460V equipment, 208V supply 8

[QGeek]

Our machinery normally uses 460V, 3-phase supply. We have come across an instance where customer only has 208V, 3-phase, and not a hope of changing. I am in the process of converting the equipment. Someone suggested wiring the 460V motors in the delta configuration. Any advice?

 

[jraef]

Someone suggested wiring the 460V motors in the delta configuration. Any advice?

Spurious advice, pay no heed to it. Likely came from someone who has no idea what they are talking about.

Either use 208V components in your machine, as it appears you are doing, or provide your customer with a transformer large enough to run your standard machine. If your equipment list is complicated and/or specialized, you may find that to be less expensive in the long run by the way.

"Will work for (the memory of) salami"

 

[QGeek]

That is a great suggestion, thank-you! Any hangups I should look for in a step-up transformer? Interference issues stepping up from the 208V supply and then (for part of the equipment) stepping down to 120V inside the equipment, etc.?

 

[X49]

I wouldn't expect any complications in using a step-up transformer.

Keep voltage regulation in mind. A transformer might drop 5-6% voltage from no-load to full load. This will be in addition to the voltage drop to the service entrance, which could be up to 5%. This could be an issue if you use a marginally sized transformer and feeder cables.

 

[jraef]

One potential "gotcha" now is that if your state has adopted the 2014 NEC, 450.11 (B) now says that you required to use a transformer that is specifically identified as being suitable for "reverse wiring" by the transformer manufacturer. No further description of how that would be any different, only a requirement that it actually be labeled for use like this. This is brand new, and many states (such as here in california) are always a few years behind on adoption of the current NEC, so YMMV.

.

"Will work for (the memory of) salami"

 

[QGeek]

Ah! Thanks. The transformer I spec'd does say that it is suitable for "reverse wiring," which brings up a question I have had. What is it about transformers that would make them suitable for only one way? Would one need to look for a step-up vs a step-down transformer?

 

[davidbeach]

I don't know the justification for that code change, but I've seen multiple occasions where someone with a 208V service and needing to hook up something at 480V went and got the most readily available transformer with those two voltages and hooked it up. That then created a grounded wye/delta connection that was likely in violation of the NEC, was certainly in violation of the utility service requirements, and resulted in damage to the customer's facility. I hope those transformers never get a "reverse wiring" label.

 

[dpc]

Jeff,

I'll have to look that one up - hadn't run into that change. I'm curious as to the original reason for the change - but not quite curious enough to look through all of the commentary.

 

[ScottyUK]

Possibly due to turns ratio: in step down mode the ratio will be chosen so that the LV bus runs slightly higher than nominal at no-load to allow for drop under load. In reverse that ratio would make the HV bus voltage low under no-load conditions and even lower when under load.

Just a thought.

 

[LionelHutz]

Sometimes, 460V motors are dual voltage 9-lead motors so you can wire them to be 230V or 460V rated. But, that still gives you a 230V motor running on a 208V supply so you're running the motor on a nominal 10% low voltage. You might get away with it if the motor doesn't run at full load but not if the motor is fully loaded.

I agree with David, you typically want to use a delta:wye connected transformer so you ground the neutral of the wye connections since code requires a 480V service to be grounded. This is rather hard to do if you connect the transformer backwards. I also understand that transformers typically have the primary winding wound first or closer around the core. Apparently, it reduces the inrush transients and losses.

 

[waross]

With the motor reconnected for 230 Volts, an auto-transformer boost will be a good option.
I prefer the two transformer 'Open delta auto-transformer boost' circuit.
Another option is the three transformer 'Wye auto-transformer boost' circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

I don't know how easy it is to get an autotransformer in your part of the World. I usually have to order one specially wound. That takes some time and is rather costly. So, I use single phase transformers fed from the ingoming voltage and then connect the secondaries in series with incoming grid. In boost direction (or buck, if needed. The rated power is no more than one third of total power times the deltaV/V fraction. If the load is, say, 30 kVA and you need to up the voltage around 10 percent, then each transformer needs to be around 1 kVA. That is three rather small standard transformers (24 V secondary is common) instead of one special unit.

If you are lucky, you can get a center tap on the secondary, then you have a coarse voltage adjustment possibility.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

Yes, Gunnar, those are very commonly available here too. Stand alone they are sold as "signal" or "bell" transformers, because of the 24V secondary. But the exact same transformer is then also sold as a "buck-boost" transformer, with a wiring diagram that instructs you on how to connect them as autotransformers in exactly the way you describe.

All of this would be predicated on the motors being reconfigurable as 240V of course.

"Will work for (the memory of) salami"

 

[waross]

Two buck/boost rated transformers rated 240:24 Volts in open delta auto-transformer boost will develop 228.8 Volts. Close enough.
Three buck/boost rated transformers rated 120:12 Volts in auto-transformer wye boost will give 132V x 1.73 = 228.6 Volts. Close enough.
Two buck/boost rated transformers rated 240:32 Volts in open delta auto-transformer boost will develop 235.7 Volts. Better.
Three buck/boost rated transformers rated 120:16 Volts in auto-transformer wye boost will give 136V x 1.73 = 235.6 Volts. Better.
Going up to 240:48 Volts develops 250 Volts. A little too high.
With four useable options the next step is to check availability of voltages and KVA ratings and select the best available compromise.
I should have made it clear in my previous post that I was referring to buck-boost rated transformers as described by Jeff, not dedicated auto-transformers. Sorry.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[racookpe1978]

A much more complex, much more detailed problem with subtle complexities than I anticipated after reading the original question!

Given those complexities of backfeeding through a "step-down" power transformer, how are the telephone-pole household power transformers handling today's new requirements from the governments to allow backfeeding the household transformer from a house's solar cells? Are those transformers at risk of early failure because of this unexpected backfeeding (step-up) of voltage and power back to the power lines?

 

[Skogsgurra]

THAT is 'nother whole story. Not applicable here. The boost transformers don't see any voltage on their secondaries unless there is a solar, wind, hydro, fuel cell car or other power source connected locally. And then, you will not have any voltage induced unless there is a reverse power flow in the secondaries of the transformers.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

The authorities may legislate the use of transformers but they can't change the basic transformer theory. As for distribution transformers, the NEC comes into force at the customers service connection.
Existing installations that have been working for years will continue to work despite the lack of a regulatory label.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DanEE]

"What is it about transformers that would make them suitable for only one way?"

Been too long to remember much detail and I believe it was ACME Electric's application literature, saying that magnetic core calculations can be somewhat different when using the secondary versus the primary winding for power input. Seems like I recall when using the secondary as the power input, there may not be as much head room with regard to core saturation.

 

[QGeek]

I feel truly enriched, can't thank you all enough. Plenty to go on (solve the problem), and plenty to look further into. Thanks again.

 

[resqcapt19]

Jeff,
The rule in 450.11(B) does not actually require that the transformer label show that it is suitable for reverse operation. The manufacturer just has to say that it is somewhere in their instructions.
For example ACME says the following on their website:
"Acme dry-type distribution transformers can be reverse connected without a loss of KVA rating, but there are certain limitations."
It would be my opinion that information is a manufacturer's instruction and would permit the reverse operation of thier transformers as long at you complied with the limitations that they specify on their website.