david n
Diamond
- Joined
- Apr 13, 2007
- Location
- Pillager, MN
I need to drop my voltage from 240(more like 248) to 220. 3ph open delta. 24 kVa. I really don't know much about them. Where to start? TIA
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Buck Boost info need
- Thread starter david n
- Start date
- Replies 20
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Jraef
Titanium
- Joined
- Aug 10, 2004
- Location
- San Francisco Bay Area
Start here.
Why?
Voltages are, in general, "nominal", in that what we call 240V from the utility can be anything from 228-252V, and when an equipment mfr says something like "220V", they SHOULD mean anything from 198 to 242V. In other words, the utility regulations call for +-5%, but EQUIPMENT design criteria (which are unfortunately not regulations) typically call for +-10%. So if you have an old machine, built in the days when there was "220V", and it was built here in North America, it likely will be fine with 240V nominal from the utility, and most likely still fine even if it's a little above the normal range, like 248V. Personally, I never worry about that.
Why?
Voltages are, in general, "nominal", in that what we call 240V from the utility can be anything from 228-252V, and when an equipment mfr says something like "220V", they SHOULD mean anything from 198 to 242V. In other words, the utility regulations call for +-5%, but EQUIPMENT design criteria (which are unfortunately not regulations) typically call for +-10%. So if you have an old machine, built in the days when there was "220V", and it was built here in North America, it likely will be fine with 240V nominal from the utility, and most likely still fine even if it's a little above the normal range, like 248V. Personally, I never worry about that.
Similar to what Jraef said, it would be helpful to know what the application and equipment specification is, to better help you select the right thing to use for the application. Like if this for a CNC that is calling for 220v, it's likely that the design voltage is more like 200 to 208V as the ideal voltage, depends on the particular machine and builder. Best to be sure before you start ordering things, electrical items are non returnable once connected.
The Acme Transformer catalog has a good deal of info in their buck boost section, to educate yourself with. Find a copy here, can be downloaded with the button on the bottom of the page.
ACM_CAT_007_0913
For the info you gave, 248V to 220V 24KVA, looking at the standard buck boost selection charts, closest match I find is, 250V to 227V (subtracting 23V) at 24KVA equates to a 1.5KVA 120x240V - 12/24V unit, Acme T-1-11684. Two of these units wired in open delta provides 27KVA capacity.
Your voltage of 248V - 23V = 225V net result. Is this where you want to be? Does it meet the equipment requirements?
SAF Ω
The Acme Transformer catalog has a good deal of info in their buck boost section, to educate yourself with. Find a copy here, can be downloaded with the button on the bottom of the page.
ACM_CAT_007_0913
For the info you gave, 248V to 220V 24KVA, looking at the standard buck boost selection charts, closest match I find is, 250V to 227V (subtracting 23V) at 24KVA equates to a 1.5KVA 120x240V - 12/24V unit, Acme T-1-11684. Two of these units wired in open delta provides 27KVA capacity.
Your voltage of 248V - 23V = 225V net result. Is this where you want to be? Does it meet the equipment requirements?
SAF Ω
TDegenhart
Diamond
- Joined
- Mar 26, 2011
- Location
- Geneva Illinois USA
One of the issues I see is that new equipment as in the case of Southwestern Industries 2OP is spec'd out at 220 volts 60 Hz. That gets to be a problem in that with the nominal voltages around 240, that with delivery tolerance from the power company, the supply voltage could easily be outside the allowable range for the equipment. This happened with my associate, the voltage out of the RPC was 250. The installation tech suggested that the voltage be dropped. Other than a buck/boost, the power company would have to lower the voltage.
Tom
Tom
david n
Diamond
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- Apr 13, 2007
- Location
- Pillager, MN
Exactly my situation................
mike_kilroy
Stainless
- Joined
- Sep 2, 2010
- Location
- Farmersville, oh (Dayton/Cinti area)
WHY? More details please.....
Jorgo
Hot Rolled
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- May 11, 2008
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- Sechelt BC
We had to drop our voltage because our 2003 Okuma won't allow itself to be turned on with voltage over 220. Our line voltage is 250.
Jordy
Jordy
Jraef
Titanium
- Joined
- Aug 10, 2004
- Location
- San Francisco Bay Area
From the manual:
Most likely, this machine was actually designed and built for use outside of North America, so when used here, the margin of error is already consumed. If it was designed for use here, making it top out at 240V is just piss-poor engineering from someone living in the dark ages...
