ECN Forum Forums Technical Discussion Electrical Theory and Applications Buck Boost Theory

I'm finding conflicting information about the application of buck boost transformers.

Most of the vendor web sites state that a common application for buck boost xfmrs is to boost 208 volt power to 230 or 240 volts.

However, i have also read "Buck-Boost xfmrs cannot be used to create a 240/120 volt single phase service from a 208Y/120 volt three phase service": the reason cited is that "unbalanced line to neutral voltages will result - one will be 120 volt and the other 130+ volts".

I interpret all this to mean that i can run a 240 volt motor or heater directly from such a setup where the neutral is not even connected, but that hooking up, say, a residential clothes dryer would be bad since it probably needs to derive a 120 volt control voltage from that neutral.

So since my manager asked me to do just what i describe above, it seems to me it would be reasonable to open up the appliance and hook up the buck boost xfmr just ahead of the heater coil, and then plug the appliance into our 208/120 power. The nameplate on the dryer actually says "240/120 OR 208/120" but the manual says that drying performance might be unacceptable on 208 volt power.

Having typed this, i seem to have answered my own question (please correct me if i'm wrong), but the new one is, can someone recommend a textbook or reference manual on this subject? I'm going to take a certification test soon; transformers, power factors and such have always been one of my weaker areas.

Thanks!

Never ever try to connect the neutral of a single buck boost transformer fed by a 208Y/120 system. the problem is much worse than a simple un-balance.

Very simplistically, assume you are bucking the 208V up to 236V. The low voltage side line to neutral is 208V/1.73 = 120V, and the high voltage side is then 236/1.73 = 136V. As you can see 120V does not equal 136V therefore the two neutral points are not the same and cannot be tied together without creating a short circuit.

Okay you say, you won't tie them together, well how will your 120V equipment like being connected to 136V?

You can connect 3 individual Transformers in a Wye AAutotransformer configuration, to achieve a 4 Wire Wye Voltage Boosting (or Voltage Bucking) setup of either 229Y/132v 3Ø 4 Wire, or 249Y/144v 3Ø 4 Wire - depending on how the Secondaries are jumpered.

Below are two Schematics for Wye connections to create a 4 wire Voltage Boosting setup (both demonstrate + 12VAC or +16VAC Voltage increase configurations).
Additional Schematics may be found in the Technical Reference Section.



and



Schematics found at page:

Voltage Boosting Transformers-Wye Connected

Be sure to observe correct polarities on all primaries and secondaries, or you will have problems with balance, loading and idling.

If possible, first just see how much the Dryer suffers being run at 208VAC, then ponder the Voltage Boosting Autotransformers option(s). The heating elements will be driven at a lower Voltage, so they will draw a lower True Power (Wattage) at 208 VAC, as opposed to driven at 230 VAC.

Scott35

The loss in performance triggered by 208Y120 vs 240/120 is too trivial to bother with.

The whole project is a waste of time.

Drying speed is going to be much more sensitive to local humidity and the progress of air blown through the dryer.

Focus on getting rid of any air flow issues.

Don't be surprised to find that the vent ducting is lousy or clogged. If so, any attempt to boost the voltage is just going to roast the clothes.