Overload relays
Overload relays
(OP)
I have purchase an overload relay which is protecting an extractor fan in a spraypainting booth.
It runs on a DOL starter with overload 2.5 - 4A. The motor tongtests at 3.7A per phase.
If I set the overload to 2.5A the motor runs for about 15 minutes before it trips!! Is this ok or is the overload faulty??
Can anyone advise me on why the overload operates this way-is it normal??
Thanks
It runs on a DOL starter with overload 2.5 - 4A. The motor tongtests at 3.7A per phase.
If I set the overload to 2.5A the motor runs for about 15 minutes before it trips!! Is this ok or is the overload faulty??
Can anyone advise me on why the overload operates this way-is it normal??
Thanks





RE: Overload relays
RE: Overload relays
The function of an overload relay is to protect the motor against overload/overcurrent conditions.
Based on present overload settings (2.4 amps) and actual current flow (3.7 amps) the overload relay will trip within the time frame you have outlined, so it is in fact working correctly.
Without knowing actual nameplate FLC or FLA values for your motor, it is impossible to guide you further other than to say;
1. The overload relay must be set to the nameplate FLC or FLA rating of the motor.
2. If the nameplate FLC or FLA rating is less than the 3.7 amps the motor draws during run, it would suggest that you have a mechanical situation that is causing the motor to draw more current than it can handle.
If you're still unsure and require further assistance, please advise motor nameplate FLC or FLA values and we'll go from there.
Regards,
GGOSS.
RE: Overload relays
3.7 / 2.4 = 1.54 x setting. Looking at a standard NEMA Class 10 OL curve (on paper using a t-square mind you, so forgive my inaccuracy), at 1.5 x setting, trip should be at about 300 sec.(5 min.). Even if it were Class 20, it should still trip in about 500 sec. (8.3 min.). 900 seconds puts you in the neighborhood of Class 30. While it is possible that you have a Class 30 overload relay, it had better be a Mill Duty motor connected to it because most standard motors cannot be adequately protected at that level.
As others have asked, please provide more details.
Quando Omni Flunkus Moritati
RE: Overload relays
Setting an overload relay to 2.5A when the load is drawing 3.7A will only prove that you overload is indeed ok.
RE: Overload relays
I guess it's time I bought a new calculator.
Regards,
GGOSS
RE: Overload relays
RE: Overload relays
Correct, except that would affect the setting one would make, not the trip time at a setting once made. I still think klh7891 has a problem.
Quando Omni Flunkus Moritati
RE: Overload relays
The nameplate details are as follows:
Motor size - 2.2kW, 415v , 5.12Amps.
RE: Overload relays
RE: Overload relays
jbartos,
Correct, except that would affect the setting one would make, not the trip time at a setting once made.
///It depends. The SF 1.15 motor may exceed the overload relay higher into the amp scale; therefore, the trip time will be shorter. However, I do agree that the trip time at a particular current setting is unaffected.\\\
RE: Overload relays
RE: Overload relays
If you want more accurate trip times than the 20% tolerance that's been quoted, or the very long 15 minutes (and since it seems you have to change the O/L anyway) try a solid-state O/L. These not being electromechanical, but rather electronic, are both more accurate and age more gracefully (besides minimizing heat loss in motor starter's enclosure, like 80 - 90 % less). Some of the solid-state O/L's have adjustable Class settings (choice of 10, 20 or 30), ground fault protection, and jam fault (will quickly sense locked rotor conditions) besides offering superior single-phase sensing.
Just keep in mind, this type of O/L does NOT break the connection 'tween the O/L's input and output, as heaters do, but rather, opens an auxilary switch, which typically is wired in series with the contactor coil.
I have seen two articles on the relative cost of motor protection vs. cost of replacing a motor recently, amortized over motor and starter life. Motor protection, even the somewhat pricey-er stuff, costs 'tween 2 - 5% of the cost of motor failure. Talk about an ROI!!!
RE: Overload relays
http://www.sea.siemens.com/contrlbu/upld/files/p266-269...
for: an IEC overload relay
http://www.geindustrial.com/products/manuals/239/239-d9...
for: a motor management relay
etc. for more info