So yes, you are going to have to use a buck-boost autotransformer. SAF's determination is what I would do too.
david n
Diamond
- Joined
- Apr 13, 2007
- Location
- Pillager, MN
david n
Diamond
- Joined
- Apr 13, 2007
- Location
- Pillager, MN
Now that I think of it, machine needs 205-235V. The +/-10% allowed by the MFGR, at 235V the machine should still be OK up to 259V.............right? My power(+/-5%), 248V, would be at most 260V. I'm riding that fine edge..................
Jraef
Titanium
- Joined
- Aug 10, 2004
- Location
- San Francisco Bay Area
No, your power being at 248V is ALREADY consuming a portion of the 240 +5% allowable by the utility service. You don't get 248 +10% on TOP of that.Now that I think of it, machine needs 205-235V. The +/-10% allowed by the MFGR, at 235V the machine should still be OK up to 259V.............right? My power(+/-5%), 248V, would be at most 260V. I'm riding that fine edge..................
And from the manual of that unit, I'd say they used a marginal design and did NOT follow the +-10% rule. As I said, it's not a requirement, it's just what's called "conventional wisdom" for designers to follow NEMA (National Electrical Manufacturer's Association) design guidelines. These guys obviously did not, they are saying that 240V is the hard upper limit. In this country, that's tough to meet, I rarely see a 240V service that isn't a few volts high or low during a work day, even higher after hours. But it shifts the burden off onto the buyers to make sure the CNC doesn't fail. Kind of a cheapskate design philosophy if you ask me.
It IS a requirement if your equipment is gonna be UL..... they specify the 10% over nominal.
mike_kilroy
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- Sep 2, 2010
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- Farmersville, oh (Dayton/Cinti area)
david n
Diamond
- Joined
- Apr 13, 2007
- Location
- Pillager, MN
david n
Diamond
- Joined
- Apr 13, 2007
- Location
- Pillager, MN
Probably grand or two......................
Buck Boost or open delta will pass transients and common mode noise. Full isolation with a closed delta to wye is much better at reducing transients and common mode noise. A lot of machinery vendors require a full isolation delta/wye for warranty. If it's a machine or two on a clean distribution system probably no problem with the open delta. If you have a building full of CNC equipment, the noise and common mode currents can build up and circulate causing problems with the equipment. With a 3 coil autotransformer the cost benefit is removed, then the full isolation is just as good.
From Acme Q&A
SAF Ω
mike_kilroy
Stainless
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- Sep 2, 2010
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- Farmersville, oh (Dayton/Cinti area)
The Grounding implications? Well lets see. Sounds like the OP was working with a RPC output, which is a center tapped delta with respect to the 1Φ system ground. So the RPC output has 2 low voltage legs (120V) and one high volt leg (208V).
Running this center tapped high leg delta output through an open delta bank for bucking 24V, produces a similar result on the output. When the hi leg is placed in the center, and the low volt leg are passed through the buck windings, you end up with the voltage on the high leg unchanged (208), and the bucked legs will be at a lower voltage (108V). In this method there is 100V difference to ground from the low leg to the high leg.
With a wye output from a full isolation unit, you have perfect symmetry from the grounded center to all three legs, equal voltage from all legs to ground, the ideal situation. And the closed delta primary will help mitigate common mode noise. The full isolation will help reduce any transients from passing through from one side to the other.
The autotransformer closed delta, and or, Isolation closed delta, for the output side, requires a corner grounded delta, one of the worst options. Even worse, no bonding of the delta to ground at all, an ungrounded system. With the corner ground, one leg is intentionally connected to ground, therefore you have full line voltage with respect to ground from 2 legs and 0 from the other.
This introduces several problems. First off it's so uncommon today that many folks just don't understand it and how to deal with it, which can make it a serious hazard to personnel, not in the know. When your tester shows 0v to ground from one leg, it can be easy to think one leg is dead. Secondly inverters and drives and their MOV protection don't like it either. I'm no drive expert, came here to learn about that part.
What I have gleaned so far is that generally the drives and their MOV protection is set up expecting a wye configuration for balance, and if it isn't, that can lead to problems. Some drive manuals that I have perused call for removing a jumper on the MOV connections, when a delta is used, others forbid a corner grounded delta altogether.
