Motor Current Signature Analysis is a diagnostic and technique that is used to analyze motors. This technology enables the testing of operating equipment to identify rotor bar and high resistance joint problems. As a preventative maintenance tool, MCSA can be used to perform a one-time test or periodic testing to track and trend motor performance. MCSA allows for remote, non-intrusive testing of the equipment being monitored. The test analyzes the current waveform using complex mathematics. (This information was provided from a paper by Dr. Bob Jones at SKF, and Technical Associates training manual along with the experience of Larry Massey.) Although there are many vendors carrying MCSA software and hardware, the SKF equipment will be discussed in this paper since it is Massey Technical Services equipment of choice.
When one is trying to make decisions on whether or not he should continue to operate critical motors, he now has a tool in addition to vibration analysis with which he can not only confirm the presence of problems, but also assess their severity. We need to note that purely Mechanical Problems can cause a motor to appear to have significant electrical problems when being evaluated by either motor current or vibration analysis. An example a severely misaligned shaft can create a variable air gap between the rotor and stator affecting both analyses. Therefore, when such tests do indicate electrical problems, it is always a good idea to first conduct a detailed vibration analysis, to check for mechanical problems, before sending out the motor for repairs.
Let’s discuss line frequency and two times line frequency (2FL), and why so many electrical problems involve 2FL rather than the fundamental line frequency. It is because the close side of an eccentric rotor will first be attracted to the north, and then to the south pole, at each pole, the force itself will vary at 2 X the frequency of the magnetic field relative to the eccentricity. It is important to have a high resolution in order to allow one to separate electrical from mechanical problems. We must be able to separate 2FL from motor running speed harmonics. It is also important to use log amplitude rather than linear amplitude scales to look for electrical problems. The reason for this is to distinguish the pole pass frequency. Pole pass frequency (Fp) = slip frequency times the number of poles.
Vibration analysis and a simple current unbalance greater than 3% can be an indication of Stator problems. Rotor problems are sometimes best seen with current signature analysis. A vibration program with current signature analysis to confirm and verify motor problems is recommended. When testing a high vibration motor, if possible, it is a good idea to stop the motor when taking vibration analysis and see if the vibration disappears immediately indicating electrical problems or a slow amplitude decrease indicating pure vibration problems.
ANALYSIS OF AC MOTOR CURRENT
The technique of evaluating the motor condition by performing an FFT of the motor current has been verified many times over the past 6 years. This technique is often referred to as a method to detect broken rotor bars, however the fact is that it detects abnormal high resistance in the rotor circuit. In other words, bad solder joints, loose connections and damaged rotor bars. The users of the new CMVA55 will note that all the mathematical functions are performed automatically by the Motor Current Monitoring Wizard which quickly provides the user with the information he needs. In all the cases, the motor must be at 75%-100% load. The closer to 100% full load the better. For users of other Microlog models, the following is a quick review of the methodology. From either the route mode or analyzer mode, the data is collected using the point setup outlined in the instruction manual or user notes. If there is a fault in the rotor circuit, then the spectrum will have two prominent features when displayed with the ‘Y’ axis as a logarithmic function. At 60 Hz, line frequency, there will be a large spike. To the left at a distance equal to the rotor slip times the number of poles will be another spike of energy. These spikes can be labeled ‘A’ and ‘B’. Note that the amplitudes will have to be obtained from the software display because it is necessary to use amplitudes to four decimal places. This is discussed in more detail, below in the (Technology facilities induction motor analysis) paragraph.
TECHNOLOGY FACILITIES INDUCTION MOTOR ANALYSIS
By utilizing velocity and enveloped acceleration in conjunction with motor current analysis, users can dramatically increase their success in trending, analyzing, and evaluating the condition of AC induction motors. Thanks to data collectors like the Microlog CMVA55, plant maintenance and reliability personnel can easily and successfully detect electrical and mechanical faults that lead to unexpected downtime. To determine the condition, perform the following calculation: Log (A/B) times (20) = amplitude in dB.
54-60 dB = Excellent
48-54 dB = Good
42-48 dB = Moderate
36-42 dB = Cracked rotor bars or other source of high resistance.
30-36 dB = Multiple sources of high resistance.
< 30 dB = Severe damage
Note that this chart applies to rotor circuit damage and that the motor must be under at least 75% load. The amplitude of the pole pass frequency is not linear with respect to reduced loads and if the amplitude from a reduced load is used, the results will not be accurate.
ENVELOPED AC MOTOR CURRENT
When the motor current is enveloped, from a motor with a damaged rotor circuit, the resulting spectrum will show energy at the actual pole pass frequency. For example, at 0.8 Hz, not as a sideband of the 60.0 Hz signal or 59.2 Hz. Initial research has shown there is a relationship between the pole pass frequency amplitude as a ratio to the overall amplitude of an FFT spectrum taken with an Fmax of 25 Hz. Typically, in a good motor, this will be a very low amplitude signal and will not be seen in an enveloped spectrum. This frequency will have to be calculated due to the low amplitude. Initial data has shown a good motor will have a ratio of 5% or less, but as damage increases, this percentage will increase. Harmonics of slip frequency are additional indicators of damage. Initial testing has shown this to be a very sensitive method and will detect very early degradation in the rotor circuit.
OBSERVATIONS OF OTHER MOTOR PROBLEMS
High efficiency induction motors obtain their higher efficiency, and use less electricity, by two methods- a smaller air gap and thinner insulation on the windings. If the owner installs these motors on a transformer circuit that has DC motors installed on it, it is possible for the DC motor silicon control rectifiers (SCR's) to back feed onto the AC circuit and induce high voltage spikes into the motors. The reduced insulation will rapidly deteriorate and lead to a reduced motor life. Field results have shown as much as a 50% reduction in the life of the motor due to such an occurrence. DC motor problems will be seen at the SCR firing frequency, 6 times line frequency. If this frequency is seen, check connections, SCR’s, control cards, and fuses.
The Motor Current Signature Analysis is a very effective tool for monitoring and trending the degradation of AC motor rotors. The MCSA can also provide savings in power consumption by providing the most efficient motors, which cost less to operate. The MCSA is a very inexpensive program that can show returns with in two years, and help to provide maintenance management with a better reliability based maintenance program.
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