What cable to use for my compressor ? advice

[meagain]

Hello i wonder if you can help me please what ive got is my back room on 2.5mm cable ring circuit and it runs 4 sockets on a 16a MCB front room is the same runs 3 sockets on a 16a MCB the kitchen is also the same which runs 5 double sockets on a 16a MCB and the oven is on a 16a MCB on a 6mm cable now heres my problem ive only got a single 2.5 mm cable on a 16a MCB and this is the only area i have not upgraded yet it runs through an inline 13a fuse which is in the house it runs washing machine and chest freezer i also have a circular saw induction motor which takes about 300w also got small bandsaw which takes about 100w and a pilar drill which takes about 100w now here comes the hard bit got a compressor which is 50 years old with an induction motor which can run up to 200 psi it draws 1500w but it kicks in at 22a so obviously the 16a MCB trips if i reset the 16a MCB compressor will fire up im considering putting 2x 2.5mm or 3 2.5mm cables as ive got about 50m of this cable left ovrer but if im better putting 6mm or 10mm through i dont mind buying some to go through the house and then an armoured 6mm or 10mm cable running on a 32a MCB to the garage the length from the meter cupboard to the outside wall is 34ft including going round corners ouside is about 38ft so i was considering leaving the washing machine and the freezer on 2.5mm and just running the 32a for the compressor and my saws so the garage can be isolated when not in use i only mention what i have in the house not to confuse anyone but i always run on the safe side and i dont want to takea spur off any of the rooms ive been a commercial fitter for 30 years and thats why i always go on the safe side only been in house 3.5 years been using saws to renovate now want to sort outside out so which is the
best option please
please excuse my grammer
David

 

[Monkeh]

please excuse my grammer

Really can't. That's pretty much unreadable.

Best option is to call a local electrician and have him advise you.

 

[ericmark]

There are times when two 2.5mm sq cables are used one common one is the feed to a kitchen distribution unit (grid switch) mainly as getting larger cables into the terminals is a problem. However with any cables in parallel one has to be very careful both cables are in fact connected. So I would not recommend doing that way.

A B16 MCB should take a start load of 22A without too much of a problem it can take 80A (5 x 16) before the magnet part should trip the device. So either well over 22A or it is drawing 22A for long enough to trip the thermal part of the trip. Looking on Fig 3.4 over 10,000 seconds.

To change the B16 MCB to a C (x10) or D (x20) will likely cure the problem but the first consideration is what is the loop impedance? The whole idea of the magnet part of the trip is should there be a short circuit then the trip will open within a fraction of a second. The thermal part of the trip is too slow to disconnect quick enough to prevent damage. Using simple ohms law if 320A is to flow at 230v then the impedance (AC name for resistance) needs to be less than 0.72 ohms. So step one is measure the loop impedance.

Now although you could either break the law or pay the LABC their fees to be safe you would need the use of a loop impedance meter which are not cheap around £350 to buy. And even then using the meter can cause danger including ionisation of the atmosphere. (i.e. Big Bang)

So I would suggest you get a scheme registered electrician to do the work as likely cheaper and safer than buying or hiring a meter.

As to the compressor it could well be the de-loading rather than plain start load which is a problem. There are many methods of allowing the motor to start and in the past I have needed to replace items like the de-load valve for it to start. I have also cheated and slacked the drive belts.

Likely there is some timer to allow the motor to get to speed before the load is applied often these are pneumatic and over time the hole allowing the air into the bellows has worn and so it needs adjusting.

There are so many methods used near impossible to guess what you have.

 

[meagain]

Thank you for your help, The motor in question used to be 415v 3 phase which used to be in the garage and then was converted to 240v 3 phase using capacitors, Then the motor will run on 240v on 3 phase. I can change the capacitors to ease the start on the motor on about 16amp kick in or alternatively i will have to buy a 3 horse 240v motor which is about £200 this would draw 2200kw with a start up of 16amp. The garage needs re-wiring anyway as there is no armored cable going outside

 

[jj4091]

Have you thought of looking at compressors on e-bay, could be easier & cheaper

 

[SimonH2]

First thing to do is buy a new keyboard - yours seems to have a duff return key The full stop also seems to be duff


Starting motors is often a problem - and I think that's why so much equipment has instructions specifying over generous supplies.

Personally I'd suggest your best bet is to have the garage/workshop supply upgraded with a decent sized cable - and fit a supplementary CU in the garage/workshop. At least, as I read that horrid block of text I get the impression this is outside.

Ideally you want sufficient difference between the breaker on the house end of the supply and the 32A on your RFC in the garage to provide discrimination. Or using different characteristics of breaker would be an alternative (eg a type C in the house and type B in the garage). That's outside my knowledge zone, so take the advice of professionals on that.

That motor isn't actually that big. There are many things you can do to help, good unloading until it's run up can help (look up soft start valves - some are electric, some just have a pip that doesn't pop out and shut off the flow until there's a certain pressure behind them). Though your motor probably isn't ideally set up and you might find a correctly sized single phase motor may be better. Or consider a spare motor (and caps) as a 3 phase converter (and rotary energy buffer) and DOL or star-delta start the existing motor in 3 phase.

 

[ban-all-sheds]

was converted to 240v 3 phase

139V per phase - that's an unusual supply.

240v motor which is about £200 this would draw 2200kw with a start up of 16amp.

That seems extraordinarily cheap for a motor of that size.

I'm a bit puzzled how it can draw 9167A when running but only 16A on startup.

