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Topic Title: Rule of thumb
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Created On: 21 January 2013 09:00 PM
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 21 January 2013 09:00 PM
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Hi all
Is there a rule of thumb calculation that I can use when selecting the correct overload for a motor, without going through manufactures tables and the motors service ambient temperature etc

 21 January 2013 09:29 PM
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I would get a contactor and overload rated for the motor. ie 2kw motor = 2 kw contactor minimum and overload set at fla.
 21 January 2013 09:40 PM
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I'd get a larger contactor than the motor rating and set the overload at or just below FLC

On feeder motors which are normally between 0.3 and 0.7 kw I always fit 9kw starters. Don't have problems then.

 21 January 2013 09:53 PM
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You are right - it is worth taking a bit of time over to make sure you get it right and to ensure that no nuisance tripping occurs, and the motor is properly protected.

A place to start (but not necessarily the best) is the FLC, which should be stated on the motor spec. Then adjust this according to ambient conditions and tolerable overload level (in glasshouses where we install vent motors high up under the glass, object temperatures can climb to in excess of 68C - more than enough to alter the behaviour of the motor currents).

Then pick a nice thermal overload relay which is sufficiently adjustable up to that level.

If the plant room/control room/etc. is hot, like 45C+, then the behaviour of the thermal trips will be altered and become slightly more sensitive at lower currents.

Measuring is often the route we use too (often to verify) - after all, 'measuring is knowing' - well for the most part .

Usually if the engineering has been done well, the motor will actually draw nothing like the FLC when measured, and a much lower overload can be chosen meaning that the motor will trip the overload without allowing catastrophic failure because of excessive forces being exerted on equipment etc. (especially useful for glasshouse vents!) despite the motor not technically overloading as such.

Also, on single phase equipment the use of a 'backup' fuse is often recommended and can be useful where the load may be reset by semi-skilled operators from 'cold' after an initial trip has occurred. In these conditions a large amount of damage can be done even in the relatively short time it takes for the overload to warm up to tripping point - especially for a jammed motor running close to FLC.

For 3ph motors the current balance between phases is critical for correct and timely operation of overloads. It is quite easy with a 20% imbalance to burn out just one leg of an overload without it tripping. Then the motor is exposed to just 2 phases which is usually quickly catastrophic and visit to the rewinders is required.

Hope this helps - there is always more to learn with this kind of topic after all!


The meaning of life is a blank sheet; write on it wisely. ~ M.Cutler.
 21 January 2013 09:55 PM
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Oh, and don't forget to make sure if it's a single phase motor you link the overloads correctly.


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