Stepping up from 240 to 480

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sullyelectric

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I have a customer who only has a 120/240 service and needs 3 phase 480. I have a 30 kva xfmr with a primary 480v delta, and secondary 240/110 delta. My first question is if this is the right xfmr for what i am tryind to do? My second question is do i still take the 480 side to the H lugs? My third question is i am hitting it with #2 copper from the 240 side, so do i come off the 480 side with # 6 copper? And my last question is there is a stinger leg will that change things?

Thanks for all the help. This is truly a great forum.
 
IMHO this is not the _right_ transformer for the job, although it might be an acceptable transformer for the job.

IMHO the _right_ transformer for the job would be a 240V delta to 480/277V step up transformer, with the secondary grounded at the neutral point. Such a transformer will have taps on the 240V side so that you can adjust for incoming voltage, will have a neutral point to ground on the secondary side, and will be designed to minimize 'inrush' current on the 240V side.

The transformer that you propose using may work acceptably, however it will not have primary side taps, will require the secondary to be corner grounded or _ungrounded_, and will show higher than expected 'inrush' current.

You ask about a 'stinger' leg; this is normally found in delta systems with the midpoint of one of the secondary coils is grounded. You won't be able to create a such a high-leg system, since you won't have a midpoint to ground. If you decide to ground the secondary, you will have to 'corner ground'. I would recommend grounding, rather than jumping through the hoops associated with an ungrounded system.

Search the forum, and you will find many threads on the issue of running transformers in reverse. It can be done, but with many caveats.

-Jon

P.S. I made the mistake of assuming that you have 240/120V three phase service, not 120/240V single phase service. If you have single phase service, then you need more than a transformer to get 480V three phase.
 
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If they are adding enough 480 volt load to make it worthwhile for the power company to add another bank of transformers on their end, it may be easier and cheaper to just add another service. The AHJ should allow it since it is for different voltage system. I have done it the other way around before too, installed a 480 volt wye service and refeed the old 120/240 delta service with a delta/delta. I have also reversed a 480 volt delta to 120/208 wye running 240 volt delta power backwards to get 560 volt to run some canadian metal working equipment. That was 12 years ago and it is still working today. (corner grounded the 560)
 
jon

jon

im helping a friend hook up a transformer,incoming power is 120 /240 he needs 480.i have the corect transformer.its a 75kva.just alittle unclear on what size primary wire to use and secondary wire to use.also can you tell me what is the output of the 75kva
 
rook81108 Reactance will be multiplied!

rook81108 Reactance will be multiplied!

Consider the other power needed on the low (240V) side. If there is high loads on the existing service, the 480V side will be doubly reactive.
Example; 240V reads 235V A to B to C and has a compressor (high draw) start up dropping the voltage to 217V momentarily all round. The 480V was reading 476V and just dropped to approximatly 440V! amperage will spike slightly as the voltage drops and possibly dammage the new equipment.

Power transmission should be like a downward ramp from utility to avoid this magnification of power fluxuation. Get a new 480V service unless there is no other way.
 
I have installed the XFMR and my step up voltage is 422 volts between phases. I did not corner ground it because im still not sure about that method. I have never done it before and am a little confused. Any Help?
 
sullyelectric said:
I have installed the XFMR and my step up voltage is 422 volts between phases. I did not corner ground it because im still not sure about that method. I have never done it before and am a little confused. Any Help?
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What is your primary voltage? My first guess is that you are feeding this stepup transformer with 208V.
 
Right now your transformer secondary is _ungrounded_. What this means is that a ground fault will not trip any circuit breakers, and an intermittent or 'restriking' ground fault could create very high voltages which will damage connected equipment.

Ungrounded supplies are permitted, however they require ground fault indication systems; since a fault won't trip a breaker you need some other way of noticing faults.

Corner grounding is exactly what it sounds like; electrically connecting one of the phase terminals to ground, just like grounding the neutral on the secondary of another transformer. However now you have a _grounded_ phase conductor, which means that it needs to follow the rules for grounded conductors, meaning white wire, no switching (with a number of exceptions), etc. Additionally, the circuit breakers on the 480V side must be 480V breakers, not 480/277V breakers.

-Jon
 
On your input side you only connect your 3 phase feeds. You do not connect any sort of neutral to the transformer primary. If your transformer has common terminal names, then you connect nothing to 'X0'.

For 'corner grounding', you use all three phases. But one of those phases is connected to ground, becoming the 'grounded phase'. You don't cap it off, you use it.

Consider an ordinary residential 120/240V single phase service. You have _3_ wires that come from the transformer, and you supply your loads with various combinations of those three wires. One of those wires is the neutral, and it is _intentionally_ connected to ground, and is called the 'grounded conductor' or 'neutral'. If you want to supply a 120V load, you run one 'hot' and one 'neutral' to the load. The physics of the situation would actually allow _any_ _one_ of the transformer terminals to be grounded...but it would be quite confusing if you had 120V from 'hot A' to 'ground' and 240V from 'hot B' to ground :)

For corner grounding, we embrace the confusion. You have three transformer terminals: H1, H2, H3. To supply a three phase load you need to have a current carrying path (wires) from H1, H2, H3 to the load. When you 'corner ground', you take _one_ and only one of those terminals and also connect it to ground. Say you connect H2 to ground. The voltage from H1 to H2 is still 480V, the voltage from H2 to H3 is still 480V, and the voltage from H3 to H1 is still 480V. But now the voltage from H2 to ground is _0_ V. The three phase load still needs the connection to H2, and that wire is still a current carrying conductor...but it is simply a _grounded_ current carrying conductor, and it is _not_ a neutral.

-Jon
 
sullyelectric said:
i have 60 volts from x0 to ground. my neutral from the 120 side is not going to anything, is that right?
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well if you have 60 volts from XO to ground that means there is no system bonding jumper. thats only a problem if oyu need the neutral for something if your feeding the transformer with 120/208 you dont need aneutral on the primary side. if you have straight 3 phase loads you dont need a neutral on the secondary side either
 
If the service into the building is conventional 208/120V three phase with a grounded neutral, and this is feeding the 240V delta winding (X1,X2,X3), then I would expect the voltage between X0 on the delta and ground (supply neutral) to be 60V. If you were to connect X0 on this transformer to supply neutral ('X0' on the supply transformer), then you would have a short circuit.

-Jon
 
The xfmr is now corner grounded and im still only getting 422 volts. I have 120 to ground on all three phases and 212 in between. My xfmr is a 480 240/120. Is it possible for me to get 480 with this xfmr?
 
If that transformer has percent taps, (the wiring diagram on the cover should show these) you can boost it some more by moving these to the highest voltage tap. (If I remember correctly I think there is a tap for 515 volts, which should boost the output) you may gain around 35 volts more. Most transformers are factory set at the 480 volt tap, in which since your backfeeding it, should boost your output. I have done this with a 480/120/208 (delta/WYE) transformer backfeeding it with a 240 delta service and actually got 600 volts and had to back the taps down to get the 560 volts I needed. You really need a 480/208 (delta/WYE) transformer and insulate XO.
 
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I know some motors are rated 440/480,(European equipment) the nameplate should show this, if it will accept this voltage, be sure to change the tap on the control transformer to 440 also. You could use a buck/boost transformer in conjunction with it, but it's going to get kinda messy now! And a 32 volt boost would still not get you all of the way to 480.
 
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