3 phase

[jmh21586]

What the hell is it in laymans terms?

I've been looking at some metal lathes and some have three phase motors.

Does this mean I have to switch motors to be able to run them on 110??

 

[nehog]

Basically yes, but any lathe big enough to need a three phase motor will be seriously underpowered with a 110 volt single phase motor, and even a 240 volt motor won't be as good.

One option is to use a phase converter (do a search!) or you can (if you are extreme) get a military surplus three phase generator set and have both backup power and three phase for your machine tools!

 

[vpd66]

Ahhh, you found the common problem that most people face when looking for used machine tools. 3 phase or not?? Like I said in your "metal lathe" post, I have a 16"x78" American Pacemaker lathe. It has a 10hp 3 phase motor. I run it off a phase converter that I built for around $50.00. Do a search (like suggested) on home built 3 phase converters. You will find lots of info on this. There are a number of ways to do it. Mine is just a couple idler motors, capacitors, and a couple switches. I aways laugh when people run from 3 phase equipment. It just makes them a better deal. I now have my lathe and milling machine running off my phase converter. When I look at used machinery I don't even care if it is single or three phase now and 3 phase machinery always seems to sell cheaper.

 

[wbrian63]

3 phase is just more efficient when it comes to motors. Think about how a motor works - it's all done with magnetism that's generated by wiring wound in a coil. Remember that like poles of a magnet repel, and unlike poles attract. A "normal" single phase motor is just two half-circle magnets that switch from N to S at the right time to cause the armature to rotate.

With 3-phase power, instead of two magnets 180 degrees apart, you've got 3 spaced 120 degrees apart. More efficient and easier to generate larger horsepower with less amperage.

Single-phase motors generally stop at 5hp. Anything larger is going to be 3-phase.

Phase converters aren't rocket science. There are lots of sources on the web for plans to build your own. They are usually expensive to purchase.

 

[Stuart in MN]

There's a reasonable primer on three phase power on Wikipedia: http://en.wikipedia.org/wiki/Three-phase_electric_power

 

[Joe92GT]

Its not just the "3 magnets" in the motor that makes three phase more efficient. Though that helps on start up a lot. It is also the fact that there is never a point when the power is zero like on single phase.

Single phase 240 is at 180 degrees apart, so one side is at +120, the other -120V. They then cross over 0 volts at the same time, leaving the motor to spin as a flywheel at the brief period. with 3 phases 120 deg. apart, there is always one phase with power available to the motor, so there is never a time when the motor free spins.

Also, 3 power wires means smaller gauge wires for the same load, so its cheaper that way also.

 

[larry_g]

What the hell is it in laymans terms?

I've been looking at some metal lathes and some have three phase motors.

Does this mean I have to switch motors to be able to run them on 110??

Three phase is basically industrial power, laymans terms. For what you need to know there are a couple of ways to run 3-phase equipment on house power. A variable frequency drive (VFD) will convert 240v single phase ( What you have in your house) to 3 phase and give you variable speed with some limitations. The second common way is to use a Rotary Phase Converter (RPC) which with a 3-phase idler motor and some capacitors tou can generate 3-phase power from 240v single phase.

In my opinion ifyou only have one or two 3ph machines then the VFD may be a simpler way to go. If you end up with a shop ful of 3ph machies then a RPC may be justified. A VFD is usually programmed to operate one machine wheras a RPC can drive multiple machines. Here is one forum that is dedicated to your subject
http://www.practicalmachinist.com/vb/transformers-phase-converters-vfd/


There are others.

lg
no neat sig line

 

[Falcon67]

I bought a 3 phase mill/drill because I liked the features better. With a VFD, I have better control of the motor and can run it on 220 single in the shop. The VFD adds about $200~250 to the cost but it is well worth the extra $.

 

[bimmer1980]

Keep in mind that there are some limitations with the variable frequency drives. generally they create power on the 3rd leg only long enough to get the motor turning. then they operate the motor on 2 out of the 3 legs. In this scenario, the motor is only operating at 2/3's of it's rated hp.

The rotary phase converters create the 3rd line from an input of 2 lines. therefore, your machine motor actually operate with all three lines and has the power.

Generally speaking, it is not recommended to use a VFD on a lathe. Also the RPC will need to be upsized accordingly. Lathes are considered "hard start" applications and need a phase converter sized accordingly.

Within reason, go for a larger phase converter vs a smaller one. I had the opportunity to purchase a 2 hp phase converter, but passed on it to wait awhile to find a 7.5 hp rotary phase converter.

best of luck.

 

[Falcon67]

The 2 HP VFD I use on the mill is rated to deliver 100% power without any loss. Braking at full power requires a dissipation resistor, but with my 1HP motor I have no problem stopping it in less than .5 second from full speed.

 

[larry_g]

Keep in mind that there are some limitations with the variable frequency drives. generally they create power on the 3rd leg only long enough to get the motor turning. then they operate the motor on 2 out of the 3 legs. In this scenario, the motor is only operating at 2/3's of it's rated hp.


best of luck.

I heard this about static converters, but this is the first time I think that I have heard that VFD's drop a leg after starting...can you cite a source?

lg
no neat sig line

 

[rwhite692]

Build a rotary phase converter, you won't regret it...


http://homemetalshopclub.org/projects/phconv/phconv.html

 

[garagetech]

Hi guys,
I just came across this forum and thought I could help with this topic. I seem to have found a niche as a retired electrician in doing a lot of industrial wiring in garages. Guys are buying these used CNC machines really cheap and running them on rotary type phase converters. This is where I think I can help on this thread.

