Primary/Secondary Current or Voltage 1

[timjturner]

I'm not sure how to distinguish when people are talking about primary or secondary current or voltage? I'd just ask but I feel like its too simple of a question to ask one of my co-workers without the embarrassment factor. So I came here to ask...

In general how can I tell where in a circuit primary/secondary current/voltage can be measured?

 

[rbulsara]

You need read up on basic of electrical engineering.

But here is a clue:

http://www.kappaelectricals.com/technical.html#principle1

 

[rbulsara]

timjturner:

Also there is nothing wrong with not knowing something and acknowleding it. Its perfectly OK to say, I do not know, but I will find out. Do not hesitate to go the senior engineers and ask them questions, while some are more helpful than others, most will appreciate your willingness to learn.

On the otherhand do not use their expertize to minimize your own leg work necessary to find out answers.

 

[dpc]

Sometimes it's obvious from the current magnitude, but not always. I've been doing this for a long time and I don't have any trouble asking for a confirmation that current is referenced to primary or secondary. Sort of like asking "So we're sure this is de-energized?"

 

[rbulsara]

Tim:

Re-reading your question, I am not sure whether you meant primary/seondary as in intrument transformers or the primary and secondary sides a regular power transformer.

Could you be a little more clearer on your question?

 

[timjturner]

well sometimes my co-workers will speak of circuit breakers providing protection on the primary side or secondary side... what prompted this question is that I was asked to add a volt trap to read primary voltage. I can usually use common sense and figure out what they are talking about, but its never as clear as it should be in my mind.

 

[rbulsara]

You need to take some courses on electrical power systems and read some good text books.

Plus you need to take more time to provide more details.

Primary/secondary side of what? Transformer?

What is voltage "trap"? You need to avoid using totally non-technical terms. Who are you working with? Shed some light on your qualifications and work.

While folks here are very helpful, this forum is no substitued for formal learning, and definitely not a class for learning 101 of engineering.

 

[waross]

The primary side of a transformer is the side that is supplied with power. The secondary side is the side from which power is taken.
A breaker on the primary side will disconnect the transformer.
A breaker on the secondary side will disconnect the load from the transformer but the transformer will remain energised.
Sometimes transformers are reverse connected.
That is, a transformer that is normally always connected with the high voltage side as the primary may occasionally be used with the low voltage side as the primary.
For example, it is desired to supply power to two small 120 volt pumps. The pumps are 2000 feet away from the panel.
To comply with the code required 3% or 5% maximum voltage drop
The cables would have to be extemely large.
It would be cheaper to buy two identical dry type transformers rated 120/240:600 volts. These transformers are generally use with the 600 volt winding as the primary.
In this example, one transformer would be supplied with 120 or 240 volts and supply 600 volts. The 600 volt winding is the secondary winding.
The second transformer, at the location of the pumps, is supplied with 600 volts. It delivers 120 volts or 120/240 volts. the 600 volt winding is the primary.
Why 600 volts?
At a given load, say 1.2 KVA, the current at 600 volts will be 20 % of the current at 120 volts.
1.2 KVA at 120 volts will draw 10 amps.
1.2 KVA at 600 volts will draw 2 amps. That's a 5:1 ratio.
3% voltage drop at 120 volts is 3.6 volts.
3% voltage drop at 600 volts is 18 volts. That is also a 5:1 ratio. The total voltage drop advantage with the higher voltage is 25:1
When a circuit is long enough that voltage drop rather than ampacity is the determining factor in sizing the cable, you will have approximately a 25:1 ratio in cable size.
With our 1.2 KVA load on #14 AWG wires at 120 volts, we can run about 100 feet with 5% voltage drop.
With our 1.2 KVA load on #14 wires at 600 volts, we can run about 2500 feet with 5% voltage drop.
The point of this is that sometimes there are good reasons to use a transformer in reverse of its normal usage.
Please try to find a way to ask questions of your co-workers.
If you suspect that they are giving you bad information as a joke on you, ask us to verify it for you.
Good luck friend.
yours

 

[itsmoked]

Nice synopsis waross!

You should copy this out, add a few blank lines and drop it into a FAQ. "Swapping Transformers for Wire Size"

Then we can re-use it a bunch.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Thanks Keith;
How about "The effect of increased voltage on voltage drop and wire size."?
Comments?
Respectfully

 

[ScottyUK]

A "volt trap" is an alternative name, possibly a Westinghouse brand name prior to breakup of the company, for a high energy suppressor. We have a number of these components in the power stage of our Westinghouse-built generator AVRs where they are intended to stop transients getting into the AVR from external equipment. It's not a familiar term over here in Europe but I think they are broadly similar to the 'Metrosil' non-linear resistor. I'll have a look at the datasheet when I get back in to work (next year!) if anyone is that interested.

How you would use one to measure voltage is beyond me - confusion reigns!


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[itsmoked]

That works for me Bill!

??? V?olta?ge?? tr?ap.... ????



Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[timjturner]

Thanks for all the help, I appreciate it. I've always understood the difference between primary and secondary side of a transformer. My confusion comes from this having nothing to do with a transformer. It has come up before in 480VAC motor control circuits where there is no xformer involved. I work for a small company that engineers large industrial control systems. My education and background is in electronics/computer engineering rather than electrical engineering. (i.e. <= 24V)

 

[nsv]

Most often transformers are used to reduce the high voltage at which it is delivered from the power plant to a more modest level which can be used by the consumer. In that case there is not much doubt what is primary and what is secondary.

But then consider any power plant, eg. a wind turbine, with an asynchronous generator. The active power produced in the generator enters the transformer at the low volt winding (usually 690V) and leaves at 10, 20 or 30kV, whereas the reactive power (if power factor correction is missing or out of service) is supplied from the grid entering the transformer at the high volt- and leaving at the low volt winding.

I used to work with a wind turbine manufacturer as a transformer specialist, and every time somebody from outside talked about 'primary' and 'secondary' I had to ask if he meant 'high volt' or 'low volt'. Rather confusing, so I recommend the 'voltage' terminology. The terms 'primary' and 'secondary' contain no information and I always avoid them


NSV

 

[dana1028]

"To comply with the code required 3% or 5% maximum voltage drop."

Just a minutia comment - there are no code requirements to limit voltage drop. The code has Fine Print Notes [which are not enforceable] which 'suggest' sizing conductors to limit voltage drop in branch circuits and feeder conductors.

This voltage drop limitation is a 'workmanship' or design issue; typically on engineered drawings we [electricians & elect. inspectors] see conductors sized to compensate for voltage drop. On non-engineered drawings most electricians will size conductors with voltage drop considered.

The Fine Print Notes [FPNs] in the 2005 NEC are as follows:
Branch circuits - 210.19(A) - FPN No. 4
Conductors - 310.15(A)(1) - FPN No. 1
Feeders - 215.2(A)(3) - FPN No. 2
Sensitive Electronic Equipment - 647.4(D) - as a P.S....this is the only code section in the NEC that does in fact mandate voltage drop controls.

 

[peebee]

Re "there are no code requirements to limit voltage drop."

Sure there are, the ASHRAE 90.1 energy code includes maximum permissible voltage drop requirements. ASHRAE 90.1 has been adopted by most US jurisdictions.

While you're correct that NEC/NFPA doesn't specify maximum permissible voltage drop for most general installations, other codes certainly do. And by the way -- NEC does specify an even tighter voltage drop requirement for fire pumps.

 

[dana1028]

"Sure there are, the ASHRAE 90.1 energy code includes maximum permissible voltage drop requirements. ASHRAE 90.1 has been adopted by most US jurisdictions."

Thank you for the reply...however -
1. ASHRAE is not a code...it is a standard
2. US jurisdictions typically do not adopt standards...to do so would require that jurisdiction to buy that standard....adding 10's of thousands of dollars to the building/planning dept. annual budget...just doesn't happen - way too many standards.

And without a jurisdiction adopting a specific standard, there is no requirement.

This is not just my opinion, I have checked with building officials in CA to confirm this comment.

 

[peebee]

US jurisdictions typically DO adopt ASHRAE 90.1 and/or the International Energy Conservation Code (IECC) as their energy code. For example, "The State of Michigan, in April 2002, adopted ASHRAE 90.1-1999 standard as the new commercial portion of the MUEC [Michigan Uniform Energy Code]".

In addition, the IECC references (and therefore codifies) ASHRAE 90.1. I believe the IECC also includes its own voltage drop limits, but I do not have a copy handy to positively confirm.

To say that code does not address voltage drop would be an overly general statement, which would be invalid in most US jurisdictions.

 

[peebee]

Not to beat a dead horse, but according to this site,

http://www.energycodes.gov/implement/state_codes/state_status_full.php

, Arizona, Delaware, Guam, Louisiana, Maine, Michigan, North Dakota, NewJersey, Ohio and Tenessee have adopted ASHRAE 90.1 as code.

It also goes on to say that California's "state-developed code, Part 6 of Title 24, which meets or exceeds ASHRAE/IESNA 90.1-2001, is mandatory statewide as of Oct. 1, 2005".

Most other states have adopted IECC or have their own state specific energy code.

 

[peebee]

And look what California has on the table (from

http://www.energy.ca.gov/title24/20...orkshop/public_comments/2001-10-19_Behzad.doc

):

Proposals for 2005 Building Energy Efficiency Standards Amendments

1. Problem:

One element of the building power waste is the excessive continuous voltage drop of an electrical system. Voltage drop causes heating of conductors, the larger the voltage drop the larger is this heat, thus increasing the amount of wasted energy. Currently, the energy efficiency standards has no limitation on the amount of power that can be wasted through voltage drop. This waste can be as much as 10% to 20% of the power consumed in any building power line resulting in significant cost.

Proposed Change:

In order to reduce this energy loss, it is recommended that in an electrical system voltage drop be limited as follows:

a. Building Branch circuit wire voltage drop limited to 3%.
b. Building Feeder circuit wire voltage drop limited to 3%.
c. Total accumulated Building branch circuit and feeder wires voltage drop is limited to 5%.
d. a or b can be less than 3% by design.

This proposed change will reduce the wasted power in any building wire by 15%, resulting in significant savings through out the life expectancy of the installation. This proposed change would also be in line with recommendation (but not required) National Electrical Code.