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  #1  
Old 09-05-2006, 05:37 PM
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Bob Warner Bob Warner is offline
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3phase motor compared to single phase

Does anyone know if a single phase moter can directly replace a 3 phase motor?

What I mean is, if a machine has a 1HP 3 phase motor, can I use ANY single phase 1HP motor as a replacement without loosing performance?

Since torque is not printed on a motor there is no way to know if the machine will perform the same after a motor replacement.

I need to replace a 1HP motor on a small mill I just got and do not have a motor to put on it. My search for a motor must start soon.


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Old 09-05-2006, 06:13 PM
Jerry Shorter Jerry Shorter is offline
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Hi Bob, you won't lose much in the way of performance, The single phase will draw more current but thats about it. You do have to make sure that the motor is reversible. you will find that info on the motor info plate. the wiring will be different on your drum switch for a single phase motor. also get a motor that you can wire for high voltage,(240) you will find out that a higher voltage helps with performance. You can go with a phase converter and keep your 3-phase motor however you will lose about 30 percent power. best bet would be to see if you can find a V.F.C (variable frequency control) Single phase in 3 phase out. A 2 H.P. V.F.C. would be Ideal. That way you could use the motor you have.

Jerry
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  #3  
Old 09-05-2006, 06:46 PM
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Bob Warner Bob Warner is offline
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Thanks Jerry,

I am not much on static phase converters due to the power loss.

A rotary would be OK but would cost as much as a motor tobuild I would guess because Iwould not want a "push start" kind of thing and the capasitors to get on going as needed would be expensive.

The VFC is different than a rotary phase converter or the same thing?

Thanks


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Old 09-05-2006, 07:00 PM
Rob Frink Rob Frink is offline
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Bob, no, you will not lose any performance. 1 hp is 1hp regardless of the motor...AC, DC, 1ph...3ph...whatever. As far as torque, that is in the hp spec. When comparing motors of the same hp and same rpm...they will have the same torque. This is because the fundamental definition of HP is Torque X speed. If both motors have the same speed and same HP...then they will have the same torque. I have replaced 3ph motors with single phase motors on several machines...just match the frame size and speed. Its a great way get high quality, hi-performance industrial machinery for the home shop. At industrial auctions, most home shoppers will stay away from bidding on equip that has 3ph motors on it.


The HP, torque, speed, voltage, current and number of phases is all related mathmatically by the concept of "power in = power out". Basically, to make 1 hp of mechanical energy, you need 1 hp of electricity....and so on.

This is given that we are truly talking about industrial motors with true ratings...not some crazy marketing scam like a 7hp shop vac.


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  #5  
Old 09-05-2006, 07:50 PM
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Bob Warner Bob Warner is offline
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Good to know Rob, thanks.

Now, let me add to the confusion a little. I "may" be getting a hydraulic surface grinder. I have been helping a guy sell the equipment that I show in another thread. In return I asked for this:



I saw it briefly and do not know what size the motor is. What do I do if it is a large motor? The largest motor I saw in single phase is 7.5hp on an air compressor and cost about a billion dollars. If the motor is over 5hp (which I should be able to find fairly easily) in the surface grinder I am not sure if I can get this thing running.

What do you guys think?


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Old 09-05-2006, 08:43 PM
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Ray Rogers Ray Rogers is offline
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Surface grinders don't seem to have large hp motors on them in my (admittedly limited) experience with them. Instead, they have some kind of motor (don't know the definition) that can deal with being brought to a stop by over zealous use without suffering any damage (at least, mine does). The larger problem is that on my machine and, I think, on the one in your picture, the motor is a custom frame special built kind of thing that you will never find a direct replacement for. That doesn't mean you can't replace it, it just means you'll have to work harder to build a way around it ....


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  #7  
Old 09-06-2006, 09:28 AM
Jerry Shorter Jerry Shorter is offline
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Bob, I have a Surface grinder just like that one, it is a Norton. The hydraulic feeds are really handy. There is a motor inside of the machine body (remove front cover to see it) this motor drives the hydraulic pump. it is 3/4 hp, I think that the spindle motor is 1/2 hp, but is a special kind and will not be easy to find a 1 phase replacement. good news is this unit will work just fine using a static phase coverter. You will have to make sure that the motors are wired for low voltage (220) If this machine works decent you will be very happy with it I'm sure. also the hydraulic cylinder that moves the table can easily be un-coupled to use the machine as a manual grinder (2 small clips on the ends of the cylinder) it is part of the design. Good machine don't pass on it.

