Discuss Sizing and renting a 3 phase generator in Wales (in the UK) in the DIY Electrical Advice area at ElectriciansForums.net

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Hello everyone, I really hope this isn't an idiot question but boy, I'm out of my depth! I've spent about 3 weeks (not solid mind) trying to work out how to size a generator to power a lathe for a test.

Situation is, I'm buying a house in Wales (UK) with a restored lathe in it. It's 3 phase, but there's no 3 phase at the property. So just for testing purposes, approx 2 hours of running, I need to power it up. I've settled on renting a generator because it seemed like the easiest option, but I can't find enough information to be able to rent the right one, and when I'm put through to someone who can help at the rental place, I can't answer their questions because the info isn't there. They want to know the kW and kVA of the machine, but that's not in the manual or stamped on the lathe. Also wanted to know what size of plug it was.

The lathe is a Colchester Triumph from the 60's, the power plate on it states: Volts: 400/440, Cycles: 50, Amps 5-10 and that there are two motors, the main one is 5hp and the lube pump is 0.05hp, so total hp is 5.05hp.

I've rattled through all sorts of calculators and THINK I've got it right but need a sense check. I don't want to blow this thing up! I think 5.05hp = 4kW, which = 5kVA. When I then look on the rental site they rent one that's 15kVA but the voltage is stated as 110/230/415V, none of those voltages are 400/440V. I'm confused, it feels like it should all be an exact science so this worries me.

Can anyone help me please? Apologies if I broke any rules (I did read them), like I say, I am way out of my depth with electrical matters but respect them and the life changing/ending mistakes numpties like me can make!

Longer term I'll get 3 phase installed, there's a pole nearby, or buy a converter.
 
TL;DR
I need to rent a generator for a 3 phase 5.05hp lathe but I can't work out which one.

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The 15kva will be fine

infact, anything rated at 8kva or more and has a 400v output (red plug) will be ok.
dont be put off by things you read online about having to rate generator at 2 times the main motor load etc.
the lathe you are powering starts the motor with no load on it so the generator will not need to be massively over specified to run this machine.
 
Have edited post to give a little insight as to why it is ok
 
Have edited post to give a little insight as to why it is ok
Thanks James! That's bang on my concerns, I was looking at a 10hp VFD for a bit because of those posts, but wow they're pricey. Ultimately I'll either get 3 phase into the building because there's a pole nearby, or if that's a non starter, I'll get a converter or buy a single phase motor. I'm not a heavy user, probably never will be, but this is a thing of beauty.

I was a little concerned as the generator talks about voltages up to 415v but the lathe is 440v, but then I haven't found any 440v generators anyway. It's Stuart Power that I'm talking to.
 
When that machine was built, the nominal uk voltage was 240v single phase and 415v 3 phase.

nominal voltage now is 230v single phase and 400v 3 phase.

in reality, nothing has actually changed because of the tolerance allowed in the electricity supply has always allowed both of the above voltages.

a vfd will not help you with this machine.
a vfd with a single phase input, will supply a 3phase 230v motor, NOT a 3 phase 400v motor.
some motors are dual voltage and can be configured for either but the nameplate on the machine suggests this is not the case here.

A single phase motor or a rotary phase converter is probably your most cost effective solution.
(think motor and generator) both in the same case.
they are not normally a true motor generator but have the same effect.
 
When that machine was built, the nominal uk voltage was 240v single phase and 415v 3 phase.

nominal voltage now is 230v single phase and 400v 3 phase.

in reality, nothing has actually changed because of the tolerance allowed in the electricity supply has always allowed both of the above voltages.

a vfd will not help you with this machine.
a vfd with a single phase input, will supply a 3phase 230v motor, NOT a 3 phase 400v motor.
some motors are dual voltage and can be configured for either but the nameplate on the machine suggests this is not the case here.

A single phase motor or a rotary phase converter is probably your most cost effective solution.
(think motor and generator) both in the same case.
they are not normally a true motor generator but have the same effect.
Ah! Ok, that's useful info, I'd been expecting to use a VFD, but will drop that route. I could get a Clarke PC60 converter, but at £400 just to test something for a couple of hours, I'm thinking the rental is the best way, and then follow that up with 3 phase to the building from the pole nearby.

