To set overload for motor

Yes, I agree that you could set the overload to less than 100%. zai_jnr could measure the normal current over some time with the conveyor transporting the maximum amount of material, and set the overload to 110% of that (but not higher than FLA of course). The overload would then become a "poor mans mechanical problems detector". The overload will act faster. If the normal operating current is substantially less than FLA you will get something like 1-2 seconds tripping time in stead of 5-10 seconds.

edit: Leitmotif's suggestion about a mechanical safety is best if you can possibly manage that.

An overload relay is not designed to function as a torque limiter and should not be used for that purpose.

It's intended purpose is to provide THERMAL protection for the motor. It is with that purpose in mind that the overload is set up generally with the motor nameplate Full Load Amps. If the motor has a service factor (it seems that this is pretty much unique to NEMA motors) then the overload relay setting can be increased a small amount over the FLA, the exact amount being determined by the amount of service factor and the local codes in effect.

Altho it is often overlooked, it is essential for proper motor protection to set the correct Class which is actually a time/overload curve which approximates the thermal heating curve of the motor. A very light weight extruded aluminum motor heats very quickly so it requires a Class 10 overload curve. A common cast iron motor (a cast iron T frame motor would be typical in North America) would use a Class 20 overload curve, and a very heavy duty cast iron motor (a cast iron U frame motor would be typical in North America) would require a Class 30 overload curve.

To provide reliable torque limiting without compromising motor protection or mechanical integrity, you would need to use a mechanical torque limiter such as a shear pin or torque limiting coupling or an electronic device which watches for peak currents and trips on those without much time delay.

Again, an overload relay is not suitable for torque limiting duty because of the time delay built into it.

zai_jnr said:

well.. the machine is slat conveyor..in this system there have side lifter to synchronize with slat when to load or transfer the part from side lifter to slat c/v attachment..side lifter, there have arm to detect attachment on slat c/v before side lifter can syncro with slat..accidentally during trial, arm close early then make attachment on slat damage..during this occur..slat still running until there have someone press emergency stop button...

Click to expand...

If I'm understanding this correctly, There was some sort of failure in the arm that detects the attachment. As DickDV mentioned a motor overload is not the right tool for the job of detecting a instantaneous motor jam. A detent style overload clutch http://tsubakimoto.com/tem/pdf/torq-gard-coupling-axial-guard-faq-2007-07.pdf or a friction style unit (less desirable). http://www.dodge-pt.com/products/pt_components/torque_tamer/torque_tamer.html
Would work much better. Both of these units can be placed near the final drive so that inertia has almost no effect on the final torque.


To try to use a motor overload relay to prevent this type of mechanical damage. Is like trying to use a sledgehammer to windup a wrist watch. You might be able to do it, but it will not be easy.

Why are you obsessed with the O/L unit it sole purpose is to protect the motor and should not be used for any other purpose than that . the problem lies in the machanical workings of the machine ,no shear pin ,no detent clutch ,no motion detection,no torque control etc .an invertor would be the easy option with programable parameters such as torque ,overload ,speed,ramp etc, etc

OK so you have a conveyer with some other mechanism that detects or feed parts onto the conveyer. Between the two mechanisms something got fouled up and you jammed the system. Operator hit the E stop before system really tore itself apart.

Given the above understanding
a standard motor overload will only protect the motor and is NOT fast enough to detect, shutdown and protect from mechanical jams and machine tearing itself apart.

Could you detect this problem using motor current - YES
- will it be perfect NO
You would need another sensor that would not be affected by motor current (at the minimum) I think the big problem here is lots of false trips caused by normal peaking loads.

BEST BET make double sure all mechanisms are adjusted properly. Pain in the axx - worked with food industry conveyer systems. You may have to do this at start of each shift. Customer will not like that

IF all the system is driven with one motor use a V belt drive and adjust belt just tight enough to provide normal torque - anything more than that belt will slip. Not a perfect solution but will work in meantime.

Use brass shear pins which will break easily in case of jams. Use brass because no rust, easier to drive out, easier to drill and you wont mangle the shafts due to its being softer.

Write BRASS in BIG letters so that when maintenance guys use steel it voids the warranty. The maintenance guys have a choice either inspect and align the machine or replace BRASS shear pins. If stupid they will use steel to make sure the pin does not break. They are also the same type that if a 5 A fuse blows use a 10 - I know I worked for one - praise the almighty I am out of there.

Dan Bentler