CHAPTER 2 Laboratory Rig and Experimental Procedure for
2.2 Laboratory Rig and Experimentation
2.2.1 Machine Fault Simulator
MFS is an innovative tool used for studying signatures generated by various machinery faults.
This system weighs close to 59 kilograms and is compact in dimensions. All the components in a MFS are machined to high precision so that it can operate in a wide range of speeds without any conflicting vibrations. Depending upon the requirement various individual or combined faults can be simulated on this set-up in a completely controlled environment. The technical details of MFS and its allied components are given in Table 2.1.
Table 2.1: Technical specification of various MFS and allied components S.No Component name Technical specification
Electrical equipment:
1 Induction motor Three phase, 0.5 HP, pre-wired, self aligning mounting system for easy installation
2 VFD Variable frequency drive with multi-featured front panel programmable controller, rpm range 0 to 10,000 rpm (short duration) variable speed
3 Voltage Supply voltage 115/230 V AC, single phase, 50 Hz
4 Tachometer Built-in tachometer with LCD display and one pulse per revolution analog output for DAQ purposes. Requires DC power supply (± 30 V/ 2 A)
5 DAQ NI PXI-8108 Core 2 Duo 2.53 GHz controller with NI PXI-4472, 8 channels, 24 bits, 102.4 kS/s sampling rate for accelerometer module Mechanical equipment:
1 Centrifugal Pump Oberdorfer™ 60P, pump body: bronze, inlet: 0.019 m, outlet: 0.127 m, flow: 5.45 cubic meter per hour, pressure: 0.16 MPa max, 16.15 m of head
2 Shaft 19.05 mm diameter, turned ground and polished steel
3 Bearings Two rolling element bearings mounted in aluminum horizontal split housing
4 Rotor base 360 mm long, completely movable using jack bolts
5 Belt mechanism Two double groove ‘V’ belt drives with one set screw mounting and one set bush/ key mounting
6 Foundation 12.7 mm die cast aluminium base, base stiffenners and 8 rubber isolators
Measuring equipment
1 Accelerometers Details in Table 2.2 2 AC line current
probes
Frequency range: DC to 100 kHZ (-3 dB with current de-rating) Current range: 100 mV/A: 100mA to 10A, 10 mV/A: 10A to 100A The construction of the MFS comprises of an induction motor, a split bracket bearing housing, a sliding shaft, a rotor with split collar ends, a flexible coupling, ‘V’ belt pulleys, belt tension
devices, a variable frequency drive with multi-featured front panel programmable controller, a photovoltaic sensor and a sliding platform with a central jack screw. All these components can be mounted and dismounted with ease, based on the requirement. The AC variable frequency drive is used to adjust the speed of the motor. The photovoltaic sensor is mounted near the coupling to measure the actual speed of the shaft via a reflecting tape on the shaft.
3
1 4
5 7
10
11 12 8
9
2 14
13
15 16
17 18
1 – CP
2 – Induction motor 3 - Coupling
4 – Bearing and bearing housing
6
5 – Pulley 6 – Tank
7 – Inlet modulating valve 8 – Water supply
13 – Motor power supply 14, 15, 16 – Current probes 17 – Data acquisition system 18 - Computer
9 – Water discharge 10 – Outlet modulating valve 11 – Tri-axial accelerometer – 1 12 – Tri-axial accelerometer – 2
4
Figure 2.2: A schematic diagram of the experimental set-up
Various sensors can be used for acquiring the signals from the operating equipment. Whenever a fault appears in a CP system, there are changes in its operating conditions, which reflects as a change in load on the induction motor that is driving it. Therefore, it is expected that the line current of the motor may serve as a good metric to understand the CP fault state. In continuation, vibration signals are versatile and are sensitive to the dynamics of the rotating machinery. Hence, in the present study, the vibration and motor line current signals acquired
from the CP and the motor, respectively are used to monitor the CP. The accelerometers used, measure vibration dynamics in three orthogonal directions (axial, radial and vertical transverse). Two tri-axial accelerometers are mounted on the CP housing and the CP bearing housing locations. Three AC current probes are clamped around the power leads of the motor, which are very easily reachable. These sensors are connected to a central DAQ via proper cable connections. LabVIEW software supplied by the National Instruments, USA is used to configure the data collection. Figure 2.3(a) shows the LabVIEW architecture and Figure 2.3(b) shows the sampled data. Details about some of the components used in the current experimentation are given in subsequent section. This is followed by the details of experimentation in Section 2.3.
2.2.1.1 Variable frequency drive
A motor speed controller or a VFD is used to alter the speed of the motor by varying the frequency and voltage supplied to it. This is a power electronics based device, which converts a fixed frequency and fixed voltage sine power wave to a variable frequency and variable voltage output, which is used to control the speed of the motor. It comprises of a rectifier bridge converter, a direct current link and an inverter which allows it to increase or decrease the speed as per the user’s application requirement. In the present work a variable speed AC drive (0.746 kW, single phase, 9.2 A/3-phase, 5.1 A, 200-240 V and 50/60 Hz, Output: 3- phase, 4.2 A, and 0-240 V) is used. Using a knob, the speed can be manually adjusted between 0-400 Hz.
(a)
(b)
Figure 2.3: (a) LabVIEW environment, (b) samples raw data plot
2.2.1.2 Photovoltaic sensor and a constant DC power source
A photovoltaic sensor, which acts as a tachometer, is mounted near the MFS coupling. It measures the rotational speed of the shaft. This sensor needs a constant DC power source ± 30 V/2A to function.
This equipment converts the AC power supplied by the mains to constant DC voltage. Thus, it protects the probe from overload or short circuit. It has an adjustable current limiter and a digital display for the voltage and current. Thus, the high resolution voltage and current outputs can be acquired. The purpose of the tachometer is to measure accurately the rpm of the shaft, which is recorded as one pulse per revolution in the DAQ. The probe is assembled near the shaft end of the motor. It strikes a laser beam onto the shaft, which is reflected by a small reflecting strip attached to it.