Description
Bently Nevada’s Air Gap monitoring system provides monitoring and diagnostic capability for all new and existing hydro generators and motor/generators. Air gap monitoring is fully integrated with the 3500/System 1 platform and utilizes
the 3500/46M Hydro Monitor. The 4000 Series air gap sensor system is available in 20 mm and 50 mm ranges for optimal
resolution across a wide range of measurement requirements.
Criticality
Air gap is a measure of the distance between rotor and stator in a hydroelectric generator. Monitoring of air gap is critically
important because both the stator and the rotor on large hydroelectric machines can be quite flexible and their shape
and location are affected by operating centrifugal, thermal, and magnetic forces. Off-center or out-of-round conditions
will at least reduce operating efficiency and, in more severe cases, can lead to damage from magnetically induced heating or a rotor-to-stator rub.
Measuring Air Gap
Air gap monitoring utilizes multiple specially designed capacitive proximity probes that are mounted on the inner bore of the generator stator in one or more planes. Measurements are made from the stator bore to each rotor pole as the rotor turns. Minimum air gap dimension and location along with rotor shape are directly measured during unit operation. Stator shape is calculated utilizing the multiple air gap probes. The air gap monitoring system provides operations and maintenance personnel with early warning of impending problems with shape and concentricity of the generator rotor and stator.
Typical Benefits
Air gap dimensions along with rotor and stator shape cannot be effectively measured with the generator out of service because of the combined effects of centrifugal, thermal, and magnetic forces. Early detection of air gap anomalies will facilitate condition-based maintenance by allowing the user time to plan for repairs before scheduled outages. Long
term trending of gap and shapes can be correlated with operating conditions and used in operational and rehabilitation planning. Knowing the rotor and stator shapes and minimum air gap dimensions will provide the operator with the information needed to remove a machine from service before serious damage like that from a rotor stator rub occurs.
Air Gap Probe and Extension Cable
The 4000 series Air Gap probe is designed for maximum survivability in the harshest hydro generator environment. It can continually operate and maintain its accuracy in temperatures up to +125 ºC (+257 ºF). The probe can also continually operate and maintain its accuracy in the 1.5 Tesla magnetic field often experienced between the rotor pole and the stator. ClickLoc connectors come standard on both the probe and extension cable to prevent loosening of the connectors.
Proximitor Sensor
The 4000 series Air Gap Proximitor Sensor is based upon our best selling 3300 XL Proximitor Sensor technology. The Proximitor Sensor uses a DIN rail connection to ease installation and to minimize the number of cumbersome screws
required when working around the hydro generator. The grounding stud on the Proximitor Sensor features a safety thread in order to ensure the locking nut does not inadvertently fall off during the installation of the ground wire on the extension cable.
