ISSN 1068-3712, Russian Electrical Engineering, 2017, Vol. 88, No. 10, pp. 692–696. © Allerton Press, Inc., 2017. Original Russian Text © N.R. Safin, V.A. Prakht, V.A. Dmitrievskii, 2017, published in Elektrotekhnika, 2017, No. 10, pp. 87–91.
An Investigation of the Influence of Bearing Failures on the Efficiency of an Induction Motor N. R. Safin*, V. A. Prakht, and V. A. Dmitrievskii Yeltsin Ural Federal University, Yekaterinburg, 620002 Russia *e-mail:
[email protected] Received September 15, 2017
Abstract⎯The influence of the technical condition of bearings on the efficiency of induction motors (IMs) with a squirrel-cage rotor was considered. The experiments were carried out using induction-motor tests in the rated nominal regime with one operable bearing and two faulty ones, which had defects of the outer and inner parts. The bearing defects increase the energy loss, which leads to increased heat dissipation in the contact zone of its components, overheating, and reduction of motor efficiency. More accurate data on changing the efficiency of IMs with faulty bearings taking into account the heated state of the machine that is close to actual operating conditions were obtained. Thermal-imaging inspection of motor was performed, the graphic data characterizing the changes of temperatures of units and the machine efficiency when operating with a faulty bearing are presented. A diagnostic technique of the bearing condition by means of integral parameters of vibration acceleration that makes it possible to analyze the potential defects taking into account the weak level of their activation and the shift over the frequency response is considered. Keywords: induction motor, efficiency, thermal-imaging inspection, bearing failures, heat losses, diagnostic parameter, vibrational acceleration DOI: 10.3103/S106837121710011X
Induction motors (IMs) with squirrel-cage rotors are the most mass-produced items of the electricmachine industry. They account for more than half the world’s energy consumption. The energy efficiency of IMs is determined by their efficiency, which depends on the losses in a machine, in particular, of mechanical losses, which are largely associated with the operation of bearings. Bearing defects can cause vibrations, heat, changing the rotational speed, increased noise, etc. Moreover, local overheatings are increased and mechanical losses increase. All of this ultimately leads to decreasing IM efficiency. Despite this obvious fact, analysis of available data shows that the influence of failures of bearings on IM efficiency has been investigated insufficiently. Thus, the influence of bearing failure on efficiency depending on the time of machine operation and so forth has not been studied. For consideration of these problems, IMs with various bearings were tested and the obtained data were analyzed. During the tests, a motor of AIR71A2U2 IM1081 type (rated capacity of 0.75 kW, rated rotational speed of 2820 rpm) was supplied directly from a three-phase mains, with the tests being conducted on a test bench on which a retarding torque was created with an induction dynamometer. During the tests, thermal images were obtained using a thermal-vision camera
of Testo 875-2i type. The currents and voltages were recorded using an analog-to-digital converter (ADC) (of USB3000 type) with a sampling rate of 50 kHz and a measuring board with three current sensors of LEMHX 02-Р type and three voltage sensors of LV25PSP5 type. The rotational speed of the IM being tested was recorded using a photo tachometer of Mastech DT-2234A type. The output torque of the IM was set equal to rated torque Mr = 2.5 N m. The instantaneous values of the currents and voltages were digitized by the ADC and were recorded to a computer for processing in the MATLAB software package. The IM tests conducted alternately with each bearing took the form of a series of stages with durations of 2, 15, 30, 60, and 120 min. At the end of each stage, the thermograms were read and the instantaneous values of currents and voltages were recorded. The IM tests were conducted with each bearing after cooling the machine to environmental temperature. Bearings of 6204 brand were used in the machine being tested. The tests of IMs were conducted with an operable bearing, with a bearing with a hole on the outer race 4 mm in diameter (faulty bearing no. 1), and with a bearing having a hole on the outer race 6 mm in diameter (faulty bearing no. 2). In the latter two cases, there were also small hollows in inner races. The methods used for artificially obtaining a defect for investigation
692
AN INVESTIGATION OF THE INFLUENCE OF BEARING FAILURES
Frame temperature, °C 80 71.5
(a) 51.9 50
70
45
60
40
50
76.5
78.5 78.0
76.2
65.5
693
70.5 62.5
30
40.5 39.5 40 37.0 30 35.0
25
20
35
23.8
39.0
43.5
43.2
10
(b) 94.8
0
90
20
80 70
40
60
80
100 120 Time, min
Fig. 2. The temperature of the IM frame under the rated load: (1) IM with the operable bearing, (2) IM with faulty bearing no. 1, and (3) IM with faulty bearing no. 2.
