Proactive Maintenance Techniques
An Introduction to Motor Current Signature Analysis
Condition Monitoring of Machinery Using Motor Current Signature Analysis (MCSA) Summary
This article explores the operating principles of motor current signature analysis (MCSA) and several case studies that relate to its use. Parts of this article are based on the SKF Condition Monitoring publications CM3005 and CM3029.
1
Proactive Maintenance Techniques Motor Current Analysis Motor current signature analysis (MCSA) is a powerful monitoring tool for electricmotordriven equipment. It provides a non-intrusive means for detecting the presence of mechanical and electrical abnormalities in motor-end driven equipment, and altered conditions in the process that may be “downstream" of the motor driven equipment. MCSA is based on the recognition that a conventional electric motor powering a machine also acts as a transducer of variations in the driven mechanical load (the latter are converted by the motor into electric current variations that are transmitted along the motor power cables). These current variations, though very small in relation to the average current drawn by the motor, can be extracted reliably and non-intrusively at a location remote from the equipment and processed to provide indicators of condition (signatures) that may be trended over time to give early warning of performance degradation or process alteration. Although MCSA technology was developed for the specific task of determining the effects of aging and service wear on motor-operated values used in nuclear power plant safety systems, it is recognized to be applicable to a much broader range of machinery. MCSA is used to analyze various pumps, blowers, compressors, and airconditioning systems powered by both AC and DC motors. A recent study by the Oak Ridge National Laboratories (ORNL) shows that motor current signatures, represented in both time and frequency domains, provide very sensitive diagnostic indicators of the condition of both the motor driven operator and its subcomponents downstream of the driver. The means by which "trendable" parameters are extracted from the raw motor current signal and related to physical processes including degradation is termed Motor Current Signature Analysis (MCSA). This article explores the operating principles of MCSA and several case studies that relate to its use.
Working Principles : The use of devices (Figure 1) to obtain spectrum and time waveform data is also beneficial for non-invasively obtaining motor current signals. A single split-jaw current probe (Figure 1, on the right) placed on one power leads is sufficient to obtain data. (Because no electrical connections need to be made or broken, shock hazard is minimal.) The resulting raw current signal is amplified, filtered, and further processed as appropriate to provide a sensitive and selective means for extracting motor current noise information that reflects instantaneous load variation within the drive train and ultimate load.
Fig 1 : SKF Microlog and AC/DC current Clamp
2
Proactive Maintenance Techniques Two separate diagnostic signals are developed from the single probe input: 1. Time domain analysis using a waveform recorder. 2. Frequency domain using a Fourier transform spectrum analyzer. Although the details of signal conditioning embodied in the custom electronics package are proprietary, the basic objective of the optimization is maintenance of dynamic range in subsequent data analysis processes. This is accomplished by eliminating those portions of the signal that lend nothing to the analysis process employed. For field use, it is most convenient to combine the two diagnostic signals into a single unit and emulate the results on a personal computer equipped with special hardware and software for further analysis.
Motor Basics Since the following discussions require a basic understanding of motor functions, a brief illustration and example are given. Figure 2 shows the basic cross-section of a motor with the shaft protruding from the right side of the figure, and the fan blades located on the upper lefthand section of the illustration.
Figure 2. Electric Motor Cutaway. An electromagnet is the basis of an electric motor. You can understand how things work in the motor by imagining the following scenario: You create a simple electromagnet by wrapping 100 loops of wire around a nail and connecting it to a battery. The nail becomes a magnet and has a north and south pole while the battery is connected. Take your nail electromagnet, run an axle through the middle of it and suspend it in the middle of a horseshoe magnet as shown in Figure 3. Attach a battery to the electromagnet so the north end of the nail appears. The basic law of magnetism tells you what will happen: The north end of the electromagnet will be repelled from the north end of the horseshoe magnet and attracted to the south end of the horseshoe magnet. The south end of the electromagnet will be repelled in a similar way. The nail will move about half a turn and then stop in the position shown.
