Hi all, we have some AC electrical motors (1800cpm-30hp)that shows frecuency of 21600 cpm. We would like to know why it is about.

If someone has been this kind of experience or knowledge. I thank your comment.

I Attached graph.

Regards.

Fabián

ELECTRICAL_MOTORS_21600cpm.doc (128 Kb, 45 downloads)

Fabuzon,

I'm sure you know that 21,600 is a DC firing frequency for a full wave rectifier.
Are these motors on a VFD?

Dave

What are these motors driving. Can you strobe and provide an exact speed?

Hi Mr. RRS_Dave, They are AC motors without VFD. They are wearing 6208-2Z/C3 bearing but their bearing frequency don't corrrespond with the bearing.

Mr. VIbeguy, i am gonna check speed with the strobe but how do i use this.


Regards.
Fabián

If the nameplate on the motor says 1800 and it the motor is not on an inverter, then the speed should be somewhere close to 1800 unless your power is 50 Hz then it would be somewhere below 1500. Just try to stop the shaft with the strobelight close to the speed that you suspect the motor is running. Just remember that the shaft is still moving. Believe it or not, there have been people that have tried to touch the shaft when it appeared to be stopped with a strobelight.

The exact speed would help understand the vibration that you are asking about.
Can you provide the frequency of the sidebands around the peaks of the vibration in question?
It would also be nice to know what the motor is running and what type of coupling.

We had a non-vfd induction motor that showed this frequency once (21,600). As mentioned above it is normally associated with ripple voltage on the output of a 3-phase full-wave rectifier.

In our case it went away after one measurement.
http://maintenanceforums.com/eve/forums/a/tpc/f/3751089...581006072#9581006072

The logical conclusion would seem to be that it came from some external source that happened to be transitory. I notice your magnitude goes up and down... does this frequency come and go in your case? If so I would tend to suspect that it's interference associated with some external electronic equipment. We use the dual rail rare earth magnet mounts. I've never heard it mentioned or documented anywhere, but I would guess that the magnet could be easily be influenced by ripple in a dc magnetic field and could cause the acceleromater to vibrate.

EPete,

I work in a steel furnace building each month and notice my magnet resonates when I walk by a particular electric arc furnace. It will buzz in your hand. I have never punched the store button to see what kind of vibes it was. Never thought too much about it.
Now I think I will next month just to see.

Dave

SCR Firing Frequency on a near by or inline VFD: 60Hz * 6 SCRs = 360 Hz = 21,600 CPM