3ph run to garage.

There are many ways to start a three phase motor.
1) Direct on line.
2) Star/Delta
3) Auto transformer
4) Resistor
5) Soft start
6) Inverter
The starting method alters how fast it starts, what start current it takes, and torque on the shaft, your questions will have different answers depending on start method, also with a compressor we look as de-loading, again different methods are used.

I had a small compressor at work part of a batching plant, it would blow the fuses, we had done all the normally tests and could find no electrical fault, the fitters could not find a mechanical fault, so since I had electrical and mechanical training I was sent to watch it, to see if I could find the fault, it ran all morning no problem, then not much concrete needed in afternoon so it was idle for a long time, then I saw the drive pulley turn a little, and it clicked as to fault, the star/delta starter in star opened a air solenoid which if there was air in the tank opened the inlet valve so the compressor has very little load, however the exhaust valve was leaking, so it would turn the crank until bottom dead centre, then fill cylinder with air, as a result the solenoid could not open the inlet valve as too much pressure, so compressor trying to start on load.

The cure was a normally open solenoid instead of normally closed, and wired to delta so all the time the compressor was idle the inlet valve was open, so no air pressure could build up, what was really annoying was rather than a pat on the back from fitters at saving them stripping down the compressor and lapping in valves, they said there you are we told you it was an electric problem.

However this interlinking between the starting system and compressor makes it hard to alter start methods, and also hard to work out start current, in the main it was a clip on ammeter and actually measure, as volt drop can alter time needed to start, in latter years there was a move to screw compressors which have a completely different set of problems.

But it is a case fools rush in where angles fear to tread, so on a forum I would not attempt to say how to wire a compressor be it single or three phase, there are so many variables. I don't think I have seen direct on line used with compressors, but seen most of the other options.

Hi Eric,

It not my field of expertise for sure, I'm a gas heating engineer but I can't help but think this is being over thought... The incoming supply can be seen as in the photo, the spec for the compressor in posted above, just need to join the two together with suitable sized cable, the loads are as per the spec, the distance in at tops 40m. The compressor has WIS built in, the electric company ask for an isolator switch by the meter.

Clarke SE36C270 (WIS) 30cfm 270Litre 7.5HP Industrial Air Compressor (400V)

The compressor is sold in the 100's probably 1000's I can't see that it would present such a problem for buyers to have them connected to a 3ph supply.

I'm happy to go over the top with cable size which gives the option of adding another piece of machinery at a latter date even if it's not used.

Cheers,

db.

rekab6969 said:

Clarke SE36C270 (WIS) 30cfm 270Litre 7.5HP Industrial Air Compressor (400V)

The compressor is sold in the 100's probably 1000's I can't see that it would present such a problem for buyers to have them connected to a 3ph supply.

I'm happy to go over the top with cable size which gives the option of adding another piece of machinery at a latter date even if it's not used.

Cheers,

db.

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Yeah as said compressors [and for that matter fluid pumps] are a strange breed and I for one will always install larger cable and control gear than is required to allow for future problems. 7.5HP will be 11A, I will supply that with with a minimum of D20 MCB, if the Z readings don't allow it I'll go to C25 or B32 MCB and fit a 40A contactor with appropriate sized overload to cover the 11A such as: https://uk.rs-online.com/web/p/cont...zy8EM9m0zJFWPqMV77kaAihpEALw_wcB&gclsrc=aw.ds

or a manual motor starter such as: https://uk.rs-online.com/web/p/motor-protection-circuit-breakers/3313668/

But all of this aside I don't know how much of this is already included on that particular machine.

EDIT: Ah the pictures have come up now, It looks like all of the auto and protection stuff is fitted.

Just need to fit a suitably sized MCB.

rekab6969 said:

as the compressor runs total 30amp which is split into three I calculate the cable size on 10amp not the full 30amp..?

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Yes the running current will be around 10A, but you have to size the cable based on the size of the upstream protection - so if (say) a 32A MCB the cable must be sized to that. For this type of load, the upstream protection must be significantly higher than the 10A or it will keep tripping when the compressor starts.

Multicore 90°C Armoured Thermosetting Insulated Cables

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Possibly not the right table to be looking at. To start with some cables are thermosetting, some are thermoplastic. And you can only use a 90˚C rated cable at full load if the equipment and terminations at both end are rated for 90˚ (most aren't). This is where knowing what you are looking at comes in

Cable Size: 1.5mm²

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Without doing any calculations, I strongly suspect that would have excessive voltage drop, and I think I can be fairly certain will not be adequate for a 32A upstream breaker.