Figure 3. Illustration of Electromagnetic Charge.
3
Proactive Maintenance Techniques This half-turn of motion is simple and obvious, as magnets naturally attract and repel one another. The key to an electric motor is to go one step further so that, at the moment the halfturn is complete, the field of the electromagnet flips. The flip causes the electromagnet to complete another half-turn of motion. You flip the magnetic field simply by changing the direction of the electrons flowing in the wire (you do that by flipping the battery over). If the field of the electromagnet flips at just the right moment at the end of each half turn, the electric motor spins freely. The armature takes the place of the nail in an electric motor. The armature is an electromagnet made by coiling thin wire around two or more poles of a metal core. The armature has an axle, and the commutator is attached to the axle. The commutator is simply a pair of plates attached to the axle. These plates provide the two connections for the coil of the electromagnet. Two parts accomplish the flipping the electric field part - the commutator and the brushes. The commutator and brushes work together to let current flow to the electromagnet, and to flip the direction that the electrons are flowing at just the right moment. The contacts of the commutator are attached to the axle of the electromagnet, so they spin with the magnet. The brushes are just two pieces of springy metal or carbon that make contact with the contacts of the commutator. When these parts are all together, you get a complete electric motor (Figure 4).
Figure 4. General Overview of a Complete ElectricMotor In Figure 4, the armature winding are left out so it is easier to see the commutator. Be aware that as the armature passes through the horizontal position, the poles of the electromagnet flip. Thus, the north pole of the electromagnet is always above the axle so it can repel the field magnet's north pole and attract the field magnet's south pole. Most small electric motors contain the same pieces, as described above: two small permanent magnets, a commutator, two brushes, and an electromagnet made by winding wire around a piece of metal. Almost always, however, the rotor have three poles rather than the two poles as explained in this article. There are two good reasons for a motor to have three poles: It causes the motor to have better dynamics. In a two-pole motor, if the electromagnet is at the balance point, perfectly horizontal between the two poles of the field magnet when the motor starts, the armature gets stuck there. That never happens in a three-pole motor. •
Each time the commutator hits the point where it flips the field in a two-pole motor, the commutator shorts out the battery (directly connects the positive and negative terminals) for a moment. This shorting needlessly wastes energy and drains the battery. A three-pole motor solves this problem. •
4
Proactive Maintenance Techniques It is possible to have any number of poles, depending on the size of the motor and the specific application it is being used in. •
Time Domain Data Time domain data is a valuable tool to use when analyzing MCSA signatures in detail. It is important to capture the time-domain data so that further analysis of motor components can be seen. For the purpose of general analysis information, most key features are determined from the frequency spectrum.
Frequency Domain Data The frequency domain of the MCSA spectrum displays most of the valuable data concerning the motor, such as: Uneven air gap between the rotor and stator Damaged Stator or Rotor Bars Damaged Windings Deteriorating Insulation These common faults are explained in the next section along with some typical spectra examples. • • • •
One other common way electrical faults show up in a motor circuit is twice line frequency (2x line frequency). An example of 2x line frequency: if a motor is running at 3600 RPM or 60 Hz, is 7200 RPM or 120 Hz. In Europe, it is common that motors operate at 50 Hz. Therefore, 100 Hz is 2x line frequency. 2xfrequency can indicate several problems in the motor.
