Transformer Question

[GalwayElectric]

Hey ya'll,

Question. I understand the process of sizing over current protection and conductor sizing for transformers. My question confusion is this:

If you have a 50kva transformer and we do our calculations for single phase and three phase we get a current of:

1) 50000/240 = 208 amps

2) 50000/240*1.73 = 120 amps

Does this mean:

Why does it seem that scenario 1 offers more amperage than scenario 2? How is the power split up per phase?

 

[kwired]

Hey ya'll,

Question. I understand the process of sizing over current protection and conductor sizing for transformers. My question confusion is this:

If you have a 50kva transformer and we do our calculations for single phase and three phase we get a current of:

1) 50000/240 = 208 amps

2) 50000/240*1.73 = 120 amps

Does this mean:

Why does it seem that scenario 1 offers more amperage than scenario 2? How is the power split up per phase?

The whole reason you multiplied by 1.73 is because this 50kVA is split between three paths instead of two. Total power is still 50 kVA in both cases.

I don't know your intentions here but you can't just choose if you wish to go single or three phase with your 50 kVA transformer. It will be designed for one or the other but not both. I will say that 50kVA is typical standard size for single phase and 45kVA would be closest typical standard size for a three phase unit.

 

[GalwayElectric]

Thanks Kwired.

I don't really have an intention. Just hypothetical.

So if at 220volts single phase I'd get (50000 / 240) = 208 amps per phase

and

at 220 volts three phase I'd get (50000 / 240 * 1.73) = 120 amps per phase


This is what I don't get. IF this is the case then it would appear you get more power from single phase than three phase? The power should be the same no?

 

[augie47]

This is not mathematically they way to look at it, but for explanation sake lets look at you 50kva 240 single phase transformer as (2) 120 volt transformers in one box. Your 50 kva 3 phase is like (3) 120 volt transformers in one box. You have one whole additional phase on which to connect your loads.

 

[GalwayElectric]

Thanks Augie.....

I get what you are saying. I understand that part of the transformer. Using your description let me ask you this:

In a box with two 120 volt phases how much power can I derive from each phase?

In a box with three 120 volt phases how much power can I derive from each phase?

Does this mean it doesn't matter as long as the total of those phases add up to more than (in this example) 50000 when adding my "V" and "A" together?

Thanks

 

[mivey]

Thanks Kwired.

I don't really have an intention. Just hypothetical.

So if at 220volts single phase I'd get (50000 / 240) = 208 amps per phase

and

at 220 volts three phase I'd get (50000 / 240 * 1.73) = 120 amps per phase


This is what I don't get. IF this is the case then it would appear you get more power from single phase than three phase? The power should be the same no?

Should read (50000 / 240 / 1.73) or (50000 / (240 * 1.73)). However that is the voltage you would have for a delta connected unit with 120 amp line currents.

Anyway, you get higher current, not more power. For the single-phase unit you get 50 kVA. For the three-phase unit you get 50/3 = 16.67 kVA per phase. The L-G voltages would be 138.56. So we could also check by 138.56 V * 120.28 A * 3 = 16.67 kVA * 3 = 50 kVA.

 

[mivey]

This is not mathematically they way to look at it, but for explanation sake lets look at you 50kva 240 single phase transformer as (2) 120 volt transformers in one box. Your 50 kva 3 phase is like (3) 120 volt transformers in one box. You have one whole additional phase on which to connect your loads.

Not really because of the delta configuration he posted. If you want to sum, what you have is like three 138 volt transformers in a box.

 

[mivey]

Thanks Augie.....

I get what you are saying. I understand that part of the transformer. Using your description let me ask you this:

In a box with two 120 volt phases how much power can I derive from each phase?

In a box with three 120 volt phases how much power can I derive from each phase?

Does this mean it doesn't matter as long as the total of those phases add up to more than (in this example) 50000 when adding my "V" and "A" together?

Thanks

Single phase: volts * amps. For the three phase, it depends if you hook them up in delta or wye and that is because you get different L-L voltages.

