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Shurafa2

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Reply with quote  #1 
Subsynchronous, Subharmonic and the Difference between Them

I've had a few discussions over the years about this point and it seems that different people consider them differently. What is subsynchronous and subharmonic in your book?

Assume you have a running induction motor at 1780 rpm and X is the motor running speed.

A. 2670 (3/2 X)
B. 1800 (1.011 X)
C. 1780 (1X)
D. 1335 (3/4 X)
E. 890 (1/2 X)
F. 880 (0.494 X)

Which of these frequencies is subsynchronous but not subharmonic in your usage?


Regards- Ali M. Al-Shurafa
trapper

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Reply with quote  #2 
Subsynchronous = D, E & F
Subharmonic = E

Technically, they could be considered the same. However, to me, the subharmonic is not really "talking the talk" in the predictive maintenance field. It's either synchronous, non-synchronous or subsynchronous. Subsynchronous means it's less than the running speed of the component you're talking about. Subharmonic, again to me, is more appropriate in the music field.
Shurafa2

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Reply with quote  #3 
How about 1/3, 1/4, 1/5, 1/6, 1/7, 1/8 X in your opinion?

Regards- Ali M. A-Shurafa
John from PA

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Reply with quote  #4 
Quote:
Originally Posted by Shurafa2
How about 1/3, 1/4, 1/5, 1/6, 1/7, 1/8 X in your opinion?

Regards- Ali M. A-Shurafa


These are sub-harmonic.  Sub-harmonic is defined as frequencies of 1/n where ā€œnā€ is an integer.  In the referenced case ā€œnā€ is 3, 4, 5, 6, 7 and 8.  

A sub-harmonic is always sub-synchronous, but something sub-synchronous is not always a sub-harmonic.
Walt Strong

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Reply with quote  #5 
I prefer to call subharmonics as Fractional Harmonics (John's description). There are other fractions of shaft speed that may related to nonlinear behavior for example 2/3 and 3/4, which is why I prefer the Fractional Harmonics designation. We could Google (Bing) it and see what the rest of the world thinks!

https://www.bing.com/search?q=definition+of+subharmonic&form=EDGTCT&qs=DA&cvid=86e14af9953d40e88965b56a3e6895f1&refig=86fb769297b74689acb733eb43166ee0&cc=US&setlang=en-US&elv=AXK1c4IvZoNqPoPnS%21QRLONr2%213u8ysOKPulYRMe1M4%21uenQWQEPLqm4VhmHTYB9M0Aq5iJj5foCjRRYQyzYLr5Mrd9cWK3wxAR8YGtTTrwL&plvar=0

Just to keep my mind closed, I have not read these!

Walt
Shurafa2

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Reply with quote  #6 
I tend to agree with you.

I've checked an old edition of ISO Vibration and shock - Vocabulary 2041 (Second 1990-08-01) and it states the below:

 ------------------------

2.26 harmonic (of a periodic quantity): A sinusoid the
frequency of which is an integral multiple of the fundamental
frequency.

NOTES
1 The term overtone has frequently been used in place of harmonic,
the n* harmonic being called the (n - 1 )* overtone.
2 In English, the first overtone and the second harmonic are each
twice the frequency of the fundamental. In French, the distinction
between harmonic and overtone does not exist, and the second harmonic
is twice the frequency of the fundamental. The term "overtone"
is now deprecated to reduce ambiguity in the numbering of the components
of a periodic quantity.

2.27 subharmonic: A sinusoidal quantity the period of
which is an integral submultiple of the fundamental period of
the quantity to which it is related.
--------------------------------

There is no mention in this standard about synchronous or subsynchronous.

In my opinion, 3/2 and 5/2 are not subharmonics not subsynchronous but I saw this in training materials from a recognized agency.


Regards- Ali M. Al-Shurafa


John from PA

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Reply with quote  #7 
Quote:
Originally Posted by Shurafa2

In my opinion, 3/2 and 5/2 are not subharmonics not subsynchronous but I saw this in training materials from a recognized agency.

Regards- Ali M. Al-Shurafa



IMO, 3/2 and 5/2 would be referred to as super-synchronous (sometimes written "supersynchronous").

Curran919

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Reply with quote  #8 
A. 2670 (3/2 X) - interharmonic
B. 1800 (1.011 X) - synchronous
C. 1780 (1X) - shaft speed
D. 1335 (3/4 X) - subharmonic + subsynch
E. 890 (1/2 X) - subharmonic + subsynch
F. 880 (0.494 X) - subsynchronous

Sometimes i refer to C as synchronous.
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