OK.
How does explain the 45A difference in current between a good motor and the bad motor?
Two speed one winding motor troubleshooting advice
- Thread starter phineascage
- Start date
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How does explain the 45A difference in current between a good motor and the bad motor?
that reading is meaningless
we have no idea what the shop test conditions were
voltage, motor temp
but even if identical, no load readings are meaningless
how do you explain
exact same issue for years before and after rebuild
if we had the motor nameplate, mfg, pn, etc we could pull up the data sheet
You can call it meaningless if you like.
But it's the crux of the problem.
has zip to do with it
how do you explain:
1 exact same issue for years, before and after rebuild
2 what changed between the shop and field to account for the avg 42 A unloaded delta
3 what was the shops pf vs fld pf?
That difference both off load and on load is, as I said, the crux of the issue.
I fail to see how you can't grasp that.
I'm done with this with you.
again, you ask questions and get responses (you may not like or agree)
but you never reciprocate
on load = mech/load issue
of course you are done before the courtesy of a response
1 explain same issue for years, before and after rebuild
2a what accounts for the unloaded delta
2b loaded delta
3 could a 5% (90 vs 85) pf delta acoount for a 25% i delta for an
loaded 10% pf motor?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I think I earlier suggested power factor could be high on an unloaded motor, leaving current draw higher then one might expect, but at same time the input kW is going to be significantly less compared to when driving something at rated full load capacity. Output is zero, but you still need input kW to drive the rotor.
Power factor is usually low on an unloaded motor and higher on a loaded motor. My modelling that I gave earlier had a full load PF of 0.85.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Oops, that is what I meant, but for some reason didn't come out that way, and was text instead of verbal, don't know how to validate that mistake.:blink:
Not a problem. You have clarified.
milldrone
Member
- Location
- Northern California
Gentlemen,
Do we know if the same meter was used at the shop, as was used in the field?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Well no, but are assuming some reasonable tolerance in accuracy of both.
consider
no load, 152 vs 195 A, 470 vac
shop kva 124 = S + jQ
field 159
the P must be the same for both: same R, rotational mass, friction, bearings, windage, etc, are assumed to be the same ie same motor
assume a pf of 0.10 for the motor at the shop, maybe lower or higher, but in that range
this means losses of 10% so eff is 90%, not a crazy assumption
so P = 12.4 in both cases
new motor pf at site
159 = sqrt(12.4^2 + Q^2) or Q = 158.5
new pf = cos(arctan(158.5/12.4)) = 0.078
delta i ~ 195/152 or +28%
from a net pf change from 0.10 to 0.078 or -22%
this can happen easily if the shop net pf > site net pf
the site has numerous large motors, chiller, clg towers, etc
so may have a pf approaching 80% whereas the shop may be > 90%
envision the shop had 80 but added caps to 90, would that not lower the motor i/kva?
if you look at the percentage of the motor/total load ratio it is apparent that the lower site pf could drag the motor pf down
comparing unloaded kva in this case is an apple to orange thing
no load, 152 vs 195 A, 470 vac
shop kva 124 = S + jQ
field 159
the P must be the same for both: same R, rotational mass, friction, bearings, windage, etc, are assumed to be the same ie same motor
assume a pf of 0.10 for the motor at the shop, maybe lower or higher, but in that range
this means losses of 10% so eff is 90%, not a crazy assumption
so P = 12.4 in both cases
new motor pf at site
159 = sqrt(12.4^2 + Q^2) or Q = 158.5
new pf = cos(arctan(158.5/12.4)) = 0.078
delta i ~ 195/152 or +28%
from a net pf change from 0.10 to 0.078 or -22%
this can happen easily if the shop net pf > site net pf
the site has numerous large motors, chiller, clg towers, etc
so may have a pf approaching 80% whereas the shop may be > 90%
envision the shop had 80 but added caps to 90, would that not lower the motor i/kva?
if you look at the percentage of the motor/total load ratio it is apparent that the lower site pf could drag the motor pf down
comparing unloaded kva in this case is an apple to orange thing
Russs57
Senior Member
- Location
- Miami, Florida, USA
- Occupation
- Maintenance Engineer
Motor wasn’t rebuilt. It had a bearing replaced and the shaft built up under that bearing.
As far as past history we have IR readings going back to 2002. Other than that we have second hand info of a coworker saying he “felt” it had been going on for a long time.
What we know for sure is motor draws 45 amps more, unloaded, on site than it did at motor shop. Motor also draws 45 amps more loaded, than the two other tower motors on site.
As far as past history we have IR readings going back to 2002. Other than that we have second hand info of a coworker saying he “felt” it had been going on for a long time.
What we know for sure is motor draws 45 amps more, unloaded, on site than it did at motor shop. Motor also draws 45 amps more loaded, than the two other tower motors on site.
And that's the crux of it. Site/shop power factor is an irrelevance.
rebuilt not rewound
so no elec changes
only mech
but we have no idea of performance before the changes
but wires look like thermal damaged
which would seem to indicate it ran hot before rebuild
plus the coworkers observations, same as before/after
WHY was it rebuilt?
how was it detrrmined that it needed it? i readings? noise?
were the others rebuilt?
post a pic of the nameplate when you can
thnx
wrong
the only thing that could account for the unloaded kva delta is pf
same motor at both places
no load
primarily reactive i
same losses so P ~ the same
~ same supply v (shop might be a few% higher)
what could change between shop/field for the same motor?
the windings did not change
the iron did not change
???
That's the $64,000 question.
But it isn't site power factor. I thought you might have known that.
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