a signal that is inherently linear over the useful range. This allows sensors and signal conditioners to be easily interchanged, simplifying stocking and usage of spare parts. In contrast, the operating principles underlying competitive designs may require considerable linearization in the signal conditioner. In some cases, this can introduce interchangeability constraints and necessitate individual tuning and matching of sensors to signal conditioners, adding complexity and cost to spare parts strategies.

15 CEATI HPLIG Project T122700 0381 (May 2015), Hydroelectric Turbine-Generator Units Guide for Erection Tolerances and Shaft System Alignment. Montreal, Canada: Centre for Energy Advancement through Technological Innovation (CEATI)

16 Rotor Circularity Calculation for Hydro Turbines, vibro-meter application note (2016).









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