ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals

ECN Shout Chat
Top Posters(30 Days)
Admin 16
Recent Posts
ESA Arc flash course
by TheShockDoctors. 03/24/17 10:15 AM
fuse rejectors
by HotLine1. 03/24/17 07:53 AM
Another Forum Update
by Admin. 03/22/17 03:04 PM
Dining room plugs
by watersparkfalls. 03/21/17 10:31 PM
by jraef. 03/21/17 09:13 PM
New in the Gallery:
SE cable question
Popular Topics(Views)
231,331 Are you busy
166,070 Re: Forum
160,532 Need opinion
Who's Online Now
0 registered members (), 88 guests, and 12 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate This Thread
#130519 - 07/13/06 10:02 PM high amp draw by motor  
ds247  Offline
Joined: May 2003
Posts: 18
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

Tools for Electricians:

#130520 - 07/14/06 11:14 AM Re: high amp draw by motor  
Radar  Offline
Joined: Apr 2004
Posts: 349
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).


There are 10 types of people. Those who know binary, and those who don't.

#130521 - 07/14/06 06:07 PM Re: high amp draw by motor  
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
ds247, it sounds like you are describing a motor like that at 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.

#130522 - 07/15/06 12:28 AM Re: high amp draw by motor  
ds247  Offline
Joined: May 2003
Posts: 18
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?

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

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA


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.


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.


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

#130524 - 07/15/06 12:45 PM Re: high amp draw by motor  
ds247  Offline
Joined: May 2003
Posts: 18
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.

#130525 - 07/17/06 03:34 PM Re: high amp draw by motor  
jraef  Offline
Joined: Jul 2004
Posts: 95
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.


#130526 - 07/18/06 10:02 AM Re: high amp draw by motor  
sabrown  Offline
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA

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.


Member Spotlight
L.I. New York
Posts: 94
Joined: August 2003
Show All Member Profiles 

2017 NEC and Related
2017 NEC
Now Available!

Shout Box
Powered by UBB.threads™ PHP Forum Software 7.6.0
Page Time: 0.012s Queries: 14 (0.002s) Memory: 0.7930 MB (Peak: 0.9522 MB) Zlib enabled. Server Time: 2017-03-24 17:54:45 UTC