Gene Vogel
EASA Pump and Vibration Specialist

When a motor is test run in the service center, the two most common vibration frequencies that occur are at 1x rotating speed (1x rpm) and at 2x line frequency (2x lf). High 1x rpm is often corrected by balancing, and the 2x lf is traditionally attributed to air-gap anomalies or voltage or winding unbalance. However, there are those cases where the traditional approaches are unsuccessful and technicians and managers are left scratching their heads. In these difficult cases, there is often a combination of electrical and mechanical vibration. Being able to separate electrical and mechanical vibration is necessary to efficiently arrive at a solution.

Although electrical and mechanical vibration can be challenging on any mo­tor, two-pole motors are especially chal­lenging. For these motors, the line frequency and syn­chronous speed are equal (60 Hz = 3600 CPM = 3600 rpm or 50 Hz = 3000 CPM = 3000 rpm). So vi­bration at one and two times rotating speed will be very near one and two times line frequency – but they are not exactly equal for induction motors. When the result of electrical forces can be separated from mechanical sources, arriving at a solu­tion becomes much more likely.

Beat frequencies
The interaction of two vibratory forces which are very close in fre­quency produces what is called a “beat frequency.” An example is two guitar strings that are just slightly out of tune. The sound they produce has a pulsating volume or amplitude. But amplitude is only half the picture since vibration also has phase angle. Let’s relate this to an electric motor. The synchronous magnetic field cre­ated by the stator “rotates” at exactly line frequency for a two-pole motor. Due to friction and windage losses the no-load speed of the rotor is just slightly less than synchronous speed.

Now imagine there is a slight mag­netic unbalance in the stator due to a voltage unbalance, and a slight mass unbalance of the rotor. The vibration from these two sources will differ by the slip frequency. The slip frequency will be the difference between the rota­tion of the synchronous magnetic field and the rpm of the rotor. For a no-load, two-pole motor, the slip frequency may be as low as 1/10th of one rpm; that is, it would take 10 minutes for one slip cycle. As the load increases, the slip cycle will become shorter so that the beat frequency becomes audibly noticeable.

Phase angle
Remember that both vibration forces have phase angle. If the phase is referenced to the shaft, the mechanical vibration will have steady phase. But the vibration from the electrical force will have a phase angle that is moving relative to the shaft. So at one instant the two forces will be aligned in the same direction, and half a slip cycle later they will be in opposite directions. So they will alternately add together to create a higher amplitude and then be in opposite direction and create a lower amplitude. Thus the beat frequency is created, with amplitude that rises and falls at the slip frequency (see Figure 1).

At this point it is important to know that there are conditions other then beat frequencies that cause vibration intervals. A single vibration fre­quency can be modulated. Such a con­dition occurs when a rotor has an open bar. The open bar will alternately be centered on the magnetic pole creating higher vibration, and then slip between energized poles so that vibration is re­duced. It is not possible to discern the difference between a beat frequency and a modulated frequency with am­plitude alone. Phase must play a role in making the distinction between them.

When amplitude and phase are taken together, they form a vector. There will be one vector for the me­chanical vibration and one for the vibration from electrical forces. And while the amplitude of each may be steady, the phase angle must vary between them at the slip frequency. So when both sources are present, the resulting vibration will be the sum of the two vectors. Figure 1 shows the resulting sum of the vectors for one slip cycle.

Change in cyclic manner
Figure 1 shows the beat frequency has amplitude and phase that change in a cyclic manner. Again, it is impor­tant to know that a single modulated frequency may also have amplitude and phase that change in a cyclic manner. Here is the key to telling them apart: For the beat frequency, the amplitude and phase change with a syncopated motion; for the modulated frequency, they change in a synchro­nized motion.

Analog vibration analyzer
To picture this, imagine using a strobe light type analog vibration analyzer (or balancing instrument) to measure the amplitude and phase (see Figure 2). In either case (beat fre­quency or modulation), the amplitude meter would be going up and down and the phase would be shifting back and forth. 

However, for the beat frequency, the amplitude would increase as the phase paused at one extreme. Then the am­plitude would pause at the maximum while the phase shifted back, and so on (syncopated). For modulation, the two would be synchronized. Keep in mind that for the beat frequency, the cycle is the slip frequency, so for an unloaded motor that could be as much as 10 minutes to complete a cycle.

Digital vibration analyzer
The phenomenon presented for an analog instrument with a strobe light can also be observed with a digital vibration analyzer that has peak-and-phase capability (most spectrum ana­lyzers have this capability). These in­struments use a reference pulse pickup, usually a laser pickup, to “see” a mark on the shaft. Since the amplitude and phase are displayed as digital values, it is more difficult to picture the synco­pated vs. synchronized motion. It may be necessary to manually plot vectors to visualize the motion.

Digital spectrum analyzer
Digital spectrum analyzers may have other features that can also be used to separate the electrical and mechanical vibration. See Figure 3. Using very high resolution (3200 lines or more) may resolve the two source frequencies so that each can be seen in the spectrum. Placing a load on the motor will help to sepa­rate the two frequencies. If the slip frequency is such that it takes more than 30 seconds to complete a cycle, it is unlikely high resolution will separate the two sources.

Synchronous time average
An alternate and more effective technique with digital spectrum ana­lyzers is synchronous time averaging. This technique uses the reference pulse pickup to synchronize the collection of a series of digital samples. The samples are then averaged together and any vibration frequencies that are not syn­chronous to the shaft are averaged out. Thus a comparison of the synchronous time average spectrum to a standard spectrum will reveal the presence of any non-synchronous vibration fre­quencies. (Phase is at work here though it is not read directly.)

Finally, the above discussion focused on vibration at or near 1x rpm. Often the vibration of interest is at or near 2x rpm. For 2x vibration, the strobe phase method will show a double reference mark, which when carefully observed can discern the syncopated vs. synchronized motion. With digital spectrum analyzers, some instruments can be set to track the 2x frequency from a single refer­ence mark on the shaft. Often, better results can be produced by placing two reference marks on the shaft, at exactly 180 degrees from each other. The two reference mark method works with any spectrum analyzer which has synchronous time averag­ing capability.

The above discussion highlights the sometimes complex technical na­ture of motor vibration. The details of the specific techniques may be known only to the vibration technician, but knowing the fundamental concept of electrical and mechanical vibration and how they interact to produce varying vibration amplitude and phase may save many hours of frus­tration when trying to correct motor vibration problems.

Categories: Troubleshooting & failure analysis, Vibration

Tags: Vibration

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