Transformer Lowering Impedance

[QuestionGuy]

The System Discription:
Need to provide 480V/3ph with less than 160 milli-Ohms of resistance.

The installed system
Building service = 120/208V 3ph 4w --> 75KVA X-former (5.11% @ 170C) --> 480/277V 3ph 4w

I am measuring 11 m-ohms on primary side & 261 m-ohms on secondary.

What %impedance do I need to achieve < 160m-ohms?
Will a 112.5 KVA X-former achieve this?
Do I need a 150KVA ?

 

[jim dungar]

The System Discription:
Need to provide 480V/3ph with less than 160 milli-Ohms of resistance.

The installed system
Building service = 120/208V 3ph 4w --> 75KVA X-former (5.11% @ 170C) --> 480/277V 3ph 4w

I am measuring 11 m-ohms on primary side & 261 m-ohms on secondary.

What %impedance do I need to achieve < 160m-ohms?
Will a 112.5 KVA X-former achieve this?
Do I need a 150KVA ?

I have no idea what you are trying to accomplish. Usually no one cares what the resistance is of a transformer winding.

Could you please clarify?

 

[QuestionGuy]

I amproviding 480V 3ph for a piece of X-ray equipment. They will not hook it up until it tests less than 160 milli Ohms of resistance. They are reading 271 milli Ohms of resistance a X-Ray terminals on their ohm/load meter. I have a 208 to 480V 75KVA x-former with (5.11% impedance) I need their test to read less than 160 milli Ohms. How can I achieve this.

Larger X-former? what size?
Lower impedance? How low?

 

[QuestionGuy]

The load on the X-ray equipment is 80A 480/3ph

 

[mivey]

I amproviding 480V 3ph for a piece of X-ray equipment. They will not hook it up until it tests less than 160 milli Ohms of resistance. They are reading 271 milli Ohms of resistance a X-Ray terminals on their ohm/load meter. I have a 208 to 480V 75KVA x-former with (5.11% impedance) I need their test to read less than 160 milli Ohms. How can I achieve this.

Larger X-former? what size?
Lower impedance? How low?

Z(ohms)=Z(in %) x (10 x kV^2)/kVA
Using the transformer you have:
Z = 5.11 x (10 x 0.48^2)/75 = 0.157 ohms = 157 milliohms

[edit: the resistance will be less than that]

 

[QuestionGuy]

mivey

mivey

Z(ohms)=Z(in %) x (10 x kV^2)/kVA

In your equation you used the secondary voltage for the [kv^2] so just to verify if I switched the primary and secondary I would use .208

Thanks for the help Mivey

 

[mivey]

Z(ohms)=Z(in %) x (10 x kV^2)/kVA

In your equation you used the secondary voltage for the [kv^2] so just to verify if I switched the primary and secondary I would use .208

Thanks for the help Mivey

correct. you're welcome.

 

[zog]

I have no idea what you are trying to accomplish. Usually no one cares what the resistance is of a transformer winding.

Could you please clarify?

AS long as they are all the same

 

[jim dungar]

I amproviding 480V 3ph for a piece of X-ray equipment. They will not hook it up until it tests less than 160 milli Ohms of resistance. They are reading 271 milli Ohms of resistance a X-Ray terminals on their ohm/load meter. I have a 208 to 480V 75KVA x-former with (5.11% impedance) I need their test to read less than 160 milli Ohms.

Ohms based on what reference?

Line-Line?
Line-Ground?
Primary to secondary?
Impedance or resistance (AC or DC)?

I have never heard of this type of specification. I cannot imagine why L-L would be important.

Most equipment cares about the available short circuit current and voltage regulation neither of which is determined solely by a specific amount of general ohms.

In reality transformers do not have %Z, they have %IZ, which we abbreviate. %Z is determine by using Ohm's Law, based on the % of voltage required to drive %100 current (therefore in %IZ I=1) through a shorted secondary.

 

[coulter]

Z(ohms)=Z(in %) x (10 x kV^2)/kVA
Using the transformer you have:
Z = 5.11 x (10 x 0.48^2)/75 = 0.157 ohms = 157 milliohms

[edit: the resistance will be less than that]

OP is asking about an AC resistance?

 

[mivey]

OP is asking about an AC resistance?

I have no idea.

 

[jghrist]

Z(ohms)=Z(in %) x (10 x kV^2)/kVA
Using the transformer you have:
Z = 5.11 x (10 x 0.48^2)/75 = 0.157 ohms = 157 milliohms

They are reading 271 milli Ohms of resistance a X-Ray terminals on their ohm/load meter.

For what it's worth, 157 x sqt(3) = 271.

Coincidence? I think so.

The measurement is meaningless. If done with a regular ohmmeter (dc), then you get the dc resistance of one secondary winding. It would not include the resistance of the primary winding or the resistance of the source.

The transformer resistance is related more to the transformer losses than to the impedance. Getting a lower impedance transformer may not get a lower resistance. As noted by mivey, the resistance will be less than the impedance because the impedance include both X and R.

I have no idea how the transformer resistance would affect the operation of the x-ray equipment. I suspect that "they" do not either and are interpreting something in the x-ray equipment instruction manual wrong.