Overload relays

[klh7891]

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

 

[jbartos]

Suggestion: Please, post the motor nameplate data. In the meantime, try 1.25 x Full Load Current appearing on the nameplate or in the device literature, e.g. instruction manual.

 

[GGOSS]

Hello Klh7891,

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.

 

[jraef]

Nothing wrong with what has been said, but I interpret your question differently. If you are asking about the validity of your relay taking 15 minutes to trip when set for 2.4A, and the motor is pulling 3.7A you have reason for concern.

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

 

[skiier]

Set the overload to the current draw you measured and you will be fine. I hope you do not think that the overload relay will "limit" the current drawn by the motor. On a DC drive you have a "current limit" adjustment that allows you to set the max current the drive will give the motor. The DC drive is an "active" device that can control current while an overload unit is a "passive" device that reacts to current.
Setting an overload relay to 2.5A when the load is drawing 3.7A will only prove that you overload is indeed ok.

 

[GGOSS]

Well picked up jraef,

I guess it's time I bought a new calculator.

Regards,
GGOSS

 

[jbartos]

Suggestion: The motor service factor is important when the overload relay is being set. The high SF motors can have the overload relay setting higher. See NFPA 70-1996 NEC Art. 430.34 Selection of Overload Relay

 

[jraef]

jbartos,
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

 

[klh7891]

Tnanks to all for the help.
The nameplate details are as follows:
Motor size - 2.2kW, 415v , 5.12Amps.

 

[jbartos]

Suggestion: Please, would you also post the overload relay and heater data, e.g. manufacturer, type, catalog number, and options, if any. Based on the original posting, the overload up to 4A is small even though the motor current is less, i.e. 3.7A per phase. The heater may have a ±20% tolerance, i.e. ±0.8A.

 

[jbartos]

Suggestion to jraef (Electrical) Jan 28, 2004 marked ///\\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.\\

 

[buzzp]

Set the OL to 5.2 amps and the motor will be protected. There is no problem except the OL is set to low since FLA is 5.12A.

 

[bklauba]

Since the motor's FLA is 5.12 at 415 VAC, but the O/L has a max setting of 4 amps, looks like one problem is the wrong O/L is being used. You might want to check with the motor mfr. as to what they recommend in the way of Class 10 or 20 for protection (unlikely Class 30, as pointed out by jraef).

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!!!

 

[jbartos]

Suggestion: Visit

http://www.sea.siemens.com/contrlbu/upld/files/p266-269.pdf

for: an IEC overload relay

http://www.geindustrial.com/products/manuals/239/239-d9_1.pdf

for: a motor management relay
etc. for more info