460V equipment, 208V supply
460V equipment, 208V supply
(OP)
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?





RE: 460V equipment, 208V supply
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"
RE: 460V equipment, 208V supply
RE: 460V equipment, 208V supply
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.
RE: 460V equipment, 208V supply
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"Will work for (the memory of) salami"
RE: 460V equipment, 208V supply
RE: 460V equipment, 208V supply
RE: 460V equipment, 208V supply
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.
RE: 460V equipment, 208V supply
Just a thought.
RE: 460V equipment, 208V supply
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.
RE: 460V equipment, 208V supply
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
RE: 460V equipment, 208V supply
If you are lucky, you can get a center tap on the secondary, then you have a coarse voltage adjustment possibility.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: 460V equipment, 208V supply
All of this would be predicated on the motors being reconfigurable as 240V of course.
"Will work for (the memory of) salami"
RE: 460V equipment, 208V supply
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
RE: 460V equipment, 208V supply
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?
RE: 460V equipment, 208V supply
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: 460V equipment, 208V supply
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
RE: 460V equipment, 208V supply
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.
RE: 460V equipment, 208V supply
RE: 460V equipment, 208V supply
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.
RE: 460V equipment, 208V supply
racookpe1978,
As I understand it, the utilities do not have to let just anyone willy-nilly connect a grid-tie solar system to their pole pigs anyway. You must apply for, and be granted, a "Net Metering" contract/license/permit (not sure of the vehicle), in which you must show compliance with all the regulatory issues involved in putting power back on the grid. In that process, the utilities must be also evaluating their equipment themselves, because I know of a couple of people who were denied Net Metering because the utility claimed their equipment was incompatible with reverse-feed and would be too expensive for them to replace. Might have just been a straw-man, wouldn't put it past them to do that, but it also might be real.
"Will work for (the memory of) salami"
RE: 460V equipment, 208V supply
Since the inverter has to have a hot line to start production, inrush from the low side doesn't happen.
In the past, the amount residential solar was very small compared to the capacity of the distribution circuit, so the simple solution was to just have the inverter trip and stay offline during any power system disturbance, thus avoiding the issue of providing a ground path for the high side of the service transformer. As the amount of solar on a feeder increases above 15%, everything gets more complicated.
Voltage regulation at the customer's premises can be an issue. If the utility feeder is at 105% voltage on the medium voltage side and an inverter is backfeeding through the service wire and service transformer, the customer's equipment could be damaged by voltage well above 105% on the 120/240V side.