I'm sweating on this one...

[Lady Engineer]

My friend wants to know something, and I can't really answer it, so I thought I'd ask.

He has an application where the building is 240V-1ph. However, he has a 480V 3ph load that must be added.

Can you put in a transformer to step it up? Or should he take a 480V 3ph to 240V -ph XFMR and put it in backwords? First off, is it legal?

Please help?


Lady

 

[jtester]

You can't get 3 phase out of a single phase transformer.
His best bet, if the load is small is to get a 3 phase generator run on a single phase motor, or in some cases, a VFD with a single phase input will run a 3 phase motor.

Jim T

 

[Lady Engineer]

can't he use a converter if that's an option?

 

[LarryFine]

can't he use a converter if that's an option?

Possibly, but it's not that simple. Remember there's one constant: You can never get more KW out than you put into a system or conversion.

Even assuming 100% efficiency and no losses, when stepping 240v up to 480v with a transformer, every amp required at 480v will need two amps at 240, and that's still single-phase to single-phase.

Then there's the issue of converting the single-phase 480 to three-phase 480v. If the load needs 10 amps @ 480v, 3ph, it'll require at least 1.732 times that much current at 480v, 1ph.

So, our fictitious 10a @ 480v, 3ph load will require at least (10 * 1.732 * 2) 34.64 amps @ 240v, 1ph. It'll probably end up requiring more like a 50- or 60-amp feeder.

See: http://www.mikeholt.com/code_forum/archive/index.php/t-52690.html

 

[rr]

My friend wants to know something, and I can't really answer it, so I thought I'd ask.

He has an application where the building is 240V-1ph. However, he has a 480V 3ph load that must be added.

Can you put in a transformer to step it up? Or should he take a 480V 3ph to 240V -ph XFMR and put it in backwords? First off, is it legal?

Please help?

I would suggest a new service. If the client is adding a 480V/3ph load now, is there a good chance that they'll add more in the future?

Will the local power company allow 2 separate meters?

 

[petersonra]

around here it is so expensive to get a 480V/3ph service that many people put in 208/3ph and step up transformers for their 480v loads. it seems very strange to me.

i would go for the single phase to 3 phase vfd, even if I had to add a stepup transformer for the input, unless the size of the motor made that unrealistic.
<added> or change out the motor which might be even cheaper.

 

[boater bill]

Once a genius I knew took a 480/208 3 phase transformer and installed it backwards to step up a service from 208 to serve several 480 v motors. The problem was the transformer manufacturers design the transformer so that the inrush in the primary side is limited with high impedence. and the load side has low impedence to limit the losses. Install it backwards, and now there is a low impedence on the input side and breakers trip, wires rattle, etc.

Either change the motors or get a new service for the additional loads. The local POCO will help if the new motors are high amp loads.

 

[steve66]

Lady:

Do you know the current or HP rating of the 480V load? And do you know what the service size is and existing load is? Given those three things will determine if this is feasable using LarryFine's example.

As far as installing a transformer backwards goes, that won't help with the single phase to three phase issue. But assume the load and service were both single phase or three phase. My understanding is that a transformer can be installed with either side as the primary if the markings say something "High" and "Low". However, if they are marked "line" and "load", or "input" and "output", then you can't reverse the primary and secondary.

Steve

 

[Lady Engineer]

Lady:

Do you know the current or HP rating of the 480V load? And do you know what the service size is and existing load is? Given those three things will determine if this is feasable using LarryFine's example.

As far as installing a transformer backwards goes, that won't help with the single phase to three phase issue. But assume the load and service were both single phase or three phase. My understanding is that a transformer can be installed with either side as the primary if the markings say something "High" and "Low". However, if they are marked "line" and "load", or "input" and "output", then you can't reverse the primary and secondary.

Steve

Is there a diagram, I'm a visual person. Is there a scientific reason, going from single to three would not work with a transformer? What's the reason behind that?


Lady

 

[LarryFine]

Is there a diagram, I'm a visual person. Is there a scientific reason, going from single to three would not work with a transformer? What's the reason behind that?

You need to understand the fundamental differences between single- and three-phase systems; 3-phase consists of more than simply having 3 hot wires. Remeber singing songs like 3 Blind Mice or Row-Row-Row Your Boat?

