32A single phase output from 3 x 16A three phase supply. Is there a way?

[Mark42]

I have a workshop in a remote location which I rent out. The incoming supply is 3 x 16A three phase on 180m of 16mm SWA.

An upcoming customer has a machine which needs a 32A single phase supply. (Actual load about 25A.)

The energy is there, but it's split across the three phases. Is there some magic way - perhaps some kind of transformer - of making this work?

 

[Julie.]

I have a workshop in a remote location which I rent out. The incoming supply is 3 x 16A three phase on 180m of 16mm SWA.

An upcoming customer has a machine which needs a 32A single phase supply. (Actual load about 25A.)

The energy is there, but it's split across the three phases. Is there some magic way - perhaps some kind of transformer - of making this work?

Yes, there are two ways, the first is just a single phase transformer 400/230V placed across two phases, should just about do 25A (should get 28A ignoring losses).

The second is a special 3 phase to 1 phase transformer, there are a few winding connections that can do it, but it would be a custom build I guess.

Even a single phase transformer would be expensive though


(Of course, you could also have a motor-gen setup, but that's going to be even more expensive)

 

[Lucien Nunes]

Agree with Julie, simplest is to use two lines as a 400V single-phase supply and step down from 400-230. A 6kVA single-phase isolating transformer might set you back £500-1000. Depending on the nature of the equipment, it might be possible to use an autotransformer which will be smaller and cheaper, although neither output terminal would then be neutral and unless the machine is designed with that in mind, with double-pole isolation and suitable fault protection for internal circuits, it probably wouldn't be acceptable.

 

[Vortigern]

Cheaper way would be to get a tenant who doesn't need an upgrade. Or...get them to pay for upgrade. What is usually done when I upgrade for new tenants, they pay on commercial letting site.

 

[Lucien Nunes]

Two more thoughts. One is that it would be undesirable to run 3-phase motor loads (even very small ones) at the end of the submain when two phases are loaded up to what I assume is a VD-imposed limit while the other phase is light. The voltage assymmetry will cause increased stator current and heating.

If there are no 3-phase loads to power, then depending on the machine, if it is tolerant of low input voltage it might be possible to convert the SWA submain to two 2-wire single-phase circuits. One loaded to the 16A limit to serve lighting and general loads with its original VD spec, and the other uprated to 25A with higher VD permitted for the single machine load.

 

[pc1966]

Is it feasible simply to put in another cable?

I.e. is the supply capable of 32A and the current cable is in duct, etc, so pulling through another is manageable without major digging effort, etc.

 

[Julie.]

Two more thoughts. One is that it would be undesirable to run 3-phase motor loads (even very small ones) at the end of the submain when two phases are loaded up to what I assume is a VD-imposed limit while the other phase is light. The voltage assymmetry will cause increased stator current and heating.

If there are no 3-phase loads to power, then depending on the machine, if it is tolerant of low input voltage it might be possible to convert the SWA submain to two 2-wire single-phase circuits. One loaded to the 16A limit to serve lighting and general loads with its original VD spec, and the other uprated to 25A with higher VD permitted for the single machine load.

Good point about the voltage unbalance, applicable in all cases of course, but you're right this would be a particularly severe case with two lines right up to their limit, and the other potentially fairly unloaded.

I would rather double up the four conductors for a higher current rating, two for the Neutral and two for the line, but it all depends if the rating limit is due to cable, or fuse/supply limitations.

The former would be easier, no good if you have 3ph loads, or fuse/supply limits.

In that case it is either a no-go, or you need to use all three phases equally, so a three phase to single phase transformer would be a must, you could either use a Scott connection, or ideally a Le Blanc, both giving proper 2 phase - where you need to use both lines, ignoring the 2 phase neutral, so voltage wise you need a 163V ph-n supply give 230V ph-ph (1.41 factor).

The most simple solution of three 400/115V transformers primaries in delta, two secondaries in open vee, and the third reversed on one end of the open vee; wouldn't really be suitable as it presents quite an unbalanced load between phases. Not quite as bad as just a single line-line transformer, but you could get the full 32A.

 

[pc1966]

A quick check on the Doncaster calculator has 25mm OK for a single-phase 32A run of 185m, so @Julie. suggestion of simply converting to a single-phase run with 2*16mm for each of the L & N would be reasonable.

Question is do they need 3-phase anyway for something else?

