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ELECTRICAL MALFUNCTIONS VIBRODIAGNOSIS
AT THE DIRECT CURRENT MOTORS
Authors: CRISTIAN BANARIE, PADURARU DUMITRU and DORIAN DIMA
Published: BULETINUL INSTITUTULUI POLITEHNIC IAŞI,TOMUL L (LIV), FASC. 5, 2004, ELECTROTEHNICĂ, ENERGETICĂ, ELECTRONICĂ
Vibrodiagnoza defectiunilor electrice la motoarele de curent continuu (Rezumat)
Identificarea si localizarea defectiunilor prin vibrodiagnoza conduce la eliminarea operatiunilor inutile de demontare si verificare a utilajelor, cu efecte benefice in directia reducerii costurilor. In sensul celor de mai sus, prin prezenta lucrare se vor prezenta si evidentia modul de manifestare si de identificare a unor defectiuni de natura electrica la motoarele de curent continuu in spectre de vibratii mecanice.

Abstract. The identification and finding of malfunctions through vibrodiagnosis leads to the elimination of useless de-mounting and checking operations, having positive effects in cost decreasing direction. In this work we present and highlight the manifestation and identification way of some electrical malfunctions at direct current motors in mechanical vibrations spectra.
Keywords: vibrodiagnosis, vibrations spectra.

1. Technical considerations

The direct current motors have a large application domain in transports and industry and especially in pulp and paper industry. These applications were imposed by the advantages offered by these motors in regard to those in alternating current: a rigid mechanical characteristic, the easy speed adjusting in terms of maintaining energy transformation efficiency at high levels, good excess current capacity, etc.

As any dynamic plant these motors can be diagnosed through vibrations analysis – through vibrodiagnosis. If the special literature presents consistent information regarding the mechanical malfunctions identification way at this types of motors (mechanical looseness on the arbors, in the bearings and in fixing the motors on bases, misalignments between the motors and the driven plants, mechanical unbalance of the rotor, etc.), it is not generous with information related to electrical malfunctions diagnostic way, certain domains completely missing.

In this direction some experimental checked information will be presented, through which we can easily diagnose a diverse range of motor’s electrical malfunctions, but also feeding and command systems malfunctions. The authors had in view, when analyzing and classifying these malfunctions, the close connection between the effects produced by the electromagnetic fields over the motor’s magnetic parts, and also the results of their interaction according to the analysis concept cause-effect.

There are four main vibrations spectra analysis methods met and used:
- Analysis through the monitoring of vibrations amplitudes average values – offers clues upon malfunctions appearance without locating it;
- Temporal variation analysis of vibrations levels – offers presuming information about malfunctions and their causes;
- Simple Fourier Transformers analysis – offers precise information about the causes of malfunctions appearance, especially the malfunctions which induce repetition oscillations;
- Simple Fourier Transformers analysis of a filtered and demodulated vibration signal for malfunctions, which manifest into vibrations through stochastic spectra.

We must mention that once an electrical malfunction occurs, others can occur, too, of mechanical type, causing anomalous motor function and in consequence affecting the plant. For this reason a lot of malfunctions types manifested on different frequencies, identified during the analysis giving us clues related to malfunction cause and place, can occur in the vibrations spectra collected during the motor’s drive. We also make mention of the thing that the electrical malfunctions are more difficult to be identified in regard to the mechanical ones, requesting professionalism from specialized technique and maintenance staff side.

2. Describing of malfunctions identification solution through vibrations analysis

For the collection of direct current motors vibrations spectra we used a collector-analyzer type MICROLOG CMVA 60, a product of SKF – fig.1 – which through portative magnetic transducers (a large range of transducers are used according to the parameters which need to be measured) measures,collects and memorizes automatically or with operator’s intervention the plants vibrations spectra and allows a deep analysis in the hostile industrial medium. It is capable to display on a screen (LCD) the frequency spectra FFT (Fast Fourier Transform) as well as the temporal spectra.

The vibrations parameters collected from plants are: speed (FFT), acceleration (FFT and temporal variation) and filters acceleration 50-1000 Hz, 500-10000 Hz with signal’s demodulation, the obtained parameter being called by the special literature “enveloping”.

After collecting and stocking of spectra by the collector-analyzer CMVA 60, these are transferred into a calculation unit where by the help of PRISM4 soft, intended for monitoring and analysis of vibrating and wave phenomena, which join plants driving, are analyzed and the plant’s technical state is diagnosed.

Based on the previous considerations four electrical malfunctions types will be presented, identified and classified for continuous current motors.

The vibrations spectra analysis begins with the acceleration parameter analysis, as data collecting is realized with accelerometers, this being the first parameter to be measured, the others being calculated on this base.

Case 1.

