# 240/ 480 trans sizing



## MDShunk (Jan 7, 2007)

100 kva is 100 kva no matter voltage you step up or down to, and no matter if you give yourself a manufactured phase. 100 kva is still your functional limit. 

On a side note, you need not size your phase converter for all the 3 phase loads (or even for the largest load) if any of the 3 phase loads will only run partly loaded or idle. The counter-EMF from underloaded 3 phase motors will bolster the 3-phase bus.


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## brian john (Mar 11, 2007)

Will the converters run all the time? If so consider a motor generator 240 to 480/277 3 phase.


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## MDShunk (Jan 7, 2007)

brian john said:


> Will the converters run all the time? If so consider a motor generator 240 to 480/277 3 phase.


I like the rotary phase converters (motor generators) much better than the static (capacitor) type. They're more reliable, make more accurate 3 phase, and are easier to service. The static type are a pain to work on, since it can often take a half a day to get the oil cleaned up inside them after a cap blows before you can start testing all the pieces and putting it back together again. The rotary are dirt simple. A single phase motor with the shaft coupled to a 3 phase motor, and the output from that 3-phase motor is used just like a generator output. All that normally needs serviced is the starting components of the single phase motor.


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## user4818 (Jan 15, 2009)

MDShunk said:


> I The rotary are dirt simple. A single phase motor with the shaft coupled to a 3 phase motor, and the output from that 3-phase motor is used just like a generator output. All that normally needs serviced is the starting components of the single phase motor.


I thought the 3-phase converters were nothing more than a 3-phase motor operating on single phase, in effect self-generating the 3rd phase. Now I'm "cornfused." 

Obviously a motor-generator will do the trick, but I didn't think the rotary converters operated that way.


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## kbsparky (Sep 20, 2007)

There ain't no way you are going to get anywhere close to 400 [email protected] Volts 3 phase from a single phase 400 Amp 240 Volt source.:no:

You're going to need a lot more kVA capacity for that to occur.:blink:


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## brian john (Mar 11, 2007)

There are rotartary converters and then motor generators.


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## buck123456 (May 9, 2009)

The power company will only put in 100 kva on 400 amp service. The only other way to get more power is go to 600 amp service at 240v. They won't supply me 480 unless its 3 phase. I will have to convert with a rotary phase converter. I'm to far from the lines to make that feasible. The motors I would be using are 480v 3 phase. Any suggestions that I can do be able to run more capacity.


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## MDShunk (Jan 7, 2007)

Peter D said:


> I thought the 3-phase converters were nothing more than a 3-phase motor operating on single phase, in effect self-generating the 3rd phase. Now I'm "cornfused."
> 
> Obviously a motor-generator will do the trick, but I didn't think the rotary converters operated that way.


Yeah, the little one's are. They are 3-phase motors with starting components to start them with single phase. Get much bigger than 10 or 20 horse, normally, then you're into a motor-generator type rotary phase converter.


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## kbsparky (Sep 20, 2007)

buck123456 said:


> ... The motors I would be using are 480v 3 phase. ....


How many and what size motors are you dealing with? How many hours per day, etc?

Many POCO's have limitations on the largest single phase motor that can be connected to their system.

You may want to buy and install your own diesel generator if you need more capacity than the poco can deliver. Many farmers do that with 3-phase 480 Volt irrigation systems.


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## MDShunk (Jan 7, 2007)

kbsparky said:


> Many POCO's have limitations on the largest single phase motor that can be connected to their system.


I know they do, but that's outdated thinking on their part. You can normally always get a pass on that if you're not starting it across the line. Start it with a VFD, or at least a soft starter, and nobody will even know.


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## MDShunk (Jan 7, 2007)

Speaking of VFD's, sometimes you can just use a VFD and forego the use of a phase converter. The last milling machine I wired up I did that way, and the customer loved it. The VFD I selected had single phase input and 3-phase output. The VFD was really nice for that application because the machinist could leave the machine mechanically set on high speed and ues the VFD to change the speed of the machine digitally. Sometimes you get lucky and get get a VFD with single phase input with a 3-phase output in a horsepower rating that matches your machine. You just need to make sure that it's a pure motor load if you do that, and not a machine that has additional loads connected in single phase across all three phases, or between each phase and neutral.


