three phase question

[K Effective]

I was bidding on a late-1980's vertical milling machine today as it was in the scrap-metal value price range. It was listed as running, but not operating correctly. Three phase 220V motor.

My question, if the motor was indeed bad, could I have had it replaced by a single phase motor of the same size (2hp)? Is current draw the downfall, or something I'm not thinking of? I was planning on using a VFD if the motor was good, but could justify putting that money toward a new motor instead.

 

[Roger Tomfohrde]

Three phase is cheaper to run. Start components on something like that would need to be considered. Might have to upsize as three phase is more efficient at work.

 

[Jeff NWOH]

I've seen single phase motors swapped on
to mills. The only real drawback I could
see is that a single phase motor won't
instant reverse like a 3 phase. I don't
really see that as an issue unless you
want to power tap with it.

 

[Oliver 500 Wa]

Even if the motor was bad, I'd plan on replacing it with a 3 phase and add a VFD. Even if it cost an extra couple hundred $. It's just too dang handy to have speed control by turning a knob instead of flipping belts around and instant reverse for power tapping.

 

[John T]

Electrical question, here come the answers lol so I may well as get mine in and see if others agree??

I would opt for a Three Phase motor: They are more efficient,,,,,,,,,,Self Starting (no start capacitors or separate start windings)
,,,,,,,,Easily reversible,,,,,,,,,

Sure if money is an issue including the cost of a VFD, an adequate rated single phase would work YOUR MONEY YOUR CHOICE

It will be fun to see how many agree or disagree as EITHER will work

John T Longggggggggggg retired and when I was with The Century Electric Motor Company so no warranty

 

[Stephen Newell]

More than likely the motor could be replaced however in machinery some motors are made for that machine only and not replaceable. I bought a machine onetime that I planned to replace the three phase motor and after I got it found out a replacement motor wasn't
available. It had something like a shaft 1 1/2" in diameter and about 8" long. Had to get a phase converter. In the end was a blessing. Once I had a phase converter three phase wasn't an issue. You can get some great old machinery cheap that runs on three phase and
I ended up with three more machines.

 

[sotxbill]

Very first question is.... do you have 3 phase power?? If no, then convert.

 

[bc]

I assume you already have 3 phase? I watched some industrial size welders go cheap at auction wishing I had one. Mentioned something to the power company, Evergy, and was told all I have to do is pay for and about a mile of extra lines and new transformer at the pole plus the pay for getting the wiring and boxes out to the barn. Welding a couple times a month, I would breakeven about the time I turn 150 years old. Guess I'm stuck with single phase.

John T, we are working through the electrical questions. I grew up in small town construction where we built houses from the ground up. I was the 5 year old in the footing ditch with a spade squaring up corners and the 5 year old pushing and pulling a hoe through the mortar box and running the cement mixer when I was strong enough to flip the barrel over into the wheelbarrow. My grandpa was a master carpenter who built all the cabinets and finish work. My uncle was a carpenter who did all the electrical and plumbing. They could hand saw a 2by8 faster than I can use a power saw. They hammered spikes like nothing and I don't think anyone uses spikes in construction anymore.
Continuing education and being involved in code enforcement got me following all the electrical and plumbing forums along with the new know nothing building inspectors asking questions at the forums. Was going to take the electrical and plumbing tests cause my cousin who took over the family business and knew everything but wasn't very good at passing tests. He had a heart attack and slowed down and found a friend who could get his freon for him.

Anyway I've seen countless posts on welder circuits and then then everyone goes nuts when an electrician brings up the NEC exception allowing 12 gauge wire with the right temperature rating to be used for short runs on a low duty cycle welder in certain circumstances. I don't have the number on the tip of my tongue but I prefer to follow the general rules on wire size, length, and amperage.

 

[K Effective]

Thanks for the replies- I'll fill in some of the info. This mill was on an auction site I frequent, but was more of a consignment than normal. The seller said it worked, but the power feed was broken. The site added that it may not even run- it was not powered in the pictures they posted.

I do not have three-phase, which is why I said I was planning to use a VFD to run it. But my question was more along the lines of "If the motor needs work, why not just switch it to single phase rather than fix the three phase and still need to convert it."

The answers that included the instant reverse is the key- it would have been better to keep it 3P and use the VFD assuming the motor wasn't complete trash.

I got outbid at $350, the $355 got it. Still pretty cheap (plus tax and 15% sellers fee, of course) for a 1988 AcraMill. I'm not really ready for a mill yet, but need to replace an old craftsman drill press, and figured for $300 are you kidding?

 

[Dusty MI]

I've been retired for almost 20 years, so I'm not up to date, but VFD changes the speed on 3 phase motors, and last I knew does not convert single phase to 3 phase.

Dusty

 

[David G]

I would be happy if my mill had 3 phase motor, then easy to put on VFD.

