Transformer resistance

[Besoeker]

I recently had a hospital with a Philips CT scanner failing components. An engineer at Philips told me that the cause for component failures was voltage sags wearing down components and causing them to fail, and I am not talking steady state sags, the sags were from utility faults and motor starts from a common subpanel, which are both short duration. The sags from the motor starts did get the voltage down to about 81% of nominal though...

Due to impedance rather than DC resistance I suspect.

 

[arnettda]

How much do I need to oversize my wire to cover voltage drop. When the ct fires my voltage droped to 464volt for a brief second. 2/0 were allready installed but I believe I would have to increase my awg 2 0r 3 sizes to cover it. I have not been given a solid answer for a current draw of the ct.

I am beginning to see issues like this more and more. CT machine manufacturers are requiring ridiculous voltage tolerances that pretty much require the customer to buy a separate power conditioner for the machine.

I recently had a hospital with a Philips CT scanner failing components. An engineer at Philips told me that the cause for component failures was voltage sags wearing down components and causing them to fail, and I am not talking steady state sags, the sags were from utility faults and motor starts from a common subpanel, which are both short duration. The sags from the motor starts did get the voltage down to about 81% of nominal though...

 

[zog]

Due to impedance rather than DC resistance I suspect.

Yep, the absorbsion and excitation currents dissipate quickly when you test DC resistance, they are always present for AC. Hence the DAR and PI ratios that are recorded for DC testing of inductive equipment.

 

[mivey]

How much do I need to oversize my wire to cover voltage drop. When the ct fires my voltage droped to 464volt for a brief second. 2/0 were allready installed but I believe I would have to increase my awg 2 0r 3 sizes to cover it.

Check the service voltage first to see if it is OK when the CT fires. If it is too low, you need a service upgrade (transformer and/or service drop). Once the service is OK, then worry about the feeder.

I have not been given a solid answer for a current draw of the ct.

Ammeter.

 

[arnettda]

My service and transformer are fine. The voltage drop is in my feeders. I was unable to get a ammeter reading.

Check the service voltage first to see if it is OK when the CT fires. If it is too low, you need a service upgrade (transformer and/or service drop). Once the service is OK, then worry about the feeder.Ammeter.

 

[mivey]

My service and transformer are fine. The voltage drop is in my feeders. I was unable to get a ammeter reading.

Voltage drop is a function of current and wire resistance. If you do not give use the run length and current, we can't answer this question:

How much do I need to oversize my wire to cover voltage drop.

BTW: Here we normally see replies following the quotes, (unlike email). It gets readers up to speed on the topic before they read the reply.

 

[mivey]

Voltage drop is a function of current and wire resistance.

There are some other things, like temperature, eddy currents due to the raceway, proximity of other wires, power factor etc.

It would be best to have source voltage & phasing, load current, load power factor, ambient temperature, raceway type, wire type, and length.

 

[arnettda]

Voltage drop is a function of current and wire resistance. If you do not give use the run length and current, we can't answer this question:

2/0 cu wire roughly 100 ft long. 480 volt have been told 100 amp load. It is a CT so it is for a short duration.



BTW: Here we normally see replies following the quotes, (unlike email). It gets readers up to speed on the topic before they read the reply.

Sorry and thanks for telling me something I have not realized.

 

[mivey]

Sorry and thanks for telling me something I have not realized.

Also see these for multiple & embedded quoting tips:
http://forums.mikeholt.com/showpost.php?p=1104079&postcount=15
http://forums.mikeholt.com/showpost.php?p=1104164&postcount=23
http://forums.mikeholt.com/showpost.php?p=1104376&postcount=32

PS: if you embed your answer in a quote, normal practice is to highlight it with a different color.

 

[arnettda]

Please ignore I do not know what I am doing

 

[mivey]

[/color]

will I ever get this. Practice makes perfect.

.................

 

[arnettda]

Is there a way to calculate the DC resistance in ohms of a utility transformer line to line? It's a 1500 KVA transformer with 5.6%Z. The reason I'm asking is there is a piece of equipment downstream where the resistance must be a below a threshold. I can calculate that back to the transformer, but what would the transformer contribute?

To calculate your impedance of your conductors to the equipment can you use ohms per thousand ft and figure out the ohm value per ft and multiply it by the length of run and then multiply it by 2 for 2 wires and use this as part of your overall impedance.

 

[mivey]

Using "{" instead of "[", you had:

{COLOR=#0000ff}{quote=ronmath;1103396}Is there a way to calculate the DC resistance in ohms of a utility transformer line to line? It's a 1500 KVA transformer with 5.6%Z. The reason I'm asking is there is a piece of equipment downstream where the resistance must be a below a threshold. I can calculate that back to the transformer, but what would the transformer contribute?


{/COLOR}To calculate your impedance of your conductors to the equipment can you use ohms per thousand ft and figure out the ohm value per ft and multiply it by the length of run and then multiply it by 2 for 2 wires and use this as part of your overall impedance.

But might should have had:

{quote=ronmath;1103396}Is there a way to calculate the DC resistance in ohms of a utility transformer line to line? It's a 1500 KVA transformer with 5.6%Z. The reason I'm asking is there is a piece of equipment downstream where the resistance must be a below a threshold. I can calculate that back to the transformer, but what would the transformer contribute?{COLOR=blue}To calculate your impedance of your conductors to the equipment can you use ohms per thousand ft and figure out the ohm value per ft and multiply it by the length of run and then multiply it by 2 for 2 wires and use this as part of your overall impedance.{/COLOR}{/quote}

Then your post would have looked like this:

Is there a way to calculate the DC resistance in ohms of a utility transformer line to line? It's a 1500 KVA transformer with 5.6%Z. The reason I'm asking is there is a piece of equipment downstream where the resistance must be a below a threshold. I can calculate that back to the transformer, but what would the transformer contribute?To calculate your impedance of your conductors to the equipment can you use ohms per thousand ft and figure out the ohm value per ft and multiply it by the length of run and then multiply it by 2 for 2 wires and use this as part of your overall impedance.

You can use the " A/A "in the upper right corner of the editor to switch editor modes and see the formatting codes.