60 50 40 30 23.1 Fig. 1. Thermograms of IM operation (operating period is 15 min): (a) operable bearing; (b) faulty bearing no. 1.
of IM operation regimes with faulty bearings were described in [1, 2]. The control of the temperature of machine units using a thermal-vision camera as the means of diagnosis of mechanical failures was considered in [3, 4]. A number of factors can influence the quality of thermal images [4]: the environmental humidity, the distance between the camera and the subject of measurement, the environmental temperature, and the relative radiating capacity of the subject under study. Thermal-imaging inspection makes it possible visually to determine the mechanical defects and temperature distribution of an IM during operation. The existence of a section with higher temperature is evidence of the appearance of a defect. As an example, Fig. 1 shows thermograms of an IM after 15 min of operation. The maximum temperature corresponds to the inner bearing race. The bearings are closed with shields, and seals, i.e., armored collars, are placed from the outer side. As a rule, rubber collars with spring are intended for shaft sealing and protection against dust and dirt and can operate at temperatures in the range from –60 to 170°C. Correspondingly, the maximum temperature in the presented thermograms corresponds to of sealing the IM (collars with a spring). RUSSIAN ELECTRICAL ENGINEERING
Vol. 88
The machine being tested with the specific bearing was operated continuously. With an increase in the operating period, with a faulty bearing, the thermal loss in the region of the bearing unit increases, which leads to increasing temperature and decreasing efficiency. The rated operating temperature of the used bearings of 6204 grade is in the range from –20 to 120°C. Thermal-imaging inspections make it possible to evaluate the power of heat releases in the defect [5]. Based on the obtained data (Fig. 2), one can note that the difference between the temperature of the machine frame and the environmental temperature under operation of an IM with mechanical defects becomes much greater. The relative deviation of temperature after the final tests of 120-min duration under IM operation with an operable bearing amounted to 19.7°C (the temperature of the machine frame is 43.5°C), and in IM operation with faulty bearing no. 1 the relative deviation of temperature amounted to 53.8°C (the temperature of machine frame being 78°C), while during IM operation with faulty bearing no. 2 the relative deviation of temperature amounted to 54.5°C (the temperature of machine frame being 78.5°C). The environmental temperature during these experiments was 23.8, 24.0, and 24.2°C (that is, the difference did not exceed 2°C). It follows from the experimental data (Fig. 2) that a bearing defect leads to continuous growth of the machine-frame temperatures. At each test stage, the electrical resistance of the winding was not measured, and so it is impossible to evaluate the winding temperature by the resistance method. The output power associated with the presence of a defect can be considered dissipation-energy consumption. Therefore, some of the input power will used in defect activation,
No. 10
2017
694
SAFIN et al.
