What is Flux Testing?
Air-gap flux testing is a non-intrusive diagnostic technique used to assess the condition of synchronous generators and motors. It provides direct insight into the electromagnetic behaviour of the rotor by measuring the magnetic flux density in the air gap between the rotor and the stator.
The technique works by installing a sensor on a stator tooth, where it measures the radial magnetic flux density of each passing pole. This enables the magnetic field produced by the individual poles to be recorded and analysed while the machine is operating at nominal speed.
Rather than relying on indirect measurements, air-gap flux testing observes the magnetic field generated by the rotor field winding directly. By comparing the flux of each pole, deviations can be identified that indicate developing faults.
Field winding faults, such as inter-turn short circuits, lead to a reduction in magnetic flux density for the affected pole. These deviations can be detected and trended over time. In addition to winding-related issues, the method can also reveal mechanical phenomena such as dynamic rotor eccentricity through characteristic variations in the measured signal.
Because the measurements are performed at operating speed, faults that are dependent on centrifugal forces can also be detected.
Faults in the rotor field winding are among the most critical issues in synchronous machines, yet they are often difficult to detect using conventional monitoring methods. Air-gap flux testing provides a direct means of identifying these faults at an early stage by observing how they influence the magnetic field.
In addition, an asymmetric magnetic field in the air gap can lead to secondary effects such as increased vibration and circulating currents within the stator.
By detecting these issues early, operators can plan maintenance in a controlled manner and avoid unplanned outages or more serious damage to the machine.
The technique is particularly valuable for:
Traditional diagnostic methods such as vibration analysis or electrical measurements (such as rotor pole drop test) can indicate that a problem exists but often provide limited information about its root cause.
Air-gap flux testing, by contrast, gives direct insight into the magnetic behaviour of the rotor poles. This makes it possible to distinguish between different types of faults, such as field winding issues and mechanical effects like dynamic rotor eccentricity, i.e. a whirling rotor.
For this reason, air-gap flux testing is often used alongside other diagnostic methods such as partial discharge testing, to provide a more complete understanding of machine condition and to support more reliable maintenance decisions. Read more about our testing services.