So if I run say 1 6mm 5x core happy days...??

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As previously mentioned, would need to do a number of calculations to check. But I'd be surprised if 6mm² SWA wouldn't do everything you wanted.

ericmark said:

There are many ways to start a three phase motor.

Click to expand...

In this case, the manual says that it's star-delta with the starter already wired in - it's on the end of a bit of flexible conduit.

I had a small compressor at work part of a batching plant, it would blow the fuses, we had done all the normally tests and could find no electrical fault, the fitters could not find a mechanical fault, so since I had electrical and mechanical training I was sent to watch it, to see if I could find the fault, it ran all morning no problem, then not much concrete needed in afternoon so it was idle for a long time, then I saw the drive pulley turn a little, and it clicked as to fault, the star/delta starter in star opened a air solenoid which if there was air in the tank opened the inlet valve so the compressor has very little load, however the exhaust valve was leaking, so it would turn the crank until bottom dead centre, then fill cylinder with air, as a result the solenoid could not open the inlet valve as too much pressure, so compressor trying to start on load.

Click to expand...

As it happens my mates twin compressor had a similar arrangement for the second compressor. The first to start was unloaded by the mechanical pressure switch, the second one was unloaded by a solenoid valve that was powered while the first was running but the timer hadn't run out to start the second one. At some point the solenoid failed and so the second unit could not start unless it had been idle for long enough for the pressure to bleed down.
Instead of replacing it, we just fitted a tee with a little ball in it (sold for non-return applications in multi-tank LPG installations) so the pressure switch unloaded both. When the first unit started, instead of blowing air through the lines and pressuring the feed line from the second unit, it just blew the ball across and blocked the port. With the dump valve in the pressure switch open, it would allow air from both units to vent.

Shopping list..

16mm double insulated tails x4.
10mm earth tail.
40amp external isolator switch external to the meter box.
40mt of 6mm 5x core swa.
swa gland kits.
swa cleats.
4 module wide for 3 phases and neutral. 32amp.

Looking ok...??

Cheers,

db.

No - your list has a number of errors and omissions.

You probably don't need 40m of the 6mm² cable. From the garage to the compressor can probably be smaller.
You'll need at least one more service connector block (often just referred to as a Henley block, but that's just one make).
You can use smaller tails - if you can get them.
Your "isolator" switch needs to be either an MCB or a switch-fuse - rating according to what the calculations say.
And what is "4 module wide for 3 phases and neutral. 32amp" ?
You'll need an isolator for the compressor end of the cable, and a means of connecting between that and the starter for the compressor.

Before you start shopping, you need a competent electrician to design the installation (which includes calculating the correct combination of cable size and circuit protection). You cannot design even such a basic circuit by picking random cables from an online calculator, and guesswork based on incomplete understanding of what people are telling you on a forum.
Having designed the installation, he will be able to come up with a shopping list of the right materials.

Shopping list..

16mm double insulated tails x4. Smaller if available. Tick..

10mm earth tail, one size below above ? Tick.

40amp external isolator switch external to the meter box. requires MCB. Ok. Size to be confirmed. Tick.

40mt of 6mm 5x core swa. Ah yes, 20m to garage then 20 to compressor.. Tick, tick.


4 module wide for 3 phases and neutral. 32amp. From earlier reply by Ericmark. ???



[QUOTE="ericmark.
Next way is a poor description we normally refer to module, but I am reasonable sure you will need at least 4 module wide for 3 phases and neutral, and will not be a consumer unit, as they are single phase not three phase, you may be able to utilise the box, and use the DIN rail mount, but three phase normally looks like this OK this one has an isolator in it not a MCB or RCBO but it is not called a consumer unit.


Cheers,

db.

operating and maintenance instructions said:

On automatic machines 10 H.P. & above, an Auto Star Delta (ASD) starter is fitted, ready to be mounted to a wall or panel etc. It is IMPORTANT to note that NO ADJUSTMENTS should be made to this component. The ASD should be connected to a 3-phase supply by a qualified electrician.

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operating and maintenance instructions said:

Suggested fuse ratings 7.5 kW, 10 HP Star/delta 25 amp

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The PDF is here

Until you gave make and model which allowed me to down load the manual I did not know for certain it needed 25 amp supply, it says fuse, and motor rated fuses will give quite a lot over 25 amp on start, but with 25 amp at 40 meters, even 2.5 mm² with a B25 would do, however we don't actually know the inrush and although they say a 25 amp fuse, it is likely it will need a C25 or even D25 MCB, and step one is measure the loop impedance.

