Identifying Mechanical Faults with Motor Current Signature Analysis

Dallas Fossum, Allied Services Group
Tags: condition monitoring, predictive maintenance, vibration analysis, infrared thermography

 Motor current signature analysis (MCSA) has proven to be a highly valuable predictive maintenance tool. Although it is a relatively young, rarely utilized technology, it is rapidly gaining acceptance in industry today. Mechanical faults related to belts, couplers, alignment and more are easily found through the use of a demodulated current spectrum.

MCSA is simply the process by which motor current readings are recorded and analyzed in the frequency domain. It has been around since 1985 and proven itself well over the years in locating rotor faults and air gap problems in motors.

The motor current signature is recorded in a time domain format. The current is represented in a graph form with the amplitude shown on the “Y” axis and the time on the “X” axis. The result is a typical current sinewave shown in Figure 1.

In order to analyze the data, a Fast Fourier Transform (FFT) is performed. An FFT is a mathematical operation designed to extract the frequency information from the time domain and transform it into the frequency domain. An example of an FFT spectrum is shown in Figure 2.

While the FFT spectrum is a great source for identification of rotor bar problems in motors, it proved difficult to analyze most other frequencies. In order to address this problem, the demodulated current spectrum was developed. 

 

Figure 1. Time Domain Format                              Figure 2. FFT Format

 

 

 Demodulated Current Spectrum

In recent years, one of the most exciting advancements in PdM technologies is the demodulated current spectrum. In order to better understand demodulation, the concept of modulation should briefly be addressed.

Modulation is when lower frequencies are merged on top of a higher frequency. In other words, lower frequencies ride on the higher frequency signal. This makes the carrier frequency the dominant peak in the FFT spectrum, and most of the information is lost in the noise floor of the spectrum. Although they have always been present in the current spectrum, the repetitive load variation frequencies have been difficult to identify and trend.

Demodulation is simply the process of taking the carrier frequency out of the spectrum. In this case, the carrier frequency is the fundamental electrical frequency being used. The fundamental frequency in the United States is 60 Hz. In many other countries, it is 50 Hz. After removing the carrier frequency, the frequencies related to repetitive load variations are left behind and shown on the demodulated current spectrum.

Frequencies such as speed, pole pass, belt pass, vane pass, gears and bearing frequencies can be identified and trended in the demodulated current spectrum. In effect, the motor is acting like a permanently installed transducer.

Due to the technology being relatively new, trending remains the most accurate method of identifying a problem in a machine. The ability to have baseline data when the machine is in good health is ideal, while comparing data to similar machines is also very effective. In the future, as historical and statistical data is compiled, there will be more alarming guidelines established for different types of equipment.

Locating Belt Frequencies

The basic current signature of a belt-driven machine is shown in Figure 3. It shows the fundamental 60 Hz frequency being the dominant peak. Note the sideband peaks on each side of 60 Hz that are labeled with their frequencies. This type of signature has the potential of being misconstrued as a potential rotor bar problem. In this example, the sidebands are actually related to belt pass frequency. The mechanical frequency in rotations per minute (RPM) is calculated using the following equation:

Mechanical frequency = (the change between line frequency and peak of interest) x 60

The mechanical frequency of interest in this case would then be calculated by multiplying 6.5 Hz by 60. The result would be 390 RPM, which is the belt pass frequency of this machine. Rotor bar problems show up at pole pass frequency, with the peaks much closer to the fundamental frequency. Pole pass frequency is calculated with the following equation:

PolePass= synchronous speed – (slip frequency / # of poles)

Figure 3. Belted machine current spectrum      Figure 4. Demodulated spectrum

 

Although the mechanical peaks in Figure 3 are prevalent, this is often not the case. Usually, the mechanical peaks will be lost in the noise floor of this type of spectrum. This is where the demodulated signal becomes so important.

Figure 4 shows the demodulated spectrum derived from the current signature in Figure 3. Notice how much cleaner and easier to read this example appears. Without the 60 Hz electrical frequency present, the remaining mechanical frequencies become much more prevalent.

