buck123456

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.

MDShunk

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

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

MDShunk

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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Peter D

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.

kbsparky

There ain't no way you are going to get anywhere close to 400 Amps@480 Volts 3 phase from a single phase 400 Amp 240 Volt source.

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

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-KB

Life is uncertain -- eat dessert first!

brian john

There are rotartary converters and then motor generators.

buck123456

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.

MDShunk

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

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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-KB

Life is uncertain -- eat dessert first!

MDShunk

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

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

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.

MDShunk

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

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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-KB

Life is uncertain -- eat dessert first!

MDShunk

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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LawnGuyLandSparky

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

MDShunk

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

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LawnGuyLandSparky

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.

drsparky

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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