single vs 3 phase question

Tribsa · Jun 19, 2020

Hi Folks
I bought a Boxford Model C lathe but it has a knackered motor - a compton parkinson 3 phase motor BS Frame no VMA544CM1A
My question is this....

From an efficiency / performance / cost point of view what is better
a - buy a replacement 3 phase motor and a VFD
b - replace it with a new single phase 240v motor

My electrical knowledge is pretty basic but I'm happy to have a go at stuff with a bit of guidance.

Things I'm unsure of are, if i go down the single phase will it affect the power/torque and if so can I go for a bigger HP? Do I need anyother electrical gubbins if i'm going down the single phase route.

Thanks in advance for all the help.

cheers

JST · Jun 19, 2020

The 3 phase is smoother, and the VFD gives speed control (you won't want to try to do the whole range with it, though).

Neither are true of single phase.

It's all in what you want to have as a result.

Phil in Montana · Jun 19, 2020

Go with the vfd and a 3 phase motor,you will have low inrush current, veri speed , and a read out on speed, amps, ect. and a soft rev if you want it...Phil

jim rozen · Jun 19, 2020

There's really not difference in efficiency, single vs 3 phase. Either one will do the job. If you want
to get the machine running on the cheap, single phase is fine.

If you don't mind the marginally larger expense there are many advantages to using a VFD. This is
my choice for new lathes these days.

beckerkumm · Jun 19, 2020

Keep in mind that a change to single phase may require a change in starter. for sure the overload size will change but you might need to upsize the whole starter and be sure the single phase motor can reverse. Once you go three phase you will prefer it though. Dave

ezduzit · Jun 21, 2020

Simplest is to convert to a single-phase motor. But if you replace with a 3-phase motor, which is cheaper, you could then get a rotary phase converter which might power other future 3-phase machines.

9100 · Jun 21, 2020

This keeps coming up. If a single phase motor is running and you switch to reverse, it will keep running forward because the direction is controlled by the starting winding and capacitors, which are out of the circuit once it gets up to speed. I won't run a single phase lathe and neither should anyone else. There is no choice to make here.

Bill

JST · Jun 21, 2020

There actually are (they are rare) instant reverse single phase motors.

They simply throw in the start with it set for the reverse direction. I do not know if there is any modification such as a different size capacitor, etc.

Seems like a rough job for the start cap, but apparently in smaller sizes they were made. There is an added relay box to handle the reversing.

ezduzit · Jun 21, 2020

Instant reversing is unnecessary.

JST · Jun 21, 2020

ezduzit said:
Instant reversing is unnecessary.

Say that again when you are getting wound up by a machine.....

It has its uses, such as for tapping etc, if you don't use a tapping head.

ezduzit · Jun 22, 2020

JST said:
Say that again when you are getting wound up by a machine.....

It has its uses, such as for tapping etc, if you don't use a tapping head.

No doubt it can be useful. But not for extracting yourself from a huge screwup; that's what "stop" is for.

thermite · Jun 22, 2020

ezduzit said:
Instant reversing is unnecessary.

Broad statement? VERY!!!

I'll play.

"Instant reversing", let alone even "disk brakes" on 16"-50 Mark 7 & 8 "super-heavy" projectiles were not necessary, either.

All they asked for was a target for the "Stop" function.

Messy?

POTD: Big Guns Make Very Big Holes -The Firearm Blog

And then? The "Stop" function FAILED!!!

Extract;

No damage to projectile indicated, though projectile had considerable remaining velocity and ended up in the Potomac River, never being recovered.

Your move?

JST · Jun 22, 2020

ezduzit said:
No doubt it can be useful. But not for extracting yourself from a huge screwup; that's what "stop" is for.

I believe the original issue was that with a drum switch, going too far , i.e. past the "stop" position on a machine set up WITH reverse, the machine, if single phase, will keep going. At least with 3 phase, getting into reverse will stop the machine rather faster than just cutting power, at which point it may be advisable to recover the switch to the "stop" position that you thought you wanted.

Then there are other cases where it is handy to have, cutting metric threads, especially on an inch lathe, cutting LH threads in some situations, etc, etc.

Blanket statements are rarely accurate, or called for.

thermite · Jun 22, 2020

JST said:
Blanket statements are rarely accurate, or called for.

Agree "rare". As to "called for", a classmate of mine was actually ORDERED to make a derogatory comment about another classmate.

Remarked that he thought the individual's "..Mother dresses him oddly."

Might class it as "accurate" depending on sense of style.

As to "blanket?" Right close to one million sojers wearing the same US Army uniform that particular day, the critic among them.

sealark37 · Jun 22, 2020

What is the horsepower of your motor? Single phase motors over 7 1/2 HP present obstacles not present in the three phase motor. If this machine is your only three phase, the VFD is a good option. If you foresee any other three phase machines, a rotary converter makes better sense.

thermite · Jun 22, 2020

sealark37 said:
What is the horsepower of your motor? Single phase motors over 7 1/2 HP present obstacles not present in the three phase motor. If this machine is your only three phase, the VFD is a good option. If you foresee any other three phase machines, a rotary converter makes better sense.

Point 75 HP (or .87). Fractional. as-in "less than one". Many here have coolant pumps more powerful.

Not 7.5 HP

If not "OEM" frame-size, 3-P? Then a skinny "PM" DC motor is what "makes better sense". Look at the photos of his lathe, Tony's web site.

Limited space!

jim rozen · Jun 22, 2020

JST said:
I believe the original issue was that with a drum switch, going too far , i.e. past the "stop" position on a machine set up WITH reverse, the machine, if single phase, will keep going.

There's a lot of discussion about that fact here. Native american story: you can tell a kid not to walk into the fire about a thousand times. But if you let him do it once, he never
will do it again.

I had a single phase motor on both of my first lathes, switched with a drum switch. I admit I flipped the handle straight from forward to reverse, once, just to convince myself that it
would behave the way I thought it would. It did. Never had to do it ever again.

I think the hazard of a setup like this is over-rated. Once the owner understands how it works. all will be well.

JST · Jun 24, 2020

Knowing the right way is all very good, I am sure.

What is much better is having the switch work in such a way that when you have other important things on your mind, the switch does not let you screw up easily.

Things you know have a habit of leaving the brain when the person concerned is under pressure. At that time, rote training or mechanical protection features are the only things that work reliably to keep the shit mostly out of the fan.

That is why on a pushbutton control, the "on" switch is generally guarded, and the "off" switch is prominently sticking out. All you have to do is slap the control station and odds are that you will turn off the machine.

As Bill (9100) says, the single phase drum switch is set up to easily fail in most cases.

jim rozen · Jun 24, 2020

JST said:
What is much better is having the switch work in such a way that when you have other important things on your mind, the switch does not let you screw up easily.
.....

Yes, that is a good idea; somebody who mixes up the switch on a lathe, though, is probably going to have other more serious problems with sharp tools and rotating spindles.

9100 · Jun 26, 2020

When my father set up my first lathe, a 9" Logan, he just installed a simple switch. It didn't take long to realize that reversing was useful, so I added a drum switch. The first time I tried to reverse it was a "whisky tango foxtrot" moment. I was only 13 and not yet all that versed in motor designs. I used it carefully for a while but swore to never get in that position again. If I simply had to set up a single phase lathe, I would do it as Jerry says, a guarded start and a big mushroom stop with a separate forward-reverse toggle switch. Dynamic braking by putting a little DC on the motor wouldn't be a bad idea.

For a couple of years I ran someone else's Clausing with a lever switch that had a lockout. You had to push a button to go over to reverse. Worked great until it wore a bit.

Bill

single vs 3 phase question

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