Transformers keep exploding

[Tim01]

Hi all,

I fished my existing wires for one self serve bay through a new conduit due to roof replacement and reconnected to the wires coming from the equipment room. I made a mistake in the reconnection and ended up shorting the transformer (100 VA). After fixing that connection, I thought the transformer was still good since the bay was operational until the next morning when the transformer exploded. I replaced that transformer with a similar one, but after turning on the circuit breaker it exploded almost immediately again. I checked the voltage and it was 211 V. I thought perhaps the last 2 explosions damaged the contactor (30 amps), so I used another contactor (20 amps) and another transformer (40 VA). This time I only have the wires from the circuit breaker connected to the contactor and the transformer and nothing else. Again upon power on of the breaker, the transformer exploded. I disconnected the burnt out transformer and tested the voltage multiple times and I got a range from 211 to 240 Volts. I manually activated the contactor to see if it works and I was able to start the pump motor. So the wiring from breaker doesn't affect the contactor, but why is it causing my transformers to explode? Thanks.

 

[MEP001]

I assume you're using a multitap 40va transformer and wiring it for either 208 or 240 volts?

I was called to look at a wash bay that had two transformers fail like that. Turned out the transformers were all installed upside-down, and he had installed the new ones right-side up, putting the 240V power into the 24V out. In case you've ever wondered, you get 2400 volts briefly to the Dixmor timer when you do that, which can burn it out and cause a direct short. I had to put in a fuse, unwire everything, then hook up one component at a time until I found the problem.

 

[Earl Weiss]

Don't know if this is your issue but a few year ago there was a burnt spot on the breaker panel and employee moved breaker to an empty position which somehow was supplying 240 instead of 120V. Did not even know that was possible.

 

[I.B. Washincars]

Don't know if this is your issue but a few year ago there was a burnt spot on the breaker panel and employee moved breaker to an empty position which somehow was supplying 240 instead of 120V. Did not even know that was possible.

Me neither. I hooked up a vacuum like that. Turns out that if you have service with a wild leg, that wild leg is 277V, or at least it was in my instance.

 

[Tim01]

I assume you're using a multitap 40va transformer and wiring it for either 208 or 240 volts?

I was called to look at a wash bay that had two transformers fail like that. Turned out the transformers were all installed upside-down, and he had installed the new ones right-side up, putting the 240V power into the 24V out. In case you've ever wondered, you get 2400 volts briefly to the Dixmor timer when you do that, which can burn it out and cause a direct short. I had to put in a fuse, unwire everything, then hook up one component at a time until I found the problem.

Hi MEP,

It looks like with the 40VA transformer I wired it incorrectly by connecting both the 208V (red) and 240v (orange) wires to the L1 and L3 of the contactor. That explains the explosion for that transformer. Since my voltage range was 211 V to 240 V, should I have connected to the black wire (common) and orange wire (240 V) of the transformer?

For my other 2 100 VA transformers, I think the wiring was correct. Hooked up L1 and L3 to H4 and H2 (208V) of transformers. The X1 and X2 are the 24 V output lines. I don't know why my 2nd transformer blew up then. Perhaps since I had a range of volts from the breaker I should've connected to the H1 (230V)? I have 5 other bays and they're all connected to the H4 and H2.

 

[mjwalsh]

Me neither. I hooked up a vacuum like that. Turns out that if you have service with a wild leg, that wild leg is 277V, or at least it was in my instance.

I fried a small transformer with my first encounter with the "unusable as a 110VAC HOT" wild leg. When I put both an analog(needle) & a digital multimeter probes on to check voltage between the leads the needle would alternatively jump back & forth from 277 to 0 etc. On the digital multimeter it would flash the 277 on & off. I guess a small enough in line fuse on both the primary side & secondary (two total) would maybe have blown in time to have prevented damage to the transformer ... not sure.

I am kind of curious to know how many of us & other small businesses have the "wild leg" delta setup for our 3 phase power panels??? I was told that to have brought in non wild leg 3 phase would have been cost prohibitive. When we had a complete redo of our car wash equipment back in 1987 we had the 3 phase brought in from the nearest UTILITY POLE. From 1968 to 1987 we only had single phase 230VAC. From 1987 to present we have had 230VAC delta wild leg 3 phase.

