Why line frequency vibration on fractional HP AC motor

[dano1]

I have a small fractional HP 4-pole AC motor that is direct driving a 14" 3-blade fan. I measured the torsional natural frequency of the fan and rotor only and found a natural frequency at 60Hz. The resonance condition is audiable when the motor and fan are running at 60Hz and if I vari the input frequency by a few hertz, the noise goes away, which makes sense. My question is what in the motor is causing the fan and rotor to be excited at 60Hz? My motor only vibration test shows a spike at 120HZ (with 60Hz supplied) and a small spike at the running speed.

 

[electricpete]

Just thinking out loud.

As you know, the normal torsional oscillations from assymetry between stator winding poles would be 120hz.

Trying to think what could cause 60hz... seems like requires some form of dysymmetry between the positive half cycle and negative half-cycle wavevorm. Perhaps current or voltage could be checked with a scope to see if it has symmetry above and below zero. Or else check with another supply.

That sounds pretty far out. Maybe there is something more basic that I am missing.

 

[BigMax]

Also thinking out loud, assuming a single phase motor, my understanding is that during a single rotation, whilst outputing an average of 100% mechanical power, the 1 ph motor receives (approximately) +120% followed by -20% electrical power, resulting in the typically annoying vibration associated with such devices.

Perhaps the vibration is related to the single phase motor characteristic of delivering constant average mechanical power whilst receiving pulsating electrical power?

Cheers!

BigMax

 

[electricpete]

Hmmm.
Maybe this is a normal expected torsional oscillation frequency in presence of assymetric stator poles. Let me think again.

 

[GusD]

Hello Point dano1
you say that when you remove the fan, the motor vibration shows a spike at 2xLf (120 hz)which would be expected.
Does not show anything at 60Hz.
If you are using "velocity transducers" you may be picking up the magnectic field of 60hz.Electromagnectic pick ups are known to do that.
If you use "accelerometers" this won't be a problem.
If the fan dynamic mass, behaves like an overhang large load, it may cause the torsional rotor forces to resonate at 60hz.
I would try to change the "fan,or its mass,"if it is possible,and see what your vibration says.
A system natural frequency at or near 60 hz can be problematic at any time.
We had a similar problem on 900 hp motors that had large brake drums.Coincidentally, the torsional resonant vibration of the rotor was exactly 3600 cpm(60Hz).When we had the right conditions,vibration levels would go from approx .15 in/sec to .8 in/sec at exactly 60 hz.
We changed the brake drums form heavy steel to aluminum drums.Problem was gone.The heavy break drum was causing an extreme overhang load condition.This extreme OH may change changes rotor rigidity and excite large resonant torsional forces in the rotor .
At times,not often, we find 60HZ vibration (1xLf)when we have bad electrical connections either in the power system to the motor or the motor itself.

Good Luck

GusD

 

[electricpete]

I have never seen 60hz vibration on induction motors before.

Back to the question of expected torsional resonance excitation for unbalanced stator pole-groups... 120hz is expected frequency. Theoretically I have a hard time imagining where 60hz comes from.

 

[jbartos]

Suggestion: Check the power supply for the second harmonic content. It tends to cause problems to motors. The second harmonic could approximately surface at about 120Hz mechanical vibration frequency. Sometimes, the second harmonic comes from half-wave rectifiers. Often, when someone is trying to safe money on rectification hardware.

 

[electricpete]

jb - the question is about 60hz excitation of a torsional resonance.

 

[dano1]

Thanks for all the information and ideas. I used an accelerometer to measure the velocity of vibration. I will be checking my source with the scope in the next few days to make sure the positive and negative half cycles are equal.

My real problem is audible noise caused by the 60Hz torsional resonance. So I'm going to alter the fan to shift the natural frequency away from 60Hz. However, I'm still interested as to what in the motor is exciting my rotor and fan combination. An experienced co-worker says you never want to have a natural frequency at line frequency, but couldn't give an explanation as to why. So, I don't believe I'm the first to encounter this problem.

 

[electricpete]

We have large 4-pole steam-drive syncronous generators... and during outages they have conducted lateral and torsional bump tests. The frequency they look for are 120hz. I asked our generator engineer today and he told me they don't even look in the neighborhood of 60hz. Still curious if anyone else has ever heard of 60hz torsional excitation.

Another possible experiement which probably would be easier and more comprehensive than the o-scope: Try running the motor from a non-variable speed drive at 60hz. This should rule out the assymetry that I suggested. Also if the vfd is creating the vibration in any way, this will rule it out.

 

[GusD]

Hi Electricpete.

As for the 60 hz Vibration (1FL)it isn't as uncommon as you may think.I don't know if you have access to Ilustrated Vibration Analyses charts,compilled by IMISensors,Update Int or Entek Ird.Any of these charts show typical problems with inverter fed systems and the frequencies you may encounter depending on the faults.
SCR faults,phase control cards problems, Power Bridge Diodes,ground Faults and loose/burnt connections
all can show at 60 HZ. EPete,these links might be helpful

http://www.imi-sensors.com

===

http://www.updateint.com

good luck

GusD

 

[electricpete]

I have never seen it in power equipment, but my power equipment is not fed off of inverters or vfd's.
If it is in fact common in inverter-fed equipment, it suggests maybe it has something to do with the power supply. Again it sounds like this theory could be tested by seeing if the sound persists when motor driven direct from clean 60hz power supply (not from inverter). Or if we are smart enough to know what we are looking for maybe it shows up in the o-scope view of the voltage or current.

 

[electricpete]

Also worth noting that the charts are probably referring to radial vibration. In vibration monitoring we don't tackle torsional oscillation often. Of times I have heard of torsional oscillation on large electric machines (not vfd/inerter), it has always been 120hz.

 

[jbartos]

Suggestion to electricpete (Electrical) May 4, 2003 marked ///\\jb - the question is about 60hz excitation of a torsional resonance.
///I addressed the following part of the original posting: "My motor only vibration test shows a spike at 120HZ (with 60Hz supplied) and a small spike at the running speed." The spike at 120HZ is at 2 x 60Hz. The 2 implies the second harmonic of 60Hz fundamental. The second harmonics pose a concern to motors as I mentioned in my previous posting.\\\

 

[electricpete]

jb - Some 2*lf radial vibration is normal in induction motors, even with no 2nd harmonics in the voltage/current. In contrast, 1*lf torsional oscillation is not. Some clues as to the intent of the question:

Subject: "Why line frequency vibration on fractional HP AC motor"

Text: "My question is what in the motor is causing the fan and rotor to be excited at 60Hz?"

 

[dano1]

I have taken care of the immediate problem, by recommending we change the fan and rotor system to shift it's natural frequency to something other than 60Hz. I still don't know what is exciting the torsional resonance at 60Hz, but is it possible that it could be the second harmonic of the running speed? The motor is a four pole motor, therefore the running frequeny is near 30Hz and I can slightly reduce (58Hz) the torsional natural frequency by changing the position of the fan on the on the shaft.

 

[jbartos]

Suggestion: Some fans have 3-blade propellers others 4-blade propellers. The mechanical resonances are supposed to be different from electrical resonances by the machine proper electromechanical design.