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View Full Version : Bought 3/4hp 3phase motor by mistake, what to do with it



dvo
09-13-2011, 10:30 PM
Other than a door stop what can I use this thing for? rotary phase converter?(I know nothing about the subject matter)

Hal
09-13-2011, 10:32 PM
Buy a VFD and you have great motor for most anything.

Hal

Duffy
09-13-2011, 10:33 PM
Buy a phase convertor to go with it and carry on. A teco convertor will cost about $120.00, and you will NOT be sorry.

Forrest Addy
09-13-2011, 11:02 PM
I vote for the VFD (it will cost you about $100). Put the motor on your drill press and you can plug the VFD into a 115V wall outlet. Here's a link:

http://dealerselectric.com/item.asp?cID=28&PID=1104

You will kiss the ground I walk on for making this suggestion.

danlb
09-14-2011, 12:54 AM
Would it be worthwhile to put one of these on a 9x20 lathe, to replace the 3/4 horse that is on there now?

Dan

Forrest Addy
09-14-2011, 02:53 AM
Would it be worthwhile to put one of these on a 9x20 lathe, to replace the 3/4 horse that is on there now?

Dan

Hell yes, but you might be happier with a little more HP.

rws
09-14-2011, 07:29 AM
You just can't beat a VFD on a lathe.....

Deus Machina
09-14-2011, 07:50 AM
I vote for the VFD (it will cost you about $100). Put the motor on your drill press and you can plug the VFD into a 115V wall outlet. Here's a link:

http://dealerselectric.com/item.asp?cID=28&PID=1104

You will kiss the ground I walk on for making this suggestion.

Well, hello...

Do those work alright on 110, 15-amp circuits? And do they have a 115 -> 220 single-phase?

BigBoy1
09-14-2011, 07:58 AM
I used a small 3-phase motor to build my own rotary phase converter so I can run a larger HP 3-phase motor on my lathe. Your 3/4 HP just might be just the ticket to do something like that.

J Tiers
09-14-2011, 08:05 AM
use more than 3/4 HP on a 9 x 20?

OOOOOOKAAAAAYYYYY

Seems like that is enough to take two turns and a half hitch on the bed of a 9 x 20.

I have a 1/3 HP on a 10" Logan , and have NEVER had any limitation from the motor whatsoever. And I KNOW that machine can stand a larger motor just fine.

JT



I used a small 3-phase motor to build my own rotary phase converter so I can run a larger HP 3-phase motor on my lathe. Your 3/4 HP just might be just the ticket to do something like that.

The usual rule is the reverse..... A 1 HP RPC for a half horse load, 2HP for 1 HP load, etc.

I am not sure how you have the "small" RPC set up to start the "larger" motor on the lathe. if it works, I guess it works, but...

jep24601
09-14-2011, 09:46 AM
My 13" swing LeBlond came with a 3/4 HP motor.
(Though maybe that was to limit damage as it was originally supplied to a high school)

lakeside53
09-14-2011, 11:25 AM
3/4 hp on a vfd will only give 3/4 hp around it's nominal frequency of 60hz. At say 30hz, you'll get about 3/8 hp..

If it worries you (shouldn't) then just set the VFD motor current so that it can only get 1/2 hp out.


Yes, you can buy a VFD that runs from 120v, BUT... it won't run on a typicial GFCI (if that bothers you). There is no "fault" - just the vfd noise currents being filtered to ground inside the vfd.

dvo
09-14-2011, 12:06 PM
I used a small 3-phase motor to build my own rotary phase converter so I can run a larger HP 3-phase motor on my lathe. Your 3/4 HP just might be just the ticket to do something like that.


This sounds like a better use for my little motor. Please elaborate? What was the hp of your rpc and hp of the lathe motor? and what the max hp the rpc can drive?

Forrest Addy
09-14-2011, 03:11 PM
dvo. I don't think I'm overselling when I suggest you research the VFD ad get some referrals. You havve a several speeds on a lathe, mill, drill whatever. The variable speed feature of the VFD fills in between - an important thing when you need lower speeds than you can gear for.

You may wish to avoid what seems like complication but installing a VFD is no more complex than building a phase converter.

I'm sure a few will chime in with VFD testimonials. I have four on my machine tools.

ironnut
09-14-2011, 03:29 PM
Actually Forrest's suggestion is an excellent one. I replaced the 1 hp Taiwanese motor on my 30 year old Grizzly mill with a 3/4 hp 3 phase 1725 rpm motor. It is so sweet. The VFD was just over $100. It will drive a 1 hp 3 phase 220 volt motor while being fed by 120 volt single phase power. It has a surprising amount of torque at low speeds as the VFD can push more current to the motor in an attempt to maintain the speed setting. Reversing is very easy. I love not having to change belt positions to get correct speeds. I can run 3/16" to 1/2" drills with good results, even larger if I take it easy. I would never go back to the single phase motor.

