electrical problem with old 3-phase milling machine

[POD]

I have recently bought an old British milling machine that was built in about 1962, it's 3-phase with a 4hp motor. I'm running it via a rotary phase converter. I have been busy stripping it down and cleaning out all the 1962 grease etc and have just begun to use the machine.
I'm having an electrical problem with it and hoping for some advice from someone who knows their way around 3-phase electrical issues.
The main contactor is a big, clunky old affair (as is all the switchgear on the machine). It is activated by a solenoid that pushes the contacts up from underneath and drops out under gravity when de-activated. The contactor incorporates bimetal strips for overload protection.
The machine is started from a lever-operated switch that has 3 positions; it is spring-loaded into the ?run? position, which closes 2 contacts. The 'start? position closes a third contact, and the 'stop? position breaks all three contacts.
When I operate the start switch, the contactor 'bounces' in and out for several seconds before it 'catches' and maintains the contact. The bouncing is worse with load, in fact if I have the machine in a high gear it will just keep bouncing and arcing and not hold the contact at all, and after several seconds the circuit breaker will blow. The motor starts with minimal arcing / bouncing of the contact if it is not in gear. I have tried disconnecting the motor from the load side of the contactor, and it closes without any of the bouncing. While the contactor is bouncing, it arcs severely and I can see the solenoid bouncing in and out.
I have taken the contact points out of the contactor and cleaned them up, this seemed to improve things but only very briefly and I'm sure they are all carboned up again now.

I have exhausted my limited knowledge of this stuff, would like some advice on whether the problem might lie in the motor, or whether replacing the old contactor with a modern one might fix it.

[Blknight.aus]

playiing with sparky stuff is dangerous be bloody careful...... or better yet have all your work checked over by a live sparky before powering it up....

is the solenoid putting enough drive on the contactor?

if you've picked the weak phase from the rotary (and Im assuming you have the setup that takes single phase in then uses a 3 leg output and capacitors to "replicate" three phases and not a single phase motor driving a 3 phase alternator) to drive the solenoid it may not have enough grunt under load to hold the contacts in place. try switching the wiring for the solenoid to one of the other "phases"

if the capacitors that offset the phases are dicey you'll get the same problem.

assuming its not an overload taking down the rotary and that all else is ok and your not looking at a star delta start controller.... and that you're trying to use a 240v solenoid on a 240v circuit and not a 415v circuit


if you wire the solenoid to operate off of the single phase that feeds the rotary and then have the contacts switch the output of the rotary to the motor.

if that works and nothing else is electrically faulty and the solenoid pulls in correctly on proper mains voltage your on or beyond the limit of the converter on startup.

as a guess and without all the details to drive you 4hp 3 phase motor up near full load start conditions you'd want to be looking at about 10 HP worth of converter. for reliable no load starts youd probaly get way with about 5 and maybe something like 7 for no load starts but full torque at rated RPM.

just be careful to watch your voltages under load, the lazy phase can drop low enough and slip sync enough to cause problems if you run near the limits for too long.

[POD]

Thanks for the reply Dave.
Something I forgot to mention; I have some contactors at hand- modern, compact units- and I wired one in in place of the existing contactor as an experiment- it operated flawlessly with regard to starting, but I am ignorant as to how to wire the thing in so that it latches in the 'run' position. The old contactor is a bit of a maze by comparison and the circuit diagram for the machine makes my head spin.

The phase converter is the rotary type with a 240-415v transformer and a 8hp idler motor, it uses capacitors to start the idler motor and balance the phases but is different to the type that just uses capacitors to generate the third phase. It's rated to 8hp. I'll try swapping the solenoid to a different leg of the 3 phase output, one of the phases is direct through the transformer from the 240v input.
The solenoid on the contactor is marked 415v.
You lost me with the star-delta start controller; does that swap between star & delta for start & run configuration? I'm 99% sure this contactor does not do that. I don't know which way the 4hp motor is connected and it is very difficult to get to the cover as it's inside the base of the column on the mill.

I can check the voltages of the 3 phases under load, they were set up so that the 'phantom' phase is 20v higher than the others under no load, I assume that is to compensate for some kind of loss under load.

