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Topic Title: motor thermal overload setting
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Created On: 10 January 2008 10:00 AM
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 10 January 2008 10:00 AM
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dodgy1

Posts: 1
Joined: 15 October 2003

i am replacing a 18Kw motor panel and was wondering if there was a genral calculation for the setting of the thermal overload,i know this would depend on the load etc i was thinking of maybe a ballpark figure..
thanks in advance

Edited: 10 January 2008 at 10:01 AM by dodgy1
 10 January 2008 10:14 AM
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electricman

Posts: 830
Joined: 17 November 2005

hi
assuming it is 3phase then

18kw/3= 6kw per phase

at 4A per kw then ......
6kw X 4A = 24A.....
then i would put the thing on, load it up mechanically and pop a clamp meter on the supply. this would then allow you to set the o/l more accurately.

does this help?
alan

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Werner Von Braun 1969
 10 January 2008 10:18 AM
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walden

Posts: 102
Joined: 03 October 2006

The FLC of a 18kW motor is about 36A on a 415V supply.

If, as is likely, the motor is started star/delta and the overload relay is "behind" the main contactor then the overload will "see" about 21A and should be set accordingly.
 10 January 2008 01:04 PM
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Peter71

Posts: 54
Joined: 25 January 2005

memonline has a good motor flc table:-

http://www.memonline.com/ams_4.html

For an 18kW 415v the current will be approx 36 amps. As walden says if it is a star delta starter then the overload should be set at 36/root3 which is approx 21 amps (this is because some of the current doesn't pass through the overload as there are six cables to the motor with three from the overload) but the overload should really be set to the motor flc marked on the rating plate.

Peter
 10 January 2008 01:27 PM
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Jaymack

Posts: 4706
Joined: 07 April 2004

The selection of the overload rating, would depend upon the duration and magnitude, of the starting torque requirement. e.g. a fan is probably the most arduous starting duty.

The supplier of the new panel, should advise on the correct overload for the duty, if given the type of driven equipment - pump etc. Star delta starters can have overloads, for starting and running. The starting current varies from about 7-10 times FLC for a DOL motor; and a third of this current approximately, for a star/delta motor.

Jaymack

Edited: 10 January 2008 at 01:29 PM by Jaymack
 10 January 2008 01:44 PM
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Jaymack

Posts: 4706
Joined: 07 April 2004

Originally posted by: electricman

hi
assuming it is 3phase then
18kw/3= 6kw per phase
at 4A per kw then ......
6kw X 4A = 24A.....
then i would put the thing on, load it up mechanically and pop a clamp meter on the supply. this would then allow you to set the o/l more accurately.
does this help?


P = 1.73 × V (line) × I (line) × PF × Efficiency.

I line = 18000 ÷ [1.73 ×400 (Say) × 0.85 (Say) × 0.9 (Say)] = 34A, this is the FLC.

Jaymack
 10 January 2008 03:39 PM
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mstaple

Posts: 324
Joined: 23 July 2004

You don't say what the starting circuit is. If it is star delta, the location of the overload determines the setting i.e. in main line, after main contactor, after delta contactor.

The setting of the overload is always = FLC rating of the motor, which is stamped on the rating plate of the motor.

The setting only changes for positioning in star delta because the current is split (as correctly stated by walden) but you need to know where the overload is in the circuit.

The setting of the overload has nothing to do with starting current or time. If the starting current or time is excessive, then the overload is normally bypassed or CT operated overloads are used.

Edited: 10 January 2008 at 03:42 PM by mstaple
 10 January 2008 04:15 PM
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mstaple

Posts: 324
Joined: 23 July 2004

Have a look at this link, it's the bible!

http://www.moeller.net/en/support/wiring_manual.jsp

Download the complete manual, about 6M.
 10 January 2008 09:13 PM
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Buz

Posts: 167
Joined: 15 December 2007

Motor rating plate current = motor overload device setting.

Any complicated calculations have already been done by the designers of these devices.

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 11 January 2008 07:06 AM
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walden

Posts: 102
Joined: 03 October 2006

Originally posted by: Buz

Motor rating plate current = motor overload device setting.

Any complicated calculations have already been done by the designers of these devices.


Not necessarily! It depends on how the motor is started, DOL or star/delta and the position of the overload device in the circuit as described above by other posters.

Edited: 11 January 2008 at 07:08 AM by walden
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