low voltage transformer

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brother

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trouble shot a roll up door and found the contactor had melted on the inside (the coil was shorted as well). Had to replace it and also the low voltage transformer (120 to 24 v). unless the transformer had gotten shorted somehow, i dont see these go bad that often. I was thinking maybe the switch somewhere was bad that kept power to the coil and caused it to short and melt the contactor. Problem is i did not find anything wrong with the switches. Any suggestions??
 
Brother,
With all that damage it appears it may be a lightning hit! If the contacts themselves are ruined, It would be more apparent that lightning might have
done it. Only an opinion from experience.
jahrtrix
 
I've heard that low voltage transformers are a version of saturable reactors; if they are shorted they just shut down and so they don't care if the short stays in place forever.

Regular transformers don't like to be shorted, and these doorbell type transformers do seem to be constructed differently than regular transformers, for the same voltamp rating.
 
jahrtrix said:
Brother,
With all that damage it appears it may be a lightning hit! If the contacts themselves are ruined, It would be more apparent that lightning might have
done it. Only an opinion from experience.
jahrtrix
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I work at this place and dont recall any lighting hit. I do know there was a test on the generators and when they were tested this is when it happen. Im thinking some how some switch or something got stuck but i just couldnt find any thing. Ill just have to wait to see what happens later and see if this happen again.
 
I might guess that the contactor coil just went bad and shorted by more natural means, and the short persisted for a sufficiently long time to toast the transformer. I know you can ruin the typical 40va, 24 volt HVAC transformer with an overload or short-circuit in as little as 30 seconds.
 
The electrics in roll up doors isn't really top quality, then add vibration and it will fail. I've seen metal shavings and bearing lubricants that fell down onto contactors and limit switches. It is definitely not unusual for contacts to go bad, or coils to burn out.
 
mdshunk said:
I might guess that the contactor coil just went bad and shorted by more natural means, and the short persisted for a sufficiently long time to toast the transformer. I know you can ruin the typical 40va, 24 volt HVAC transformer with an overload or short-circuit in as little as 30 seconds.
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thats exactly the type of transformer this is, 40 va 24 volt. I didnt see a fuse on the secondary side of the transformer, I think if we had a fuse it would helped save it. so thats 1.6 amps? 40/24 = 1.6
 
brother said:
thats exactly the type of transformer this is, 40 va 24 volt. I didnt see a fuse on the secondary side of the transformer, I think if we had a fuse it would helped save it. so thats 1.6 amps? 40/24 = 1.6
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Yeah, but 40va transformers are typically fused at 3A, and 75va transformers are typically fused at 5A.
 
brother said:
Ok, is my math wrong?
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No


would not the 40va secondary side need to be fused at 1.6 amps to protect the secondary side??
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No

With a 3 amp fuse it is protected from short circuits and it can not be overload unless the load is changed.

Check out table 450.3(b)
 
brother said:
Ok, is my math wrong? ohms law, would not the 40va secondary side need to be fused at 1.6 amps to protect the secondary side??
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I don't think so. I'm just telling you what is common to see as far as fuse sizes in equipment manufactured with both 40 and 75 va transformers. I suspect that this is to correspond better with the available automotive type fuses, but I have no idea for sure. These wee transformers are supposed to be internally thermally protected, I believe, so any additional fuse protection is not for a safety enhancement but to save the transformer itself. Feel free to put in a 1-6/10 amp fuse if you want to. That is a manufactured size.
 
iwire said:
With a 3 amp fuse it is protected from short circuits and it can not be overload unless the load is changed.

Check out table 450.3(b)
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ok 450.3(b) sends me to 430.72(C) 1 thru 5. Since this is a motor control circuit rated less than 50 va and located in the enclosure. C(3) says it can be protected by the primary overcurrent devices, which in this case was a 20 amp single pole breaker.

So i guess its not required. Thanks for the reference article.
 
Your welcome, but honestly I did not remember about the exception to 450.3(B).

What I was trying to point out was that transformers are often protected well above their capacity.

For example a 75 kVA 480-208Y/120 unit.

Rated primary current 90 amps, allowable primary protection up to 225 amps.

Rated secondary current 208 amps, allowable secondary protection up to 250 amps.

It is really the calculated load that protects the transformer from long term overcurrent.
 
Many listed Class 2 transformers have an internal non-replaceable fuse on the secondary. This is often just a fusible link in series with one of the secondary leads under the first couple layers of the outer wrapping.
 
iwire said:
Your welcome, but honestly I did not remember about the exception to 450.3(B).

What I was trying to point out was that transformers are often protected well above their capacity.

For example a 75 kVA 480-208Y/120 unit.

Rated primary current 90 amps, allowable primary protection up to 225 amps.

Rated secondary current 208 amps, allowable secondary protection up to 250 amps.

.
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Ok help me with the math here again. how you get 90 amps from 75,000 va / 480?? I get 156.25 amps for primary and i get 75000/120 = 625 amps for the secondary. am i missing something here??
 
brother said:
Ok help me with the math here again. how you get 90 amps from 75,000 va / 480?? I get 156.25 amps for primary and i get 75000/120 = 625 amps for the secondary. am i missing something here??
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Yes, you are missing the square root of 3 factor that enters into 3 phase calculations. You need to divide your answers by the square root of 3 to get the correct amps. Also you only divide by 208 and not 120 for the secondary current.
 
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brother said:
Ok help me with the math here again. how you get 90 amps from 75,000 va / 480?? I get 156.25 amps for primary and i get 75000/120 = 625 amps for the secondary. am i missing something here??
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I do it this way although I am sure there are other ways.

Primary 75 * 1000 / 480 / 1.73 = 90.317 amps * 2.5 = 225.79 amps or a 225 OCPD

Secondary 75 * 1000 / 208 / 1.73 = 208.426 amps * 1.25 = 260.53 amps or actually up to a 300 amp OCP (Your can round up to the next standard size)
 
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