Remember how the second singer came in after the first, and then the third? If there are the same number of singers as there are lines in the song, and they keep restarting, the result would be a smooth, symmetrical repitition of the pattern.

3-phase power is very much like that. There are 3 single-phase systems, offset from one another in timing by 1/3 of the cycle. If you could see just the positive peak of each line, you'd see 1, 2, 3, 1, 2, 3, etc.

The only difference between 3 single-phase systems timed like that and a 3-phase system is that the 3 phases share wires. You can take one phase from a 3-phase system, which is how every single-phase system is derived, because that's how the POCO's generate it.

Once you have derived a single-phase syetem, you can't just use a transformer to "re-derive" the three separate single-phase timing waves. You either have to use electronics to synthesize it or a rotary converter (or a 1-phase motor coupled to a 3-phase alternator).

Hope this made sense, as well as has some sense of accuracy.

 

[Johnmcca]

Larry, the musical analogy was interesting, nicely done:grin:

 

[Lady Engineer]

don't get me wrong, I've had the electrical engineering theory in college, not much as much as an EE but some...however, I just didn't think this because the phases are separated by angle...3 phase 120 degree, and single 180 degrees.

I guess I wasn't seeing because it's all alternating current, AC.

Nevermind, I have to look for myself.

 

[peteo]

To answer your friend's question,
1) A 10 HP single phase motor is almost always three phase.
2) Before 1990, asides from specialty equipment, there were two main choices:
2a) Static inverter, uses capacitors to simulate a third phase
2b) Rotary inverters, uses motor windings (transformer-like) to generate third phase

Among those two, rotary inverters were preferred by people with motorized equipment. In addition, some equipment manufacturers supplied generator sets which could, for example, allow german machine tools to run from US power - a rarity.

3) Today there's the option of using an electronic inverter, which produces three phase AC from a DC buss. Many inverters will run the logic and produce a buss voltage from single phase. What we used to call inverters are now referred to on this forum as VFDs. It is even concievable that the 10 HP equipment in question uses a VFD and has the capability of being run from single phase; the manufacturer probably fields questions of this nature daily.

 

[kingpb]

You can get a 10 Hp single phase motor.

Baldor, Marathon, and TECO-Westinghouse all have them.

 

[LarryFine]

. . . I just didn't think this because the phases are separated by angle...3 phase 120 degree, and single 180 degrees.

In my opinion, it's better to avoid thinking of single phase as 180 degrees of anything. That merely describes the opposite ends of any single-phase windings. It's like saying North is 180 degrees opposite South. Duh!

240/120 is merely a single secondary that just happens to have a center tap that just happens to be grounded. On 3-phase, there are genuinely three separate 1-phase secondaries that happen to be interconnected.

A Y system has one end of each of three singlephase secondaries connected together at a single point. (Picture the spokes of a Mercedes-Benz star.) This is a genuine neutral, and is almost always grounded.

Just to throw it in, on a Delta system (3 secondaries in a triagle) with a grounded-center-tap secondary, that secondary is identical, and I mean identical, to a grounded-center-tap single-phase system of the same voltage.

Anyway, my point is that a single phase is a single phase, whether you have a center tap or not, whether it's grounded or not. Apologies to Rattus aside, if you choose to measure from the center tap (120-0-120) instead of one end (0-120-240), it's just semantics. (See 1st paragraph)

 

[beanland]

Rotary Converter or VFD

Rotary Converter or VFD

There are rotary converters that will convert single-phase to 3-phase fo small applications. If the load is a motor, there are VFD designed to take 240VAC single-phase inputs and generater 3-phase outputs for the motor. For very small loads, there are capacitor phase shifters that can generate a pseudo-3-phase, but not for 3Hp.

 

[Johnmcca]

Larry, are you trying to start the single phase two phase debate here?

 

[LarryFine]

Larry, are you trying to start the single phase two phase debate here?

Nooooooooooooooooooooo!!!!!!!! Boo! Hiss!

 

[Johnmcca]

Hey it was fun for the first 1 or 200 posts then I couldn't follow the math anymore, oh well........................

 

[kingpb]

It's like saying North is 180 degrees opposite South. Duh!

Wait a second, I thought the south pole was -180 degrees from North?