The malfunctions from the motors commutation system have as an effect the introduction of specific vibrations in the entire system. The first identifiable malfunction through vibrodiagnosis is caused by an incorrect trailer plate position towards the neutral motor axis. This acts through the appearance of sparks or even of a fire circle at the contact between brushes and collector, and provokes an overheating and a dark-brown coloring of the collector. Hereby, these induce effects acting at frequency high values (in collector’s commutation frequency domain), in the motor’s magnetic elements. The vibration spectrum associated to such malfunction is represented in fig.2. The electrical motor from which the vibrations spectrum was sampled has a power of 75 kW and Un=440 V. The electrical motor has the speed of 882 rot/min and the associated fundamental frequency of 14,7 Hz. It has a collector made of 36 bars and a plate with two pairs of brushes. The commutation frequency is the product between the fundamental frequency and number of collector’s bars, reported to the number of brush pairs on the trailer plate. In this case a simple mathematical calculation indicates a commutation frequency situated in the frequency domain of 265 Hz. This type of malfunction manifests into the vibrations spectrum (FFT acceleration) through amplitude increasing at high values in the commutation frequency domain (fig.2), dominant towards any other spectrum amplitude, and the appearance of its sidebands at quotes of over 25% from fundamental’s amplitude. We mention that at normal drive the harmonics do not pass over 10-15% from the commutation frequency amplitude. Likewise in the adjacent areas of the commutation frequencies there are no spectral bands, at normal drive being only in the commutation area, increasing easily to its value then decreasing, the number of spectral sidebands being of 4-5 bands on each fundamental side.

Case 2.

From the same category of malfunctions related to commutation on continuous current motors, we present in fig.3 the electrical motor specific spectrum on which the collector is clogged up with black-lead powder resulted from the friction between brushes and collector’s bars. The electrical motor from which the spectrum was sampled is identical to that presented in the previous case with the same functional and constructive parameters.

This type of malfunction acts in the vibrations spectrum (FFT acceleration) through the appearance of sub harmonics belonging to the commutation frequency fundamental fc with values of 1/3*fc, 2/3*fc, fc. We also identify in the vibrations spectrum the side spectra belonging to the commutation frequency, which are more centered to this one, and sidebands on both commutation frequency sides with increased amplitudes. The cause of their appearance is exactly the increased rotor current value as a result of the collector’s channels clogging, but also the aggravation of the contact between brushes and collector after the accretion of black lead particles and copper oxides.

Case 3.

We can also identify and diagnose through vibrations spectra the malfunctions from the category of imperfect contacts between brushes and continuous current motors collectors. Hereby, the vibrations spectrum (fig.4) in acceleration – FFT - for a continuous current motor, is of the same type with those presented previously, and has imperfect contacts between brushes and collector. The recognition of this type of vibrations spectrum malfunction is realized in the domain of the harmonics superior to the commutation frequency. For this the vibrations spectra collecting domain was extended for this case at 800 Hz, towards the previous cases when it was at 400-500 Hz. This malfunction acts through the appearance of additional harmonics in the vibrations spectra, belonging to the commutation frequency up to 3 – 4 * fc and without important sidebands. The additional harmonics have values close to the commutation frequency, their level decreasing with the increasing of their multiples. We mention that in fig.4 is not accentuated but the second harmonic, as this domain offers enough information regarding the malfunctions cause.

Case 4.

Another malfunctions category, which can be identified and diagnosed through this method, is related to feeding tension unconformities of continuous current motors. Therefore, due to the fact that on adjustable drives the continuous current motors are fed from controlled rectifiers or dc - dc converters, malfunctions may appear on these equipments, which affect well motor’s functioning. In this case the most usual situation is improper motors feeding due to conduction relative periods of the devices operated through rectifiers or converters.

In the spectrum represented in fig.5 this type of malfunction acts through the appearance of the feeding frequency fundamental (50 Hz) and its harmonics (up to the 10th harmonic), mentioning that one of 2nd, 3rd or 6th harmonics higher in amplitude than the network fundamental, a particularity related to the tension rectifying way. The motor is of the same type as those presented previously and it presents another malfunction, acting through superior harmonics of the rotary frequency fundamental, caused by a light bearings loosening (backlash between shaft and ball or ball and plate) through the appearance of the rotary frequency 15 Hz (900 rpm) and its multiples.

3. The advantages of the dc motors vibrodiagnosis

The diagnostic through the method of continuous current motors malfunctions have the following advantages:

  1. Motors maintenance and repairing costs reduction;
  2. Repairs planning avoiding the accidental plants shut-downs;
  3. Forecasting the optimum intervention moment;
  4. Plants life and reliability increasing;
  5. Company’s end products costs reduction.

References
1.     Constantin Bala, Electrical Machines, EDP, Bucharest, 1982.
2.     SKF Reliability Systems, San Diego 2002 – Condition Monitoring Ins.
3.     PRISM4 for Windows - Quick Start Manual, San Diego 2000 – Condition Monitoring Ins.
4.     http://www.dliengieneering.com/vibman
5.     Banarie Cristian, Dima Dorian, Vibrating diagnosis of the mechanical loosening on axles.B.I.P.Iasi, Tomul L(LIV), Fasc.Vb, 2004.

 
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