Flow Diagnostics in Centrifugal Pumps by Motor Current Signature Analysis

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1. Introduction

In industry as well as in domestical appliances centrifugal pumps are most widely used and many of the centrifugal pumps are driven by induction motors. Either of the induction motor failure or the centrifugal pump failure would result in a preplanned unexpected shutdown leading to loss of production as well as loss of revenue. Therefore, early stage detection of faults and isolation and take necessary steps against their causes are of great interest. Basically fault detection in early stage would minimize maintenance costs and increase reliability of the pump-motor system, is a part of predictive maintenance.

Kenullet. al(1)investigated the specific frequency ranges of the energy content of the motor current signal to detect abnormalities that occur in centrifugal pumps like partial flow operation, pressure pulsation generation during the flow, cavitation and circulation formation in the impeller passage. Budris and Mayleben(2)have shown experimentally about the occurrence of cavitation and suction pressure pulsations above the required NPSH of a centrifugal pump and its relation with the intensity of suction energy level. By monitoring the inlet pressure pulsations for the varying flow rate the experiment has been carried out. Perovic et. al(3)extracted the fault signatures from the motor current electrical spectrum to detect cavitation, blockage and damaged impeller condition by relating the spectral features to the individual faults. Schmalz and Schuchmann(4) calculated the spectral energy within the band of about 5 Hz to 25 Hz. To detect the presence of cavitation or low flow condition in centrifugal pumps, spectral energy within the specified band is used. Harris et.al(5)describe a fault detection system for diagnosing potential pump system failures using fault features extracted from the motor current electrical spectrum. Abdel-Rahmanet.al(6) have performed vibration analysis which offers a comprehensive method of identifying a variety of pump problems. Hernandez-Solis and Carlsson(7) analyzed the motor current and power signatures when cavitation is present in the centrifugal pump. The starting point of cavitation has been found out through the motor current and power signature analysis. Al Thobiani and Faisal(8) investigates methods for the detection of incipient cavitation in centrifugal pumps.

2. Design of Experimental Set-up
2.1 General requirements
The aim of this study is to diagnose the flow condition at different flow rate in centrifugal pumps.In order to achieve this objective, it is necessary to design and construct a suitable test-rig for the experimentation shown in Figure 1 and Spesification of Pump , motor etc. is given in table 1.

2.2 ± 15 volt D.C. Power Supply for Current Sensor (LA 55-P)
Current sensor LA 55-P will be connected between the power supply and induction motor in series connection. A ± 15 volt DC power supply will be connected with the current sensor to activate it. This DC power source will supply power to this Hall Effect current transducer to make this sensor workable.

3. Experimentation and Data Acquisition
3.1 Linearity Test of Current sensor (LA-55P)
Before connecting the current sensor to the experimental set-up set it is required to calculate the range of current for which the operation of current sensor is linear. So in order to check this, a current ranging from 1amp to 10 amp is passed through the current sensor using a variable resistor and ac voltage source. The voltage across RM i.e. measuring resistance connected with current sensor is observed in DSO (digital storage oscilloscope) and its variation is plotted with variation with sensor current.

In the above Figure6 the circuit diagram of the sensor LA 55-P has shown where Ip is the primary current consumed by the induction motor connected with the centrifugal pump which has to measure. RM is the measuring resistance which is 100 ohms in present case. Vc is the voltage supply (+15V/-15V DC) to the current sensor LA 55-P. M is the measuring point.

The above Figure 8 shows the voltage across RM for the sensor current of 6amp. The abscissa represents time and ordinate represents measured voltage. By changing the current flow through the current sensor LA 55-P, variation is observed in the measured voltage. It helps to check the linearity of the current sensor. The data has been recorded throughout the experiment of the linearity check of the current sensor and represents above through the table 5

From the Figure9 we have come to a conclusion that the current transducer is linear upto current of value 6 amp. The respective rms value of the voltage is 5.78 V. For the current above 6 amp it is observed that the transducer saturates and do not give expected result. Beyond 6 amp current the obtained result contains error. So it is required to measure current upto 6 amp in order to obtain accurate result. In the present experimental set up the normal current consumption of the induction motor is about 2.6 amp which is within our linear operation zone.

3.3 Graphical Programming
From the figure 10 the data, which is conditioned through the NI PCI 6251 DAQ card, has been processed by the LabVIEW software. In LabVIEW a graphical program has been made to capture the raw data. This raw data has been analysed by doing power spectrum analysis (FFT). Different tools like DAQ Assistant, Spectral Measurements are used to execute this program.

4. Results and Discussions
4.1 Experimental diagnosis of centrifugal pump
The diagnosis of centrifugal pump has been carried out with the measured data during the running condition of the centrifugal pump for different flow rate and by doing analysis of the harmonics of the power spectrum of the motor current signature captured by current transducer.

4.1.1 The characteristic curves of pump_1
The characteristics curve of the pump_1 when running at 1496 & 1906 rpm and when
water depth in the storage tank is 32 cm have shown below Figure11& 12respectively

4.1.3 Comparison between pump_1 and pump_2
Now a comparison has been shown in head &efficiency of the two pumps for their almost same 1498 rpm.in figure 15 and figure16.

4.2 Diagnosis of the centrifugal pump by current signature
Current consumption data has been captured by current transducer LA-55P for different cases for both pumps. These cases are (1st, 2nd, 3rd, 4th, 5th, 6th, 7th, 8th) Rotation opens of discharge valve and 9th Rotation opens of discharge valve or Full discharge.
The frequency, which is highlighted in the spectrum window, has shown below table 6.

The experimental results and motor current signature analysis results have shown in
same graph to understand the variation of the flow parameters and electrical parameters and there mutual dependence. This kind of plot helps us to make a decision about their health condition of both pumps.

4.3 Analysis of the current spectrum and graph
From the above electrical spectrum and graphs the analysis can be made
 Current spectrum for the both pump has shown in above figure19. White colour spectrum is reflecting pump_1 current spectrum and red colour spectrum is reflecting pump_2 current spectrum. Both the pump is working in same flow conditions. There is a minor variation in current between the two pumps. In each variation of the flow rate it has been observed that pump_2 is always consuming more current than pump_1 also the disturbance in harmonics is more in case of pump_2 which reflects that in pump_2, some flow problem is occurring due to which the efficiency of pump_2 is less than pump_1.
 The region with the lowest efficiency corresponding to the operating points with high head and low flow rate in case of pump_2 there is immense effect of noise, is the highest one for normal operating conditions of the pump, as a result the input power consumption is high as compare with the other operating zone.
 The harmonic component corresponding to the operating point with high flow rate and low head gets higher.

5. Conclusions
In this study, experimental MCSA is used to diagnose centrifugal pumps. It is found that, for the flow diagnosis this method is very reliable to detect the abnormality in the flow. Certain harmonic components in the stator current spectrum are developed when any kind of problem generate in the pump. Based on this approach it can be explained very clearly the reason behind the abnormality in the pump.The characteristic curves and current signature of the induction motor both are used to determine the most suitable operating zone of the two centrifugal pumps in present case. During this suitable operating zone, it is clearly shown that the efficiency of the pump is maximum and current consumption is comparatively low as compared with the other operatingzone. The location of the best efficient zone is properly recognized through the motor current spectrum which is the uniqueness of this study.

Posted by: Subhashis Nandy. in Science | Date: 01/04/2016

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