Beat noise is the result of two vibratory forces having very close, but not identical, driving frequencies (vibratory forces generally produce noise). These frequencies alternately add and subtract from each other. The end result is an audible beat with a maximum amplitude
equivalent to the sum of the individual maximum amplitudes, and a beat frequency which occurs at the difference in the two individual frequencies.
Beat noise occurs most commonly in two pole motors. The primary reason for this is the closeness of the twice line and twice rotational frequencies (i.e. 2X and 2f). The two graphs on the next page should clarify the addition and subtraction the 2X and 2f frequencies. This beat frequency is essentially twice slip frequency. In the case of two pole, 50Hz ANEMA motors, typical full load speed is 2985 rpm. The 2X frequency is 5970 RPM, or 99.50 Hz, and the twice line frequency is obviously 100 Hz. The beat frequency would then be 0.5 Hz. Another way to state this is to say that the beat noise will have a period of 2 seconds, or, that the noise will go from zero, to some maximum noise level, and back to zero, in two seconds. While two polemachines are more susceptible to beat noise than slower speed machines, 50 Hz machines are not impacted any more or less than 60 Hz motors. The existence of an audible beat is common in these machines, and in and of itself, does not detract from performance or reliability in any way.
In addition to two pole motors being susceptible to beat noise, when similar machines operate in close proximity to each other, beat noise can occur as well. A twin engine turboprop plane
should be a familiar example of this phenomenon.
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