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Occurence of Electrical Malfunctions to Squirrel Cage AC Induction Motors
in Mechanical Vibrations Spectra
Authors: PADURARU Dumitru, BANARIE Cristian , DIMA Dorian
Published: Buletinul Institutului Politehnic Iasi, publicat de Universitatea Tehnica „Gh. Asachi“, Iasi, Tomul L (LIV), FASC. 5, 2004, ELECTROTEHNICĂ, ENERGETICĂ, ELECTRONICĂ

Abstract: The most industrial equipments uncontrollable drives are set with squirrel cage ac induction motor, their maintenance and repairing being essential for units safety function. The induction motors functioning state control by the help of predictive maintenance systems is based on vibrations measuring and analysis. Through this method the place and type of disturbance is identified, intervening at the optimum moment and eliminating the worthless operations, with healthy effects in maintenance costs decreasing.

Keywords: vibrodiagnosis, vibrations spectra

1. Technical considerations

Due to some advantages like: low boundary line and weight, solid construction, low manufacturing cost, the electrical induction motors are the most used in industrial constant speed applications, respectively in paper industry.

As any dynamic equipment, any type of disturbance can be determined by the help of vibrations analysis, also known as efficient the performance of Fast Fourier Transform in the evaluation of plant condition.

Through this method electrical malfunctions can be detected as: broken or cleft rotating coils, their damaged joints, non-balanced sides, alternative magnetic gap, eccentric rotors and fixed coils, feeding connections abatements etc. A high efficiency of motors is obtained using a magnetic gap of low dimensions and a very thin isolation of windings, thus the alternative magnetic gap and the windings defaults are mostly met in motors vibrodiagnosis.

The common characteristic of most electrical disturbances is the appearance of line frequency fl and of its harmonics into the vibrations spectra (the second harmonic being more representative).

A great part from induction motors vibrations spectra having electrical malfunctions are also characterized by side bands of rotary frequency and/or of line frequency, which increase the severity grade of the problem, along with the increasing of their number and amplitude.

In vibrations spectra mechanical and electrical malfunctions could appear, the second category being more difficult to localize. For making the difference between those two, it is necessary to take a quick scanning over the frequency spectra components and to make a clear separation between the harmonics made by the electrical part of the motor. For this we can take into consideration the relations defining certain frequencies, which need to be selected from the vibrations spectrum to detect the electrical malfunctions:

where n1 is the synchronism speed, n is the nominal motor speed , 2p poles number and b is the rotating coils number.

An identification method of a frequency spectrum induced by electrical malfunctions is the measuring and analysis of vibrations spectrum during the stop period. In this way it is known that the electrical malfunctions distort the electromagnetic fields inside the motor; when the feeding tension is interrupted, the electromagnetic field distort being instantaneously interrupted, the frequencies due to this must disappear from the vibrations spectrum. If the frequencies persist, then we have a mechanical malfunction.

Note that once the electrical malfunction appears, there can appear through induction malfunctions of the same type or of mechanical type due to the close connection between the distorted magnetic field and the magnetic elements belonging to the motors compounds.

2. Describing the solution of identifying electrical malfunctions based on vibrations analysis

The collecting of vibrations spectra from the electrical induction motors was realized by the help of a collector-analyzer type MICROLOG CMVA 60, a product of SKF, which through collected the parameters: speed (FFT), acceleration (FFT and time variation), acceleration with filters 50-1000Hz, 500-1000Hz, always with signal demodulation, the obtained admeasurements being called by the special literature “enveloping” and having the mathematical significance of the FFT function of acceleration envelope.

In the moment of analysis we have in view the monitoring of the vibrations amplitudes average values (trend), temporal variation analysis of the collected parameters and the simple Fourier transforms analysis.

Based on the previous considerations we will present four cases of electrical malfunctions identified at the induction motors. Taking into account the fact that the induction machines gap has very low values we will refer to the reason, which causes that, as eccentric fixed coil or eccentric rotor.

There are several reasons for malfunctions into the fixed coil like manufacturing errors, maintenance errors and isolation defectives. Any malfunction into the fixed coil creates a non-uniform field around the rotor inducing inside variable solicitations, speed derangement and electrical vibrations. In this way, in the fixed coil malfunctions area the magnetic field is attenuated and it has a greater intensity in the opposite side.

Casel 1.

The eccentric fixed coil is due to some mechanical unconformities (shield detritions, shield bearing backlashes) or to a bad assembling, which introduces special frequencies vibrations in the whole motor.

This malfunction determines an unequal gap and high amplitude vibrations in all directions. Where the gap is at high value the magnetic field is attenuated and has a maximum intensity in the opposite direction, as the force which appears in the rotor is not equilibrated, the result being the magnetic vibrations having as first harmonic the network line frequency. The magnetic attraction force between the rotor and the fixed coil is in inverse ratio to the square distance between these two.

