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Old January 4th, 2009, 08:17 PM   #1
zai_jnr
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To set overload for motor

Hi every one.....

I wonder how to set value of overload for motor..there have some argument regarding setting of overload... one member said should be increase 5% from current reading of motor..other member said should be 10%..other one said 20%...there also have said setting max amp motor that stated in motor plate...need senior member in this forum to clarify for this issue and the best setting for overload motor... ermmmmm......what said of survey????....
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Old January 4th, 2009, 08:28 PM   #2
controlled
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for most motors, the max ol setting is 125% of motor FLA. Overload relays that have adjustable dials already have the 125% factored in, so you set the ol setting to match the motors FLA.
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Old January 4th, 2009, 08:40 PM   #3
zai_jnr
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Quote:
Originally Posted by controlled View Post
for most motors, the max ol setting is 125% of motor FLA. Overload relays that have adjustable dials already have the 125% factored in, so you set the ol setting to match the motors FLA.
Hi controlled..erm..125% of FLA (full load amp) motor? let say my motor is 3.7kW, 3 phase, 415VAC ,so amp that stated at plate is, let say 6.4A..so i must set 125%x6.4A=8A? Like this? or let say i check actual current using tachometer, i get the reading is let say 5A, so 125%x5=6.25A..which one is true?to set 8A or 6.25A?...
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Old January 4th, 2009, 08:54 PM   #4
geniusintraining
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8Amp
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Old January 4th, 2009, 09:16 PM   #5
zai_jnr
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Quote:
Originally Posted by geniusintraining View Post
8Amp
erm..8A? in my previous project, i setting my OL motor is max amp current motor = 6.4A, but this setting already argue by customer due to motor never cut off by OL function when there have stuck with motor rotation , then make there are so many damage in the system because motor not stop..
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Old January 4th, 2009, 09:23 PM   #6
leitmotif
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Common accepted practice is to set at 125% of FLA.

The choice is really yours - it can be argued that in both cases
FLA = 6.4 with overload at 125% and set at 8
OR measured motor full load is 5 and overload is at 6.25
could be correct.

IF the motor NEVER goes over 5 amp you would be OK. This may be the case if you are worried the motor can damage the machine it is driving
ie the motor is oversized cause that is all you had.

The idea is to
set the overload low enough to protect the motor from overload AND set it high enough to prevent nuisance trips from short term overload ie starting. Kinda frustrating to try and start the motor and have the overloads trip cause they are set too low - been there dun that.

Dan Bentler
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Old January 4th, 2009, 10:10 PM   #7
Stationmaster
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Don't be frustrated about the two possibilities. What you have described is ANOTHER use for "overloads". The PRIMARY purpose of overloads is to protect the motor from "overload". Hence the name. So when you ask "At what amperage do I set the overloads?" and give us motor information, our answer will be "set the dial on the adjustable, solid state, overload to the nameplate full load amps". This will protect the MOTOR from destroying itself due to "overload".

This is entirely DIFFERENT from using an overload setting to protect the attached machinery.

Mechanical catastrophe can occur faster than overloads are designed to trip. Solid state overloads can take up to 2 full seconds at overload amps to trip (The bi-metal, heat strip, mechanical type can take much longer). Destruction of mechanical equipment can occur in MILLISECONDS.

Not that the solid state overloads couldn't be designed to trip faster, just that they AREN'T so they won't be tripping all the time due to inrush amps, momentary loads, etc.

Counting on overloads alone to protect attached machinery is something I would not recommend. However, that is where the other answer comes in. If you are trying to use overloads to protect attached machinery, then use your amp meter to determine "maximum normal amp draw" and add 25% (as a starting place).

It would be MY recommendation to protect the machinery with some sort of "mechanical" fail safe, shear pin, clutch, belt adjustment, ANYTHING besides counting on the "overloads".

I also find it curious that your motor could go to "locked rotor amps" and "never" trip the overloads. Do you mean that the machinery failed before the overloads tripped? Once the machinery fails, the "overload" or "locked rotor" condition is relieved.

Did the MOTOR survive?

Stationmaster

Last edited by Stationmaster; January 4th, 2009 at 10:17 PM.
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Old January 4th, 2009, 10:20 PM   #8
Derek
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I usually size mine just high enough to avoid nuisance trips.
125% is fine for conductor sizing but if your motor is running that
much over current consistently you got a problem.
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Old January 4th, 2009, 10:26 PM   #9
leitmotif
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erm..8A? in my previous project, i setting my OL motor is max amp current motor = 6.4A, but this setting already argue by customer due to motor never cut off by OL function when there have stuck with motor rotation , then make there are so many damage in the system because motor not stop..

I did not see this when I replied probably came thru while writing. Something is definitely wrong with your setup. It could be lots of things motor is too large for the driven load, overloads not set correctly, overloads not even connected maybe or connected incorrectly,
wrong size heaters,

OK the customer complained that motor and machine stalled and the overloads did not trip - how long was this - did they shut machine down before overloads had the time to trip - they are a time delay device after all.