I have been on calls where the customer has connected drives and machines to a reverse connected (step up) transformer 230CTΔ to 460Δ output, and left the system output ungrounded. A 480V drive on their press kept getting hiccups after its move and re-installation. I checked the feeder and transformer connections and saw there was no bonding of the Δ output. The only ground the machine was getting was through the building steel and the CTΔ of the 240V service. Not a good situation to have, the drive finally let go after about 3 weeks. The customer didn't want to hear they needed to change the 75KVA transformer. This is the dangerous part, neither the guys that installed the transformer, nor the ones that wired the machine, could understand that there was an issue.
Is this why you asked about the grounding implications? SAF Ω
Running this center tapped high leg delta output through an open delta bank for bucking 24V, produces a similar result on the output. When the hi leg is placed in the center, and the low volt leg are passed through the buck windings, you end up with the voltage on the high leg unchanged (208), and the bucked legs will be at a lower voltage (108V). In this method there is 100V difference to ground from the low leg to the high leg.
With a wye output from a full isolation unit, you have perfect symmetry from the grounded center to all three legs, equal voltage from all legs to ground, the ideal situation. And the closed delta primary will help mitigate common mode noise. The full isolation will help reduce any transients from passing through from one side to the other.
The autotransformer closed delta, and or, Isolation closed delta, for the output side, requires a corner grounded delta, one of the worst options. Even worse, no bonding of the delta to ground at all, an ungrounded system. With the corner ground, one leg is intentionally connected to ground, therefore you have full line voltage with respect to ground from 2 legs and 0 from the other.
This introduces several problems. First off it's so uncommon today that many folks just don't understand it and how to deal with it, which can make it a serious hazard to personnel, not in the know. When your tester shows 0v to ground from one leg, it can be easy to think one leg is dead. Secondly inverters and drives and their MOV protection don't like it either. I'm no drive expert, came here to learn about that part.
What I have gleaned so far is that generally the drives and their MOV protection is set up expecting a wye configuration for balance, and if it isn't, that can lead to problems. Some drive manuals that I have perused call for removing a jumper on the MOV connections, when a delta is used, others forbid a corner grounded delta altogether.
I have been on calls where the customer has connected drives and machines to a reverse connected (step up) transformer 230CTΔ to 460Δ output, and left the system output ungrounded. A 480V drive on their press kept getting hiccups after its move and re-installation. I checked the feeder and transformer connections and saw there was no bonding of the Δ output. The only ground the machine was getting was through the building steel and the CTΔ of the 240V service. Not a good situation to have, the drive finally let go after about 3 weeks. The customer didn't want to hear they needed to change the 75KVA transformer. This is the dangerous part, neither the guys that installed the transformer, nor the ones that wired the machine, could understand that there was an issue.
Is this why you asked about the grounding implications? SAF Ω
Yes.
The MOV protection is generally expected to do some good... So the MOV are selected for a line-to-ground voltage that is safe long term with the mains voltage, but will hold it as low as possible* above mains volts. Obviously the wye voltage is lower than the phase (delta) voltage, so what's safe for wye may not be for the corner grounded delta.
To do that, it is common to essentially do a "wye" to ground with them. That is fine, but then if you get not the wye voltage but rather the line voltage across them, due to corner grounding, then you may need a higher voltage and so less protective MOV, and that may not be what you selected.
One way around that is to use higher voltage parts inside, and uprated MOVs but that's expensive.
With a wye setup of MOVs, disconnecting them in a corner grounded setup may be best.
Delta, no ground, gives a lot more potential "hits" on the MOVS, and also may be a problem. With no ground, all the transients can come right through the transformer common-mode, capacitively coupled.
If you disconnect the MOVS, then that may affect the UL rating, as it changes your transient category, and the required internal spacings on PWBs etc. So disallowing the use of corner ground or floating delta may essentially come down to UL issues.
Some do a wye to a point, and then another MOV to ground from there. That helps but may be high vo,ktage
* MOVs have a fairly large difference between their voltage with a large surge current, and the voltage where they begin to break down and protect. So you usually want to select them as low as possible. But they also degrade over time with lots of transients, so you need to start pretty high.
Net result is that you have a balancing act to get them to be useful with regard to the "damage voltage" of the parts they protect. A large variation of mains volts is not helpful.
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