Some exit lights are feed with 277 and stepped down to 120. I always wondered if I could feed one of those transformers with 120 and hooked it to a 277V lay in light fixture would the insulation on the transformer windings hold or cook? Because I valued my job I never tried it but I bet it would have worked.
Transformers generally don't know which end is up!
| Some exit lights are feed with 277 and stepped down to 120. | I always wondered if I could feed one of those transformers with 120 and | hooked it to a 277V lay in light fixture would the insulation on the | transformer windings hold or cook?
The transformer has to have insulation to withstand at least the 277 volts that would be applied, wherever that 277 volts might be. Running it in reverse to step 120 volts up to 277 volts would be applying 277 volts in the same places 277 would be normally. So the insulation would not be an issue.
How the transformer is grounded might be an issue.
The capacity of the transformer (rated in VA or kVA) might be, depending on how much load you have.
| Because I valued my job I never tried it but I bet it would have worked.
I take it this is not an electrical engineering job.
Sorry. But the insulation is very important. The current difference would be the issue. The transformer would be designed to take the incoming current it takes to run the exit light. If you put the load on the other end.....would the windings be able to handle the current?
I take it you are new to engineering. How many exit lights have you wired up in your engineering job?
Don't do it. While it might be possible to get that transformer to work for your application, there is always the little 'gotcha' in the code called 'approved for the purpose'.
If you don't know the answer to your own question, you are going to have a tough time selling something to an inspector that wasn't explicitly approved for the application you intend.
I understand that this is a bad thing to do. I still would like to know if you could safely "not legally" make a 277 volt light fixture work for home (120V) use by using one of those transformers.
You can get hundreds of 277V light fixtures from left over remodel jobs. I am sure there are restrictions that permit you from doing this but I bet it would work fine.
I AM NOT GOING TO DO THIS. I don't even do electrical work any more. I don't have a transformer and I don't have a 277V light fixture. I am just bored and would like to discuss it.
As someone else said, the biggest issue I see is the capacity in VA of the transformer. A 3 lamp 4 foot fluorescent fixture would have a load of 100 watts or more. An exit light may have a load of 12 watts or less. In the interest of efficiency, the exit light probably does not have a 100 VA transformer.
Mitch Thompson
| Sorry. But the insulation is very important. The current difference would | be the issue. The transformer would be designed to take the incoming | current it takes to run the exit light. If you put the load on the other | end.....would the windings be able to handle the current? | | I take it you are new to engineering. How many exit lights have you wired | up in your engineering job?
Of course the insulation is very important. What makes you think that I ever said it was not. I said it would not be the *ISSUE*. The reason is because merely running the transformer backwards puts the same voltage on each side that it expects.
The transformer designed for 277 volts on the primary and 120 volts on the secondary will have an insulation rating designed to handle each voltage in the respective places, as well as whatever maximum difference will exist anywhere in the winding or wiring terminal. Energizing the primary with
277 volts puts 277 volts where 277 volts is expected. Then 120 volts is induced where 120 volts is expected. Reversing it, you are putting 120 volts where 120 volts is expected, and 277 volts is induced where 277 volts is expected. As long as it is wired in reverse CORRECTLY, the insulation it has as designed will protect it in reverse.If you were to energize the 120 volt side with 277 volts, then you have a problem. That will induce 640 volts on the 277 volt side, if it does not saturate the core first, and risk overvoltage on the insulation, if it doesn't burn out due to overcurrent first.
Since you are talking about stepping 120 volts up to 277, if you were to get it backwards, you could end up stepping 120 volts down to 52 volts. You'd get 18.75% of the wattage from 120 volt light bulbs that way.
Since both 120 volt and 277 volt circuits are typically configured with one hot wire and one grounded (neutral) wire, such a transformer may have the chassis grounded to the wire it expects is grounded. Back when there was not a separate ground wire, this was often done. Hopefully you won't have that unsafe situation.
The transformer _may_ be an autotransformer, meaning that the low voltage and high voltage share the same winding, and the circuits are not isolated. Since both 120 volt and 277 volt circuits are typically configured with one hot wire and one grounded (neutral) wire, this would not be a problem. But you may have only one neutral wire coming out for both sides. be sure your wiring is correctly reversed and correctly grounded.
