Transformer size question-conflicting info?
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    Default Transformer size question-conflicting info?

    I recently got a nice Emco Maximat V13 lathe. The motor is 2-speed, 2/2.4 Kw (about 3 HP?), 3-Ph, 440 V only.

    I'm collecting parts for a 5 HP RPC, but I'll also need a transformer to get from my shop 240 V 1-Ph to 440 V for the lathe.

    I've perused a LOT of articles on here and around the web, and there's a lot of variation in recommendations for Xfrmer size. I've read that 1 kVA /HP is enough, but figuring in start-up draw, motor efficiency, etc, that doesn't seem like enough. The most general opinion seems to be about 1-1/2 to 2 X the motor HP in kVA, which would put me needing about a 5 or 6 kVA unit for my 3 HP motor.

    Any in-the-know confirmation on this? (At least for a while, the lathe will be the only machine powered by this particular setup, and if I add to it's job description in the future, it should still only be running one machine at a time.)

    I actually used to be a licensed residential electrician for several years, but this is quite outside my experience.

    One other thing-the motor has a magnetic brake-any problems I should be aware of with that, as pertains to the RPC/Xfrmer setup?

    Thanks for any help.

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    Are you planning on putting the transformer before or after the RPC? Makes a difference. If before the PRC, you also have to factor in the 1 phase to 3 phase current factor, 1.732. The 3 phase current rating of the motor will need 1.732 X the current from the 1 phase source.

    Perhaps one reason you see a disparity in sizing suggestions is because there is no one simple answer. For one thing not all transformers are the same. Some, such as "Control Power Transformers" are specifically designed to provide much more starting current capability than general purpose transformers. CPTs are usually limited to about 5kVA and are always 1 phase, but in this type of application that covers a lot of machine tool motors. So you might be able to use a 1kVA CPT for a 1HP motor where you will likely need a 2kVA GP transformer to do the same task. It also makes a difference how big the motor is, how heavy the load is, how you are starting the motor and what kind of voltage drop you are willing to live with. For example if you are using a VFD, even if just as a phase converter, it is always inherently soft starting and can accelerate most motors without ever going above the FLA rating. But Across-the-Line starting can really suck up kVA and a general rule for that is 2 to 2-1/2 X the HP rating (in my experience) if you use a GP transformer, maybe more if you have a heavy inertial load.

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    Jraefl has a lot of experience in this area so you should strongly consider his comments.

    1kva/hp should be fine in my book (a slight difference in my thoughts vs Jraefl is that peaks/inrush are momentary and most decent design xfmrs can handle 2-3-4-5-6x rated for short inrush periods. therefore, 1kva/hp is fine if decent xfmr.

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    Ok-let me preface my next comment by saying, earnestly, thanks for your input. However... See what I mean? Differing opinions.

    So-I'm planning to step up the voltage after the RPC, since I foresee getting more 3-Ph stuff in the future, but probably not any more 440 stuff, but one never can tell... Should have mentioned that before.

    I found a good deal on a GE gen purp 6 kVA that should do what I need, so I think I'll go with it. That's for my afore-mentioned 3 hp motor. If I should run into trouble starting it in high gear, I'll post so all can share my dismay.

    Thanks again.

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    Default 440v 3 ph

    Clang,

    I think maybe you should consider a single phase transformer and a VFD for your EMCO, especially since you do not anticipate any further 440v machines.

    You might be interested in the following thread:

    http://www.practicalmachinist.com/vb...20-1ph-172472/

    Good Luck on your Project,

    Joe

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    3 KVA was suggested by ACME TRANSFORMER for 1 1/2 HP 440 only four speed and has worked flawlessly for over ten years. This is down stream from first RPC and later Phase Perfect.

    J.O.

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    I keep bringing this up and have been met with total silence. A three phase transformer with all the windings on a single core or three single phase units connected in wye-delta or vice versa will have output phases that are composites of the input phases, not just transforming each separately. That should blend the output of an RPC to give more uniform phases. I have everything to test that, but since I have three phase in the shop, there is little reason aside from curiosity to do it.

    Bill

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    Quote Originally Posted by 9100 View Post
    I keep bringing this up and have been met with total silence. A three phase transformer with all the windings on a single core or three single phase units connected in wye-delta or vice versa will have output phases that are composites of the input phases, not just transforming each separately. That should blend the output of an RPC to give more uniform phases. I have everything to test that, but since I have three phase in the shop, there is little reason aside from curiosity to do it.

