# Single Phase or 3-Phase?



## MWCurl (Feb 11, 2019)

Hi Guys

I am in the process of purchasing a new lathe.  I have the option of 1-phase or 3-phase power.  My shop is wired with single phase. I am aware of the benefits of a VFD ..... but that adds about $500 to the cost of the lathe. And, based on the anticipated use of the lathe, I do not place much value on having a variable speed motor.

I saw a review where the person stated that threads cut with a 1-phase motor would not be as smooth as threads cut with a 3-phase motor.  The rational is that 3-phase power produces a smoother current than 1-phase power.  This difference in the current will translate into a difference in how smooth the thread is cut.  I understand the theory, but question how much difference it makes as a practical matter.

Does anyone have experience using the two different sources of power for cutting threads? Is there a distinguishable difference?

Any comments on the topic are welcome.

Mike


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## benmychree (Feb 11, 2019)

There would likely be no discernable difference in thread cutting smoothness with the different motors; there are so many other factors involved, such as motor balance, gear irregularities, material being cut, tool geometry and sharpness, cutting fluids, and the relative ridgidity of the machine and setup, not to mention the skill level of the operator.


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## P. Waller (Feb 11, 2019)

Polyphase has attributes that single phase lacks, if possible use a polyphase system.
If not practical do not.
It is a simple as that.


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## benmychree (Feb 11, 2019)

One good point about 3 phase is instant reversibility.  I have this on my 19" lathe, which has a direct belted drive, and I use it to slow the machine down and also for some threading work, such as metric and coarse leads up to shoulders.


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## pacifica (Feb 11, 2019)

Also you can cut threads with geometric style die  heads(instant class 3 threads) or 3/4  of the way single point threading and finish with an appropriate die . Those 2 methods will produce very nice threads.
 Having said that I still believe in 3 phase and VFD's for a lathe.


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## Cooter Brown (Feb 11, 2019)

Always use 3 Phase if its available..... You lose horsepower running a 3 Phase motor on Single Phase and most VFDs....


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## higgite (Feb 11, 2019)

MWCurl said:


> .
> .
> I saw a review where the person stated that threads cut with a 1-phase motor would not be as smooth as threads cut with a 3-phase motor.  The rational is that 3-phase power produces a smoother current than 1-phase power.  This difference in the current will translate into a difference in how smooth the thread is cut.  I understand the theory, but question how much difference it makes as a practical matter.
> .
> .


Hi Mike,

I think the person that stated that is either overthinking the issue or pulling someone's leg. If it made a discernible difference in how smooth the thread is cut, it would also make a discernible difference in how smooth it makes finish cuts. IMHO, it's a non-issue.

Tom


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## P. Waller (Feb 11, 2019)

higgite said:


> Hi Mike,
> 
> I think the person that stated that is either overthinking the issue or pulling someone's leg. If it made a discernible difference in how smooth the thread is cut, it would also make a discernible difference in how smooth it makes finish cuts. IMHO, it's a non-issue.
> 
> Tom


No one would ever pull any ones leg on the internet, go on pull the other one (-:


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## Chipper5783 (Feb 11, 2019)

There is nothing wrong with using single phase.  For a small lathe, it will work fine.  Anything over ~3 hp will be a bit harder to find in single phase, and above 5HP it starts to make for a more expensive motor.  3 phase works better (simpler motors) and so most commercial equipment is going to be 3 phase.  It is really up to you and what you plan to do with your work shop activities - if you are looking at one or two machines and plan to get pretty small machines, or stick with machines that you know will be single phase - then go with single phase.  If you get into anything a bit bigger (still not a "big " machine), or used commercial machines - the single phase options get pretty sparse.

If you end up with a "few" machines - then a whole shop solution really works well (the most popular being an RPC).  Of course a VFD can provide the phase conversion and variable speed - but it is a bit of work to convert the electrics over so that the control is on the VFD (typically have to gut the existing controls).  The VFD option is motor specific.  Each of the motors on your machine needs to have the power requirements met.

I  have not used a VFD powered lathe, and I can't say it has ever held me back.  I have a VFD on one drill press and a variable drive on one mill - they are nice features, but IMO it is not a game changer.  For a lathe, there are a number of features I would place ahead of a VFD.  Perhaps others could comment on the contribution of a VFD on a lathe.

I went 3 phase off the bat on my first lathe (with an RPC) - turned out to be an excellent choice, it opened up options for more machines that were then easy to power (reduced the competition at the auctions).