 

[marconi]

Mark42: What is the machine? I'd want to know more about it and its electrical design, its current/time plot and how often it is used. For example does it only need 32A to start or does it use 32A when running and loaded or up to 32A when running and dependent on how heavily loaded?

 

[Mike Johnson]

How about a 3P input to 1P output Variable Frequency Drive? I think Invertek make a few: Variable Frequency Drive Manufacturer | Variable Speed Drives | AC Drives | Invertek Drives - https://www.invertekdrives.com

 

[pc1966]

A quick check of the cable resistance values (Prysmain BS5467 data sheet) has R1 for 16mm as 1.15 mOhm/m and R2 as 3.1 mOhm/m, paralleling the conductors leads to R1+R2 of about 3.68 mOhm/m so for the 185m run around 0.68 Ohms.

For a 16A MCB that is usable to D-curve.

But for a 32A MCB that already (even before looking at the supply DB Zs) puts the supply MCB limit at a B-curve, and that might be too trip-happy on the machine's start up current.

So you might be looking at a 100mA delay RCD to meet sub-main disconnection times (unless you look at a 32A or 40A switched-fuse, etc)

 

[Mike Johnson]

We seem to have lost our way on this thread, the OP wants 32amps single phase from a 16amp/P three phase supply, or is it me that's lost the plot?

 

[pc1966]

We seem to have lost our way on this thread, the OP wants 32amps single phase from a 16amp/P three phase supply, or is it me that's lost the plot?

Well, the issue was feeding a 32A single-phase load from the end of a long 3-phase cable that only meets VD, etc, for 16A.

• Converting 3P => 1P via transformer(s) of VFD is one route, but not cheap,
• Reconfiguring 4C 16mm to avoid a new sub-main is another possibility, but loss of 3P options
• Or the simple, obvious, but potentially expensive iv civil work needed to put in a dedicated 1P 31A feed.
• Possible, but maybe not feasible for cost, difficultly & warranty reasons, etc, would be to convert the load to use a 3P motor, etc.

Any other options?

 

[Mike Johnson]

Post 10?

 

[pc1966]

Post 10?

Was first in list, 3P->1P conversion.

I did look at the site, but it sucks in terms of finding stuff. They did list a 1P -> 1P VFD but I did not see 3P -> 1P mentioned. Do you know of a part?

I know some "3 phase" UPS are really 3P charger, 1P inverter (you can tell as they tell you on bypass the L1 needs 63A instead of the 25A MCB for the L2/L3), but they are hardly a cheep option.

 

[Mike Johnson]

When I bought from Invertek the site was much easier to navigate they seem to have retro graded the site somewhat, used to have a list down the left side of all the drives they had available, I can't find a link to even the one I have let alone a 3P->1P conversion.

 

[Mark42]

Hi, OP here - sorry for the disappearing act, I've been on 16 Hr days building even more sheds for an upcoming job. All great ideas, will digest and respond over the weekend. Thanks all.

 

[Mark42]

Thanks again for all the advice.

The machine is some kind of funky super-powerful x-ray machine for scanning vehicles. I don't have the full specs but I have to assume it does need constant current, not just inrush. However, I doubt it would be energised and generating radiation for very long periods.

The rental is only for a couple of weeks for a research project (that’s what I offer here), so there’s no question of digging trenches or otherwise messing with the direct-buried underground infrastructure.

The solution is so obvious I missed it! As several on here said, just temporarily swap a few cables and breakers at each end to single-phase the DB on 2 x 16mm paralleled conductors.

Physically lock off any three-phase breakers in the building’s DB (supplying only a few 5P sockets, and two small roller shutters which can be opened manually.)

Sorted in an hour at zero expense and at least another hundred quid a day on the rental for bespoke 'supply upgrade' ?

 

[Lucien Nunes]

Completely OT but curiously relevant. After WW2 there was an upsurge in cases of TB and the (then new) NHS launched a 'mass radiography' programme, sending dozens of mobile chest X-Ray screening vans out to hotspots (e.g. industrial towns) to locate cases before people developed symptoms. The X-Ray machines needed a 30A single-phase supply for which they re-purposed diesel generators from redundant Type 14 radar sets. These were actually 3-phase generators but rated to deliver full load on any one phase. They decommissioned the 3-phase outlets and partially rewired the control panels, and off they went around Britain.