I don't believe a rotary phase converter based on a 3/4 motor can safely run a 3 phase motor larger than itself. I thought that typically if you wanted to run a 3 hp 3 phase motor, that you needed a rotary converter based on a 5 hp motor.

gordon

gellfex
09-14-2011, 03:40 PM
I vehemently vote with the VFD crowd. If I had to do 1 tool it would be the drill press. insanely easy to dial in the right speed, can overspeed the motor somewhat if you really need that, and makes tapping a breeze.

BTW, I have a 1/2hp on my 10" logan and the belt will slip long before the motor strains. I like that actually, keeps things safer.

MaxHeadRoom
09-14-2011, 04:22 PM
You didn't mention the RPM, if it is a 4 pole motor 1740rpm something, then you can run in up to 120hz and double the rpm.
I wouldn't suggest going to 120hz with a 2 pole, however.
Max.

Tony Ennis
09-14-2011, 06:58 PM
I have one of those $100 VFDs on a 1/2 HP 3ph motor.

I power my Atlas with it.

Best.
Thing.
Evar.

dvo
09-14-2011, 07:14 PM
I already have variable speed on my lathe and drill press, mill is a gear head. So the post about the rpc perk my ears as I shy away from a lot of equipment that I want but are 3phase. I did read somewhere about diy rpc and it did say small to drive larger load motor. I need to do more research on the matter. Seems like a lot of variation and opinion on what I think should be "one" schematic(circuit). Not a different one for each person that builds one. My original door stop is starting to sound pretty good :D

PixMan
09-14-2011, 07:19 PM
I used a small 3-phase motor to build my own rotary phase converter so I can run a larger HP 3-phase motor on my lathe. Your 3/4 HP just might be just the ticket to do something like that.

I agree with J Tiers, it's the other way around. I have a 15HP 3-phase motor as an idler for the rotary phase converter, to run a 5 HP lathe, 1 HP Bridgeport and 2 HP surface grinder...and all at once if I need to.

MaxHeadRoom
09-14-2011, 07:53 PM
I did read somewhere about diy rpc and it did say small to drive larger load motor. I need to do more research on the matter. Seems like a lot of variation and opinion on what I think should be "one" schematic(circuit).

I based my RPC on the old Fitch Williams design http://metalworking.com/dropbox/_1998_retired_files/
Look for the FRW- files.
But I think the way to go is use a VFD.
Max.

flylo
09-14-2011, 10:14 PM
Put in a 10HP Rotary Phase Converter & liked it so much I just bought a 7.5 HP RPC & a 440V Transformer. Now I can run anything I want. 3 phase equipment in Michigan is less than scrap price,especially 440, because they don't have the hassle of hauling to the scrap yard. So much stuff wasted in the US.:(

justanengineer
09-14-2011, 11:06 PM
Well, hello...

Do those work alright on 110, 15-amp circuits? And do they have a 115 -> 220 single-phase?

I have one of the small Teco's running the original 3ph 1hp motor on my Bport. It will run off both 110 and 220, but I have had no problem with running it off of 110 at 15 amps and have never had it trip the breaker. Though I cannot wait to buy a house w/220 again (planned for next summer) for my welders, for my J-head and 12x36 Clausing I have no need for it. Having always run 220 to machines previously, I was rather amazed how switching to 110 yielded no noticeable difference for me.

3 phase motor? I prefer it!

J Tiers
09-15-2011, 12:19 AM
Get a VFD for it, and put it on a machine. Lathe gives the best bang for buck, but DP is another good one.

3 phase is, in my experience, much smoother and less apt to start chattering on lathes in particular.

Everything you read on how to stop chatter will actually work..... With single phase, you will be left wondering what those guys were smoking when they wrote all those things.... because many of the speed changes, feed changes etc won't do a thing about the chatter.

And, if you have a flat belt drive.... with single phase, if it starts to slip, that's it, it's usually coming off the pulleys. Very hard to recover gracefully.

With 3 phase, slipping is more gradual, and due to the smoother pull, if it starts to slip, you can usually catch it and recover by slacking the feed for a short time.

Try it, you won't go back willingly.

BigBoy1
09-15-2011, 07:47 AM
This sounds like a better use for my little motor. Please elaborate? What was the hp of your rpc and hp of the lathe motor? and what the max hp the rpc can drive?

The motor I used in the rotary converter was a 3/4 HP, 220 volt 3 phase, as it was a freebie. The 3 ph. motor in my lathe is 220 volt, 1 HP. I had a friend who understands electrical stuff, design the circuit and I built it to his specifications using the 3/4 HP motor. The 220 volt 3ph lathe motor runs like a champ using 220 volt single phase power. I'm sorry I can't answer as to the max. HP motor my rotary converter will run as I haven't a clue. As long as my lathe runs and works, l'm happy.

