Transformer

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Brian Blackadar

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new hampshire
I have a 240 to 480 single phase transformer that I am using to step up the voltage - then 1200' away I am usin g the same transformer to step back down to 240.
Do I run 3 or 4 wire between the 2 transformers?
 
roger

roger

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Tell us what you think and why.


BTW, if you're going to use the same transformer at both ends you're going to have to be real fast

Roger
 
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templdl

Senior Member
Location
Wisconsin
Brian Blackadar said:
I have a 240 to 480 single phase transformer that I am using to step up the voltage - then 1200' away I am usin g the same transformer to step back down to 240.
Do I run 3 or 4 wire between the 2 transformers?
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Strange that you gave no information with regard to what the actual nameplate of the transformers are.
The HV of buth transformers have 2hv connections and as such there are only 2 line conductors required. Is addition, though, you do need and EGC run between the transformers.
 
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templdl

Senior Member
Location
Wisconsin
Brian Blackadar said:
they are 25kva, I will run an EGC - I don't see a need for a neutral conductor?
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Please advise the connection that are present is the HV side of the transformer..is there an H1 and H2 on both?..if so have you identified as mneutrasl connection?
This is why it is best to include a picture on the nameplates of the transformer. If included the would be no doubt as to the transformer connections.
 
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templdl

Senior Member
Location
Wisconsin
ActionDave said:
No need for a neutral.
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Since there is no forgiveness in electrical mishaps that can and do occur does the OP actually understand why a neutral is not required? That is why my last post was stated the way it was. Does the OP actually understand transformer connections? The there are the HV taps which are probably provided where the transformer being used as a stepup the use of those taps can be confusing and the taps on the step down transformer.
As such my concern is if the OP is questioning the use of a neutral conductor is he going to understand taps?
 
ActionDave

ActionDave

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templdl said:
Since there is no forgiveness in electrical mishaps that can and do occur does the OP actually understand why a neutral is not required? That is why my last post was stated the way it was. Does the OP actually understand transformer connections? The there are the HV taps which are probably provided where the transformer being used as a stepup the use of those taps can be confusing and the taps on the step down transformer.
As such my concern is if the OP is questioning the use of a neutral conductor is he going to understand taps?
Click to expand...
Tricky stuff indeed.

Most of my posts are made unencumbered by the thought process.
 
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jap

Senior Member
Occupation
Electrician
roger said:
Tell us what you think and why.


BTW, if you're going to use the same transformer at both ends you're going to have to be real fast

Roger
Click to expand...


That's funny right there,,,,, I don't care who you are that's funny.....:)


JAP>
 
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jap

Senior Member
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Electrician
templdl said:
Since there is no forgiveness in electrical mishaps that can and do occur does the OP actually understand why a neutral is not required? That is why my last post was stated the way it was. Does the OP actually understand transformer connections? The there are the HV taps which are probably provided where the transformer being used as a stepup the use of those taps can be confusing and the taps on the step down transformer.
As such my concern is if the OP is questioning the use of a neutral conductor is he going to understand taps?
Click to expand...


Doubtful, and if it's hooked up on the "Step Up" side of things incorrectly, the transformer will let you know it's wrong in quick fashion.


JAP>
 
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meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
A long time ago, I posted about testing a 480 to 120v step down control transformer for ratio as a nuclear plant test technician. We had many that failed because we applied 120 to the secondary and measured for 480 on the primary. We got MANY wrong ratios. Someone here explained why. Reverse it to apply 480 and measure for 120 and they were correct. You might search for the thread. Just because they're rated for step-down, does not mean they'll work as step up and vice versa. Eye opener for me.
 
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GoldDigger

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Placerville, CA, USA
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Retired PV System Designer
As I understand it power transformers generally (always?) have turn ratios that exactly match their nominal voltage ratios.
Control transformers, for whatever reason, are "compensated" to adjust for expected voltage drop. When used in reverse the output voltage will be low by twice the expected voltage drop.
But that does not mean that power transformers can always be used in reverse. (NEC 2014 prohibits that when the manufacturer does not explicitly approve).
Among the potential issues are extremely high inrush currents when the "wrong" winding supplies the magnetizing current, and high hysteresis losses on other than nominal input voltages because the voltage adjusting taps are on the wrong winding.
 
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templdl

Senior Member
Location
Wisconsin
meternerd said:
A long time ago, I posted about testing a 480 to 120v step down control transformer for ratio as a nuclear plant test technician. We had many that failed because we applied 120 to the secondary and measured for 480 on the primary. We got MANY wrong ratios. Someone here explained why. Reverse it to apply 480 and measure for 120 and they were correct. You might search for the thread. Just because they're rated for step-down, does not mean they'll work as step up and vice versa. Eye opener for me.
Click to expand...

Machine tool and control power transformer in most cases will not have a turns ratio as one would expect to have such as 4:1 for a 480-120 transformer. The windings are compensated for a voltage drop due to coil inrushes for control circuits. Instead of the expected 120v no load secondary voltage the secondary voltage will be higher as is will drop when a coil is energized. When you attempt to use one of these transformers as a sterp up from 120-480v you will highly disappointed when you get a voltage noticably less than 480v.
 
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