[Blknight.aus]

what you might have is a solenoid that has a pull in and hold in winding.

if its marked as 415 then it has to have 415v on it to pull in and hold in.

if you have it across the "lazy" phase when youve got a big load on during start up it may not have enough grunt to hold in, if youve got it on 240v from only the lazy phase it will do the same thing.

Be careful with the replacement if you have the 2 part solenoid and contactors, as the solenoids come in 240+415 flavors and the contactors the same you can drive one voltage from the other but putting 415 through 240v stuff rarely works for long.

as a side thought if its a really old set of contactors they wil have some adjustable srpings inside that set the make and break tensions, if the make springs hav gotten weak that could be making the difference.

the phase thats 20v over at idle is the "lazy" phase on your setup and under the "optimial" load for your converter will deliver the same volts and amps into a properly balanced load. cheaper units (or home made units) intended to drive non critical more tolerant loads like welders wont have the gubbins wound up to have the voltages raised.

the winding that comes straight through from the single phase is the one I reccomend you tap for all the 240v control side stuff and the phase that isnt the lazy one should be the additional one you pick up for 415v control.

star delta start motors run the wires back to a change over contator set which is typically discreet sets but are sometimes ganged onto a single solenoid with a seperate main power contactor. Youd kow it if you saw it.

The reason I asked about that is a star delta will start and sometimes run with a phase down but under load as the RPM drops off the contrller can flick it back between the start and run configurations.

[Homestar]

I see a solution to your problems mate. You know how we were talking about a small job I need a milling machine for, and you know I'm a Sparky with years of motor control experience.... :)

Only problem at the moment is working out when I could get there to help - it may be a week or so. I would wire in one of the new contractors you have. Sounds like the old clunker is too weak to hang in there with the setup you have - the old contractors are much less tolerant to low voltage situations than newer units.

Call me when you can and we can chat about what shifts your working and when I may be able to get there if this sound like something that is workable for you. :)

[p38arover]

I would wire in one of the new contractors you have. Sounds like the old clunker is too weak to hang in there with the setup you have - the old contractors are much less tolerant to low voltage situations than newer units.

Old contractors are unlikely to work at all. I know I don't. :angel:

[Blknight.aus]

Old contractors are unlikely to work at all. I know I don't. :angel:

Thats only because you're too busy being the AULRO spelling/grammar police

[POD]

I see a solution to your problems mate. You know how we were talking about a small job I need a milling machine for, and you know I'm a Sparky with years of motor control experience.... :)

Only problem at the moment is working out when I could get there to help - it may be a week or so. I would wire in one of the new contractors you have. Sounds like the old clunker is too weak to hang in there with the setup you have - the old contractors are much less tolerant to low voltage situations than newer units.

Call me when you can and we can chat about what shifts your working and when I may be able to get there if this sound like something that is workable for you. :)

Sounds like a very workable solution to me Gav.:)
I'm on nightshift tonight (at work now) then off til Tuesday, will give you a call.

[Homestar]

Old contractors are unlikely to work at all. I know I don't. :angel:

Goddam it! Contactors is automatically changed to contractors by this bloody machine! I get a feeling your stalking me as well Ron... :D

[scarry]

Thats a very old contacter.I haven't seen one for years.

I would leave it until bacicat can have a look as a chattering contacter won't do the motor or the contacter any favours.In fact it is a pretty good way to wreck things.

Low voltage at the contacter coil may be the issue,as the motor starts the voltage drops,chattering starts.

And as Dave has said,be very careful,you don't get many chances playing with power.

Good luck,hopefully a new contacter will sort it.:)

[POD]

Had a bit of a go at it this afternoon, I tried changing the solenoid feed to the phase that is direct through the transformer, made no difference. That is about as far as I am game to go.
I can't get the machine to start at all now except in neutral, which is a pain as I have a job in the vice that I wanted to get finished.:(
Will give you a call this evening Gav. I have about half a dozen contactors here left over from the converter build, I'm sure one of them should do the job.

[POD]

Happy times, I caught up today with the friend who designed my phase converter and supplied me with the bits. I described the contactor problem to him and he said it sounded like the solenoid was running on the phantom phase, suggested swapping the feed wires, rotating positions to avoid reversing the motor. I did that and the machine now starts just like it should.
There is still one minor issue in that there is a momentary switch on the gearbox that is used for lining up teeth when changing the spindle gears; it now operates as a start switch instead of a momentary. Something for Gav to do when he brings his milling job!:D