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Spot Welder


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#1 Scott Brown

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Posted 10 March 2019 - 08:58 AM

Has anyone ever run a spot welder off a generator. If so what generator do you have.

#2 Homersimpson

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Posted 10 March 2019 - 03:36 PM

I always run mine off the mains and its a 3kVA unit.  Its a hell of an inrush current and I doubt that a small generator would do it and provide a consistent weld, either the generator would stop or the weld will be poor.

 

If I have a 25A Type C MCB on the garage it trips after 4 welds so I've changed to a 32A which just holds.

 

I used to size large industrial generators but don't have much knowledge on the baby sets but I think you would want at least 10kVA single phase (assuming your welder is single phase) to stand a chance.



#3 Scott Brown

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Posted 10 March 2019 - 03:53 PM

Thanks for your help so if I was to change my mcb it would run a spot welder and not trip.

#4 Homersimpson

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Posted 10 March 2019 - 04:16 PM

It depends on what source you are using.  I should explain i'm an electrical design engineer by trade so this is my bread and butter so to speak so excuse the long winded response.

 

Are you considering a generator because your house supply keeps tripping? If so then basically if your using the mains supply even at home and your spotwelder is reasonably small (i.e. 3kVA or similar) then it should work fine.

 

Does the spotwelder trip the MCB or the RCD?

 

If your problem is that the MCB feeding the garage is tripping then you could fit a larger one or a different type, however, you need to be careful that the cable feeding the garage is large enough for the increased MCB otherwise the cable could become overloaded and in the worst case start a fire.

 

You have to consider that an MCB, fuse or any type of electrical protection device does not trip at its rated current if the load is slightly larger than the rating.  Each device will have a time current characteristic curve which will show how many amps it will withstand for how long.

 

All electrical design is based on a dead short circuit between live and earth and all disconnection time are calculated on this.  This is fine as a dead short will cause a very quick disconnection.

 

If on the other hand you overload the device it may not trip/blow before the cable is compromised.

 

Imagine filling a bucket up from a tap with a hole in the bottom where the bucket is the cable and the water coming out of the hole is heat.  When the water entering the bucket is less than going out the hole everything is fine, If on the other hand you put too much load on the cable (i.e. more water into the bucket) it starts to fill up and eventually it will overflow.

 

In the case of a cable too much load means it will eventually fail possibly with catastrophic results.

 

The spot welder applies a large load for a short period but if you do lots of them in succession (which is usually the case when welding on sills, rear quarters etc.) you could put so much heat into the cable that it fails.

 

Can you advise whether you have an available mains supply, what size and type of cable you have from your consumer unit to the spot welder, what rating and type of MCB feeds this (it will say something like B32), what plug and socket arrangement you have on the welder and of course the electrical characteristics of the welder.

 

If its your RCD thats tripping however then this indicates a fault with the welder.






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