Gene Vogel
EASA Pump and Vibration Specialist

Resonance is a property of all mechanical structures. It can be described as a sensitivity to a certain vibration frequency. For machinery such as electric motors, pumps, turbines, etc., it becomes a problem when small vibratory forces from the machine operation are amplified by mechanical resonance. The result can be very severe vibration levels, even when the exciting forces are small. Often resonance is encountered when a speed change has been implemented, as with retrofitting a VFD or operating a 50 Hz motor on 60 Hz power.

The most common example of resonance is when the structure supporting a machine is resonant at or near the rotating speed of the machine. Even slight vibratory forces from residual unbalance and misalignment will excite the resonant base structure, resulting in severe vibration. The machine components can also be resonant. There are many examples of 2-pole electric motors where a resonant endbracket caused very high axial vibration at 1 x rpm or 2 x rpm.

A second category of resonant conditions occurs when the resonant component is the rotating element of the machine. This is common with gas and steam turbines, centrifugal pumps and 2-pole electric motors. While the result is similar (high vibration when a certain operating speed is reached), this is a more complex phenomenon. When the operating speed reaches the resonant frequency of the rotating element, the rotating element actually distorts and the vibratory forces increase significantly.

There is a need to distinguish between these two types of resonance. The first, where a supporting structure or non-rotating machine component is resonant, is usually referred to as a “structural resonance.” The second, where the rotating element is resonant, is known as the “rotor critical speed.” This leaves the term “critical speed” (without the word “rotor”) somewhere in limbo.

Technically, a critical speed could be either a structural resonance or a rotor critical speed. For the sake of clarity it’s best to avoid using that term. The simple term “resonance” can be applied to both conditions to avoid confusion.

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