Figure 5. Twice line frequency (2x line frequency) at 119.79 Hz, second major peak from the left with circular marker. Twice line frequency also includes multiple harmonics at 239.58 Hz, 359.37 Hz, etc. Measurement is taken in Enveloped Acceleration. Running speed is 1792.4 RPM. Uneven Air Gap Between Rotor and Stator
Another common problem indicated by 2x line frequency is an uneven air gap. As the poles of the motor pass the narrow gap, the magnetic pull is greater compared to the pull on the 5
Proactive Maintenance Techniques opposite side (180 degrees away) where the gap is widest. The number of motor poles does not change, and as a result, an uneven air gap in a velocity spectrum signal at 2x line frequency (120 Hz or 100 Hz) is displayed. The cause of this air gap is often a ‘soft foot’ caused by an uneven base plate. As the motor is mounted to the base, the motor housing and stator are distorted, which results in an uneven gap of air between the stator and the rotor. Empirical data seems to indicate that a twice line frequency signal appears when the gap clearance exceeds 10% variance. Tightening and loosening bolts, one at a time, on the foundation while observing the changes in the vibration spectrum, can help diagnose soft foot. Damage to the Stator, Windings, or Insulation
There are numerous causes of stator damage - manufacturing, environment, or flaws in the insulation. Any damage to the stator creates an uneven magnetic field around the rotor. This uneven field generates an uneven pull on the rotor, regardless of the motor speed and cause a mechanical vibration at twice line frequency. It is often possible to locate the area of damage with either an infrared or thermal detector (See An Introduction into Thermography). Often there is an area on the motor housing where the surface temperature is 20-30 degrees F hotter. A damaged stator can also generate a mechanical vibration signal at a frequency equal to the number of rotor bars, multiplied by the rotation speed. Again, in the area of stator damage, the magnetic field is weakened, and therefore stronger 180 degrees away. As each rotor bar passes this area of higher strength, the bar mechanically pulls in that direction. Typically, induction motors have between 45- 55 bars in the rotor, but this varies by manufacturer. For this reason, troubleshooting a motor vibration, it is very important to set the Fmax at least 100 times rotation speed. Please note this is for TROUBLESHOOTING only. Since the number of rotor bars varies, it is most important to establish a procedure of counting and recording the actual number of rotor bars. It is also important to record the full bearing model number so the bearing frequencies can be accurately determined when analyzing bearing degradation. To verify the vibration is electrically induced, shut off the motor while observing the velocity spectrum to see if the distorted magnetic field instantly collapses. This can be indicated by the disappearance of the twice line signal. If the signal does not disappear, but rather slowly degrades, this is an indication of some type of mechanical problem. There are no agreed upon amplitudes of concern if the twice line frequency signal is present in the velocity spectrum. It is generally agreed that it is not desirable to have any signal at 2x line frequency, however it is often seen. Generally accepted limits are between 0.04 - 0.06 IPS at 2x line frequency. Sidebands
As in most vibration signals, the presence of sidebands around fundamental frequencies is a
6
Proactive Maintenance Techniques measure of increasing severity as the sidebands increase in number and amplitude. Some sideband energy that may be seen is pole pass frequency, (number of poles times slip) and slip, (nominal speed minus actual speed). At the rotor bar pass frequency (number of rotor bars times actual motor speed) it is possible to see sidebands of 2x line frequency. In troubleshooting, the user may find it necessary to increase the resolution to 1600 or 3200 lines of resolution to separate the sidebands.
If sideband amplitudes are greater than 0.5% (40 dB difference) of the center (line) frequency peak, then there is concern that the motor is developing rotor problems or has another source of high resistance.
Figure 6. Illustration of MCSA signature with sidebands. Sidebands indicate possible motor damage. In this example, it was determined after inspection that there was rotor bar damage and a broken end ring. Calculation for severity level is Log 0.0908/8.777 X 20 = 39.7 dB. This corresponds to the 0.5%, indicating possible rotor damage.