You'll need to talk to the DNO about getting a supply that size laid in.

 

[ericmark]

Small motors often have 230v windings so in delta runs on 230v in star runs on 400v. However using capacitors to generate a three phase supply is a very old idea. Often a motor drives just a flywheel to produce the three phase and each motor added tends to reinforce the three phase.

Today the normal method is to us an inverter which will also give a soft start and in some cases even an output when full power is reached to operate the de-loading valve.

 

[bernardgreen]

A soft start invertor will help to reduce the voltage dip that will affect neighbours when the motor starts. Too many dips and complaints to the supply company ( DNO ) may result in a ban on the use of the motor.

 

[SimonH2]

was converted to 240v 3 phase

139V per phase - that's an unusual supply.

No, it's a 240V line-line supply, not at all uncommon, we've got such a supply here, and I suspect you have there. He didn't say it was a 3 phase supply running it, nor that it was balanced around a neutral point.
As Ericmark says, not at all uncommon for small motors to have 240V windings. If you are lucky enough to have that, then adding a capacitor to create some phase shift is enough to make many 3 phase motors run on 1 phase - though often with reduced capacity and poorer starting (depends on motor design and load characteristics).

In our garage we have a commercial 1-3phase converter. I got it when it was being "disposed of" from my last place - someone had bought this rather than run a new 3 phase supply about 10 feet along the wall !. It has an auto transformer to step up the voltage to 415, a stack of caps, and a rotary switch to select the amount of caps to match the load. It also has a voltage controlled switch to pull in more caps while a motor is starting, and drop them out again when the 3rd line voltage recovers. If the motor can run on 240V then the transformer can be omitted and the same principal applies.

I believe it's not uncommon to have one such converter in workshop, and run up a largish motor with a flywheel attached. The motor then acts as an induction generator and largely creates a fairly decent 3rd phase on it's own. Then the other machines can just be started DOL to this "3 phase" supply.
I was looking to create just such a setup a few years ago - to run a 3 1/2t car lift at my mates house (we take DIY very seriously ). Step up transformer (recused from scrap), large motor (ditto), flywheel off one of my old engines, and a manual start-delta starter to ease running up of the flywheel (also rescued from scrap). Then my mate fell out with his missus, got divorced, and we had no garage or outdoor space and the project got cancelled

If you have a lot og gear, it can be cheaper than buying 1 phase - 3 phase kit is almost always cheaper, and often has better starting characteristics.

A soft start invertor will help to reduce the voltage dip that will affect neighbours when the motor starts.

Ah yes, forgotten about that option. But rather than a soft-start, a regular inverter speed control might be better. 1P in/3P out converters are quite common and cheap now but only give 240V 3P out (which the OP already has his motor connected for) and don't step up to 415V. We have one in the garage to run the lathes and such. As you say, configure one with a "full speed" output to close a soft start valve and it should work very well.

BTW - several of the above options only give a 3 wire supply, so no neutral and nowhere to connect 240V controls to. Something to bear in mind.

 

[ban-all-sheds]

As Ericmark says, not at all uncommon for small motors to have 240V windings. If you are lucky enough to have that, then adding a capacitor to create some phase shift is enough to make many 3 phase motors run on 1 phase

But would you then describe the motor as having been converted to run on 240V 3-phase?

 

[SimonH2]

As Ericmark says, not at all uncommon for small motors to have 240V windings. If you are lucky enough to have that, then adding a capacitor to create some phase shift is enough to make many 3 phase motors run on 1 phase

But would you then describe the motor as having been converted to run on 240V 3-phase?

I probably wouldn't, but it is a fair description of how it's running - so I wouldn't say it's wrong either.

 

[ericmark]

Years ago DC motors were used everywhere when speed control was required however today with inverters standard three phase motors have replaced these DC motors being cheaper. So for motors the size required for conveyers and like many more are being made and the price drops as a result. The same with the inverters these have also dropped in price. It is cheaper to get a small single phase to three phase inverter than a three phase to three phase inverter so the idea of having 230 volt windings is again becoming popular.

I realise for a compressor speed control is not required but still inverters may be best option.

A softstart does not convert to 3 phase where often an inverter does. Softstarts often have a bypass contactor once running and with a generated three phase not sure a softstart would work.

 

[SimonH2]

Of course, in the best tradition of forums, it's drifting a bit off topic ...
The "simplest" option for the OP if it's practical would be to simply upgrade his supply to match what his compressor needs. I strongly suspect that by the time he gets to a "standard" Type B 32A breaker he'll be OK.

That seems to be the approach taken by most equipment manufacturers - pay little attention to controlling start-up demands, and just require an oversized supply.

 

[Taylortwocities]

please excuse my grammer

Really can't. That's pretty much unreadable.

Best option is to call a local electrician and have him advise you.

Good grief.

Dear meagain.

Here is a bit of quick advice. Don't take it personally, it may help you get more assistance next time.

Please break up your writing into readable sized chunks. They are called sentences. Try speaking what you have written out loud. Put a full stop in the text every time you have to take a breath.

Now and again, put in an extra line space. The best time is usually when you start to talk about something slightly different. This is called a paragraph.

It makes your writing easier to read and understand. It's something like bite sized chunks of food instead of somebody pouring a bucket of spaghetti down your throat in one go and expecting you to eat it.

You see, I would have helped answer your query, but I can't be bothered to read it the way you have deluged it.

Again, nothing personal. I'm just trying to help you.