resulting in heating and vibrations [5]. Moreover, the consumed energy dissipates as heat, the temperature of the bearing unit and machine frame grows, and heat dissipation increases. It is known that the operation of an IM with faulty bearings leads to decreasing efficiency. This is confirmed by the conducted experiments. Figure 3 shows the dependences of efficiency on the operating time of the IM. One can note that in 120 min the efficiency of an IM with an operable bearing decreases by 1.64% as compared with the initial value, the efficiency of an IM with faulty bearing no. 1 decreases by 5.9%, and the efficiency of an IM with faulty bearing no. 2 decreases by 6.4%. Decreasing the efficiency, in turn, leads to the excessive consumption of electric power. The decrease of IM efficiency after 120-min operation with a faulty bearing is 5.9–6.4%. In a long-term regime of IM operation, a bearing defect will increase the torque of resistance during machine rotation. Only a few rolling elements of bearing receive a direct static load during the IM operation, while others are rolled between the races. In addition, the arising variable loads reach a maximum and minimum in the upper and lower points of the rolling element. Accordingly, the defects in the bearing races over time cause variable changes in their inner radial clearance, which leads to fluctuations in the machine air gap in which the magnetic field of the IM is distorted. To evaluate the technical condition of bearings, the instantaneous values of vibration accelerations were analyzed using an accelerometer of ACH-01-04/10 type installed on the bearing shield of the machine. Vibratory control of the condition of machines makes it possible to evaluate the necessary information on the technical condition of an operating machine for subsequent maintenance. As a rule, the root-meansquare (RMS) values of accelerations or speeds are used to measure the vibration. It is appropriate to use acceleration for measurement during control of the condition of the roller bearings and gears, because effects of these units can appear in the high-frequency band.
Efficiency, % 74 71.3
73
72.5
72.2
72.0
72
71.9
71 70.0
70
69.1 69 69.7 68 67
68.0
68.3 0
68.0
20
67.6
67.6 40
60
67.3 100 120 Time, min
80
Fig. 3. IM efficiency under the rated load: (1) IM with the operable bearing, (2) IM with faulty bearing no. 1, and (3) IM with faulty bearing no. 2.
Each of the main bearing elements has a characteristic frequency, at which the vibration energy is concentrated as a result of cyclic stresses and periodic blows. The vibrations of the frequencies of bearings are determined as follows [1, 2]: ⎯the rolling frequency of the rolling elements in the inner race is
(
)
(
)
(1) f i = n f r 1 + d cos α , 2 D ⎯the rolling frequency of the rolling elements in the outer race is (2) f o = n f r 1 − d cos α , 2 D ⎯the rotational speed of the rolling elements is
fb =
)
(
2 ⎞ Df r ⎛ d ⎜1 − cos α ⎟ , ⎠ 2d ⎝ D
(3)
Table 1. Frequencies of bearing vibrations Bearing Operable
Faulty no. 1
Faulty no. 2
Frequencies characterizing the bearing defects
Frequencies fi, fo, and fr defined by vibrations, Hz
Defect in the inner race Defect in the outer race Defect of the rolling elements Frequencies characterizing the bearing defects Defect in the outer race Defect of the rolling elements Frequencies characterizing the bearing defects Defect in the outer race Defect of the rolling elements RUSSIAN ELECTRICAL ENGINEERING
229.8 141.8 92.5 228.8 141.2 92.1 227.6 140.4 91.6 Vol. 88
No. 10
2017
AN INVESTIGATION OF THE INFLUENCE OF BEARING FAILURES
695
(a)
Amplitude, m/s2 ×10–3 fr 1.0
fb
fo
fi
0.5 0
40
80
120
160 200 Frequency, Hz
(b)
Amplitude, m/s2 ×10–3 fr 1.0
fb
fo
fi
0.5 0
40
80
120
160 200 Frequency, Hz
Fig. 4. Comparative spectrogram of vibration accelerations: with (a) the operable bearing and (b) faulty bearing no. 1.