The induction motor from which we have the vibrations spectrum (fig.1) has the power of 55 KW, the feeding tension of 380 V and the speed of 2950 rpm, from which results the rotary frequency first harmonic of 49.1 Hz. The collected spectrum (FFT acceleration) is characterized by the rotary frequency first harmonic together with the superior harmonics (to the fifteenth harmonic) and by the line frequency first harmonics with their respective superior harmonics. The number and the amplitude of the harmonics superior to the rotary frequency first harmonic is closely related to the motors bearings technical state like the internal ball backlash, the eventual bearing abatements, mechanical abatements when fixing demountable elements (cap pieces, shields etc.).

Case 2.

Among the malfunctions, which provoke a variable gap, is also the eccentric rotor, which manifests itself through the magnetic forces disequilibrium.

The phenomenon manifests into the vibrations spectra through the appearance of motor rotary frequency harmonics and through the significant increase of the line frequency second harmonic attended also by sidebands at equal distances with poles frequencies. The calculation relations of these frequencies are (1) – (6) from the previous paragraph.

The inductional motor from which we took the spectrum (fig.2) has the power of 55 KW, the feeding tension of 6 KV and the synchronism speed of 1000 rpm and the result is the frequency fp = 1.02 Hz. Among the causes which create the gap irregularity we can also distinguish abatements from the foundation smoothness directly referring to the weak leg or loosening of a coupling pin into the foundation attended by the motor’s sliding. A big part from the empirical data indicates the fact that the manifestation of variable gap appears in the frequency spectrum when it has a radial distance variation higher than 10%.

Case 3.

Another type of malfunction, which could appear in the fixed coil’s area is the starter, covers disequilibrium (different number of turns on wire), resulted from the production process, the rewinding process and the deficient maintenance.

This gap irregularity leads to the absorption of an asymmetrical rotary current system which creates an irregular rotary magnetic field around the rotor, inducing pulsating currents and, as a consequence, mechanical vibrations.

This type of malfunction is distinguished in the acceleration frequency spectrum through the appearance of a specific frequency equal to the product between the number of turns and the rotary frequency. This characteristic is accentuated on a vibration spectrum (FFT acceleration – fig.3) sampled from a motor with the power of 55 KW, feeding tension of 380 V and nominal speed of 1470 rpm, where the malfunction specific frequency is of 836 Hz.

Case 4.

Another malfunction met on large scale at the induction motors with fault condition rotor is the rotating coils weakening or cleft. These malfunctions can be produced at the motors having many on-off cycles. During the building-up start-ups, when the current induced in the rotating coils is at high values, the heating and the expansion of the rotating coils produces, by forcing the lead rings on their ends.

The malfunction appeared in the incipient faze develops itself very fast because of the power dissipated in a continuous increase on the coil and of the transfer gradually reduced by the heating to the rings after the thermal barrier introduced by the cleft.
This type of malfunction has a specific behavior in the mechanical vibrations spectra through the appearance of superior rotary frequency harmonics (to the fifteenth harmonic) accompanied by sidebands at equal distance with the rotating coils frequency. This type of malfunction is accentuated in the vibrations spectra from fig.4, sampled from a motor with the power of 75 KW, feeding tension of 380 V and nominal speed of 1460 rpm.

Note that the motor can function with such malfunctions but the rotating coils irregular hardness will provoke the irregular heating and finally their cleft, the motor’s turnover and repairing being requested.

3. The advantages of the diagnosis methods

The diagnosis through this method of all the electrical motors malfunctions has the following advantages:

  1. Reducing of motors maintenance and intervention costs;
  2. Optimum moment and right place intervention;
  3. Time planning of all repairs avoiding the accidental plant stops;
  4. Plants life increasing;
  5. The integration of the electrical malfunctions identification system together with the mechanical ones, in a singular concept, with effects on the reducing of malfunctions diagnosis costs.
MANIFESTAREA DEFECTIUNILOR ELECTRICE LA MOTOARELE ASINCRONE
cu rotorul in scurtcircuit in spectre de vibratii mecanice
(Rezumat)

Actionarile nereglabile ale majoritatii utilajelor industriale se realizeaza cu motoare asincrone cu rotorul in scurtcitcuit, intretinerea si repararea lor fiind esentiala pentru functionarea in siguranta a instalatiilor. Controlul starii de functionare a motoarelor de inductie cu ajutorul sistemelor de mentenanta predictiva este bazat pe masurarea si analiza vibratiilor. Prin aceasta metoda se identifica locul si tipul defectiunii, intervenindu-se la momentul optim, eliminandu-se operatiile inutile, cu efecte benefice in reducerea costurilor de mentenanta.

References

1. Constantin Bala - Masini electrice, Editura Didactica si Pedagogica Bucuresti 1982.
2. SKF Reliability Systems, San Diego 2002 – Condition Monitoring Ins.
3. Jason Mais - Vibration Monitoring and Current Analysis of A.C. Motors. SKF Reliability Systems, San Diego 2002.
4. Victor Duarte - Control of rotors condition in A.C. motors 2003
5. http://www.dliengieneering.com/vibman.

 
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