Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

Autotransformer Rebuttal

“The main disadvantage of an autotransformer is that it does not have the primary to secondary winding isolation of a conventional double wound transformer.”  Well it’s a disadvantage only if you need isolation.  Where is the need for isolation to operate a 220v drill press via a step-up autotransformer from 110v? 

“Then autotransformer’s can not safely be used for stepping down higher voltages to much lower voltages suitable for smaller loads.”  Patently false.  Variable autotransformers have been safely used for years to step down voltages.  Any electrical device used incorrectly can be a safety hazard.  When used properly an autotransformer is completely safe.  One primary use for autotransformers is as a variable AC voltage source.  Here are examples of commercial autotransformers versions of which have been used for years.

http://www.variac.com/staco_Variable_Transformer_Map.htm?gclid=CPKuw8b7kcsCFQctaQodlooPpQ

http://www.danaherspecialtyproducts.com/uploadedFiles/Siteroot/Superior_Electric/Products_and_Solutions/POWERSTAT%C2%AE_Variable_Transformers/powerstat%20cat.pdf

One use for small autotransformers “sometimes called Variacs (a tradename)  is in the restoration of old radios.  It is recommended that you connect the radio to a “Variac” and slowly bring up the applied voltage so as to not overstress the old components.  The problem here is that some of these old radios have the chassis connected to one side of the AC line (mains to some non-US folks).  This can result in a hazard if the power plug is inserted incorrectly leaving the radio chassis at line potential and a shocking hazard should you touch the chassis and a ground.   This is an instance where an autotransformer doesn’t provide protective isolation if the radio is connected improperly. 

“If the secondary side winding becomes open-circuited, current stops flowing through the primary winding stopping the transformer action resulting in the full primary voltage being applied to the secondary terminals.”  Well in the case we’re talking about (an autotransformer that steps up the 110v line to 220v for the drill press) that would not result in any overvoltage damage since the “full primary voltage” of 110v would be applied to the 220v drill press.  In addition, the remaining autotransformer winding would present a series inductance that would reduce the applied voltage by an amount determined by the load.

“If the secondary circuit suffers a short-circuit condition, the resulting primary current would be much larger than an equivalent double wound transformer due to the increased flux linkage damaging the autotransformer.”  That’s why we use fuses.  Transformers are generally very tolerant to overload so a short period of overload (before the fuse blows) is unlikely to damage it. Wellllll that assumes the autotransformer is fused which it should be.

Since the neutral connection is common to both the primary and secondary windings, earthing of the secondary winding automatically Earth’s the primary as there is no isolation between the two windings. Double wound transformers are sometimes used to isolate equipment from earth  In the US power wiring has three connections, LINE, NEUTRAL, GROUND.  I think most countries follow this standard now.  It appears that the drill press in question has a three prong plug so that it adheres this standard.  Any autotransformer used to power this drill press from US standard power should carry through all three connections, LINE, NEUTRAL, GROUND.

mnothdurft