208 to 230 in small commercial

[synchro]

Got to remember that a modern autoclave is likely fully computerized, as such the power supply is very likely to have MOVs and many MOVs are rated 150v RMS MCOV. If it has a L1-G-L2 MOV and you consider the allowable continuous variation of +/- 5% and occasional allowance of up to +10%, you could have problems with cooking MOVs.

... If one needs a voltage boost that uses the series-connected secondaries, one could separate the two secondaries and wire one at each end of the primary, obtaining the boosted voltage while maintaining a balanced line-to-neutral output.

If the buck boost has two separate secondary windings each outputting 11V at a 208V input, then Larry's suggestion to wire one at each end of the primary will produce 129.6V L-G at both line outputs. Adding the +10% variation mentioned by Flicker Index would give 142.6V L-G.
By comparison if 22V boost is applied at one end of the primary winding then a 10% increase above the 139.5V nominal would be 153.5V L-G . So Larry's idea is something to consider if the transformer allows it.

 

[LarryFine]

I've never seen or heard of doing it, but it just popped into my mind earlier, when I suggested it, so I went looking.

Here's a normal arrangement, with the secondaries in series:




Here's what I envisioned, with the secondary coils split:




Of course, the source neutral can not be connected to H3-H2, because there's only one transformer core.

Which means, while the load will receive equal L-N voltages, this "derived neutral" can not be grounded.

 

[synchro]

Of course, the source neutral can not be connected to H3-H2, because there's only one transformer core.

Which means, while the load will receive equal L-N voltages, this "derived neutral" can not be grounded.

Yes, H3-H2 will be at 60V relative to the 208Y neutral.

 

[petersonra]

For such a small amount of power why screw around with a buck boost transformer. Just get a 208-240 single phase transformer. Make sure whatever you buy has a center tap on the secondary in case you need it.

 

[James L]

For such a small amount of power why screw around with a buck boost transformer. Just get a 208-240 single phase transformer. Make sure whatever you buy has a center tap on the secondary in case you need it.

Makes the most sense, especially considering he was asking about having 120v on each leg, and doesn't seem to understand the concept of buck/boost

 

[James L]

I've never seen or heard of doing it, but it just popped into my mind earlier, when I suggested it, so I went looking.

Here's a normal arrangement, with the secondaries in series:

View attachment 2555167


Here's what I envisioned, with the secondary coils split:

View attachment 2555166


Of course, the source neutral can not be connected to H3-H2, because there's only one transformer core.

Which means, while the load will receive equal L-N voltages, this "derived neutral" can not be grounded.

I thought we were trying to keep it simple

 

[LarryFine]

I thought we were trying to keep it simple

In this forum?

 

[MTW]

It works FINE Larry. On 3 phase as well as single. Often used on rotary phase converters to adjust voltage down for CNC that require a 220V balanced input.

 

[drcampbell]

1 leg will be 120v to ground
1 leg will be 139v to ground

That would yield only 224 volts L-L. (vector arithmetic)
To achieve 240 volts L-L, one phase would need to be boosted to 156 volts. (or both phases would need to be boosted to 139 volts)

None of this sounds like a good idea without a written guarantee from the manufacture that the proposed circuit is acceptable.

 

[James L]

That would yield only 224 volts L-L. (vector arithmetic)
To achieve 240 volts L-L, one phase would need to be boosted to 156 volts. (or both phases would need to be boosted to 139 volts)

None of this sounds like a good idea without a written guarantee from the manufacture that the proposed circuit is acceptable.

Yep. I did a little bit of squirrely math. Thanks

This configuration is so common that nobody thinks to ask a manufacturer