J Tiers
09-15-2011, 08:23 AM
The typical ratio is to have an RPC of 1.5 to 2 x the largest single motor it will have to start. So if your machine has a 1 HP motor, 1.5 or2 HP would be a good size for the RPC.

The issue is really voltage drop. The RPC adds "impedance"* in series with the "manufactured" leg of the resulting 3 phase. It has to, there is "electrical stuff" in series with it that doesn't exist on the other two wires.

The RPC must take in power on the single phase wires, and output that power on the "manufactured" leg (or "phase"). That process includes some losses, which show up as a voltage drop generally proportional to current. Larger motors will have less impedance in series with the manufactured leg, and so will produce less voltage drop.

When you start up the load motor, it draws a large amount of current, as much as 6 times the normal full load current is common. There must be sufficient voltage on the manufactured leg to provide the turning torque required.

If the voltage drops too far, the current will also be very low, and starting will be sluggish, or perhaps the motor will not start at all.

The larger the RPC motor used, the less fussy you will need to be about "balancing".... adding capacitors to even out the loaded voltage among the three wires.

Since you have ONE motor, your best bet for the moment is not to get yet another motor, but rather to get a VFD for the motor you have, and use it.

That motor isn't large enough to make a good RPC, and getting another larger motor to make an RPC could cost more than a VFD, unless you can get it cheaply used.

* Impedance is an AC property, similar to DC resistance, but includes the AC effects of capacitance and/or inductance along with the resistance.

vincemulhollon
09-18-2011, 11:51 AM
Other than a door stop what can I use this thing for? rotary phase converter?(I know nothing about the subject matter)

No one else mentioned that 1 phase VFDs do exist, make sure not to buy one, since you need a 3Ph, and with your luck you'll end up with a 3ph motor and 1ph controller... In the last "couple decades" they've gone from aerospace prices to practically as cheap as the electrician wiring the wall outlet it'll plug into.

Second problem is gonna be the cooling fan doesn't keep the motor cool when running slow speed and hogging it, so may as well plan to rig something up with full speed computer fans and air ducts right away.

Or blow lots of money on a controller smart enough to shut down when the motor gets hot; Most (all?) controllers will shut off when the controller gets too hot, so you can proudly do the 3rd world engineering thing and install a bunch of high powered old fashioned lamps in the controller enclosure, thus making the controller overheat and shut down before the motor overheats, at least in theory. Also great for preventing condensation corrosion.

J Tiers
09-18-2011, 12:04 PM
No one else mentioned that 1 phase VFDs do exist, make sure not to buy one, since you need a 3Ph, and with your luck you'll end up with a 3ph motor and 1ph controller... In the last "couple decades" they've gone from aerospace prices to practically as cheap as the electrician wiring the wall outlet it'll plug into.

Second problem is gonna be the cooling fan doesn't keep the motor cool when running slow speed and hogging it, so may as well plan to rig something up with full speed computer fans and air ducts right away.




1) the one-phase VFDs are uncommon enough not to be much issue..... Although several companies make them.

2) one does not throw away the back gears etc..... Slowing the motor to less than perhaps 1/3 normal RPM loses so much power that it is impractical, unless the motor and VFD are way over-sized for the basic need.
You keep the belts and back gears, and use the VFD for adjustment of speed.

3) Most any home shop use will not stretch the cooling capacity of the motor, particularly if you do not insist on going to 1/10 rpm.....
The thermal time constant of most motors is an hour or so, they don't heat quickly unless well overloaded, which a matched VFD will not allow

I have a 1/3 HP totally enclosed 3 phase motor rated intermittent duty (garage door opener) on a Logan 10" . I have done fairly heavy stock removal making mill arbors from 4140, and the motor surface has never gotten hot enough to make me suspect the hottest internal temps are too hot.

vincemulhollon
09-18-2011, 12:19 PM
Slowing the motor to less than perhaps 1/3 normal RPM loses so much power that it is impractical, unless the motor and VFD are way over-sized for the basic need.

Agreed 90% of the time with exception of VFDs widely marketed as "now you can slowly tap in your drillpress"...

J Tiers
09-18-2011, 01:36 PM
Agreed 90% of the time with exception of VFDs widely marketed as "now you can slowly tap in your drillpress"...

They take advantage of "torque", but not HP. Doesn't change the power..... but if they have full torque (current) available, due to being "vector" and not regular V/Hz, then it works.

And then perhaps some heating issues may occur, if you do that a lot.

Some totally enclosed VFD-rated motors are rated for 10:1 speed, without temperature issues, no added fan needed.