Bucking 240 -> 208v residentially

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wil

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A friend who wants to install some commercial 208v appliances residentially and has typical North American 3-wire, single-phase, midpoint neutral (aka split-phase) service. He asked, knowing I work for the hydro-co. I advised it wasn't a great idea but he wants to know if it is at all possible as he apparently already bought the machines. A few back-and-forth with a service guy and I got some schematics. A quick look suggests he could buck his 240 down to 208 as one leg is going to the electronics/motor while the other hits the heating elements. I'm a little unsure of running the motor with unbalanced legs but I recall it being ok.

So, it looks technically possible. Am I correct in saying it's still a bad idea?
 
augie47

augie47

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From your description I appears the equipment is 208/120 ie: a neutral connection is required, correct ?
 
infinity

infinity

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IMO the introduction of any transformers into a dwelling is a bad idea. If straight 208 volts is need wouldn't an auto-transformer work?
 
Smart $

Smart $

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Ohio
wil said:
.... A quick look suggests he could buck his 240 down to 208 as one leg is going to the electronics/motor while the other hits the heating elements. I'm a little unsure of running the motor with unbalanced legs but I recall it being ok.

So, it looks technically possible. Am I correct in saying it's still a bad idea?
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Need better details on the electronics, motor, and elements. Which are powered L1-N, L2-N, and L-L? If only the heating elements connected L-L, the appliance may easily "tolerate" being run on 120/240V without a buck trannie.
 
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hillbilly1

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You can do it, but if it is a cord and plug connected piece of equipment, and a neutral is required, there is no guaranty that another piece of equipment plugged in there will have the 120 load on the same leg. In an industrial setting, that is not so much an issue because you would assume that qualified personnel would be supervising the changeout.
 
kwired

kwired

Electron manager
Location
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Little Bill said:
A buck/boost is an auto-transformer isn't it?
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Yes.

480sparky said:
The cost of installing the transformer(s), and the fact that they will run 24/365.
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24/365 depends on how it is installed. Depending on machine there could be way to control the autotransformer along with the load, if autotransformer is needed ahead of control components then it must be energized whenever the machine has power to it.




Just what do we have that may be very complex of a machine that is used in a dwelling? Is it worth the cost to supply it with rated voltage? Most typical equipment you would commonly find in a dwelling that has a 208 volt rating will usually also have a 240 volt rating.
 
weressl

weressl

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wil said:
A friend who wants to install some commercial 208v appliances residentially and has typical North American 3-wire, single-phase, midpoint neutral (aka split-phase) service. He asked, knowing I work for the hydro-co. I advised it wasn't a great idea but he wants to know if it is at all possible as he apparently already bought the machines. A few back-and-forth with a service guy and I got some schematics. A quick look suggests he could buck his 240 down to 208 as one leg is going to the electronics/motor while the other hits the heating elements. I'm a little unsure of running the motor with unbalanced legs but I recall it being ok.

So, it looks technically possible. Am I correct in saying it's still a bad idea?
Click to expand...

One of the difference between the 240/120 system and the two legs and neutral of the 208/120 3 phase system is the phase angle difference between the hot legs to neutral. so in the case of 'bucked' derived system would maintain the 180* instead of the 120* native three phase system. If that is not an issue, you should be OK. Unballanced phase loading happens all the time, in this case you may voltage regulation issue between the two different 120V legs. If the load is constant you can buck one leg less than the otherif you have sufficient number of taps, but it is probably not a big deal.
 
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hillbilly1

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I do a lot of those for pottery kilns installed at residences, they claim the heating elements last longer because there not running as hot. They are a straight 240 volt load, so voltage imbalance is not an issue.
 
kwired

kwired

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NE Nebraska
hillbilly1 said:
I do a lot of those for pottery kilns installed at residences, they claim the heating elements last longer because there not running as hot. They are a straight 240 volt load, so voltage imbalance is not an issue.
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Makes sense, just how many hours does the average kiln in a residence see? Most people I have seen that get into crafts that involve something like that, they may get into it big time initially, but then they find it is not that profitable and slow down a lot or quit altogether. So is the installation of the autotransformer worth the offsetting of changing the elements once, maybe twice during its period of ownership and use? In a commercial setting where it is used daily - maybe so.
 
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hillbilly1

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Location
North Georgia mountains
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Owner/electrical contractor
kwired said:
Makes sense, just how many hours does the average kiln in a residence see? Most people I have seen that get into crafts that involve something like that, they may get into it big time initially, but then they find it is not that profitable and slow down a lot or quit altogether. So is the installation of the autotransformer worth the offsetting of changing the elements once, maybe twice during its period of ownership and use? In a commercial setting where it is used daily - maybe so.
Click to expand...

Yeah, my baby sister is an Art teacher, she has one that I wired for her about three years ago, didn't go the buck/boost route either. She has used it maybe once.
 
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gar

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Comparing a buck-boost transformer with a true auto-transformer, and using the same core the losses will be slightly less with the auto-transformer because:
1. The core loss won't change.
2. The available window space will have better utilization. One coil instead of two coils.

If the load is a pure resistance heater and nothing else, then use of a phase shift control to adjust voltage is probably the lowest cost approach. Further it could be used for temperature control and further increase the heater life. If there is other stuff besides just a heater, then this same approach could be used by only controlling the heating element with the variable voltage, and connecting control circuitry and motors separately.

If the load is a CNC machine and not basically a resistor, then phase shift control probably won't work.

Need to really know more about the load.

.
 
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