I agree with Stationmaster IF the machine is valuable I would have a sacrificial link in mechanism to prevent tearing machine up ie brass shear pin or set a v belt drive a little loose so it will slip. For sure I would have something in there to give before tearing up machine in overloaded condition.

I would test this whole setup out to ensure everything is the right size and installed correctly. I would test run to double check that the overloads will trip on an actual overload.

Sure would be nice to know what kind of machine we are talking about here.

Dan Bentler
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Old January 4th, 2009, 10:30 PM   #10
Gil47
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You are in Malaysia the daily temperature average is high.
You are in Malaysia that tend to follow the European Standards ( England )
as also do New Zealand where I am.
Motor overloads tend to heat on a similar relationship to that of a motor so if the air temperature around the motor is similar to that of the overload then there actions work similar.
A test I do with motors is if the motor is so hot that you can not grip the colling fins then check that overload is not set to high .
Now my answer is, I do not set overloads on Din standard motors any higher than that written on the name plate, in your case 6.4 Amps.

The overload is there to protect the motor not the machine that it drives.
Now you mention the motor causing damage, then my view is the motor is over sized and by using a clip on ammeter I would set the overload to a lower setting maybe but not lower than its normal running amps.
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Old January 4th, 2009, 11:47 PM   #11
Derek
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Quote:
Originally Posted by leitmotif View Post
I would test run to double check that the overloads will trip on an actual overload.


Dan Bentler
Exactly, with IEC type starters you never know.
Amazing how many NEMA starters around from 60's
that never have had a problem.

IEC starters have made me a lot of money over the years.
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Old January 5th, 2009, 12:08 AM   #12
zai_jnr
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Quote:
Originally Posted by leitmotif View Post
erm..8A? in my previous project, i setting my OL motor is max amp current motor = 6.4A, but this setting already argue by customer due to motor never cut off by OL function when there have stuck with motor rotation , then make there are so many damage in the system because motor not stop..

I did not see this when I replied probably came thru while writing. Something is definitely wrong with your setup. It could be lots of things motor is too large for the driven load, overloads not set correctly, overloads not even connected maybe or connected incorrectly,
wrong size heaters,

OK the customer complained that motor and machine stalled and the overloads did not trip - how long was this - did they shut machine down before overloads had the time to trip - they are a time delay device after all.

I agree with Stationmaster IF the machine is valuable I would have a sacrificial link in mechanism to prevent tearing machine up ie brass shear pin or set a v belt drive a little loose so it will slip. For sure I would have something in there to give before tearing up machine in overloaded condition.

I would test this whole setup out to ensure everything is the right size and installed correctly. I would test run to double check that the overloads will trip on an actual overload.

Sure would be nice to know what kind of machine we are talking about here.

Dan Bentler
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...
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Old January 5th, 2009, 05:37 AM   #13
jschapansky
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I didnt read all the posts in detail but I pulled out my code book...Canadian Electrical Code...

28-306 Rating or Trip selection of overload devices

1. Overload Devices responsive to motor current, if of the fixed type, shall be selected or rated or, if of the adjustable type, shall be set to trip at not more than the following:
A. 125% of the full load current rating of the motor having a marked service factor of 1.15 or greater; or
B. 115% of the full load current rating of the motor which does not have a marked service factor or where the marked service factor is less then 1.15.

So if your 6.4 Amp motor has a SF of 1.15 or greater then MAX OL is 8A.
If SF is less then 1.15 or not labelled, then MAX OL is 7.36A.
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Old January 5th, 2009, 05:56 AM   #14
JesperMP
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I say that the overload shall be set to 100% FLA. The OP never asked about service factor or anything like that.

As to service factor higher than 1.0, then that will reduce the expected life of the motor. Even if it is allowable, I know of noone that would actually do that except for in an emergency and you need to run a smaller motor for some time before you can install the proper size motor.

Apart from that, then there are some other extreme cases where you may be forced to set the overload to something else than 100% FLA. But for normal use, it is 100% FLA.

Quote:
Originally Posted by zai_jnr
erm..8A? in my previous project, i setting my OL motor is max amp current motor = 6.4A, but this setting already argue by customer due to motor never cut off by OL function when there have stuck with motor rotation , then make there are so many damage in the system because motor not stop..
Again. This tells us that you definitely should not set the overload higher than FLA.
As to zero speed protection, then using an overload relay is not recommended for that. At locked rotor, a normal thermorelay will trip after approx 5-10 seconds. If that is too long time for you, you should install a genuine zero speed detection device.
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Old January 5th, 2009, 06:28 AM   #15
BryanG
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First thing is to check the documentation that comes with the overload, it should tell you what setting to use. If you use that setting and have a problem then you can complain to the supplier.

If the overload is measuring Line current then in the UK maximum should be 100% FLA. You can set it lower to give greater protection but you shouldn't set it higher. If it is measuring Phase current in a Star/Delta system the you divide the Name plate current by 1.732.

You need to investigate why it didn't trip, is the Auxiliary on the Overload connected and working? What current is being drawn at the time of the fault?

Bryan
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