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## CluelessNewB (Feb 11, 2019)

Cooter Brown said:


> Always use 3 Phase if its available..... You lose horsepower running a 3 Phase motor on Single Phase and most VFDs....



This is not correct.  You lose horsepower with a static phase converter, not a VFD and not with a rotary phase converter.


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## Mitch Alsup (Feb 11, 2019)

CluelessNewB said:


> This is not correct.  You lose horsepower with a static phase converter, not a VFD and not with a rotary phase converter.



Err, no::

A single phase motor has 3 windings (just like a 3-phase motor) and operates like a 3-phase motor but with the phases running 0 degrees, 180 degrees and 90 degrees and the voltage at 90 degrees is only 71% that of the other two. In this orientation, the motor produces 86% of the input power as output power, whereas the 3-phase motor with 0 degrees, 120 degrees, and 240 degrees, runs at 100% input=output power. {Both neglection thermodynamic losses,...}

It is possible to make a static power converter that happens to develop 0 degrees, 120 degrees, and 240 degrees, but only if you use the motor at constant HP. This typically requires a box full of big capacitors an a few inductors.


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## tq60 (Feb 11, 2019)

Vfd allows for rpm as well mpr in back gear.

Makes threading easier as it is easier to stop.

Vfd needs to be proper size for motor.

Ours is 2 hp motor and 3 hp vfd. 

Sent from my SAMSUNG-SM-G930A using Tapatalk


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## mksj (Feb 12, 2019)

You can see a discernible difference in surface finish between single phase and 3 phase lathe motors, the degree is highly variable even within the same model of lathe. See these old threads, where the issue went away when converting to 3 phase with a VFD or modifing the single phase motor/mounts. There are may be other variables such as anchoring, rigidity, oscillation, etc. that exacerbate the phenomenon.








						1340gt surface finish problem
					

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					www.hobby-machinist.com
				











						G4003g Baldor Motor With Isolation Mounts - Surface Finish Fix 2.0
					

1. The isolation mounts I had purchased were not hacking it with the motor mounted on its side so I ordered the much larger isolation mounts another forum member ordered.  2. It was getting really annoying trying to hold this heavy motor up with one hand while trying to start the three bolts...




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A VFD is a practical option with motors up to 3 Hp, beyond that Hp the cost goes up significantly and typically one has to buy a 3 phase input VFD and derate it for single phase use above 5 Hp. They do have many benefits, including controlled acceleration and deceleration, the latter usually requires a braking resistor in this application. I have installed VFDs on quite a few machines and lathes up to 1640, braking times of around 1-2 seconds are easily achieved, but spin a very heavy chuck/material at high RPM and try to brake too quickly and you will most likely get an over voltage error and the VFD will free wheel to a stop. Some VFDs have the ability to circumvent this by dynamically modifying the braking rate to prevent an over voltage error. Cheap VFDs are often lacking the braking circuity despite having the connection terminals for an external braking resistor. A VFD only need to be rated for the Hp/amps for a particular motor, so up sizing a VFD is not needed if it meets the motor specs.

As far as Hp output of motors, my understanding is that a 2 Hp rated motor is 2 Hp independent of being single phase or 3 phase. The efficiency/PF for a single phase motor is less (typically around 65%, but newer motors are higher) vs. 3 phase which are in the 80 - 90%, so although both motors have the same rated Hp, the single phase will draw more power. Out of curiosity I checked out the performance data for two Baldor 2 Hp motors, one single phase the other 3 phase, the major difference is at 100% rated output (2 Hp) the single phase motor is pulling 2400 watts, the 3 phase is 1700 watts. Given that an ideal motor 100% efficiency would be ~1500 watts. The 3 phase motor has more break-down, pull-up and locked rotor torque.


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## CluelessNewB (Feb 12, 2019)

Mitch Alsup said:


> It is possible to make a static power converter that happens to develop 0 degrees, 120 degrees, and 240 degrees, but only if you use the motor at constant HP. This typically requires a box full of big capacitors an a few inductors.


 

So for practical use on a lathe we are basically talking about a spherical cow? 

_*Milk production at a dairy farm was low, so the farmer wrote to the local university, asking for help from academia. A multidisciplinary team of professors was assembled, headed by a theoretical physicist, and two weeks of intensive on-site investigation took place. The scholars then returned to the university, notebooks crammed with data, where the task of writing the report was left to the team leader. Shortly thereafter the physicist returned to the farm, saying to the farmer, "I have the solution, but it works only in the case of spherical cows in a vacuum". *_


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