Thanks for all you help James!
 
Some good advice there from @James. I feel your frustration with the initial enquiries so I'll expand a few points for the sake of completeness.

110/230/415V, none of those voltages are 400/440V.
The voltages quoted for the generator are options: you can have 110V or 230V or 415V (probably more like 400V) according to which flavour of socket you use. In contrast, the voltages on the lathe data plate are minimum and maximum of its working range, i.e. anywhere between 400 and 440 is OK but you can't select another range e.g. 230-250. As the generator 415V output option lies in the range quoted for the lathe, all is well. The lack of a 230V option, as James mentions, is the reason a single-to-3-phase VSD probably won't work. Technically to rule that out 100% one would need to check the plate on the motor rather than on the lathe but it is extremely unlikely to be suitable.

I think 5.05hp = 4kW, which = 5kVA.

There's some practical truth in that but you shouldn't equate those figures directly as kVA and kW are different things.

Horsepower and watts are different size units of the same quantity (power). 1hp = 746W. Therefore it is true to say that 5hp = 3.7kW.
This is the rated mechanical output power of the motor, i.e. the 'size' of motor. These days motors are sized in kW but that still refers to the mechanical output; to order a replacement you would ask for a 3.7kW motor. This is a rated maximum that the motor can deliver continuously; in typical use it might only be called upon to deliver 1hp or less. In the context of a 15" x 30" lathe, it takes a hefty job to use 5hp unless you have the tool in backwards. After half an hour using 5hp chewing up a 50kg billet of stainless, you will feel the heat radiating from the machine and job and the coolant sump will be steaming.

The electrical power consumed by the motor is greater than the mechanical output because it is not 100% efficient. If the efficiency is 90% then the electrical load will be 3.7/0.9=4.2kW. That is the 'real power' consumed that would register on your electricity bill.

The current that flows through the motor winding is actually higher than would be calculated for an ideal resistive load of 4.2kW. This is because the motor is slightly inductive, which puts the current and voltage out of phase with one another, resulting in a certain fraction of excess current flowing that transfers no power (called wattless or reactive current). The ratio of the 'apparent power' (in kVA calculated as the product of volts and amps) to the 'real power' (the 4.2kW actually consumed) is called the power factor, which is somewhat less than unity for a fully-loaded motor and significantly below unity when running light. If we generously assume the pf is 0.9 at full load, the apparent power consumed is 4.2/0.9= 4.6kVA. This is what the generator's alternator windings will need to carry, in order to get 5hp out of the motor.

There's more to talk about but this post is getting long!
 
Some good advice there from @James. I feel your frustration with the initial enquiries so I'll expand a few points for the sake of completeness.


The voltages quoted for the generator are options: you can have 110V or 230V or 415V (probably more like 400V) according to which flavour of socket you use. In contrast, the voltages on the lathe data plate are minimum and maximum of its working range, i.e. anywhere between 400 and 440 is OK but you can't select another range e.g. 230-250. As the generator 415V output option lies in the range quoted for the lathe, all is well. The lack of a 230V option, as James mentions, is the reason a single-to-3-phase VSD probably won't work. Technically to rule that out 100% one would need to check the plate on the motor rather than on the lathe but it is extremely unlikely to be suitable.



There's some practical truth in that but you shouldn't equate those figures directly as kVA and kW are different things.

Horsepower and watts are different size units of the same quantity (power). 1hp = 746W. Therefore it is true to say that 5hp = 3.7kW.
This is the rated mechanical output power of the motor, i.e. the 'size' of motor. These days motors are sized in kW but that still refers to the mechanical output; to order a replacement you would ask for a 3.7kW motor. This is a rated maximum that the motor can deliver continuously; in typical use it might only be called upon to deliver 1hp or less. In the context of a 15" x 30" lathe, it takes a hefty job to use 5hp unless you have the tool in backwards. After half an hour using 5hp chewing up a 50kg billet of stainless, you will feel the heat radiating from the machine and job and the coolant sump will be steaming.

The electrical power consumed by the motor is greater than the mechanical output because it is not 100% efficient. If the efficiency is 90% then the electrical load will be 3.7/0.9=4.2kW. That is the 'real power' consumed that would register on your electricity bill.

