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Thread: Lathe wiring questions.
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24th Sep 2020, 10:50 PM #1Most Valued Member
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Lathe wiring questions.
I have bought a second hand lathe, (ex Bendigo Water Board I am led to believe), namely a CD6250B, which is a lathe of Chinese parentage. Upon inspecting it after unloading it from the truck, the first thing I notice is the 5 pin 20A 3 phase plug. Upon further inspection of the electrical box, I found that the 5th pin is in fact connected, (which I wasn't expecting to be honest). after tracing the wiring in the box, I discovered the following.
1/ There are four contactors, all with 240V coils.
2/ The neutral wire connects to both the contactors and a 63VA step down transformer with both 240V and 415V primary inputs (240V conected) and 12V and 24V secondaries (24V connected).
3/ The step down transformer supplies only a work lamp.
4/ The neutral wire also has a jumper wire attaching to the Earth terminal. This jumper wire appears to be a different type of wire to the rest of the wiring in the box.
As far as I am/was aware such an arrangement was not kosher. I suspect that some person/s with only 4 pin 3 phase outlets have put the jumper wire in and done the old live to earth trick to get the 240V to run the contactors and the step down transformer and before the lathe was shipped to me, the original 5 pin plug has been fitted.
I have contacted the importer of the machine and they confirm that it was designed with a 5 wire connection.
My workshop is wired for 4 wire 3 phase (L1, L2, L3 and earth), so frankly 5 wire is a PITA as I would need to run new wiring and a new 5 pin outlet. To me, a better and more cost effective way to proceed would be to swap the step down transformer input to the 415V primary terminal and supply it with 415V which takes care of the 24V supply for the work lamp. Next step would be to source a 415V to 240V step down transformer to supply the switching coils on the contactors. my initial research indicates that a 200VA 415V to 240V step down tranny can be had for under $200 and a 100VA for $130 odd. This seems far simpler and better than rewiring the workshop supply. I'm thinking that 100Va would be probably be adequate as I'm guessing (to be verified by wire tracing) that there would be a main contactor switched by the E stop, interlock switch on the gear cover and the foot brake, one contactor for FWD, one for Reverse and one for the coolant pump, this would mean only one contactor being switched at a time (inrush current load of 35 or 40VA per contactor) while the other one or two would only have their respective hold in load (10VA per contactor or thereabouts).
Even using the larger $200 unit will be a lot cheaper than rewiring the workshop.
Naturally I will use suitably qualified persons to perform any mains wiring.
What say the brains trust?
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24th Sep 2020, 11:14 PM #2Senior Member
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Ill say you've got it covered.
Get rid of the neutral earth connection.
Make sense to do away with the neutral on the supply and fit a 200VA 415/240v transformer to power the contactors. They shouldnt draw too much. Its a easier approach to utilise the existing gear as much as possible.
Shouldnt be too big a job. Can you get a wiring diagram from the supplier, Just be handy to check whats there and then modify to match any wiring changes. Nothing worse than someone having to work on it in future and wiring has been changed from original. A few years back had to do similar work on a thread rolling machine my BIL was repairing and trying to work out what someone years ago had thought of with wiring mods.
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24th Sep 2020, 11:58 PM #3Most Valued Member
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I've got a diagram and owners manual organised, but it will be some weeks away as it is coming out of China.
The jumper wire from earth to neutral floored me somewhat as I can remember from my electrical fundamentals course done during my Boilermaker Welder apprenticeship that connecting live to earth to get 240V was smacky fingers material.
I do wonder why they didn't just use a transformer in the first place? If they used a higher rated 415V to 24V transformer then they could have just used 24V coils and had 24V switching which would be safer into the bargain.
Sounds like a bit of wire tracing and an online shopping expedition at RS components may be in my future.
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25th Sep 2020, 12:32 AM #4
Hi Guys,
Running the neutral to the machine frame is not a good Idea. The machine frame should be earthed anyway !
It used to be that the neutral was connected to the machine frame, as you said to get a 240 volts feed from one of the phases.Best Regards:
Baron J.
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25th Sep 2020, 12:49 AM #5Senior Member
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I once worked on a 3 phase outlet in a garage at a engineer s house. Had me stumped because out of back of it were a red black and green going off on their own to a single phase outlet. Red to one phase, black and green were connected to the 3 phase outlets earth. Questioned the guy and he said he wired it up that way because there wasnt a neutral on the outlet and anyway the earth was connected to the neutral at the switchboard so what was the problem. This was a electrical engineer.
They didnt go down path of changing coils probably because of cost. Also if contactors are large there are sometimes problems with 24v coils having enough power to properly pull in contacts. 240v coils is the way to go for reliability.
Another option which isnt really Kosher is to wire in a separate 240v single phase supply from a gpo. Wire it so it possibly goes through contacts on the lathes isolator and use that to separately power the control circuit ( use the original 3 phase wiring to power the motors and nothing else). At least you can prove that the controls work and then you can source a transformer to be hard wired in.
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25th Sep 2020, 07:40 AM #6Most Valued Member
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I did briefly consider running a separate 240V supply to run the contactors, but I'm pretty confident that all works as it should (even though it appears that all is not wired as it should be!). I'll just source a 200VA step down transformer and do it properly from the start.
It does just show that you should check any second hand machine for questionable in field modifications before powering it up.
Thanks to all.
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25th Sep 2020, 10:06 AM #7Diamond Member
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It's a shame that your step down transformer for the work light isn't a bit more robust. With windings for 415 and 240V, you would be able to get your 240V right there, but at only 63VA it would probably be a bit light on (no pun intended) for 4 contactors, even without running the work light.
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25th Sep 2020, 10:52 PM #8Most Valued Member
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Decision made. Step down transformer ordered, $172 +GST.
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25th Sep 2020, 10:58 PM #9Senior Member
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Your electrician will probably do it but make sure you use a double pole circuitbreaker for the transformer supply, seen similar setups where separate c/b's were used and one side was still live.
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28th Sep 2020, 06:45 PM #10Member
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I would also change the 5 pin three phase plug to a 4 pin and fit a 4 wire cable from the plug to the lathe.
This will keep the wiring similar to any other 3 phase machines you have and avoid any confusion.
I have a number 3 phase machines, all are 4 wire connected. I do not have any 5 pin sockets in the workshop.
I use step down transformers installed inside the machines to run work lights.
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29th Sep 2020, 07:05 AM #11Most Valued Member
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30th Sep 2020, 10:46 AM #12Most Valued Member
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there is nothing wrong with running a single neutral to the 5 pin outlet machine.
.and correct internal wiring accordingly
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