Motor Air Gap Check 3

electricpete · Jul 26, 2001

How do you perform a rotor air gap check on a small (100hp) horizontal motor with one-piece end-housing. Removing the housing to get to the gap means we'd remove bearing from it's housing and position of the rotor would not longer be controlled.

teco · Jul 26, 2001

electricpete:-all our motors 110kw and above come with removable plugs mounted in the end cover. by undoing them gives you access to the air gap, using long feeler guagers.
for motors below this size then 16mm plugs can be fitted to the end covers.
this modifacation can be done to any machine providing care and attention are undertaken.
kind regds.

THooper · Jul 26, 2001

electricpete,

Your air gap can very from 3% to 5% overall. The only way I know how to measure this 100hp air gap is by pulling the rotor out of the stator and measure your rotor core OD vs your stator core ID with micrometers. I have seen large horsepower motors that have removable inserts knockouts on the endbells that a guy can measure the air gap through with a very long set of feeler gauges, but I have never seen these on smaller than 500hp, and not all large motors have them. The rotor core must be concentric or you will loose torque. If the rotor core is not round it would require restacking the core. The rotor laminations should not be loose on the shaft, look for signs of rust at the edge of the rotor core this would indicate movement and looseness, Always measure rotor alignment. Rotor iron can be longer than stator iron. If your rotor alignment is off, when the motor is under load it will try to pull and find magnetic center and can cause preloading to the bearing. The rotor core should not runout more then .003, most are within .001 You did not say if this 100hp is ac or dc? I have not seen very many air gap problems with ac motors,only a few that was large horsepower, but I have seen a lot with dc motors. Good question electricpete.

rhatcher · Jul 27, 2001

Good answer motorhead1. electricpete, we normally don't check air gap on small motors like yours, is there a reason that you suspect a problem?

electricpete · Jul 27, 2001

rhatcher - We're troubleshooting 7200 cpm vibration on this two-pole machine. Have exhausted the normal range of troubleshooting... (soft foot checks, alignment checks, bar current analysis check for rotor bar problems).

I should mention it's not limited to one machine. We have 3 different groups of 3600rpm 100 hp vertical motors with 6 motors in each group of sister units. All of these seem to be having chronic vibration problems of one type or another. Often the problem is 2x and 7200. We're chasing around a lot of potential causes in different cases (in one case the pump fluid is 350 degrees... lots of potential for thermal growth affecting alignment)

I'd like to be able to rule out at least this one potential cause (air gap problems). It also strikes me that the air gap on these motors is very tight... perhaps 50 thousands of an inch, and it wouldn't take much movement at all to disrupt things. We've replaced bearings on all of them at some point in last 10 years... I'm wondering if there is some way we might have slightly moved the rotor somehow. I don't remember there being any rabbet fit of the end bells onto the stator... how do they control the position of the endbells with respect to stator so that the position is reproducible to within a few thousands of an inch?

THooper · Jul 28, 2001

Formulas do exist for approximating the minimum advisable air gap depending upon the motor's physical size and number of poles. Thes values aren't based on precise theory but reflect actual design and mfg experiance. Most motors use somewhat larger gaps for mechanical reasons and tolerance. Here's one commonly used formula:

GAP, inch= 0.005+ 0.0003 D+ 0.001 L+ 0.003 V

In which D is the stator bore diameter in inches, V is the rotor peripheral speed in thousands of feet per minute, and L is the core stack length inches. For example, assume an 1800 rpm motor with a 10 inch bore and 8 inch stack length. The minimum gap equals 0.005+0.003+0.008+0.014, or 0.03 inch on a side.

2-pole motors are very sensitive and require precision repair practices when recondition is being performed for smooth operation. Having high 2x could be several problems which most you have ruled out. Troubleshooting these bad boys, what I would try now if possible, uncouple the motor from the pump and perform vibration analysis. If this is not possible due to the motor and pump are one unit? Have your data collector on the motor and turn off the power to the motor. If vibration stops instantly when power is off, we know we have an electrical problem. If vibration comes down gradually with rpm slow down, we know we have a mechancal problem. Most common would be a bearing housing worn our of tolerance, which could cause air gap problems, alignment problems, premature bearing failure etc.. Or it could be a worn casing ring, worn bearing housing on the pump side sending resonance down to the motor. Where you are seeing multiple applications that resemble each other, I think this is an internal mechanical problem with these motor pump combos and will probably find this vibration to come down with the motor and pump when the power is shut off.

Don't quote me on this, I am not an Engineer, just a motorhead.

Kind Regards,

rhatcher · Jul 29, 2001

motorhead1 is proving to be a very knowledgeable source of motor info. A star from me makes three...I hope we see more from him.

electricpete, take motorhead's third paragraph ("2-pole motors are...) as the gospel. See if you can implement any of his recommendations as they are proven steps for isolating the type of problem you have. Let us know what you find.

jbartos · Aug 2, 2001

Suggestion: The air gap check is timely, when the bearings are experiencing wear. This would also call for maintenance or a replacement of bearings. There is normally not other factor that can change the size of air gap. However, there are a few exceptions to this, perhaps the ship propulsion motor aligned with the ship shaft and propeller.

rhatcher · Aug 5, 2001

electricpete,
I am curious to know if you have any more info on your problem.

electricpete · Aug 5, 2001

rhatcher - Not much to add at this time. As I mentioned, it's a chronic problem which we haven't been able to get our arms around. There may be a few different factors at work although I tend to lump the machines together based on similar size, speed and symptoms. I have heard that small 3600 rpm machines are just plain more sensitive to small problems with alignment, gap, frame distortion, imbalance etc.... and some newer plants specify a max of 1800 rpm for this reason.

For the first set of machines we're looking carefully at our alignment practices since process fluid is approx 350F and we don't account for thermal growth.

A second set of machines has had "Fabreeka Pads" installed for the last ten years... installed in an apparent attempt to detune resonance. I think these Fabreeka pads hurt our cause because they allow the alignment to shift. This second set of machines also has approx 0.1 ips vibration on the bearings of a pump when it isn't running (vibration transmitted thru suction and discharge pipes). We're working up a modification which will remove the Fabreeka pads, stiffen the base, add expansion joints in the pipe. Not a lot of concrete evidence to show that'll solve our problem, but that's where we're headed. We're also have some of the "failed" (removed based on vibration symptoms) bearings from these machines sent off to see if there is evidence of false brinneling.

For the third set of machines, we have seen pretty concrete evidence that we can reduce the vibration by temporarily stiffening the base. We'll be putting in some permanent stiffening. This particular group I think we've solved. I'm not convinced on the first two.