Thread: electrical problem with old 3-phase milling machine

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.

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.

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.

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.

Originally Posted by bacicat

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.

Originally Posted by p38arover

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

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

Originally Posted by bacicat

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.

Originally Posted by p38arover

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

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