Oh! A VFC does the same thing as a rotary phase converter but is not the same thing. you will not notice any loss in performance for what you would be using it for. there are many settings to adjust the VFC to your application. as a bonus you have a great way of controling your spindle speed. also comes in very handy for power tapping with instant reverse. I replaced a 2hp (chinese) motor on my mill/drill with a 2hp VFC and can tell you that it is the cat's meow. as a side note Chinese H.P. is different that the rest of the world. Its seems to be based on (volts X amps) instead of ((speed X torque)/5252). not even close to the same animal as far as power goes. Good luck!

Jerry
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  #8  
Old 09-06-2006, 11:21 AM
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Bob Warner Bob Warner is offline
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Jerry,

Yes it is a Norton. Glad to hear you have one and are happy with it. Do you have a book I can borrow on how to operate the thing?

Is the spindal motor an odd shape or what? Why will it be hard to replace?

I will look around for plans for a static phase converter. Did you build or buy yours?

If I build a phase converter I can use it with the small mill I have also so it can do double duty.

For those that own a mill, is there an advantage to having variable speed? I am considering replacing the 1HP 3 phase with a 5HP treadmill motor. The motor is physically small but variable speed. Any opinions on this idea?

Before I do anything with the mill I need to see if I can find collets that fit this thing as there is only the one in it and it holds 1/2" end mills. I am checking that out today at my favorite store. I'll take the collet I have in and hand it to the old guy there. He can probably tell me what brand it is and I can then buy other sizes of that brand as well as the end mills themselves.


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Old 09-06-2006, 12:47 PM
Jerry Shorter Jerry Shorter is offline
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Bob, I don't have a book but I can make up an operation manual for you. It is really pretty simple. Spindle motors are percision balanced and have high percision bearings in order to reduce vibration ( any vibration will show up in your surface finish) That being said, if you can find a single phase standard motor that will fit for cheap, give it a try, you may be satisfied with the finish. also if you plan on converting it to a 2x72 belt instead of the stone, it proably won't mater what kind of motor you have, The spindle motor that is now on it is probably 3600 rpm. If you get the grinder let me know and I will help you in any way that I can. The static converter that I have is commercial (it was free) but I did look into building one and it is fairly simple and well within your skills ( you did build that etcher!) it's no more complicated than that.

Yes there is a BIG advantage to variable speed on a mill, if nothing else then at least a multi-stage pulley system. if you have a variable speed drill press then you might consider using the parts off of it on your mill then you can use it for both. but you are right in finding the collets first. If it has a half inch collet already and you can't find other collets then you might consider getting a ER or a double angle collet system with a 1/2 inch shank that will fit into your collet. see http://www.useenco.com

Jerry

Last edited by Jerry Shorter; 09-06-2006 at 05:47 PM.
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  #10  
Old 09-06-2006, 08:45 PM
smird smird is offline
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Bob you can get a VFD/VFC here fairly cheap LINK I just bought the 1hp model to hook up my Bridgeport mill I just got. Hopefully have it set up this weekend. A pretty well respected member on cnczone.com said he's installed about 30 of these and he loves them. Not bad for about $100.00.

Brad Stilley

PS: the link has a PDF of the manual incase you want to see what you'd be gettin into.
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  #11  
Old 09-06-2006, 09:29 PM
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Bob Warner Bob Warner is offline
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Thanks guys but I think I am having a "Stupid Attack" or something. The VFC appears to be similar to a controller like I have on my grinider but also acts as a phase converter allowing me to operate the 3 phase motors in the surface grinder. In the case of the grinder, assuming it is the same as the one Jerry has, will have a 3/4Hp motor AND a 1.5hp motor for a total of .5HP combined. So I should buy a 2hp VFC correct?

Now from what I see this operates the motor. This confuses me with Jerry's statement of:
"I replaced a 2hp (chinese) motor on my mill/drill with a 2hp VFC and can tell you that it is the cat's meow." Does this mean you bought a VFC and replace the Chisese motor also?