    Bill
    Bill, I'll bite..... if 3ph on 1 core, yes, different voltages on different phase will slightly combine to some extent and try to equalize all since the magnetic field for all 3 phases have to run in parallel across the top and bottom of the steel core and must cancel out in the end.

    3 SEPARATE core 1ph xfmrs wired in 3ph will NOT try to equalize voltages at all.

    delta/wye/etc has no bearing on either design above.

    As one who has designed, applied, and sold xfmrs for over 35 years, I am always happy to take extra money from someone who wants to pay me for a 3kva xfmr when a 1.5kva is all that is required. but I WILL tell them first before taking their money that they are oversizing it. then if they are happy, I am happy.

    this is sure to stir a hornets nest among the other electrical engineers here, but that is probably a good thing.

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    Joe H, thanks for the link-lot of good info packed in that one, and I hadn't seen it yet. I'd actually rather not use a VFD here, for a few reasons, some of which are admittedly a little neurotic. The gist is, though, that I like mechanical controls/machinery, I'd rather leave the lathe in, or at least close to, it's original state, and I don't really see the need for a VFD for this app. The lathe was, IMO, well thought-out as to speed range, etc. Also, I've read several places that using a VFD as a phase converter is generally a ticket to a shorter operative lifespan. And, of course, I can always use the RPC to run other stuff, presumably for little to no added expense. All that said, though, I think a VFD would be a god-send for a lot of applications.

    Interesting thoughts re the phase blending. Since most of what I know about Xfrmrs I've learned lately, how common (if) is it to find a single-core 3-Ph Xfrmr vs multi-core (3, I guess?)?

    Another, somewhat-related question: I've seen at least 2 how-to's for RPC's that suggest run cap's across 2 sets of phases, but read elsewhere that a run cap should only be across one set of phases, or it can cause phase-shifting problems. Input on that?

    As always, all replies here are appreciated.

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    Quote Originally Posted by CLANG View Post
    .......
    Interesting thoughts re the phase blending. Since most of what I know about Xfrmrs I've learned lately, how common (if) is it to find a single-core 3-Ph Xfrmr vs multi-core (3, I guess?)?............
    99% of 3ph xfmrs are 1 pc as you are used to seeing. there is no economical or engring reason to make ur own 3ph xfmr out of 3 single phase ones - except if u dont have a regular 3ph xfmr but do have 3 single phase ones in the junk box that u can use and not have to buy one.

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    Default 440v 3 ph

    Quote Originally Posted by CLANG View Post
    The gist is, though, that I like mechanical controls/machinery, I'd rather leave the lathe in, or at least close to, it's original state, and I don't really see the need for a VFD for this app. The lathe was, IMO, well thought-out as to speed range, etc. Also, I've read several places that using a VFD as a phase converter is generally a ticket to a shorter operative lifespan. And, of course, I can always use the RPC to run other stuff, presumably for little to no added expense. All that said, though, I think a VFD would be a god-send for a lot of applications.
    Clang,

    You have two issues with Emco that usually require special attention. The motor is two speed and the voltage is 440v.

    The standard static is not a viable choice. The single phase transformer to 440v with a 440v RPC will work although some balanceing may be required to get it to work with the two speed motor. The downside is if you add 240v machines you will need a 3 phase isolation transformer to run them from the 440v rpc and the step up transformer must be sized to handle the addition load. If you plan on adding additional 240v machines you may be better off to use a 240v and a 3 phase transformer to power your Emco so that any further 240v additions could be powered directly from the rpc.

    In your application a VFD would only be used as a phase converter. It can be wired up so that you use all the existing control functions of your machine if you like. If you ever need really slow rpm for something special like relieving cutters a VFD will slow the rpm to a crawl. I totally agree with you about the original lathe control set up.

    Joe

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    Ok-I think you misunderstood my intended set-up. I'm planning to run a 240 RPC, then a 240-440 Xfrmr just for the lathe, for all the reasons you just mentioned. Re the VFD, one of many things that I particularly liked about this lathe was the lowest speed of 30 rpm's.

