Question for electricians

[Cursitor Doom]

Hi there, sparkses,

I have a 2.2kw phase converter which outputs 230v AC 3 phase from a standard single phase mains supply. This converter is currently powering my 3HP lathe. But I've just bought a milling machine which requires slightly less current so I wish to power them both from the same converter (not at the same time, though, obviously!). I had planned to use manual switchgear or perhaps relays, but it seems nowadays these things called 'contactors' are all the rage. Being a bit er, "mature" in years I'm completely unfamiliar with these new-fangled devices. I will need to switch the current from the converter between the two different motors depending on whether I'm using the lathe or the mill. Let's say the peak current for both will not exceed 25A and as I've already stated, this will be 230VAC 3 phase. Now, precisely what type of 'contactor' will I need to switch between the two machines?

cheers,

cd.

[John Williamson]

"Contactor" is the currently fashionable name for a heavy current relay or switch.

The cheap and easy way to do it is to use a three pole, two way, centre off unit like the one I use to switch between shore supply and inverter supply on the boat. I paid just under £40 plus the cost of the box.

[Dave Liquorice]

AIUI a "contactor" is just a fancy word for BFO mains relay.

Why not KISS and fit a 3ph Ceeform socket to the output of the convertor and matching plugs on the input cables to the lathe and mill.

[Tim Watts]

I think it's been fashionable since 1950 :)

[robgraham]

I think it has always been 'contactor' for ac drive and 'relay' for dc driv e.

I will take the line oft repeated here that if the OP's electrical power kn owledge is that lacking, should he actually be tackling such a task ?

One thought that does come to mind is that an inverter should be fed from a 'No Volts' contactor which is linked into the emergency off circuits; this is so that the inverter is isolated from the mains if there is a trip, and doesn't restart if the trip only is reset - I've been there and nearly suf fered when working on someone else's machinery.

If the OP is not aware of what a 'contactor' is, it makes me worry that his whole system is not covered by emergency trips and, having initially thoug ht the 3ph socket solution was favourable, I now realise that it is in fact extremely hazardous unless suitable wiring from the emergency buttons is i ncorporated.

[meow2222]

3 pole 2 way

NT

[Cursitor Doom]

Too much aggro. I like simple safety precautions, like a thick rubber mat under the working area immediately in front of the machine. In an ideal world it would be nice to have safety this and safety that, but at the end of the day, you'd get no work done, so what can you do?

[Fredxxx]

Many machines have their own built in No Volts contactor.

This is something I need to look into myself. What are the requirement for emergency stop switches?

[harryagain]

As someone has said, the easy way is to put a socket on the output from your inverter and have plugs on your machines. You could use two contactors and a two way light switch to control the coils on them. However there are quite a few possible pitfalls. If you have to ask these questions, maybe you should GSI (Get Someone In)

[ARW]

There is your answer. It's the stop start switch on the machine or am I missing something - such as you have to program the invertor diferrently for each machine and want it to automatically power the correct machine depending on which program you have chosen?

[ARW]

I think it has always been 'contactor' for ac drive and 'relay' for dc drive.

No - it's a power thing.

[Brian Gaff]

Yes, contactor is just a generic name. It could be anything inside, but as long as the outcome is a switch over where both cannot be connected at the same time, you are fine. The things that get me befuzzled are starters, which seem to be able to keep from running big motors flat out at the start, gradually spinning them up, this is very handy if there is a big mass on the motor and you do not want to strip gears or burn it out! Brian

[Andrew Gabriel]

A contactor is not symetric. The construction gives very high contact pressure on the N/O contacts, but not on the N/C contacts (indeed most contactors don't even have N/C contacts). This means the high current rating applies only to N/O contacts.

Therefore, to build changeover functionality with contactors will normally require two of them. There's usually some extra circuitry to ensure both can't be energised at the same time, such as auxilliary contacts locking the other one out.

2.2kW @ 3P 400V is not a high current, particularly given this case is likely to be all off-load switching. A good relay will handle it for a reasonable time, providing its contacts are rated for 400VAC. Using a socketed relay would enable easy changing if it does wear out.

Depending on the start circuits of the two appliances, you might even manage without any switchover, but simply have each one when operating disable the other one's start circuit or supply completely. This would mean modifying the two appliances, but would make for easier use.

That's usually called a transfer switch, although in this case being used in reverse.

[Fredxxx]

I associate contactors with 3-phase.

[Cursitor Doom]

If you want to run 3 phase machinery but you don't have a 3 phase supply, then one of your very few options is to use a phase converter and change the motor box's delta/star configuration over. So far so good. However, the problem with phase converters is that you can't just connect them up to the machine's 'distribution board' in the same way you would with a standard 3 phase installation and enjoy full use of everything. They have to be wired direct to the motor(s) concerned, and this bypasses a lot of the safety features built in to the machine. For example, by using a converter, you lose the functionality of the 'big red stop button' plus any guards around rotating parts won't cut the machine out when raised, and the foot-bar emergency stop no longer works. This is not an exhaustive list! :(

[ARW]

Yes you change from star to delta.

So you are bypassing the machines built in contactor.

[Cursitor Doom]

Neither of them have one so far as I know; unless they're called something else in my universe.

[ARW]

Give a make and model number of your machines and I can probably help you more with the wiring

[Cursitor Doom]

Thanks but nothinssthaksbit pissed

[The Other Mike]

Yes, but with an inverter there is nothing to stop you wiring it up so that all the interlocks and stop buttons function as well as or maybe even better than the factory original - It's relatively easy to use dc injection braking for the motor for instance which has the potential to stop a motor very quickly.

There are also a few occasions when you might even want to bypass safety features. An interlock switch on a chuck guard on a lathe can be a real pain in the arse. Common sense informs you when you need to use it - almost but not quite 100% of the time. Multiple emergency stop switches just below the drip tray and within easy reach are IMHO better than any foot bar.