 

[David G]

A VFD converts AC to DC then back to AC at adjusted frequency and power factor. The input section must be sized for enough watts to keep the DC bus charged, there are more watts in same amperage of 3 phase. The single phase input models just suck more amps than the 3 phase so are upsized.

 

[BarnyardEngineering]

I may be mistaken but my understanding was that three phase was preferred with machine tools to reduce vibration and harmonics.

 

[David G]

It is, there is less time between the impulses, much smoother.

 

[John T]

bc, heres the deal on sizing conductors which non electricians and non engineers aren't familiar with. There are MORE THEN one table/chart
and ratings THERE ARE VAST DIFFERENCES DEPENDING ON such variables as:

Temperature,,,,,,,,Class and type of Insulation,,,,,,,Conductors in free air versus jacketed,,,,,,,,,,,versus if enclosed in
conduit,,,,,,,,,,versus how many conductors in what size conduit,,,,,,,and other such things as Duty Cycle and Maximum Continuous Current
etc etc etc.

The way I was taught at NEC seminars when I practiced power distribution was to compute the Maximum Continuous Current,,,,,,,,,Size the
Conductors (see above) to have a minimum ampacity of 125 % of the MCC,,,,,,,,,,,Size the Overcurrent Protection to protect the wire

John T Long retired Electrical Engineer

 

[John T]

That and many other reasons to choose three phase power when I was practicing. In all my career (and NEC Seminars taught) if it was
available anything much over 3 to especially 5 HP I specified three phase........

John T

 

[John T]

David, you state "there are more watts in same amperage of 3 phase"

CORRECT Power = Square Root of Three X Power Factor X Volts X Amps

John T

 

[MarkB_MI]

For the same reason an inline six cylinder engine is so smooth: Torque and power are almost constant.

 

[kevinstuff]

Most of the 3 phase motors on mills are not a common frame, shaft size and length
So the biggest advantage of 3 phase , is its ability to instantly reverse , smooth quiet cool operation
So before wasting time trying to find a single phase , I would just have the original
motor rebuilt , and use a vfd that is oversized , Also many three phase motors are dual voltage, 240 or
480 volts , and some are just 480 volt only , if you decide to get it rewound make sure its 240 volts
If you want to test the motor your self , connect 2 of the hot single phase , one to each leg of the 3 phase
3 lead motor , which will leave you 1 leg of the motor not connected , then by any means manually get the shaft
spinning fast , and then apply power , it will run or it will slow down, if it slows down change one of the leads to the open leg,and maintain an open leg Now be careful that you secure the motor so you don't drop it and break the pulley to reverse 3 phase all you have to do is swap out 2 of the three legs , in this case though you only have 2 legs , which is why you have to get it spinning fast , A 3 phase motor will run on 2 legs as long as its spinning , this is theory behind phase convertors , get a larger 3 phase motor and get it spinning and when it reaches speed the motor becomes generator exciting that open leg with a little voltage drop
then you will have 3hase power . I would just take it to a motor shop have it tested and rewound if needed

 

[Oliver 500 Wa]

It would be kind of interesting to figure out why folks think that a VFD designed for single phase input needs to be upsized for the output load.

The only time one [u:cd8b985394]might[/u:cd8b985394] need to upsize would if using a VFD designed for 3 phase input with single phase input. In theory one should, but in practice it is rarely needed when used in a machine tool application.

 

[buickanddeere]

(quoted from post at 18:40:57 11/12/20) It would be kind of interesting to figure out why folks think that a VFD designed for single phase input needs to be upsized for the output load.

The only time one [u:100261265e]might[/u:100261265e] need to upsize would if using a VFD designed for 3 phase input with single phase input. In theory one should, but in practice it is rarely needed when used in a machine tool application.

Because single phase power in is only powering two out of three of the VFD's input terminals . This limits rectification capability and reduces power available from the DC bus to be inverted in AC.
A 10Kw three phase inverter is limited to 5.77Kw output power when supplied with single phase power . Square root of the number 3.0 .
Same as a three phase motor that is operated on single phase via spinning the shaft prior to applying power. That motor is also limited to 57.7% of it's nameplate HP.
If you have a few hours , we can go through the phase diagrams .

 

[Oliver 500 Wa]

@buickanddeere
Was talking about single phase input VFDs, not really about 3 phase input VFDs.

When a VFD designed for single phase input, the input side is designed for the additional current draw. As an example the one on my bandsaw has the single phase input rated at 9A. The output side is rated 4.8A. It is sized for a 1HP 3 phase motor and can run it at FLA. NO NEED to upsize it. Every single phase VFD in my shop (3 of them, 1HP, 2HP, and 3HP) has the same characteristic.

 

[buickanddeere]

Certainly if the VFD designed for only single phase input and three phase output .
I was thinking of the ordinary boring garden issue three phase in and three phase out VFD .