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    This is a ten year old thread... but I am doing the same thing that CLANG was talking about. Any chance of an update? I am having trouble finding the xfrmr that was mentioned, but will keep trying. There is also a Emco Mill that connects to the lathe, and a 440v pump motor for the cutting fluid to run as well. Any suggestions on where to look for the xfrmr? Things to look out for? Thanks

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    Keith

    Transformers can be used in reverse. Most transformers are manufactured for use as a step down transformer such as 480 to 240. The step down transformer may be reverse connected and used as a step up transformer. The turns ratio will possibly be off a tad as it is optimized to make up for losses in the transformer.

    I would look on ebay and other auction venues for a 480 to 240 step down transformer.

    Bill

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    Quote Originally Posted by CLANG View Post
    See what I mean? Differing opinions.
    Yah well.. there IS a good deal of flexibility, y'see. Not as simple as politics, where there is only ONE right answer.

    Plus other considerations. Full, not auto- transformers have a filtering effect as well

    I use a 27 KVA 1:1 (Delta to Wye) for 10 HP RPC XOR 10 HP Phase-Perfect.

    But that's so I can add-on idlers to 22 1/2 RPC idler power..

    Wasteful, yes. But it was cheap.

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    Thanks Bill, that was what I was given to understand. I overstepped my budget on the lathe itself (sorry honey), and have to get a few items just to tool up to a larger lathe than I had previously. I am seeing basically what I think I need going for $500-$700, and I am really hoping to find a used Xfrmr for $100-$200. My query has to do with the lack of HP rating on the motor tag. I see 2/2.4 KW, 5.4/5.6 A..... would this lead you to think the motor is roughly 3HP, and consequently look for a 6KVA rated Xfrmr? Would 5 KVA work? Less? thanks for the help!

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    I am confused by the information on the motor tag. 5.6 amps three phase at 480 V is about 4.5 kW. I would double check those numbers. Perhaps the current is for 240 V operation. That would make a little more sense.

    If it really is 2.4 kW then then a 5 kVA tranformer should work fine. Also check the frequency the motor is rated for. Many of the motors I have run into that are rated in kW are rated at 50Hz. This is not a show stopper but it does change the ideal voltage for operation at 60 Hz.

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    Quote Originally Posted by Keith Gotschall View Post
    Thanks Bill, that was what I was given to understand. I overstepped my budget on the lathe itself (sorry honey), and have to get a few items just to tool up to a larger lathe than I had previously. I am seeing basically what I think I need going for $500-$700, and I am really hoping to find a used Xfrmr for $100-$200. My query has to do with the lack of HP rating on the motor tag. I see 2/2.4 KW, 5.4/5.6 A..... would this lead you to think the motor is roughly 3HP, and consequently look for a 6KVA rated Xfrmr? Would 5 KVA work? Less? thanks for the help!
    Unless.. you are IN Europe.. or find a part-out, as I did (but did not buy..) on an Italian-made transformer of 6.6 KVA IN the usa..

    What you are most likely to find in either the economically-priced, new or the used-but-good market are even steps that WE find handy in volume production.

    So 5 KVA, 7.5 KVA, etc. Or wotever else turns up most often, any range of needs.

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    Thanks Bill, I did re-check the tag and it is for 60hz, so that is good. If you look at the original post, it is also 440V 5.6A 2.4KW. In fact it is for the same machine too!
    I have looked into the ebay market again, but searched for step down instead of step up, and found a lot more listings. So thanks for that. I am waiting on some call backs from sales people I contacted on Friday to see what they come up with. Good to hear a 5KW would probably work.
    Question, it is a two speed motor, which I've been told negates the possibility that it was 440V changed to 220V. Does that sound right?

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    How I would interpret this is the motor tag is that the 2.4 kw converted to hp is 2400/745=3.2 hp. Due to motor inefficiencies the power and phase angle the current is higher and thus the transformer must be sized for a load of 440 X 5.6 X 1.7= 4.188 kVA. The 5kVA transformer is very adequate to RUN the motor but some would be concerned if it is enough to start the motor. Some say the transformers can easily withstand the surge and others recommend more head room. If would either look at manufacturers data sheet or ask the manufacturer for additional information as they know what their product can do. Looking at Dongan transformer sizing chart your are on the good size of things with a 5kVA transformer unless you have motor starts of more than one per hour and then you would be a little bit over spec.

    That said I have a single phase 3ph pump that I have been using for years on a pair of 5kVA transformers one reversed for step up and another for step down with 1700 feet of wire between them that have been working fine for years.

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