3ph Plasma cutter wired backwards

[lotus_esprit_s1]

The power cable on my old 3phase Hypertherm Powermax was past its best, so I bought a new length of four core. I’ve just come to fit it and found that the machine has been wired with incorrect phase rotation. (The power lead has previously been replaced at some point in it lifespan as it was white not the original black). I’ve triple checked against the wiring diagram printed on the inside of the casing and the 4 pin plug and L2 and L3 are definitely inverted at the 4 pin plug .

It has been working for at least the past ten years with incorrect rotation, should I rewire it as is or should I put it back to correct rotation?

 

[8ob]

Cant see how you can get it wrong, L1,L2 and L3 are all lives and it doesn't really matter which way around they are.

Bob

 

[Hood]

Unlikely that the phase rotation will matter to a Plasma/Welder/whatever.

 

[grim_d]

Cant see how you can get it wrong, L1,L2 and L3 are all lives and it doesn't really matter which way around they are.

Bob

Unless of course it's a motor...

 

[awemawson]

My big (but old) Hypertherm has a large fan that I think is 3 phase, so if wrongly connected would puff in the incorrect direction.

 

[Bornfree]

It will weld if the phases are wrong.

 

[carbon]

Ive managed to find a service manual for the Hypertherm Powermax 105 which shows the fan voltage to be 48v dc. The service manual check for a faulty fan is to measure the voltage between two points so highly unlikely that swapping the phases in the lead will make any difference to the direction the fan spins as it not directly connected to a three phase supply

​

 

[Robert Weaver]

The wiring diagram, page 343 taken from Here, shows the incoming is applied across a rectifier bridge to generate a tank of 540 v DC. This will then be chopped into lower volts to drive various systems as needed.

A 3phase rectifier doesn't care how the phases are wired.

Depends whether you want to rewire it, it won't matter.

 

[knighty]

quick question.... is incoming 3 phase mains rotation set right?

I've always assumed "it is what it is" and you deal with it from there.... all my sockets are set clockwise... (they are since I rewired years ago anyway) - but then my machines plugs are wired up to suit the machine not the plug/socket

 

[brightspark]

quick question.... is incoming 3 phase mains rotation set right?

I've always assumed "it is what it is" and you deal with it from there.... all my sockets are set clockwise... (they are since I rewired years ago anyway) - but then my machines plugs are wired up to suit the machine not the plug/socket

up to the meter from the mains incomer the phases will be correct. if i ever connected a machine up that must run correctly as soon as it runs eg would damage gears running in reverse i just put my phase rotator meter on the supply

 

[lotus_esprit_s1]

Thanks for replies - I have phase rotator I’ll check the socket tomorrow but always assumed it to be correct; will wire the machine up correctly and see what happens!

 

[Dave-sparks]

I would connect it up the same way it was previously, it presumably worked that way so there's no reason to change it.

If it ain't broke don't fix it!

 

[sako243]

Thanks for replies - I have phase rotator I’ll check the socket tomorrow but always assumed it to be correct; will wire the machine up correctly and see what happens!

As @Robert Weaver states it's irrelevant for a machine that immediately converts the AC to DC. Most plasmas and inverter welders these days will convert to DC and then if required back to AC o they've got greater control (and usually higher efficiency).

 

[Robert Weaver]

I've noticed a sharp transition away from stepdown transformers to rectified DC. Almost none of the modern electrical consumer products I get to repair use transformers, probably driven be cost of copper and efficiency gains as @sako243 says. Effectively buck converters from 325volt DC down.

 

[sako243]

@Robert Weaver possibly more of a driving factor is flexibility. At least in the higher end sets. It's quite difficult to change AC frequency without first rectifying it. Once you've built the AC to DC to AC hardware then for the vast majority of scenarios PWMd output waveforms are good enough and you don't need to worry about smoothing them thus with a software update you have easy upgrades.

 

[123hotchef]

Unless of course it's a motor...

Yes my bench grinder was spinning the wrong way until I switched a phase, previous owner had been using it like that.

 

[Woody.v8]

Yes my bench grinder was spinning the wrong way until I switched a phase, previous owner had been using it like that.

Did it add material instead of removing it ?

 

[Screwdriver]

Having worked recently on some large thee phase installations (woodworking workshop), I can see how easily that can happen. Bit like fuse spec floating up over time, it only takes one owner to swap over a couple of phases to "fix" a backwards motor and over time the whole system can migrate!

I have seen a machine with a miswired socket being fed from a miswired plug. You only discover it when troubleshooting something else(!) because everything seems normal.

As has been said, three phase is relatively tolerant of phase reversals depending on where exactly that might occur, "tolerant" as in things still work but nobody notices. If you start swapping things around to be "correct" just be careful to start out from first principles and follow the chain all the way through the system. While you can sometimes "get away" with just swapping the odd phase to reverse a motor, if you start swapping power input feed, things can get dangerous pretty quickly if the system also provides balanced single phase loads.

I had a devil of a time with some lighting which had been (badly) installed running off a poorly wired three phase supply.

 

[lotus_esprit_s1]

After checking with phase rotator it turns out that all my 3ph sockets are wired backwards, although the cutout is correct rotation, so there is a cross somewhere between the cutout and the sockets, and as a result my machines which were left in the unit when I purchased it such as the old Hypertherm are wired backwards to make them correct rotation at the machine.
I’ve just wired all the sockets for correct rotation and will rewire the machines to suit.

I’m glad I checked as I’ve got a 3ph lift on the way and would’ve wired it up assuming correct rotation at the socket

 

[knighty]

I’m glad I checked as I’ve got a 3ph lift on the way and would’ve wired it up assuming correct rotation at the socket

the vast vast majority of stuff is fine running backwards, at least for a while time while you turn it on, realise it's going backwards and turn it off to rewire it

anything that isn't will have a phase rotation relay on it to warn you it's wired backwards


my go-to for testing rotation it to turn it on and see