7
Proactive Maintenance Techniques
Figure 7. Enveloped AC motor current, slip frequency of 0.8125 Hz (first peak from left) generated by 5 broken rotor bars and a damaged end ring. Ratio of pole pass frequency amplitude to overall amplitude is 63%. Observations of Other Motor Problems
High efficiency induction motors obtain higher efficiency, and use less electricity with two methods: • A smaller air gap • Thinner insulation on the windings If an owner installs these motors on a transformer circuit with DC motors installed, it is possible for the DC motor silicon control rectifiers (SCRs) to back feed onto the AC circuit and induce high voltage spikes into the motors. The reduced insulation rapidly deteriorates and leads to a reduced motor life. Field results show as much as a 50% reduction in the life of the motor from this occurrence. DC motor problems are seen at the SCR firing frequency, 6x line frequency. If this frequency is seen, check connections, SCR, control cards, and fuses. Direct Current (DC) motors, in general, are more difficult to monitor. They require a thorough understanding of construction and component makeup of the motor. Much of the information concerning vibration monitoring of DC motors revolves around the process of monitoring the performance of circuitry, connections, and control systems. Development in the field of DC motor monitoring is an on-going process and should be considered as a higher level monitoring process that is not covered extensively in this article. Enveloped AC Motor Current
When the motor current from a motor with a damaged rotor circuit is enveloped, the resulting spectrum shows energy at the actual pole pass frequency. For example, the signal appears at 0.8 Hz, not as a sideband of the 60 Hz signal or 59.2 Hz. Initial research shows there is a relationship between the pole pass frequency amplitude as a ratio of overall amplitude of an FFT spectrum taken with an F max of 25 Hz. Typically, in a good motor, this is a very low amplitude signal and is not seen in an enveloped spectrum. So, the frequency has to be calculated to locate it. Initial data has shown a good motor has a ratio of 5% or less, but as
8
Proactive Maintenance Techniques damage increases, the percentage increases. The example of the broken rotor bars in Figure 7 has a ratio of 63%. Harmonics of slip frequency are additional indicators of damage. Initial testing shows this to be a very sensitive method and detect every early degradation in the rotor circuit.
Comparison of Motor Current and Accelerometer Signals An electric motor acts as an effective transducer for load variations both within itself (windage, bearing friction) and downstream (in the drive train and in the device driven). Thus, the comparison of motor current signature with data acquired from an (Figures 8 and 9) can be very useful. In this case it is a three-phase induction motor.
Figure 8. Vibration Signature compared with a Motor Current Analysis Signature (Figure 9). An indication of 3600 CPM peak in the MCSA spectrum indicates problem with the motor. The vibration signature also indicates the 3600 CPM peak. Notice the motor shaft rotational speed of 1200 CPM (20 Hz) and its harmonic 2400CPM.
9
Proactive Maintenance Techniques
Figure 9. Motor Current Analysis Signature (Figure 9). An indication of 3600 CPM peak in the MCSA spectrum indicates problem with the motor. The vibration signature also indicates the 3600 CPM peak. Those experienced in vibration analysis and familiar with the construction of motors are not surprised to see pronounced peaks in the accelerometer signal spectrum (Figure 8) at frequencies corresponding to the motor speed (slightly less than 20 Hz) and its harmonics. These same signal components appear prominently in the motor current spectrum (Figure 9), although the amplitude relationships are different. The two spectra also show some distinct differences. In the motor current signature there are sometimes peaks at 1.5 Hz or lower. Although not contained in this particular example, this signal component is a general characteristic of AC induction motors and reflects the rate at which the spinning armature continually falls behind the rotating electrical field generated by the motor's field windings. Since this motor slip frequency component is electrical rather than mechanical in origin, it has no vibration counterpart. Condition Indication of Other Mechanical Devices
There are many other types of devices that were analyzed from a motor current signatures standpoint. Two of those devices are listed in the following section: Vacuum pump Squirrel cage blower Each application can be indicated as a success in the sense that highly reproducible signatures were obtained, each possessing distinctive features that could be linked without ambiguity to specific physical phenomena. However, because of programmatic consideration, no opportunity existed to study the sensitivity of the signatures to implanted or naturally occurring equipment defects. Nonetheless, the following two examples are presented briefly to provide a somewhat enlarged perspective on additional areas in which MCSA may have application validity. • •
