reference for airgap tolerance average +/-15% 4-pole machine

[electricpete]

I remember hearing a long time ago that, airgap measurements should be within 10% of the average for 2-pole machines and 20% for slower machines.

Now, most of the references I see only say 10% (no regard for speed).

I realize 10% should be obtainable, but I also believe that if 10% is acceptable for 2-pole, then surely we can get away with a little more for slower machines which have dramatically lower magnetic unbalanced pull than 2-pole machines .)

Does anyone have any reference that will permit 20%? For non-2-pole machine?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[DougMSOE]

I have always used 5 to 10 % as a solid rule. However, you are in dangerous land when you use only speed. The problem is from what designers call 'Shaft Stiffness Ratio' which is strickly a function of the design of the motor not the RPM.
Another thing to remember is that as the air gap decreases the radial force increases as the square of the delta airgap.
Not uncommon to se with a 10% air gap tollerance with 'tons of radial force'.
Have seen 12 pole motor rotors on compressors with sloppy air gaps close up a 3/8 inch air gap during starting.
You should see people run then.

 

[edison123]

pete,

10% air-gap variation is the standard regardless of speed.

The air-gap in slow speed machine is lower than that of a corresponding high speed machines. So, a higher gap variation (in a smaller gap) does not sound right.

* Anyone who goes to see a psychiatrist ought to have his head examined *

 

[aolalde]

Hi Electricpete:

The information I have for DC, Synchronous and Induction machines is: “the difference between the largest and smaller air-gap measurement must not exceed 10% of the average gap”
A non uniform air-gap causes an unbalanced magnetic force to act on the rotor in the direction of the minimum gap. The force increases with the air-gap reduction and an excessive force to this nature may deflect the shaft enough to permit the rotor to rub the stator. A non uniform air-gap can cause unnecessary losses and heating, noise and excessive bearings load.
Since air-gaps are larger in 2 and 4 poles machines as compared to those with 24 or more poles the 10% proportion is a good practice limit in spite of the poles in the machine. If that target is not achievable check for rotor rubs against the stator and make a study of “shaft magnetic stability” or rotor-shaft rigidity as compared to gap reduction unbalanced magnetic pull.

 

[UKpete]

Pete, I'm not sure how you can measure an airgap with any accuracy, what with lamination build-up etc, so I wonder if it is a bit academic.

Back in the old days when dc motors were manufactured for traction applications, it was very important that the armature airgaps were all within a close tolerance; the reason for this being that in a dc motor the gap affects the speed of the motor at a given voltage and field setting (e.g. a larger airgap gives a weaker field and therefore a higher speed). This matters when the motors are effectively mechanically coupled together and running off the same dc supply, as in a locomotive, because the load-sharing has to be satisfactory.

Which is a long way around to saying that measuring the gaps just wasn't found reliable. Each machine was run at a set of fixed voltages and field currents and the speed measured. Out of tolerance motors were corrected by labouriously adding/subtracting steel shims from under the field pole bricks.