We had no before hand "heads up" as to the electrician needing to take extra precautionary steps to balance to allow for the wild leg ... leaving unusable for 110VAC breaker slots & less utilization of one of the four wires coming in.

 

[MEP001]

Hi MEP,

It looks like with the 40VA transformer I wired it incorrectly by connecting both the 208V (red) and 240v (orange) wires to the L1 and L3 of the contactor. That explains the explosion for that transformer. Since my voltage range was 211 V to 240 V, should I have connected to the black wire (common) and orange wire (240 V) of the transformer?

That should be correct. I'm curious why your voltage varies so much. Do you know if your voltage should be 208 or 240? I'd have to assume 240 since 208 wouldn't likely vary that high.

 

[2Biz]

This topic has come up before...Sounds like you have "Wild Leg" 3 phase like I do. When checking any two leads, you get 240v. But checking each leg to ground or neutral you get 2 leads @ 120v and 1 lead 208v....Doesn't add up right, but it has to do with the sine wave timing of the center Wild Leg Tap that still gives you 240v when checking the wild leg with either of the 120v legs.......This configuration allows for single phase 120v and 240v, single phase 208v for older lighting transformers with a 208v tap, and 3 phase for larger motor power.

Copied from a Google Search...

The "wild leg", also known as the "high leg" exists in any 120/240 three-phase service, where there is 120 volts from phase A to neutral, and from phase C to neutral, but there is 208 volts from phase B to the neutral. It is a necessary characteristic of a phase A to phase C transformer which has a grounded center tap, but which the two other (one other, for "open delta") are not so tapped.

 

[2Biz]

When connecting power to any transformer, always check the output voltage before connecting load. You could save yourself some grief if you have the input wires not connected properly. In addition, a transformer should always be fused. Speaking From Experience, I hate letting the "Magic Smoke" out!

 

[mac]

While there are experienced people here commenting on this, I think you all missed the most important clue that Tim01 provided. He said that he rewired the bay and pulled the wires or cable in new conduit, and THEN the exploding transformers started showing up. My guess is something is either miswired or a wire was scrapped while being pulled through the conduit. Trust me, it's easy to do.

 

[br549ms]

We are assuming everything worked before the rewiring of "one" bay?

This is probably what you have for service. Still see this type of service, mainly in older industrial areas.

Three Phase Four Wire Delta

Also known as a high-leg or wild-leg delta system. Used in older manufacturing facilities with mostly three-phase motor loads and some 120 volt single-phase lighting and plug loads. Similar to the Three Phase Three Wire Delta discussed above but with a center-tap on one of the transformer winding to create neutral for 120 volt single-phase loads. Motors are connected to phase A, B, and C, while single-phase loads are connected to either phase A or C and to neutral. Phase B, the high or wild leg, is not used as the voltage to neutral is 208 volt.

 

[br549ms]

I know this is hindsight, but always get the cell out and take photos of everything especially wiring connections before starting any rewiring or repair jobs. It will save you a lot of guessing later on.

 

[Tim01]

Yes prior to the wiring everything worked. Part of the issue with the re-wiring was that the one of the 24 V lines on the roof had a break somewhere (These wires are nearly 30 years old!), so I had to cut it and add a new wire. I made the mistake of leaving the old portion connected on one end, but not capped on the other end. That's probably what shorted my initial long lasting transformer, which I've never had to replace before. I then removed that old portion completely and thought everything was good with the transformer because it was functional for 12 hours until it blew up the next morning.

I took photos of the existing configuration and that was part of the problem w/ the second transformer. Not all transformers have the same wiring configuration for the same voltage. Thinking that they're the same, I wired the 2nd one exactly like the photo I took. The original transformer was a Southern Pride while the 2nd transformer was a Square D. Today I looked at the wiring diagram label of the broken Square D and yep, it's quite different. Lesson learned!

Today I lucked out that a local Grainger has some 70 VA transformers with a builtin mini breaker. I connected to the correct wires (white-common and orange-204V) this time to my 3 phase 211 -240 breaker wires (L1 and L3) and no explosion!

By the way, I think it's possible that I have a cheap voltage meter from Harbor Freights and that's why I see a wide range from 211 - 240 Volts in my tests.

Thank you all!