What size generator.

Hi folks,

Iam thinking of buying an old Colchester triumph gap bed lathe with a vfd.

However my main workshop is remote from main electricity,up till now i have been successful in using a large power inverter for the smaller machines.

The lathe has a 5hp 3 phase motor,what size of single phase generator would i need to power the lathe? I have considered changing the motor for single phase but don't know how successful that would be. I have been told the benefit of a vfd are considerable. Thanks. Ronnie.

Think here that a 5HP motor, partly loaded, on single phase domestic supply might well empty your mains supply at start up! Lights go dim and possibly the main supply fuse will be activated if the wife has the oven and a few other loads in operation at the same time. My large stick welder or 2HP compressor both noticeably dim the lights momentarily at switch on.

You would likely need a rolled steel magnet framed generator, not a rotating field genny, to avoid a stall situation at start up. To be safe, I would hazard a guess at ten times running current? That is why most larger motors are three phase. I would guess that a three phase motor would start with much less overload capcity but I don't know how much less.

Soft start, such as a VFD could provide, is a definite advantage. I would suggest borrowing, or even hiring, a generator to check out suitability, after seeking advice from a generator hire company.

Given that you are looking for a generator and you have a 3 phase lathe. Why a single phase generator?
A 3 phase genny will be more expensive than single phase, but you won't have the additional cost/complexity of changing the motor or adding a VFD. I echo the previous poster's comments about speaking to a hire company for advise.

Once above (say) 3kW single phase fed VFDs are much less common than 3 phase fed. There can even be bargains on auction sites for 3 phase fed VFDs (I guess less people can make use of them).

Having used machinery in the field I had a 10Kva and 27Kva generators to use. The 27Kva gen. was able to supply 3 phase and fed battery charging facility and also a Lathe and a power bench drill and also a Baby burco to provide constant hot water. Also heating bars in the machinery vehicle. The 10Kva was used just by one ECE lorry as it had air conditioning for electronic work.

In consequence I would not consider anything under 7.5Kva. Bear in mind the running costs, the 27Kva used 9 liters of diesel an hour under a middle load. the 10Kva about 5 liters an hour. You do have the use of the power for other purposes if needed as they are mobile on a trailer. If you are in need then look to ex military gear. Both gen. will give 3 phase, plus 13A sockets on the output board. There was no complicated wiring and phase adjustment was done by swopping plug in sockets colour marked

Clive

How remote is remote? It's going to cost a lot to get a generator, unless you get an old dog that needs lots of attention. If you dig your own trench then cable isn't that expensive. 6 sqmm twin and earth is about £1.50/metre, but check the voltage drop for whatever length you have