Anyone explain 3 phase switch gear to me

[JimM]

I'm trying to swap out the 3 phase motor of my milling machine to a single phase unit. The motor swap itself is pretty straight forward but could do with some help on the switch gear.

Machine has a main power on/off box and a separate motor fwd/off/rev switch.

With the motor switch in the fwd positon I can trace the continuity from the L1/L2/L3 on the power switch to the cables that connected to the original motor. However continuity is only maintained whilst I hold the 'Power Switch On' button in, as soon as I release pressure continuity is lost.

So questions are...

Is there likely to be some sort of relay that would latch if power was actually provided to the machine ??

If there is a relay any chance this will work with a 240v single phase supply?

Would I be better to just swap out the swicth unit for a single phase unit?

Thanks

Jim

 

[Archie]

I believe the answer to the first is 'yes'. Certainly mine works that way. I'm afraid that's as far as I can get you though if the box of tricks looks anything like that on mine I'd want to leave well alone!

 

[rtbcomp]

It's probably got a No Volt Release in the switch.

Pressing the button connects the power to the output and at the same time energises a solenoid which holds the contacts closed. If the power fails the solenoid drops and releases the contacts so the motor will not start on its own when power is restored.

If you're just testing with a meter there will be no power to hold the contacts closed so continuity will be lost when you release the button.

There may be a solenoid per phase so if any one phase is lost the unit will trip out, in this case you will have to connect all three phase contacts in parallel to live.

If there ism't a fourth contact for neutral I'd be inclined to fit a proper 2 pole single phase starter, but make sure you get one with an NVR if you want that feature.

 

[Archie]

I hope you don't mind my inserting a question of my own? I've just taken the suds pump out of mine as it's thrown a track. Symptoms are: it sometimes turns sometimes simply buzzes. If you give it a prod, it starts to turn but only for a short period (30 secs) before tripping some part of the switch gear. My diagnosis is a missing phase, which I suspect means the windings are shot somewhere along the line. Am I on the right lines?

 

[piman]

Hello Jim,

it's a fairly simple system. As there is fwd and reverse there may be two contactors or else the fwd\reverse switch has all three phases running through it. Yes, normally there will be an auxiliary contact that is the hold in circuit for the motor.

You can run a single phase motor from your existing switchgear, and evn, if you want run the single phase motor forward or reverse but you will need to extend the auxiliarly winding cables.

You do need to determine if you have two contactors or not.

Archie,
it may be that the supply has lost a phase, assuming a three phase suds pump? It is quite easy, if you have a multimeter to check the motor windings for resistance between all three phases. Just measure between L1 and L2, L1 and L3 and L2 and L3. All should be fairly close in resistance. If they are it's probably the supply, maybe the contactor has a bad contact on one phase. Again, with a multimeter, check the voltage to earth on all thres phases one by one. You should have 240 ish on all three.

Alec

 

[Ben m]

I hope you don't mind my inserting a question of my own? I've just taken the suds pump out of mine as it's thrown a track. Symptoms are: it sometimes turns sometimes simply buzzes. If you give it a prod, it starts to turn but only for a short period (30 secs) before tripping some part of the switch gear. My diagnosis is a missing phase, which I suspect means the windings are shot somewhere along the line. Am I on the right lines?

This problem is called phasing ! one of your phases is not connected or working therefore the machine is only running on two phases a bit like a car running on 3 pistons. by you twisting it you are effectively bump starting the machine as it requires all 3 phases at 120degree phase rotation for the machine to start and operate effective Ie in balance. As this machine is running on two phases the machine is unbalanced and will run for a short time before stopping as the field magnet will eventuallly rotate in line with the armature casing the armature to stop, high currents will flow at this time to try to make the armature rotate this is the time that the circuit protection is activated.

I hope this gives you an insite. the short answer is all three phases need to be connected for the machine to operate.

hope this helps Ben

 

[Archie]

Thanks Ben. That confirms exactly what I thought. I think I'll have a poke around with the multimeter and then hand it over to the alternator fixing man and see what he has to say.

 

[HasBean]

Some moons ago I fitted a NVR switch to my mill/drill. It was supposed to be for a single phase motor but wasn't
From what I remember (don't take this as gospel) all three feeds to the contactor like to have juice running through them.
So Earth was earthed, Neutral was connected to neutral, but I had to put the live input to the L1 input, then out of the L1 output & into the L2 input, then out of the L2 output & into the L3 input, then out of the L3 output into the motor.
As I say I can't be sure this is correct but it makes sense to me & it has been working now for some 12 + years or more so.............
If any of you gents who know the ins & outs of this please let me know

I'm guessing 3 to single phase would be just reversing the above

 

[Paul.]

Some moons ago I fitted a NVR switch to my mill/drill. It was supposed to be for a single phase motor but wasn't
From what I remember (don't take this as gospel) all three feeds to the contactor like to have juice running through them.
So Earth was earthed, Neutral was connected to neutral, but I had to put the live input to the L1 input, then out of the L1 output & into the L2 input, then out of the L2 output & into the L3 input, then out of the L3 output into the motor.
As I say I can't be sure this is correct but it makes sense to me & it has been working now for some 12 + years or more so.............
If any of you gents who know the ins & outs of this please let me know

I'm guessing 3 to single phase would be just reversing the above

Thats the way to do it, connect the 3 contactor paths in series, I usually like to fit an overload as well, so its especially important to make sure the current passes through all 3 circuits.

 

[JimM]

Guys

Thanks for all the advice. Had another look at the switch and can't see an obvious neutral connection so think I'm going to bypass the machines wiring and install my own switch gear.

Cheers

Jim

 

[Pete.]

Your mill most likely has a 415v contactor. Since you are converting to 240V the voltage is probably too low to hold the coil in. I had this problem with my lathe so I unwound a lots of turns off the contactor pull-in coil to make it hold on 240V. Worked perfectly.

http://www.denfordata.com/bb/viewtopic.php?f=14&t=1948

I have three un-used 240V 3-pole contactors if anyone needs them for a tenner each. I was going to use them but got silly and wanted to stick with the standard button-plate etc on my lathe which wouldn't let me use the new contactors, that's why I modified the 415V ones.