Example: 50 kVA units rated at 120 volts:

single-phase: 50 kVA / 120 V = 416.67 A
and
416.67 A * 120 V = 50 kVA

wye:
with L-L voltage of 120 * 1.73 = 207.84 V we have
(3 * 50 kVA) / 207.84 V / 1.732 = 416.67 A line and winding current
for the individual units:
416.67 A * 120 V * 3 = 150 kVA = 3 * 50 kVA

delta:
with L-L voltage of 120 we have
(3 * 50 kVA) / 120 V / 1.732 = 721.69 A line current but 416.67 A winding current
for the individual units:
416.67 A * 120 V * 3 = 150 kVA = 3 * 50 kVA
for L-G loads we have 69.28 V L-G and:
721.69 A * 69.28 V * 3 = 150 kVA = 3 * 50 kVA

 

[GalwayElectric]

So Mivey are you saying that a 50kva transformer is delivering 50 kva per phase thus 150 kva total??

 

[GoldDigger]

So Mivey are you saying that a 50kva transformer is delivering 50 kva per phase thus 150 kva total??

No, I think he is saying that if you build a 3 phase transformer bank using three 50kva transformers, you will get 150kva total, but the current you get will depend on whether you wire them in a wye or delta configuration.

If you find a listing for a single 50kva 3-phase transformer, you will have a little more work to do.

 

[GalwayElectric]

Thanks Gold digger,

Ok let me ask this way as this thread see a to only adding to my confusion.

Just an answer:

Solve for x

A 50 kva transformer per phase supplies how many amps???

1 phase delta x = ?
3 phase delta x = ?

1 phase wye x = ?
3 phase wye x = ?

 

[GoldDigger]

Thanks Gold digger,

Ok let me ask this way as this thread see a to only adding to my confusion.

Just an answer:

Solve for x

A 50 kva transformer per phase supplies how many amps???

1 phase delta x = ?
3 phase delta x = ?

1 phase wye x = ?
3 phase wye x = ?

Care to choose a system voltage first? Or at least a transformer secondary voltage?

 

[hurk27]

Thanks Gold digger,

Ok let me ask this way as this thread see a to only adding to my confusion.

Just an answer:

Solve for x

A 50 kva transformer per phase supplies how many amps???

1 phase delta x = ?
3 phase delta x = ?

1 phase wye x = ?
3 phase wye x = ?

I think what your missing is that the KVA is the power your assigning to amps, to get power you multiply amps times volts which is the VA in KVA and without getting into power factor, this also can be called watts (if we ignore power factor) which is what we pay for at the electric meter, so in a three phase circuit this wattage is divided between 3 conductors, this is why we use the square root of three (1.732) to calculate how many amps a given watts into a transformer will produce on each phase of the transformer, we use the term KVA versus watts to define the difference between real power and apparent power this is also called the power factor.

so when we calculate power as voltage times amps we are figuring real power, if we have a motor that can be wired to run on 120 volts or 240 volts, and lets say this motor uses 1,000 va, we will see it draws 8.34 amps on 120 volts and 4.16 amps on 240 volts, the the power it is using is still the same 1,000 va or 1kva, the same motor on three phase will use 2.40 amps @240 volts but we still are using 1kva of power so the cost of running that motor is still the same, the benefit is we now have less amps and our wire sizes can be smaller as well as the motor will operate more efficiently on 3-phase, but thats another chapter.

so we are not changing the amount of power consumed, we are just changing the perimeters of how we are using that amount of power by using 240 volts we have cut the current in half so we can use smaller conductors, but the amount of power is the same and using three phase over single we have cut the current farther by dividing the single phase current with the square root of three (1.732).

so when we look at a singe phase transformer versus a three phase transformer, both 50kva transformers will supply the same amount of power, but the current is going to be less on the three phase transformer because we have divided that power across three conductors instead of 2, this is what the square root of three does in the equation.

Remember we don't pay the utility by how many amps we use, we pay them the amount of volts times amps or watts we use, this is called power.

 

[jcbabb]

Hey ya'll,

Question. I understand the process of sizing over current protection and conductor sizing for transformers. My question confusion is this:

If you have a 50kva transformer and we do our calculations for single phase and three phase we get a current of:

1) 50000/240 = 208 amps

2) 50000/240*1.73 = 120 amps

Does this mean:

Why does it seem that scenario 1 offers more amperage than scenario 2? How is the power split up per phase?

I think there is a lot of good information in this post, but I feel like you need a simpler, more direct answer to help you with your question.

For scenario (1), you have two phase conductors at 208 amps each.
For scenario (2), you have three phase conductors at 120 amps each.

You get more amps at single phase because there are fewer phases to account for the total power (50kVA).

Thanks Kwired.

I don't really have an intention. Just hypothetical.

