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Anonymous Poster #1

Frequent Motor Housing Problems

08/01/2019 2:15 AM

I have a 3 phase, 50hz, 7.5kw, 415v ac sq cage induction motor for blower purpose. Rpm is 2900. It has two separate impeller mounted directly on motor shaft (quite long shaft than usual motors with long shaft key slot). When it was new it worked very fine for several years. But once it had a winding burnt problem, we re-winded the motor and replaced the bearings with newer one. But since then the motor front housing and both bearings are falling very frequently and sometimes winding is also getting burnt due to this. Whether it is our mounting and assembling problems or poor repair of housing, I am not so sure. If anybody can be help me on this, I will be grateful.

Thanks.
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HiTekRedNek
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#1

Re: Frequent motor housing problems

08/01/2019 5:57 AM

Check the clearances of the bearings,ID and OD in the housings.

The last two digits of the bearing,times 5 for example: (6207 npp,kdd,etc) are the shaft size in MM.This would be a 35 MM shaft.(I am just guessing about the size,so use as an example only)For a press fit in inches,multiply this number by .03937.This is for the rotor shaft race only,but provides an easy quick check.I do not know the rule-of-thumb for the OD dimensions,consult bearing specs.If the housing is worn due to spinning of a locked bearing,it must be bored and bushed.If the rotor shaft is worn,there are several ways to restore it to specifications,depending on the degree of wear.

Also check for bent rotor shaft using v blocks or lathe and dial indicator.

A bent shaft can be straightened.

There is only approximately .004" clearance between the stator and the rotor,so excessive bearing or housing clearance can cause the rotor to drag.The bearing should be a press fit on the shaft,and heated before installing,applying pressure only to the inner race.Do not drive the bearing onto the shaft.The outer race should be a close hand-press fit in the end bell(housing). Make sure there is no residual lacquer on the inside of the stator laminations.Examine the stator for signs of rotor dragging or scuffing,on either the stator or rotor.Be certain the bearing is the proper clearance as designed.ABEC1 is a loose bearing(internally),ABEC 3 is a closer fit,and the tolerances decrease at the ABEC number goes up,along with the price.The ABEC rating should be indicated on the bearing.Buy a name brand bearing,like Fafnir,or Timkin.There are some cheap counterfeit bearings out there,so be careful.

Please give some feedback on what you find.

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setlock77
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#2

Re: Frequent Motor Housing Problems

08/02/2019 7:08 AM

it sounds to me like the rotor was not correctly balanced after the winding.
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HiTekRedNek
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#4
In reply to #2

Re: Frequent Motor Housing Problems

08/03/2019 5:50 AM

This is a small 3 phase motor,not an armature motor.

Rewinding does not affect the balance of the motor.

If the fan blades have key ways,which is normally the case,they will go back in the same position as original.

The op should also check for balancing weights that may be missing from the fan blades.If there are fins on the end of the rotor,they should also be checked.

Very large motors,should be spin balanced,especially if the rotor shows signs of contact with the stator,due to the large forces involved.

However,a bent rotor shaft or loose clearances in the housing or on the rotor shaft can cause an unbalance.

Typical wear is in the end bell housing where the bearing outer race spins in the housing.(It is a looser fit). If the bearing actually locks up,both the shaft and housing can show wear.

Also check for the presence of and proper installation of the preload springs in the housings behind the bearing.

The OP does not say if they send out the stator and reassemble the motor themselves of if they are relying on the motor rewind facility to reassemble the motor.

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Circuit Breaker
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#3

Re: Frequent Motor Housing Problems

08/02/2019 9:23 AM

Is it on VFD? If so bearings may be failing due to eddy currents, causing the windings to fail. There are bearing made to prevent this and kits that can be added to help. I think balance is a real concern if the fan blades have been removed and re-installed, are they clean and or have they been bent? -- JHF

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Stef
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#5

Re: Frequent Motor Housing Problems

08/05/2019 12:02 AM

Have you checked that the LH and RH fans have not been transposed and producing an imbalance because the original balancing is now incorrect for the fan positions?

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PWSlack
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#6

Re: Frequent Motor Housing Problems

08/07/2019 6:26 AM

Please describe the insulation testing that was carried out before energising the motor.

Please describe the overload protection arrangements on each motor; <...burnt...> does not happen with correctly-selected and adjusted overload devices.

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nesubra
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#7

Re: Frequent Motor Housing Problems

10/05/2019 11:47 AM

First of all check whether shaft has any bend.2900 rpm is a high speed and even slight deflection could cause problems.When u have an extra long shaft it is better to consider using a Jack shaft arrangement that is an intermediate bearing external to motor before the impeller if space is there.

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Circuit Breaker (1); HiTekRedNek (2); nesubra (1); PWSlack (1); setlock77 (1); Stef (1)

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