buck and boost transformer

[MikeLechiara]

We recently installed a buck and boost transformer for a machine shop where we needed 5A of 3 phase 480V but had a 208V system. We fed a 9kva 208v to 480v xfmr with a 3P 20A main breaker. The machine shop wanted a disconnect on the primary side of the xfmr because they don't use the machine every day. Once we connected the xfmr to a disconnect, it tripped the circuit breaker. We upsized the breaker to a 30A but it still tripped. So we then tried a motor rated switch without overload protection, but that also caused the circuit breaker to trip. Once we take the switch out of the picture, the circuit breaker holds fine. What causes the circuit breaker to trip and how can we provide a means of disconnect that won't create this problem?

 

[jim dungar]

We fed a 9kva 208v to 480v xfmr with a 3P 20A main breaker.

Did you purchase a true step-up transformer? If you are 'backfeeding' a transformer there are several issues to be considered, not least of which is the handling of the 208V neutral.

Tell us what you have for a transformer wiring diagram.
What are you doing different when you add a switch into the circuit. How are you changing your wiring?

 

[templdl]

We recently installed a buck and boost transformer for a machine shop where we needed 5A of 3 phase 480V but had a 208V system. We fed a 9kva 208v to 480v xfmr with a 3P 20A main breaker. The machine shop wanted a disconnect on the primary side of the xfmr because they don't use the machine every day. Once we connected the xfmr to a disconnect, it tripped the circuit breaker. We upsized the breaker to a 30A but it still tripped. So we then tried a motor rated switch without overload protection, but that also caused the circuit breaker to trip. Once we take the switch out of the picture, the circuit breaker holds fine. What causes the circuit breaker to trip and how can we provide a means of disconnect that won't create this problem?

Are you actually using a buck-boost trsnsformer to boost from 308v to 480v? It can be done but there is no advantage over an idolation type transformer in doing so and you have no isolation.
If I may take a wild guess you are actually using a common 9kva 480d-308y/120 to step up from 208 3ph to 480v 3ph. NB it, to go any farther please attach a picture of the actual nameplate from your transformer in the interest of safety to assure that we understand the actual trsnsformer you are referring to. I venture to wild guess that you are using a common delta wye transformer as a step up and the you a feeding the transformer incorrectly. If so then you must coect the connections and understand the dynamics of inrush/magnetizing current when using this transformer to step up. If my suspicions are correct the solution may be simple.

 

[ActionDave]

It sounds like you have a step up transformer not a buck boost. Either way adding a switch would not make the breaker trip unless something is miswired.

 

[Ingenieur]

WHEN does it trip?
opening motor off
closing motor off
once the motor is running with the switch closed
????

2 winding xfmr
or autotransformer type

my wag
when closing the switch inrush current
rated primary current is 25 A

 

[kwired]

If you are back feeding a 480 delta to 208/120 wye transformer depending on the equipment you are supplying it may not like not having a 277 ground reference either, especially if it has a VFD. And if you are using such transformer you have to corner ground the 480 volt secondary, there is no neutral to ground.

 

[petersonra]

We recently installed a buck and boost transformer for a machine shop where we needed 5A of 3 phase 480V but had a 208V system. We fed a 9kva 208v to 480v xfmr with a 3P 20A main breaker. The machine shop wanted a disconnect on the primary side of the xfmr because they don't use the machine every day. Once we connected the xfmr to a disconnect, it tripped the circuit breaker. We upsized the breaker to a 30A but it still tripped. So we then tried a motor rated switch without overload protection, but that also caused the circuit breaker to trip. Once we take the switch out of the picture, the circuit breaker holds fine. What causes the circuit breaker to trip and how can we provide a means of disconnect that won't create this problem?

If you have a 9 kva 3 phase xfmr, FLC would be about 25 Amps @ 208 V. I would not be putting either a 20 or a 30 Amp CB on this device. Sometimes small xfmrs like this can experience residual magnetism that can cause spurious tripping on power up. You could try some 30 Amp FNQ fuses. They are made to accommodate the inrush transformers see and fit in standard class CC fuse holders.

Personally, I would probably have used a 60 CB on the primary of this thing. It makes the wire bigger but reduces the chances of spurious tripping.

Just curious when you went from a 20 A to a 30 A CB did you increase the conductor size on the primary side? Or was it already large enough?

A properly wired switch won't have any effect on whether it trips or not. I suspect it is either miswired, faulty, or the tripping is coincidental.

