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#130519 - 07/13/06 07:02 PM high amp draw by motor
ds247 Offline
Member

Registered: 05/17/03
Posts: 18
Loc: California
I replaced a psc condenser fan motor. The bad original was a half horse motor, 1050 rpm, psc, 208-230 motor, totally enclosed, rated amp draw was 2.9. The incoming voltage is 208 and the compressor is a 3 phase. The new motor has identical specs except that rla is 3.0A. I took an amp draw with 2 differnt meters and found that it was about 3.6, about 20 percent more than rated amps. I thought that maybe I was getting some interference so I connected a long wire and took the amp draw outside the cabinet and found the same 3.6. I will probably replace the motor but in the few warm days that it has run it has not gone out on internal overload. I believe the SF is 1. I presume the motor is bad but anyone have any suggestions why the motor has not gone out on overload or why I might be getting a false reading? By the way I took an amp draw of the old motor and it was 3 amps when it would run so I imagine that the readings are not being influenced by being too close to the compressor--or else that motor's reading was about 20 percent over as well. Thanks

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#130520 - 07/14/06 08:14 AM Re: high amp draw by motor
Radar Offline
Member

Registered: 04/30/04
Posts: 344
Loc: Los Angeles, CA
I would verify the SF. Also, check the rated temperature rise of the motor. If the temp rise is not over 40ºC, then the required overload setting is FLA * 125%, otherwise it's FLA * 115%. [NEC 430-32(c)]

So if your FLA is 30 amps, in all likelyhood the OL setting is 37.5 or thereabouts.

Just my opinion, but even if the OL isn't tripping, I think having a 30 amp motor with a 1.0 SF running at 36 amps is an incorrect application. I'd plan a replacement. I'd also tach down the fan speed, if possible. Something's causing the overload condition, and I wouldn't jump on a motor defect as the first suspect (although anything's possible).

Radar
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#130521 - 07/14/06 03:07 PM Re: high amp draw by motor
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
ds247, it sounds like you are describing a motor like that at http://6L6.net/bjarn/6R13.ecn.WWG.p109.catalog.pdf with the fan blade mounted directly on the shaft.

An SF (service factor) of 1.0 means that the motor can’t run with any overload for much time without damage.

I’d make sure that the installed run capacitor matches the one called for on the motor label. Also, check voltage (at the motor) to see if it’s OK.

Try taking off the fan blade and checking the motor’s amp draw. It should drop below 3 amps.

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#130522 - 07/14/06 09:28 PM Re: high amp draw by motor
ds247 Offline
Member

Registered: 05/17/03
Posts: 18
Loc: California
I believe the rated temp rise is 60 degrees and the capacitor is correct (10mf). I also checked the capacitance with my meter and it was good. Yes, the motor is similar to the webpage cited by Bjarney. I would like your opinions--do you think the amp draw reading is accurate? If it is then changing out the motor is the thing to do.
One other question. Will the amp draw be more at 230 volts or at 208 volts?

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#130523 - 07/15/06 03:11 AM Re: high amp draw by motor
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2680
Loc: Anaheim, CA. USA
ds247

 Quote:

One other question. Will the amp draw be more at 230 volts or at 208 volts?


The FLA will be higher when running the Motor on a 208 VAC Circuit - when the Motor is developing rated Horsepower.

I was wondering if the Nameplate Ratings of the new Motor show the FLA on a 230 VAC Circuit as 3.0 Amps, and the FLA on a 208 VAC Circuit to be higher - like maybe 3.6 Amps.

If not, could it be that the Ammeters are "True RMS" type, and may be measuring harmonic currents, in addition to currents flowing within the "normal" Apparent power package (just curious..).

A 10 µF Capacitor - with like a 370 VAC rating, sounds about right for a 1/4 HP - 1/3 HP PSC Motor.

Question:

Does the Motor Start "Slowly", and Stop "Quickly"?
If yes, sounds like the shaft, or blower, is binding against something; possibly the Motor's enclosure is out of alignment.

Scott35
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#130524 - 07/15/06 09:45 AM Re: high amp draw by motor
ds247 Offline
Member

Registered: 05/17/03
Posts: 18
Loc: California
The motor starts quickly. The stopping time I don't remember. I did check how freely it spun and there seemed to be no issue with that. One meter was a true RMS and the other meter was not.

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#130525 - 07/17/06 12:34 PM Re: high amp draw by motor
jraef Offline
Member

Registered: 07/21/04
Posts: 88
Loc: San francisco, CA, USA
Be careful on the True RMS issue. Some meters say True RMS on the outside, but they mean only for voltage, not current. The Fluke 87 is like that. So when you add a current probe to it, it is not reading true RMS current. Some condensers are coming with electronic motor controllers now, small VFDs designed to work with 1 phase PSC motors. They will give you trouble when trying to read current.
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JRaef

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#130526 - 07/18/06 07:02 AM Re: high amp draw by motor
sabrown Offline
Member

Registered: 12/12/02
Posts: 294
Loc: Ogden, Utah, USA
jraef,

Thanks for the information on the meters. Can you tell us how the VFD's tend to affect the meters current reading? I assume that the reading will be overall low. Please explain if otherwise.

After spending enough money on my Tektronic DMM914, I am glad to know that I should rely on my Fluke 41B when the readings are more critical.

Shane

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