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## buck123456 (May 9, 2009)

The largest motor would be provably 100hp that provably the limit with the 100 kva at 240v. Its on a sawmill and the 100hp would provably be enough on the headsaw, but there are other pieces to with maybe 50hp more. Now not all would be under load at the sametime they maybe running. I know there provably regenerative effect by having the motors run. I don't know if you step up the voltage which I would have to any way if that would let me run more hp. I don't know how much reserve capacity the 100 kva has, before we burn the pole down and have the power co mad.


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## MDShunk (Jan 7, 2007)

buck123456 said:


> I don't know how much reserve capacity the 100 kva has, before we burn the pole down and have the power co mad.


I know that they don't upsize them until they're running close to 150-185% overloaded for a good while. The tattle-tale light on the newer pole top transformers doesn't even come on until 125%. The OP says that he has a 100 kva pole top transformer. That's a huge transformer. That's the biggest one you'll ever really find on a pole.


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## kbsparky (Sep 20, 2007)

> .... 100 hp ....


That's a LOT of motor there, bud.

More than would be practical with any rotary phase converter?

I'm not sure whether Marc's suggestion of a VFD would be practical or not with such a large motor.

You might still have to look into your own diesel powered gen set here ...


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## MDShunk (Jan 7, 2007)

kbsparky said:


> I'm not sure whether Marc's suggestion of a VFD would be practical or not with such a large motor.


I just guessed that he has a variety of smaller motors, and not one large motor. Could be one big motor, I suppose.


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## LGLS (Nov 10, 2007)

How are you going to get 400 amps @ 3 phase @ 480 volt from a 120/240 v single phase 400 amp service?


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## MDShunk (Jan 7, 2007)

LawnGuyLandSparky said:


> How are you going to get 400 amps @ 3 phase @ 480 volt from a 120/240 v single phase 400 amp service?


Who has suggested that? Nothing I read so far even remotely suggests that.


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## LGLS (Nov 10, 2007)

MDShunk said:


> Who has suggested that? Nothing I read so far even remotely suggests that.


_Have 240/ 400 amp service single phase supplied 100 kva transformer on pole. I need to go 480v 3 phase at 400 amp. I have to add a converter to get 3 phase. In order to get 480 I have to step up. How big of a transformer kva can I go and should it be before or after the converter and which side would be the best for economics and power. And am I limited by the single 100 kva on the pole now. I need to install some equipment with very large motors, and am trying to see how much hp can I get out of 480v/ 400 amp, 3 phase._ 

From the OPs wording, it doesn't sound like he intends to alter the 120/240 400a service.


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## drsparky (Nov 13, 2008)

I would get with the husk supplier. They probably have done this many times before. No sense in reinventing the wheel.


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## brian john (Mar 11, 2007)

drsparky said:


> I would get with the husk supplier. They probably have done this many times before. No sense in reinventing the wheel.


 
STEP ONE...for sure.


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## wildleg (Apr 12, 2009)

actually, I think step one would be to put the horse before the cart. that is to say, what are the loads you want to supply ? you keep saying you want to supply some loads but you haven't described them. do a load calculation for the loads you want to serve, then start figuring out what you can and can't do to serve the loads.


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## brian john (Mar 11, 2007)

wildleg said:


> actually, I think step one would be to put the horse before the cart. that is to say, what are the loads you want to supply ? you keep saying you want to supply some loads but you haven't described them. do a load calculation for the loads you want to serve, then start figuring out what you can and can't do to serve the loads.


 
Actually to do this you need to know the voltage rating of the equipment which IF at all possible should match the supply voltage. And to do that you should contact the manufacture.


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## njspark83 (May 11, 2009)

i agree with wild leg, figure out your calculated load


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## randomkiller (Sep 28, 2007)

brian john said:


> STEP ONE...for sure.


 
Seconded, it normally helps to start at the beginning.


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## zgozvrm (Jun 17, 2009)

MDShunk said:


> 100 kva is 100 kva no matter voltage you step up or down to, and no matter if you give yourself a manufactured phase. 100 kva is still your functional limit.



On the other hand, KVA is calculated differently for single phase and 3 phase. With single phase, you multiply the required current by the voltage you'll be using. For example 240V x 400A = 96,000VA. So 100KVA is used.

Whereas with 3 phase, suppose you needed 240V 3 phase at 400A: The formula now is Volts X Amps X 1.73 (the square root of 3). This results in 240V x 400A x 1.73 = 166,277VA (167KVA is a standard _single phase _transformer size, but 225KVA is the next larger standard 3 phase size).


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