The current that flows through the motor winding is actually higher than would be calculated for an ideal resistive load of 4.2kW. This is because the motor is slightly inductive, which puts the current and voltage out of phase with one another, resulting in a certain fraction of excess current flowing that transfers no power (called wattless or reactive current). The ratio of the 'apparent power' (in kVA calculated as the product of volts and amps) to the 'real power' (the 4.2kW actually consumed) is called the power factor, which is somewhat less than unity for a fully-loaded motor and significantly below unity when running light. If we generously assume the pf is 0.9 at full load, the apparent power consumed is 4.2/0.9= 4.6kVA. This is what the generator's alternator windings will need to carry, in order to get 5hp out of the motor.

There's more to talk about but this post is getting long!
Thanks Lucien, great info. I've learned a lot from a single post in this forum from knowledgable and helpful people, such a great resource.

I'm leaning towards buying a Clarke PC60 (Clarke PC 60 - 5.5hp Phase Converter PC60 - https://www.sitebox.ltd.uk/clarke-pc-60-55hp-phase-converter-pc60-oclarke_6012795), a 5.5hp phase converter as it's portable (the generators I can find for hire all require towing, so now I'm into renting a van as well as the generator..). I've seen a couple of videos of people running similar machines on them, and this is only for a test. So I'll probably buy the converter, use it for an hour or so, then sell it on once the 3 phase is in the building.

Once again, thanks for all the help!
 
As you've probably discovered, these units are something of a compromise. They deliver somewhat lopsided 3-phase which can result in reduced motor performance, depending on how well the load selector matches the actual motor in use.

You will need to change the plug on the lathe as the PC60 has a 4-pin output. There is no particular reason to think that the neutral pin in the existing plug has been used; often a 5-pin plug is fitted for compatibility. But if there is a digital readout or other 230V single-phase load piggybacked on the motor supply using one line and the neutral, that must be disconnected. According to the lathe circuit diagram, the contactor supply is connected between L1 and L3 as required by the PC60. IME this cannot be taken for granted and if someone has reversed L2 & L3 in the junction box for example (to correct the rotation) the machine will not operate until you rearrange the lines. The worklight appears to use L2 and probably won't illuminate with the machine at rest.
 
As you've probably discovered, these units are something of a compromise. They deliver somewhat lopsided 3-phase which can result in reduced motor performance, depending on how well the load selector matches the actual motor in use.

You will need to change the plug on the lathe as the PC60 has a 4-pin output. There is no particular reason to think that the neutral pin in the existing plug has been used; often a 5-pin plug is fitted for compatibility. But if there is a digital readout or other 230V single-phase load piggybacked on the motor supply using one line and the neutral, that must be disconnected. According to the lathe circuit diagram, the contactor supply is connected between L1 and L3 as required by the PC60. IME this cannot be taken for granted and if someone has reversed L2 & L3 in the junction box for example (to correct the rotation) the machine will not operate until you rearrange the lines. The worklight appears to use L2 and probably won't illuminate with the machine at rest.
Thanks Lucien! Saved me buying a PC60 that also won't work, most likely! Powering this lathe for a short test is getting beyond a joke, seems my only route is renting a generator that I'll also need to rent a van to tow it with, and even then risk getting it to site to find out whatever the next thing is that prevents it being right for this application. I'm leaning towards letting this lathe go, getting into the property and buying a more known entity and having the power needs met by a qualified electrician, matched to a machine I own and is tested. Might even opt for a Myford 7 with single phase.

Thanks for your help @Lucien Nunes and @James!
 
Depending on your budget, this might be an option.
it is likley to be a lot less than having 3 phase installed and will double up as a standby generator when you Have a power cut.

 
p.s. It is what I have at home as both a standby AutoStart backup for power cuts (rural location where it happens often)
also it serves as my 3 phase supply when working on 3 phase equipment that I am building or testing.
 
p.s. It is what I have at home as both a standby AutoStart backup for power cuts (rural location where it happens often)
also it serves as my 3 phase supply when working on 3 phase equipment that I am building or testing.
Thanks James, that may well be my best bet. I'm expecting to have a fairly simple task getting 3 phase into the building as there's a pole nearby, but given my luck I reckon this is a good back up, and the added bonus of power backup too... yeh, attractive.
 

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