The sideband frequencies shown in Figure 3 now show up in Figure 4 at 6.5 Hz along with a 2x and 3x belt pass frequency in the demodulated spectrum. Looking closely at the current spectrum, the 2x and 3x frequencies are present but not as easily identified. Belt pass peaks in this type of spectrum are good early indicators of belt alignment, wear and sheave problems.

Running Speed Frequencies

Both drive and driven speeds will also be found in the spectrum if there is a problem. As in vibration, a 1x rotational speed will signify an unbalanced effect on the machine. In Figure 4, the fan speed shows up at just over 25 Hz. In Figure 5, the running speed of an 1,800-RPM pump motor can be identified at just less than 30 Hz. In both cases, trending the amplitude of these peaks can tell us many things about the condition of the machine. Normally, this peak will be at or near the noise floor of the spectrum. The two most common reasons that the amplitude will climb on direct-drive pump assemblies are due to misalignment or a damaged coupling.

The amplitude response on the spectrum is remarkably sensitive. A noticeable amplitude increase will occur with the incorrect key length in the hub. The common flexible-type couplings used will also show up when they fatigue, crack or become twisted. An example of a brittle flexible coupling with stress cracking is shown in Figure 5. Figure 6 shows the follow-up test completed after a new coupler and laser alignment was performed. Notice the amplitude of the rotational speed peak at 30 Hz relative to the amplitude of the low-frequency product flow noise in each spectrum. 

 

 

 

 

 

 

Figure 5. Pump Assembly with Problem                          Figure 6. Pump Assembly Repaired

 

Other Demodulation Opportunities

Another common frequency found in a demodulated current spectrum is the pump vane pass and fan blade pass frequency. This will help to trend and locate problems with the impeller or flow restrictions. In a demodulated current spectrum, it is calculated by using the following equation:

Vane pass = pole pass frequency x # of vanes

With the growth of the technology, knowledge and software advancements, locating bearing faults, gear mesh and other mechanical frequencies with MCSA will also become more common. 

Incorporating into a PdM program

With all that demodulated MCSA can do, there are still many questions as to how it will fit into and benefit a PdM program. Frequently asked questions might include: Why do I care about finding mechanical faults with a demodulated current signal if I can find them with technologies like vibration analysis or infrared thermography? How reliable is the data that is generated and can it take the place of vibration? How often should MCSA be completed on equipment?

With any condition monitoring technology, there are strengths and weaknesses. Each technology applied will give a more complete view of the health of the equipment. For best results, it is recommended to complete MCSA at least quarterly. If a program is testing less frequently than this, the overall results of the motor testing program will be compromised. As with any technology, it is critical to have enough data to accurately trend the history of the machine.  

As for MCSA technology looking for mechanical faults, there are many reasons why this can benefit a PdM program. For example, when it comes to belt and coupler problems, demodulation will give an earlier and often more accurate fault indication than vibration analysis. The amount of energy created by the early stages of this type of fault is relatively low. When belts or couplers begin to wear, it is often not noticed in a vibration spectrum until the fault is nearing catastrophic failure. A demodulated current spectrum has the ability to detect the fault early enough to provide plenty of time to plan and schedule the repairs. However, demodulated MCSA is not intended to take the place of a vibration program. It is best used as a complimentary technology to a good vibration program.

An added benefit of this technology would be in remote equipment locations or areas where equipment is not accessible during normal operations. On this type of equipment, visual inspections can be difficult, and the ability to perform vibration analysis is limited. Depending on the risk assessment, remote wiring transducers for vibration may be too costly. In this case, MCSA would work well due to the ability of the equipment to be tested from the motor control center.

Part of any strong PdM program is having the ability to verify a fault with more than one technology. This not only ensures the validity of the fault but also helps make a more accurate and precise repair recommendation. The importance of verification with a second technology is never more evident than on a critical piece of equipment that requires plant outages for repair. Demodulated MCSA brings an added dimension to this effort and has proven itself to be an invaluable tool for any PdM program.


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