Electricity is a hobby for me, not my job. So this means I probably know more about this than half the BSEE graduates out there (who mostly did not study power systems). The true power engineers who fully understand their field (not something a mere BSEE can always produce) are the ones to get real answers from.
|> | Some exit lights are feed with 277 and stepped down to 120. |> | I always wondered if I could feed one of those transformers with 120 and |> | hooked it to a 277V lay in light fixture would the insulation on the |> | transformer windings hold or cook? |>
|> The transformer has to have insulation to withstand at least the 277 volts |> that would be applied, wherever that 277 volts might be. Running it in |> reverse to step 120 volts up to 277 volts would be applying 277 volts in |> the same places 277 would be normally. So the insulation would not be an |> issue. |>
|> How the transformer is grounded might be an issue. |>
|> The capacity of the transformer (rated in VA or kVA) might be, depending |> on how much load you have. |>
|>
|> | Because I valued my job I never tried it but I bet it would have worked. |>
|> I take it this is not an electrical engineering job. |>
|> -- |> -------------------------------------------------------------------------- | --- |> | Phil Howard KA9WGN |
He is a legend in his own mind.
|> -------------------------------------------------------------------------
-|> -------------------------------------------------------------------------
-| He is a legend in his own mind.
At least I have a mind and can read English.
Since the current and voltage are inversely proportional, as long as you don't draw more than the original rated current, when used in reverse this should work. Grounding may be another issue. And of course, code. .....Ross :>)
On Fri, 23 Apr 2004 23:45:04 GMT, "Ross Mac" Gave us:
Another retarded response by the RossTard that did NOT address the question.
The fact is that the insulation on mag wire is in the area of 1500 volts, and will not break down. Also, the rating is for Volt Amps, not merely current. That is for "heavy nyleeze". High temp wire has much better ratings. So it would depend on how the transformer was constructed.
As long as the rating of the transformer is not exceeded, it should be able to transform in either direction up to said specified rating.
Why would it not? A 94% efficient transformer at 200 VA being used as a step up OR as a step down should exhibit the same behavior in either direction at any given consumption rate that falls inside its rating.
As far as using it in a structural power feed installation, I would say that the transformer can only be used as it is designed in that case. Which would mean NO, but that wasn't the question.
| The fact is that the insulation on mag wire is in the area of 1500 | volts, and will not break down. Also, the rating is for Volt Amps, | not merely current. That is for "heavy nyleeze". High temp wire has | much better ratings. So it would depend on how the transformer was | constructed.
The volt-amps rating is usable capacity. But the other ratings cannot be allowed to be exceed in any variant usage. The voltage rating is one (and it _may_ be different per winding). The current rating is another (and it usually is different per winding if the ratio is other than 1:1). There's also temperature rating. The various ratings work together to produce an optimal cost design at a specific designed use (particular volts and a current at those volts).
| As long as the rating of the transformer is not exceeded, it should | be able to transform in either direction up to said specified rating.
All of the ratings. The only issue with reversability is how the ratings can be affected by windings that are either on an inner core wrap or on an outer core wrap. Usually the lower voltage (higher current) winding is on the inside, and usually there is no real difference.
Reversing a delta-wye three phase transformer is a different issue.
| Why would it not? A 94% efficient transformer at 200 VA being used | as a step up OR as a step down should exhibit the same behavior in | either direction at any given consumption rate that falls inside its | rating.
The only issue I really see is if one or the other winding is grounded to the case or core.
| As far as using it in a structural power feed installation, I would | say that the transformer can only be used as it is designed in that | case. Which would mean NO, but that wasn't the question.
Exactly.
Read the reply....I said it "should work" but also said it probably would not be code...You said the same thing and embelished it....what else is new here....get life punk.....
--------------- Actually Phil is right on target. In theory the transformer doesn't care which side is the supply. In some cases where corners are cut to save cost, there may be a small difference in practice but the transformer will work quite happily.
Your correct but there will be a little less (proportionetly) magnitizing current when the coil closest to the core is the one energized.
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| Your correct but there will be a little less (proportionetly) magnitizing | current when the coil closest to the core is the one energized.
Usually, the lower voltage coil is the one closer to the core. The reasons for that include less wire length (resistance) for the winding that has the higher current. I suppose, also, that higher voltage is easier to deal with getting connected if it is on the outer coil. The typical distribution transformer for utility service is step down, and all that I have ever seen (on the inside) have the high voltage on the outer coil.
We are each experianced with different types of transformers.
Most of my transformers seldom got to a full killowat, except in a couple high powered converters. Most of my transformers are sitting out there in space rather than on store room shelves or on power poles/ I also have done quite a few magamps, which if properly applied can be useful in todays circuitry. I even have some of them in space. I made use of their open circuit volt second accumulation ability to extremely reliably control voltage ouput of converters. They can be quite small at high frequencies, wound on 1/8 mil cores. The others that I designed include a batch of current measuring transformers for short circuit shutdown. By detecting peak currents with them, you can make the output of a converter or other power supply notch way back during over load. The peak current remains constant but the pulse width contracts to reduce the output voltage, while the peak current allowed remains the same.
magnitizing
magnitizing
------------ This also extends to EHV transformers where the high voltage winding is made up of interconnected "flat pancake coils" for insulation and electric field/capacitance control. Typically these will have a higher %impedance than distribution transformers.
When I designed hv torroids (1500 volts) on small cores we also sector wound them for the same reason. They were used for traveling wave tube power supplies.
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