It may seem, that measurements of absorption and reseistance rates of the windings are sufficient to make an explicit statement about the condition of the machine. You couldn’t be more wrong.
The absorption rate determines only the level of moisturizing the main insulation, saying nothing about interwinding insulation, it is also defined at the voltage far more different than the nominal voltage, while the measurement of windings’ resistance indicates only the symmetry and continuity of windings.
That’s why, to assess completely the condition of electrical machine, Partner Serwis implmented few unique diagnostics methods.
The assessment of condition of interwinding insulation using the voltage impulses method.
One of the biggest problems of electrical machines’ diagnostics is the assessment of interwinding insulation. The main insulation or interfacial insulation (only if the system of connections allows it) can be easily diagnosed, while quick detection of short-circuit, or weakening, of the insulation between the individual windings of one coil, is impossible when using classical methods (for example the measurement of winding’s resistance).
This problem concerns both new machines (also after rewinding) and the ones in operations. In case of new and rewinded machines, the main task of the diagnostics is to eliminate the performing errors, and in case of machines in operation, detecting short-circuits in the beginning stage. The method offered by Partner Serwis uses the phenomenon of response of winding system (of inductive character) to feeded high-voltage, low energy impulse. Depending on the winding parameters, the response of the system to the impulse has a shape of fading sinusoid of various frequency and deading time. The perfect electrical machine has the same sinusoid shape for each of winding. In case, when in one of the windings the failure or other assymetry (for example one coil turned away) occurs, the shapes of courses will differ. Basing on the observed differences, it can be concluded about specific causes of assymetry – damaging.
This method is used as well to check the machines repaired by us, as to used by our clients. Periodic inspection and comparison with previous results allows to take up optimal decisions concerning previous motor exchange (for example during the next stop), reserve purchase etc.
We use to our examination an instrument of American origin, which, apart from percussive test, performs also the measurement of windings’ resistance with technical metod, and measurement of the resistance of insulation at the test voltage. The percussive impulses can be generated up to 15kV voltage. This method is successfully used as well to alternating current electrical motors, as to direct current motors, and also to transformers inspection.
Insulation examining using the reconstruction of voltage method
In using the electrical machines, it’s important, from the user’s point of view, to track the process of insulation of windings going off. This process is influenced by:
- electric field,
- air pollution,
- temperature,
- vibrations,
- mechanical tension.
The knowledge about the condition of insulating system, allows to apply optimal repair and investment economy. Maintenance services should have relatively precise information concerning the degree of insulation’s wearing and tearing and estimated time of its possibile using. In order to make the best assessment of the condition of main insulation, we propose conducting a number of measurements, which results are professionally assessed. The measurements are conducted at the constant voltage up to 15kV (the recommended voltage for 6kV motors is 13kV).
The method is to define 3 characteristics:
- R(60) = f(U) – within the range of 0-15kV, if it’s possibile,
- Ip = f(t) – time course of leakage current after supplying, for the entirely discharged insulating system, of constant voltage with nominal value,
- Uod = f(t) – charging the insulating system to nominal voltage, till obtaining the fixed state, then unplugging the powering voltage and making the circuit shorted for the time tz, next opening the circuit and reading off the rebuilding voltage course on the insulating system.
Adequate interpretation of obtained results allows to define the level of wear and tear windings’ insulation.
Measurements are conducted in the meter circuit as show in the picture below.
The meter circuit consists of direct current electronically regulated feeder, microammeter and digital electrostatic voltmeter.
Entirety of measurements was conducted in a way eliminating an influence of meter circuit for insulating system.