where n = 8 is the number of rolling elements, pieces; fr is the rotor rotational speed (with the operable bearing, 46.45 Hz; with faulty bearing no. 1, 46.25 Hz; and with faulty bearing no. 2, 46 Hz) Hz; d = 7.94 is the rolling-element diameter, mm; D = 33.5 is the pitch circle diameter, mm; and α is the minimum angle of contact of the bearing (conditionally assumed to be 0°). The calculated values of frequencies using formulas (1)–(3) are presented in Table 1. The wear of rolling bearing elements leads to the appearance of lateral bands of the high-frequency component [6] (Fig. 4). The frequency analysis of vibration acceleration signals makes it possible to analyze the harmonics coinciding with the frequencies of
possible damage in the bearing elements. This makes it possible to detect potential defects in advance. To find the level of the fault condition of the bearings of an IM, integration was carried out of frequency bands within a harmonics with lateral components that are multiples of the bearing frequencies (Table 2). Based on the data of Table 2, one can note that, during operation of an IM with faulty bearings nos. 1 and 2, the RMSs of vibration accelerations and its peaks values significantly increase. Such phenomena can occur for various reasons—failures of bearings, rotor defects, misalignment, etc. In this case, the RMSs were obtained within harmonics that are multiples of the characteristic frequencies determining the defects of bearings. The obtained results make it pos-
Table 2. The values of integrated diagnostic parameters of vibration accelerations Parameters of vibration accelerations, m/s2 RMS of total time range, m/s2 2
Peak value, m/s RMS of frequency range (45 + 55 Hz) RMS of frequency range (90 + 100 Hz) RMS of frequency range (135 + 145 Hz) RMS of frequency range (225 + 235 Hz) RMS of frequency range (95 + 105 Hz) RMS of frequency range (185 + 195 Hz) RMS of frequency range (275 + 285 Hz) RMS of frequency range (455 + 465 Hz) RMS of frequency range (145 + 155 Hz) RMS of frequency range (280 + 290 Hz) RMS of frequency range (415 + 425 Hz) RMS of frequency range (685 + 695 Hz) RUSSIAN ELECTRICAL ENGINEERING
IM with operable bearings
IM with faulty bearings IM with faulty bearings no. 1 no. 2
0.0078
0.2883
0.2825
0.0239 0.000010034 0.0000061991 0.0000055761 0.000033823 0.0000056433 0.0000074856 0.0000077111 0.0000079547 0.0000056731 0.0000069210 0.0000048701 0.0000011519
1.1834 0.000087192 0.000028502 0.000030361 0.000012040 0.000024672 0.000019741 0.000016268 0.000071208 0.000033602 0.000015146 0.000015310 0.000018857
1.1844 0.00024968 0.00011642 0.000090581 0.000051265 0.00011491 0.000070409 0.000046322 0.000061222 0.000077990 0.000039409 0.000031256 0.000040460
Vol. 88
No. 10
2017
696
SAFIN et al.
sible more accurately to reveal the defects of bearings, as the calculations according to formulas (1)–(3) may not always provide a sufficient level of diagnosis reliability. REFERENCES 1. Obaid, R.R., Habetler, T.G., and Stack, J.R., Stator current analysis for bearing damage detection in induction motors, Proc. 4th IEEE Int. Symp. on Diagnostics for Electrical Machines, Power Electronics and Drives, SDEMPED 2003, New Jersey, 2003. 2. Silva, J.L.H. and Cardoso, A.J.M., Bearing failures diagnosis in three-phase induction motors by extended Park’s vector approach, Proc. 31st Annu. Conf. of IEEE Industrial Electronics Soc. (IECON), Raleigh, NL, 2005.
3. Picazo-Rodenas, J., Royo, R., Antonino-Daviu, J., and Roger-Folch, J., Use of infrared thermography for computation of heating curves and preliminary failure detection in induction motors, Proc. 20th Int. Conf. on Electrical Machines (ICEM), Marseille, 2012. 4. Baranski, M. and Polak, A., Thermographic diagnostic of electrical machines, Proc. 19th Int. Conf. on Electrical Machines (ICEM), Rome, 2010. 5. Yaroshenko, I.V., The way to increase functioning efficiency of high voltage mechatronic modules on the base of technical state diagnostics, Cand. Sci. (Techn.) Dissertation, Novocherkassk, 2014. 6. GOST R ISO (Russian ISO State Standard) no. 133731-2009: State Checking-up and Diagnostics for Machines. Vibration Checking-up of Machines State. Part 1. General Methods, 2009.
RUSSIAN ELECTRICAL ENGINEERING
Translated by M. Kromin
Vol. 88
No. 10
2017
