Single Phase light on 3 phase machine?

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25th February 2013, 03:23 PM
camoz

Single Phase light on 3 phase machine?

Hi all,

Firstly I know I will need to get an electrician to do anything electrical, so this is not a how do I do it question, but a is it possible with what I have?

I have 3 phase power in the garage, and several old machines. For my scrollsaw I have a 240 V light that I have mounted to the shaft and plug into a 240v single phase power point, and I was thinking of doing the same for my drill press (ideally I would like to go with the lights incorporated into the machine, but at the moment both are wired with 3 core and earth).

Recently I got an old metal cutting vertical bandsaw (which is a to do project waiting in a corner), which I noticed has a light attached, and I have assumed for the last few months that it worked due to the fact that it was a 5 pin plug (I have always thought that 240 single phase stuff was run on 3 phase by wiring in one of the phases and the neutral wire, needing a 5 pin plug).

Today I noticed that the plug on the end is actually only 4 pin:confuzzled:, so now I am wondering am I wrong? I have not run the bandsaw, so I have no idea if it works yet or if the light is still connected, but it looks like a standard bayonet fitting. My understanding was phase to phase is 415 odd volts, earth to phase is 240 odd volts (and even I know phase to earth is a big no no). So do I have a machine that either has had the light disconnected and gone back to 4 pin, a machine that someone has connected the light fitting from phase to earth, or is there another option that gets a 240v single phase light working on 3 phase with only the 4 pins?

To be honest I could care less about the metal cutting bandsaw, but if I can get the scrollsaw and the drill press set up so that they can run a light while not having to go to the expense of upgrading the socket to 5 pin and the plugs to 5 pin, and run 4 core and earth, that would be great.

So in short, am I incorrect about needing 5 pins to run a single phase light on 3 phase machine, or do I need to add another 240v single phase light to my drill press if I don't add neutral?

Cheers,

Camo

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25th February 2013, 03:59 PM
A Duke

Hi,
You are correct in your assumptions. The other possibilities is that the band saw has a 5 core lead and the previous owner only had a 4 pin socket so had to do without the light and all you would have to do is have the plug replaced the other possibility is it has a transformer fitted. Some machines go overboard with safety (especially with metal working machines) and have 25V, 50V or 110V instead of 220/240V for the lighting and control circuits.
Hope this is some help.
Regards
25th February 2013, 06:14 PM
NCArcher

Probably a control transformer.
You really dont want 415V at a start button and metal working machines often have a low voltage coolant pump as well as the control circuit. Some larger machines will have more than one transformer or a multi tap transformer to give several control voltages.
25th February 2013, 07:14 PM
Stustoys

I've fround a phase to earth circuit :doh:Took awhile to work out what was going on, coz no one would do that right!!!!! I can only assume they didnt have 5 core wire. So it can pay to have a good look. At least you'd know why the RCD tripped when you turned the light on.

Stuart
25th February 2013, 07:51 PM
camoz

Quote:

Originally Posted by NCArcher

Probably a control transformer.
You really dont want 415V at a start button and metal working machines often have a low voltage coolant pump as well as the control circuit. Some larger machines will have more than one transformer or a multi tap transformer to give several control voltages.

Thanks guys for all the quick replies. NCArcher, I think you might have it, I had not thought about the fact that this machine also has a coolant pump.

I guess this is why I keep this kind of stuff to the guys who know better. I am glad you got back to me because I would probably have cut the cable, but I will padlock off the plug as a precaution anyway until it gets checked out.

Cheers,

camo
25th February 2013, 10:01 PM
mannum3

240 Volts 415 Volts

Hi Guys,

You probably know this 3 Phase 415 Volts RMS can give Single 240 Volts RMS by connecting 1 phase to Neutral.

Many 3 phase motors have a separate start circuit so that the full voltage is not connected until the motor is picking up speed.

Regards Barry
25th February 2013, 10:58 PM
camoz

Quote:

Originally Posted by Stustoys

I've fround a phase to earth circuit :doh:Took awhile to work out what was going on, coz no one would do that right!!!!! I can only assume they didnt have 5 core wire. So it can pay to have a good look. At least you'd know why the RCD tripped when you turned the light on.

Stuart

This has me thinking now, about all these times I have heard "earth shouldn't be used as a neutral" stories, having been given a power point tester to go around the house with, and having to reset the circuit breaker after turning the dial to I think it was 30 milliamps, now I am wondering what they were talking about? As Stuart said if someone was stupid enough to connect the phase to the earth, the RCD would trip, right? Is this just an old problem, before the introduction of RCD's, or do I have to think more stupid, like what do I need this green wire for, I will just use this as the neutral because I don't have 4 core and earth?

cheers,

Camo
25th February 2013, 11:03 PM
RustyArc

Quote:

Originally Posted by NCArcher

Probably a control transformer.

This. If a machine only has 415v phases available, then it'll have a transformer with a 415v primary and whatever secondary windings are required. Though for plug-in gear this does effectively isolate whatever's connected to the secondary from the protection provided by the RCD that should be on that circuit.
25th February 2013, 11:34 PM
Stustoys

Quote:

Originally Posted by mannum3

Many 3 phase motors have a separate start circuit so that the full voltage is not connected until the motor is picking up speed.

All the ones I have seen do connect full voltage to the motor. But with the motor connected in star. It is then switched over to Delta. No neutral. (star start/ delta run motors that is)
But I'm sure I havent seen it all.

Quote:

Originally Posted by camoz

or do I have to think more stupid, like what do I need this green wire for, I will just use this as the neutral because I don't have 4 core and earth?

Omg now that one never crossed my mind....I still have the plug I'll check in the morning.

Quote:

Originally Posted by RustyArc

Though for plug-in gear this does effectively isolate whatever's connected to the secondary from the protection provided by the RCD that should be on that circuit.

That depends on the type of tranformer.
As I understand it, you dont need the protection of the RCD after an isolation tranformer as there is no potentail to earth. Unless of course someone does something less than clever.

Stuart
26th February 2013, 11:35 AM
RustyArc

Quote:

Originally Posted by Stustoys

All the ones I have seen do connect full voltage to the motor. But with the motor connected in star. It is then switched over to Delta. No neutral. (star start/ delta run motors that is)
But I'm sure I havent seen it all.

Same here - start/delta start is common, but for reasonably large motors or ones starting against a heavy load - I've never seen a 3ph motor with start windings, but I guess it'd make sense for a smaller motor that needs high starting torque.