reference for airgap tolerance average +/-15% 4-pole machine
reference for airgap tolerance average +/-15% 4-pole machine
(OP)
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?
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?
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RE: reference for airgap tolerance average +/-15% 4-pole machine
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.
RE: reference for airgap tolerance average +/-15% 4-pole machine
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 *
RE: reference for airgap tolerance average +/-15% 4-pole machine
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.
RE: reference for airgap tolerance average +/-15% 4-pole machine
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.