It will likely work with a C25 and 4 mm² cable but personally I would fit 6 mm². It does state

operating and maintenance instructions said:

If using a circuit breaker, ensure it is motor rated and of sufficient size to allow for motor starting.

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however they don't actually say which so I would expect likely needs D25 MCB plus it states

operating and maintenance instructions said:

Compressors should be connected to mains electricity supply via an earth leakage protection device (RCD).

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So a D25 will need an ELI of 0.437Ω with in coming of 0.35Ω then you will need 6 mm² cable which will allow around 54 meters of cable, 4 mm² would only allow 35 meters of cable, but this is assuming the Ze is 0.35Ω if the Ze is not 0.35Ω then my calculations are not valid.

What you need to remember is domestic should not really have three phase supplies, the idea is all equipment in domestic is type tested, and you can't use type tested equipment with 3 phase, so the location of the compressor must be kept locked and it is not considered part of the house.

First of all, although you have a polyphase meter, you still don’t actually have a three phase supply available to use, and once you do it would be wise to prove that it is a proper three phase supply with a rotation tester before you get too far into the project.

Having three cutout fuses does not automatically mean that it is actually a three phase supply.

Secondly, you’re going to need some sort of steel enclosure to gland the armoured cable into, as well as an isolator and a over current protective device at the supply end somewhere near the meter. Whether this is all one unit, or separates is up to you.

Personally for this type of supply I would install BS88 or BS1361 fuses rather than a circuit breaker at the supply end as it provides better discrimination, it is less susceptible to false operation, and it is easier to achieve the required Zs value at the load end.

Morning Eric,

Thank you for your info..


RF lighting.... wise words for sure... Igloo needed to replace the meter, had a panic attack when they saw it was 3ph and sent in the EDF who said it was 3ph but changing meters wasn't their job.

Igloo then spent ages compiling a 3ph contractor to do their 3ph work, guy changed the meter was from their new 3ph team but as with the other two guys no one tested it to see if it was actually 3ph..

I'll pop in to EDF this morning and see if they can check it for me. Local and a good bunch of guys..

Thanks to one and all,

db.

1st prize goes to R F Lighting. Not true 3ph coming in after a lot of chasing to get it checked... 2x 1ph and one doubled up on the same phase.! Anyone want to buy a 3ph compressor..


It's been very informative and I have learnt a lot, a big THANKS to everyone who has taken the time to reply, all good advice too.

Update... UK power quoted £22k to upgrade me to true 3ph... This is worse case sinario and they are having a re think...

RF Lighting said:

... Personally for this type of supply I would install BS88 or BS1361 fuses rather than a circuit breaker at the supply end as it provides better discrimination, it is less susceptible to false operation, and it is easier to achieve the required Zs value at the load end.

Click to expand...

Just noticed the above from a few weeks back....

I agree with your first two reasons (for preferring a fuse to an MCB), but I don't really understand the last one (it sounds 'back to front' to me!) - can you explain/clarify?

Kind Regards, John

JohnW2 said:

Just noticed the above from a few weeks back....

I agree with your first two reasons (for preferring a fuse to an MCB), but I don't really understand the last one (it sounds 'back to front' to me!) - can you explain/clarify?

Kind Regards, John

Click to expand...

I don’t have my books with me at the moment so don’t know exactly how accurate these figures are. They are just what I found online, but the theory is the same.

I’m using an example of a 60A submain so a 5 second disconnection time. If you protect this with a BS 1361 60A fuse the max permitted Zs is 0.53Ω. If you were to protect the same submain with a 63A type C BSEN 60898 circuit breaker, you would need a Zs of 0.28Ω. Both devices offer the same level of over current protection, and a similar resistance to nuisance operation due to spikey inductive load current inrush but it is much easier to achieve the required Zs for the BS 1361 fuse.

The difference is explained when you look at the fuse graphs, as the operating current for a fuse is linear, and a circuit breaker is not as you need to operate the magnetic part of the circuit breaker to achieve the required disconnection time.

I encountered this exact situation a couple of years ago. A building I look after has sub let a small office to a taxi company. The taxi office operates 24 hours a day, and does not have access to the main building which is only open during normal working hours Monday to Friday.

We installed a local DB in the taxi office so if anything trips they can reset it themselves. Part of the design was to make sure that a fault in the office was unlikely to take out the submain as losing the computer, phone and taxi radio for an extended period would not be good.

The supply to main building is a standard 100A TP&N TN-S supply and the Ze exceeds the Zs required for a type C circuit breaker, but was within limits for a BS 1361 fuse.