This is clear as mud to me. Also at $125 for the VFC I will need to wait a while to make this all happen, still don't have a %&^$ job. Nobody wants a broken up 49 year old guy with no degree. Everything I try, fails and ends up costing me. I guess age and injuries makes you STUPID or something.

I'm just trying to get things going so I can at least do some shop stuff for small cash until I trick or blackmail someone into hiring me.

Hey, that gives me an idea, www.BLACKMAILJOBS.com
I could get a list of people that did bad things and gather the evidence on them from disgruntled employees and when I find someone needing a job I could call the guy up and say; "Hire this guy or I send the pictures to your wife."


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Last edited by Bob Warner; 09-06-2006 at 09:32 PM.
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  #12  
Old 09-06-2006, 10:34 PM
smird smird is offline
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Very simply the VFD takes 220v or 110 on smaller motors and converts it to 3 phase. So from your 220 single phase outlet goes in one side of the VFD out the other side 3phase.

Now the cool thing. our electricity it 60 hertz the VHD can out put the Hertz between say 10-90 hertz so your 3600rpm motor can slow to almost a stop to or turn faster than the rated 3600rpm. With all of the torque.

Jerry probably replaced his 2hp single phase motor with a 3phase motor and VFD
Its not a motor just a little box between your outlet and motor.

You need a VFD for each motor. One rated for the motors Hp no you don't need to go over.

For someone that fails at everything you sure have some helpfull tutorials on your site. I'm still trying to bodge together my press.

Keep asking question put all of on this site together and we're one smart sucker.

Brad
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  #13  
Old 09-07-2006, 10:16 AM
Jerry Shorter Jerry Shorter is offline
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smird is right Bob, sorry about the confusion. I replace the motor on my mill drill with a 3 phase motor and installed a VFC to run it. If your belt grinder has an AC motor the the controller is likely a VFC (variable frequency control) if you motor is DC then it is just a DC speed control and won't work on an AC motor. I would not hook up a VFC to the Norton grinder, not worth it and there is a safety issue with spinning the spindle at more than 3600 rpm (grinding wheels are only rated for 3600, at least the ones Iv'e seen. also if you spin it at less that 3600 rpm you will have to feed the table a lot slower. also the hydraulic pump motor must run at it's rated rpm in order to develope the correct pressure and volume. It is really better to just run a phase converter on the Norton. Considering your financial delimma let the Norton set while you aquire the parts necessary to build a static phase converter. it's really not much when you break it down, the simplest form is just a capacitor and a couple of switches. The cap. has to be the correct mfd and voltage for the intended hp but that is the only critical thing. check it out there is a lot of diagrams and instructions on the web. and the delay in getting the Norton running will give you a chance to clean it up and paint it. what the heck you have plenty of time.
hope this helps,

Jerry
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  #14  
Old 09-11-2006, 10:19 AM
SamLS SamLS is offline
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Three phase would be the preferred way to go if you have 3 phase power.

A 2hp single phase motor is around 9.0 Amps @ FL
while a 3 phase 2hp motor would be about 3.0 Amps at FL or full load. Its not just a little bit its 3X. Starting current is the same factor. The 3 phase motor @ the same HP will typically have about 10-15% more starting torque than its single phase counterpart with the same FL torque but the single phase motor needs 3x the current to start the same load. Thus the starting capacitors etc on the single phase motor to reduce peak current demand just on startup.

Also, When using a frequency drive or ( VFD ) and you are supplying single phase to run a 3 phase motor the VFD is typically derated to a lower current output. Depending on the brand of VFD you may have to jump to a 3hp VFD to run a 2hp motor. You really don't size a freq drive based strictly on HP you do it based on current demand for the application.

Good luck
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  #15  
Old 09-11-2006, 02:54 PM
David Peterson David Peterson is offline
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I've been reading this thread with interest. You guys are great with the info. I have a quick question. I have a 5HP 240V three phase motor that I'd like to run on my regular 240V single phase outlet. Would I need a simple inexpensive controller, or would it cost me an arm and a leg?

Sorry for taking the focus off of Bob's original question, but I thought this would be an OK time to jump in. Thanks in advance.

-Dave
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