Laboratory Vacuum Pumps
A motor current noise frequency spectrum for a laboratory vacuum pump is shown in Figure 10. This signature is notable for its large number of distinct, identifiable peaks. The singlecylinder reciprocating pump tested is V-belt driven at a speed reduction of about 4.5:1. Two harmonics of the pump pulley rotation speed are visible in addition to the fundamental at 6.5
10
Proactive Maintenance Techniques Hz. The dominance of the second harmonic (13.0 Hz) is attributable to the two-direction (and load) reversa1e that the piston and its connecting rod undergo with each revolution of the pump shaft as a result of the crankshaft construction. The irregularity produced by the periodic: passage of the joint in the V-belt is also seen to introduce strong peaks, at the second harmonic of the joint passing frequency (10 Hz) in particular. The magnitudes of the belt-generated spectral components were observed to be strongly influenced by belt tension. The data shown here were obtained with the V-belt fairly s1ack, which resulted in some belt “whip" during operation. Small Centrifugal Blower
A small (1/30 hp, 3600 RPM) squirrel cage blower was tested at various load and flow condition. Figure 9 illustrates that as the blower discharge flow area was increasingly blocked, the motor speed increased as indicated by the readily observable decreases in the motor slip frequency. The data suggests that MCSA applied to a blower of this type could provide remote indication of air now and/or pressure drop in a piping, or ductwork system without a need for conventional flow or pressure transducers or a tachometer.
Figure 10. Frequency domain motor current signature for a reciprocating vacuum pump.
11
Proactive Maintenance Techniques
Figure 11. Frequency domain motor current signature for a small blower with various degrees of flow blockage. Applicability to Other Machinery
Although application experience is presently lacking in areas outside those already cited, it is likely that MCSA will provide a highly sensitive, selective, and cost-effective means for online condition monitoring of a wide variety of heavy industrial machinery. For example: • Motor-driven compressors / pumps • Rolling mill stands • Mixers and crushers • Fans and blowers • Material conveyors Likewise, it appears that MCSA may prove useful in production pre-shipment testing of some motor-driven consumer appliances and lighter industrial equipment, such as: • Refrigeration equipment and heat pumps • Washing machines and dishwashers • Audio/Video reproduction equipment • Computer disk drives The ability to transfer MCSA technology to these prospective industries is also available.
Conclusions MCSA is a useful tool for monitoring the mechanical and electrical condition of motors, particularly in relation to their operational readiness. Experience with motor-driven machinery equipment suggests that MCSA is equally applicable to monitoring present
12
Proactive Maintenance Techniques condition and diagnosing impending trouble in a wide variety of consumer and industrial equipment. MCSA has a number of inherent strengths, the most notable being that it: • Provides non-intrusive monitoring capability at a location remote from the equipment. • Provides degradation and diagnostic information comparable to conventional instrumentation (e.g.. accelerometers), without the attendant disadvantages or added sensors and signal cables. • Offers high sensitivity to a variety of mechanical disorders affecting operational readiness. • Offers means for separating one form of disorder from another (selectivity). • Can be performed rapidly and as frequently as desired by relatively unskilled personnel using portable, inexpensive equipment. • Is equally applicable to high-powered and fractional horsepower machines, AC and DC motors. MCSA should be considered as a viable technology to assess machinery and additionally scrutinize the workings of the machine in the future by engineers from diverse industries and equipment manufacturers.
References Lawrie, Robert J., “Electrical Construction and Maintenance,” Volume 91: Acceptance tests guide electrical preventive maintenance. McGraw Hill, 1987 Legowski, Stanislaw F., “Instantaneous power as a medium for signature analysis of induction motors,” IEEE Transactions on Industry Applications Volume 32 , 1996 Renwick, J.T., “Condition Monitoring of machinery using computerized vibration signature analysis,” IEEE Transaction on Industry Applications Volume IA-20, 1984
13