If you plan on ever using any equipment over about 5 HP, just wire your shop for 3-phase. Run 3 wires and a ground to junction boxes all over the shop (should only cost you one more wire). You can use them for single phase until the day you need three-phase, then just install a converter, then a three-phase circuit breaker panel, and the rest you already did. Make sure you size the wire for single phase current and then they will be plenty big for three-phase.
My advice on the converters, run the new digitally controlled rotarys from American Rotary. Awesome, no comparison to the converters my dad used to run. I have installed probably most of them out there because my customers buy the deals on the internet and think they are all the same and I have to end up messing with them and nobody wants to pay for me to fix someone elses product. I always recommend the American Rotary now that I have installed several of them. They can also get you the three-phase breaker panels etc. too. If you are bent on building one, I am pretty sure they will sell you the components to do so, they don't advertise it, but use their stuff instead of guessing what will work.

On VFD's, they don't drop a leg, static phase converters engage the third leg for starting and then drop it out. Keep in mind that a VFD is good for one motor so if you want to power a whole shop with three-phase a rotary is the way to go. There is also a pure digital phase converter (not a vfd) which works good, but it can't do anything more than the AR digitally controlled rotary units, and you better not have neighbors that want to watch tv or listen to the radio. I have installed a few and swear that the smooth analog output of the AR rotary converters run the machines better than the pure digital. American Rotary sells both so they should have an unbiased opinion on them. Here is a link www.americanrotary.com

Rob

 

[saabman]

VFDs provide voltage on all 3 legs. The 2/3 rating you see comes from feeding it with 220v single phase. Many VFDs can work with single phase 110, single phase 220, and 3 phase inputs. From these you get 1/3, 2/3, or full rated capacity on output respectively.

 

[Norcal]

VFDs provide voltage on all 3 legs. The 2/3 rating you see comes from feeding it with 220v single phase. Many VFDs can work with single phase 110, single phase 220, and 3 phase inputs. From these you get 1/3, 2/3, or full rated capacity on output respectively.

Not correct, a 1Ø input VFD is not derated...... A static converter is derated because all it does is start a 3Ø motor then it runs single phase.

 

[lametec]

Not correct, a 1Ø input VFD is not derated......

Depends on the VFD. Some are, some aren't.

 

[Falcon67]

Depends on the VFD. Some are, some aren't.

I think that is right - I specifically shopped for a VFD that did not de-rate. The only thing mine would need to handle a full 2HP 3 phase motor would be a braking resistor for quick stops. Mine's on a 1 HP mill with no resistor and if you hit that E-Stop button, the spindle stops NOW. No trip.

Also - as noted, the VFD is really a one motor pony. If you have similar remote controls set up on two machines, you could use 1-for-2, but not at the same time. You'd have to move the motor and control harnesses. The control part is the big deal, unless you like reprogramming the VFD between machines. That's a bit of a PITA - some of that control language is Greek.

 

[bobadame]

A couple of things to add. At work I use a couple of Haas CNC machines. They can be powered by either single phase or 3 phase. I have no idea how they do this. What is interesting is the braking resistor they use for these machines. It is a common electric stove top element. They use 2 .

The other comment I have is that this week I've been wiring in a couple of phase converters. One is a rotary that includes a big heavy transformer and about a 5 hp 3 phase motor. The other system is an old static converter that I've been using since about 1975. I like the static unit because it uses no energy just sitting there idling like the rotary does. The 66% power at my mill spindle has never been a problem, ever. I'll use the rotary converter to power my CNC mill and my big lathe. Everything else will run on the static. This includes a band saw, a punch press, milling machine and a surface grinder.

 

[larry_g]

I'll use the rotary converter to power my CNC mill and my big lathe. .

When I installed my big mill with a control system in it I had a problem with the contactors dropping out on startup of the main motor. Problem turned out to be that I was powering the contactors from the generated leg of the RPC. Just something to be aware of in the cnc, the generated leg can sag when the load is high.

lg
no neat sig line

 

[bobadame]

Yes I had that problem many years ago on a Cincinnatti toolmaster mill. In fact the first time I used my static converter back in '75.

 

[dougmac]

The main advantage of 3 phase motors is that they are simpler than single phase motors.

In a single phase motor the the fields do not rotate so the motor needs a way to start the rotation in the proper direction. To do this you need a coil that is out of phase for start up. There are a number of ways to do this .... a capacitor, split phase motor, shunt coil....

Because three phase already has rotating fields, there is no need for an additional device to start the rotation in the proper direction.

 

[6768rogues]

With 220 single phase, the phases are opposite so that at their peaks, they have a potential difference equal to 220v. With three phase 208, you have 3 110V phases that are 120 degrees from each other. Because they do not peak at the same time, a root mean square equation is used to calculate the voltage. Thus, three 110V phases in three phase give 208 volts. 480v three phase has three 277v legs. The legs of three phase are labeled A, B, and C. Because the electrical field is rotating with the 3 phases 120 degrees apart, three phase motors are self-starting and do not need a starter. When wiring a three phase motor, bump start the motor and note the direction of rotation. If it is backward, swap any two phases and it will reverse. ABC turns into CBA by swapping out two of the letters, cool stuff.