Page 47 - Engineering in Kenya Mag

Fig 2: Sources of vibration in electric motors [2]
2.12. Vibration analysis
The vibration of a system can be expressed as a displacement, velocity, or acceleration. Measurement and analysis of vibration signals is carried out using dedicated equipment as is shown in fig. 3 below.
Fig 3: Process of vibration analysis
A vibration transducer or pickup is attached to or kept in proximity to the machine or structure undergoing vibration analysis. Vibration of a motor housing is measured with accelerometers, while shaft vibration is measured with proximity sensors. Accelerometers provide a voltage output whose amplitude is proportional to the acceleration of the vibration. Subsequently, the analyzer can integrate this signal to obtain the speed and displacement making the accelerometer the most versatile sensor. The signal from the vibration transducer is then amplified, filtered, and digitalized by an analog-to- digital converter. It can then be presented in time domain, or processed further, by FFT for example. Time domain analysis methods attempt to analyze the amplitude and phase of the vibration signals to detect faults. Frequency (spectral) domain analysis include Fast Fourier Transform (FFT), Hilbert Transform method, and power spectrum analysis. Frequency domain analysis achieves both objectives of fault identification and determination of the cause.
3.0 Methodology
Vibration data was obtained using a Microlog Accelerometer probe while vibration spectrum analysis was carried out using an SKF Microlog spectrum Analyzer for axial, radial, and horizontal axes. Vibration data was obtained from the motor DE and NDE bearings. The vibration analysis was carried out by Products services and solutions ltd in conjunction with KPC.
Equipment
POINT name Date 18-Jul-18
Last value
Units
Alarm status
22P01
Mtr NDE V Vel
9.538
mm/s
Overall - Danger
22P01
Mtr NDE H Vel.1
11.469
mm/s
Overall - Danger
22P01
Mtr NDE H Acc
0.766
g
---
22P01
Mtr NDE A Vel
10.468
mm/s
Overall - Danger
22P01
Mtr DE V Vel
12.017
mm/s
Overall - Danger
22P01
Mtr DE H Vel
10.675
mm/s
Overall - Danger
22P01
Mtr DE H Acc
0.7
g
---
22P01
Mtr DE A Vel
9.569
mm/s
Overall - Danger
4.0 Findings
The overall vibration readings for the motor bearings are as shown in table 1.
Table 1: Overall vibration reading for the bearings
The spectrum for the NDE bearing in the axial direction is as shown in
fig. 4.
Fig4: Spectrum for NDE motor bearing
The motor was inspected for alignment, bearing clearance, mounting, and the only area of concern was the clearance between the bearing and the bearing housing which was found to be 0.2 mm beyond the manufacturer (ABB) recommendation.
Eng. Tony Washike, PE, MIEK,B Tech (Electrical and Communication Eng), Msc (Eng), Senior Electrical Engineer, Kenya Pipeline Company.
Engineering in Kenya Magazine Issue 002
45