So if at 220volts single phase I'd get (50000 / 240) = 208 amps per phase

and

at 220 volts three phase I'd get (50000 / 240 * 1.73) = 120 amps per phase

This is what I don't get. IF this is the case then it would appear you get more power from single phase than three phase? The power should be the same no?

I think in your second post, you are simply confusing 'more amps' with 'more power'. Remember that [apparent] power is volts x amps. When comparing a single phase and three phase transformer, both scenarios have the same voltage, so the amps MUST be different to get the same power.

I'm not sure if that helped, but it seemed to me to be what you were asking for.

 

[GalwayElectric]

Thanks Jcbabb,

I thank you for you simple direct approach. Actually all was very helpful. I understand the basics. I think the best thing I learned in this thread is to think less about the "amps" and focus more on the "power" of a transformer. As electricians we tend to focus more on the amperage due to breaker sizes than anything.

Thanks again

 

[kwired]

Thanks Jcbabb,

I thank you for you simple direct approach. Actually all was very helpful. I understand the basics. I think the best thing I learned in this thread is to think less about the "amps" and focus more on the "power" of a transformer. As electricians we tend to focus more on the amperage due to breaker sizes than anything.

Thanks again

Think about how much power is available from a 200 amp 480 volt source vs. a 200 amp 240 volt source. Then consider single vs three phase in either case.

Amps alone means nothing if you don't know the voltage and number of phases driving it.

Think of how fast amps add up for say 12 volt lighting systems yet total power is not all that high.

 

[GalwayElectric]

So if I have a 6k transformer and it operates at 240/120 v this gives me

6000/120 = 50. If I have three phases because they operate or cycle at different times does this mean I could run 3 50 amp 120 volts circuits at the same time?

 

[hurk27]

So if I have a 6k transformer and it operates at 240/120 v this gives me

6000/120 = 50. If I have three phases because they operate or cycle at different times does this mean I could run 3 50 amp 120 volts circuits at the same time?

No because with the 3-phase transformers you will again divide by 1.732 which will only give you 28.9 amps per phase, 6000/120/1.732 = 28.86 amps.

three 50 amp loads is 18kva @ 120 volts, so you can see that you would exceeding the rating of the 6kva transformer, a transformer is rated for the KVA of the total load not just the amperage of the load, a 6kva transformer will only supply 6kva worth of load, this is why you must quit mixing the terms of amperage and power because the two are not the same measurement when sizing a transformer, at 18kva each phase will be 86.6 amps, this is because of the vectors involved with three phase loads, as part of each 50 amp 6kva load will put extra current on the other phase as each phase will share the 50 amp load so we have to add them vectorial by using the 1.732 to know what each phase will see.

s

 

[hurk27]

So if I have a 6k transformer and it operates at 240/120 v this gives me

6000/120 = 50. If I have three phases because they operate or cycle at different times does this mean I could run 3 50 amp 120 volts circuits at the same time?

Also keep in mind that a three phase power source does not have a 180? phase displacement which would change things and allow each phase to only supply current to one phase at a time, they have a 120? phase displacement and because of this the adjoining phase will contribute current to to a single phase load between only two of the phases, this is why we see 86.6 amps at each phase when each load only pulls 50 amps when you have three equal 50 amp loads connected to all three phases, if you had a 0? displacement you would have 100 amps at each phase since each phase really supplies two loads, this is because while phase A is at it's voltage peak 90? phase B is at 330? and phase C will be at 210 in there phase cycle, maybe someone could point to a plot of a three phase sine wave that will give you a more visual reference to better wrap you mind around, to understand how vectors work in three phase supplies.

 

[GoldDigger]

Also keep in mind that a three phase power source does not have a 180? phase displacement which would change things and allow each phase to only supply current to one phase at a time, they have a 120? phase displacement and because of this the adjoining phase will contribute current to to a single phase load between only two of the phases, this is why we see 86.6 amps at each phase when each load only pulls 50 amps when you have three equal 50 amp loads connected to all three phases, if you had a 0? displacement you would have 100 amps at each phase since each phase really supplies two loads, this is because while phase A is at it's voltage peak 90? phase B is at 330? and phase C will be at 210 in there phase cycle, maybe someone could point to a plot of a three phase sine wave that will give you a more visual reference to better wrap you mind around, to understand how vectors work in three phase supplies.

Maybe we could ask Smart $ to slow down his avatar by a factor of 10 or so?