isolators and motors

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danny7299

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EH UP!!!

been working in a factory today and they have a 3p motor , nothing fancy , and its 20amp a phase.

The op needs to be able to turn it on and off a lot and my boss has just giving me a ****ty cheap 25A 4pole isolator. http://www.toolstation.com/shop/Electrical/Industrial+Accessories/Rotary+Isolator+4+Pole+25amp+11kw+415w/d190/sd3080/p88825

now... i thought an isolator wasn't able to be switched with a load on it all the time? also with it being a motor does it not need any type of DOL starter or anything?

Cheers Chaps

 
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All motors should have a no-volt release starter. If the power goes off downstream and is then restored , that motor would restart on its own , possibly injuring someone. You should not be switching it on and off with the isolator, it is what its name suggests , an isolator.

What do you find wrong with the isolator you show ? I use a lot of them , for

 

spark-doctor

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All motors should have a no-volt release starter. If the power goes off downstream and is then restored , that motor would restart on its own , possibly injuring someone. You should not be switching it on and off with the isolator, it is what its name suggests , an isolator. What do you find wrong with the isolator you show ? I use a lot of them , for
 

danny7299

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i dont know... all i know is it 20Amp a phase? sounds silly but when buying thermal protection do you buy it to match the rating? all i can find goes up to about 18amps?

 

spark-doctor

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Sorry, i did not read the original post properly. Yes you will need a DOL with thermal overload rated to 20amp.

 
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It sounds like a 5 to 6 HP motor (4KW) seeing as its working, measure the running current before you order the starter to make sure you get the right overloads.

Your boss has no choice, motors must not be able to re-start on their own.

 

danny7299

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so how do you go about finding the right overloads, look and the side of the motor? see what it says? ? like spark doc says , its pulling 20amps per phase? so get smothing like a 18-15A overload and adjust it accordingly...

I will speak to the boss tomrrow , but i am just trying to learn for myslef in the meantime!

many thanks all!

 

spudmiester

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When you buy a self contained starter, there is usually no overload with it, you buy the correct rated one for your job and fit it in the starter box

Cant find my little black motors book, but with some VERY rough calcs, I recon your motor is about 5KW.

To avoid switching motor currents with the isolator, you could take an extra pair of cores to the isolator auxillary pole (if it has one), and series it up with the contactor coil. The auxillarys are usually 'early break' so will drop out the contactor coil and cut off the 3 phase before the isolator actually breaks the motor supply, therefor not switching under load. This is fairly common in some industrial applications.

 

KR Sol

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Evening all, I have had a bad day and this may be the cherry on top but I would think the motor is larger 13kw. kw=v*amp*pf*1.73/1000 therefore kw= 415 * 20 * 0.90 * 1.73/1000. I may have done something daft and if i am please say what. the other point is, you say they are starting and stopping the motor alot the rule of thumb (The one I was told) a motor should not be started more then 3 times an hour this is to stop damage and stress. Is this a new installation or a alteration? if they are altering it why? I get a gut feeling (it may be something i ate) they are trying to cut corners, just be careful you are putting your name to it.? :|

 

spudmiester

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Knew my calcs were rough.....and as it turns out, way off.

Found the black book and a motor with a full load current of 20A is 10KW or 13.5HP in old money.

Sol is right about the starts per hour, what is this motor driving ?

 

danny7299

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cheers for the help ... the motor is some kinda of vacum compressor thing... for sticking plastic together.

using KR farmula (kw=v*amp*pf*1.73/1000) gives approx 13Kw.... the only DOL starters i have seen go up to 11Kw... is this as high as they go or do i need to look harder?

 

KR Sol

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cheers for the help ... the motor is some kinda of vacum compressor thing... for sticking plastic together.using KR farmula (kw=v*amp*pf*1.73/1000) gives approx 13Kw.... the only DOL starters i have seen go up to 11Kw... is this as high as they go or do i need to look harder?
Hi, I have seen 15kw DOL starters in RS components. Probably find a local place like CEF etc may be able to help. Does the motor not have a rating plate on it, this may clarify its actual load in KW or HP, the formula has such bits like voltage and pf which may be different to what I used which will effect the KW value. Still a little worried about them stopping and starting a 13kw motor alot just seems a little worrying. My post said yesterday about stopping times per hour being 3 times an hour sorry it should have been 4 times, thus why things like large compressors have run down times of 15mins.

 

mysticalelectrickery

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the overload current will be on the motor rating plate

and the wiring of it will also be on the motor i.e star or delta.

 
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