 

[MikeLechiara]

If you have a 9 kva 3 phase xfmr, FLC would be about 25 Amps @ 208 V. I would not be putting either a 20 or a 30 Amp CB on this device. Sometimes small xfmrs like this can experience residual magnetism that can cause spurious tripping on power up. You could try some 30 Amp FNQ fuses. They are made to accommodate the inrush transformers see and fit in standard class CC fuse holders.

Personally, I would probably have used a 60 CB on the primary of this thing. It makes the wire bigger but reduces the chances of spurious tripping.

Just curious when you went from a 20 A to a 30 A CB did you increase the conductor size on the primary side? Or was it already large enough?

A properly wired switch won't have any effect on whether it trips or not. I suspect it is either miswired, faulty, or the tripping is coincidental.

Yes, we upsized the wiring to #10. I personally felt that it was the inrush which is hard to test with an amprobe as it trips too rapidly, but the running load was quite low. We did try a 40A main to see if that helped the problem but it tripped again the first time then held. I'm also wondering if it could be a resistance problem with the switch, though that doesn't seem right.

 

[petersonra]

Yes, we upsized the wiring to #10. I personally felt that it was the inrush which is hard to test with an amprobe as it trips too rapidly, but the running load was quite low. We did try a 40A main to see if that helped the problem but it tripped again the first time then held. I'm also wondering if it could be a resistance problem with the switch, though that doesn't seem right.

try a 60 A CB with 30A FNQ fuses downstream. If you are close enough you should be able to use the same wiring by taking advantage of the tap rules.

 

[MikeLechiara]

We recently installed a buck and boost transformer for a machine shop where we needed 5A of 3 phase 480V but had a 208V system. We fed a 9kva 208v to 480v xfmr with a 3P 20A main breaker. The machine shop wanted a disconnect on the primary side of the xfmr because they don't use the machine every day. Once we connected the xfmr to a disconnect, it tripped the circuit breaker. We upsized the breaker to a 30A but it still tripped. So we then tried a motor rated switch without overload protection, but that also caused the circuit breaker to trip. Once we take the switch out of the picture, the circuit breaker holds fine. What causes the circuit breaker to trip and how can we provide a means of disconnect that won't create this problem?

Okay, I didn't have all the correct information. What we installed is a 3phase isolation transformer:

Federal Pacific Transformer, TE4D9F, 9 KVA, 480 Volt Primary, 208Y / 120 Volt Secondary, NEMA 3R, Encapsulated, Dry Type, Three Phase Dry Type Transformer

 

[mike_kilroy]

9kva @208 v 3ph is 25 amp rating. That xfmr likely has typical 10x inrush max on turn on.

See your issue? Get breaker or fusing That can handle 250-300amp inrush. If fuses, add your on,off switch..

Sent from my SM-G900V using Tapatalk

 

[templdl]

Okay, I didn't have all the correct information. What we installed is a 3phase isolation transformer:

Federal Pacific Transformer, TE4D9F, 9 KVA, 480 Volt Primary, 208Y / 120 Volt Secondary, NEMA 3R, Encapsulated, Dry Type, Three Phase Dry Type Transformer

Now we're getting somewhere. You may anticipate the inrush of your transformer to to in the range of 650-750a. Based upon 759a and also knowing the the mag pickup of a TM breaker is 10x the rating of the breaker consider a 75at breaker. But there are no 76at breakers. Refer the NEC table 450.3(B) foot note 1 which allows you to go back up to 25a x 250% or 62.5a which you are a!lowed to use an 89a trip breake. The mag trip would be 800a +-20% know if that they are normally calibrated on the high side.
Another interesting thing to be aware of is that the LV windings are wound over the core first because the transformer taps if provided are located of the HV windings for accessibility. Bring the the LV windings are right over the core the if the HV inrush if used as a step up transformer were 15x for example using that same transformer as a step up transformer the LV in the bush will be more than 15x.the LV FLA.
Lesson learned? Don't undersized the OCLDb used upon the LV input side of the transformer. Remember that the a properly sized HV OCPD on the output side of the transformer protects the transformer from overload and the LV OCPD on the input provides short circuit protection protecting the upstream distribution system should a fault in the transformer winding.

 

[jim dungar]

Federal Pacific Transformer, TE4D9F, 9 KVA, 480 Volt Primary, 208Y / 120 Volt Secondary, NEMA 3R, Encapsulated, Dry Type, Three Phase Dry Type Transformer

Because you are back feeding this transformer, make sure you do not connect the 208Y neutral to the transformer, the X0 connection must be left un-terminated.

As temple said, a backfed transformer can easily have inrush currents of more than 15X. A small transformer, like this 9kVA, is probably really a two coil 'T' connected device, and it may even have compensating windings. It's performance, when backfed, may produce several unexpected issues.