600 Hp induction motor, 3600 rpm, driving a centac compressor. Alignment is good, soft foot measured at 5, 5, 5, 3 mils.

Overall vibration is Ok (.15 ips), but there is a strong surging sound with a frequency of about 40 cpm clocked via stop watch. Have not put a zoom on a spectrum yet, but that could well be related to slip.

I can see the shaft and it is not shuttling. Confirmed the compressor is fine and not surging. I have been told that it is an aluminum core rotor.

So what should I be looking for? Could this be related to stator distortion? How about the aluminum? I have heard of lots of problems with aluminum core rotors. Any experience?

The "pole pass frequency" you probably also know is twice slip frequency for 2-pole motor. If motor were running around 3580 rpm, slip is 20 rpm and pole pass frequency is 40 rpm.

Pole pass frequency noise can come from;
1 - beating of closely spaced peaks at 2X running speed and 2FL
2 - Pole pass sidebands around 1x, 2x etc.

Zoom spectrum of vibration is the next logical step as you mentioned, since zoom spectrum required to recognize either of the above.

1 is generally fairly harmless. Could be caused by the slight soft foot condition causing the 2*LF.

If 2 is observed, deserves investigation with current signature analysis. Pole pass sidebands around LF in current exceeding a certain threshhold (perhaps 45db below main peak) point toward rotor assymetry, possibly rotor bar problem. This can also show up as swinging of the needle of installed ammeters. Also look for increase in vib/noise after startup as indication of possible rotor problem.

You said aluminum core rotor - I think you mean cast aluminum rotor? (it still has a steel core). Cast rotors can have porosity defects that show same symptoms as rotor bar joint problem for fabricated rotors. In general I think people don't get as excited about these symptoms on cast rotor but still bears watching as cast rotors can fail also.

Motors that have frequent and/or severe starts (takes time to get up to speed and no vfd) tend to be most at risk for rotor bar problems.

This message has been edited. Last edited by: electricpete,

Is the motor running 3560 rpm? Is it possible to unload the compressor so the motor speeds up? The surging frequency would decrease as the motor speeds up, and the amplitude would decrease significantly also.

This is one of those weird jobs you run into occasionally. My office is in a large facility with research labs and offices and pilot plants etc. I was minding my own business sitting at my desk when I get a call asking me to come out to the utility area to help them with this motor. Seems someone heard there was a vibration dude on the facility.

No analyzer, but the mechanics are there messing with it, checking soft foot. I reviewed that, checked their alignment numbers, and checked the controls and readings on the Centac. They really needed the air as their demand was pretty high that day. Found out the motor is 30 years old. I suggested that maybe it was time for a new one…

The soft foot is way out of wack, and by the second hand on my watch I timed the frequency of the noise, and I am thinking the noise is related to slip so maybe the stator is being deformed by the soft foot. I suggested that at the very least the soft foot needed correcting. They called our motor contractor who had recently rebuilt the motor to come out and have a look. He should have taken some vibes yesterday…

I agree with El'Pete that it is probably the aluminum rotor pososity that causes a stray current like a rotor bar fault. The symptoms typically get worse at high load and the motor may draw more current at startup and take longer to get to full speed. If that becomes the situation, then replace motor rotor or entire motor.

Walt

How do we prove that at the motor shop Walt? What test needs to be run?

Steve,

Several years ago I had an ID fan motor with severe pole-pass fault indication. Motor was hard to start and drew high current, amp meter had periodic variations, and floor vibrations had the same period. The motor shop tried a standard "growler" test and could not find anything. They made a "super growler" and finally confirmed fault. They found slight porosity indications that looked like weld sputter on the rotor surface. I did not witness the shop test. The motor was replaced, since at that time there were few factories that would rebuild a cast aluminum rotor. The best motor shop test is with a dyno to put load on motor. The problem is that few shops have one especially for testing large motors.

Walt