en vacío = no load

Reference:

TERM tensión continua real en vacío

Reference VEI 551-6-20


Definition the actual mean direct voltage at zero direct current

Reference IEV 551-6-20
(1)
TERM real no-load direct voltage

Reference IEV 551-6-20

Note {GRM} n.p.

http://europa.eu.int/eurodicautom/Controller

You might assume the transformer's output voltage is highest at no load. It would then make sense that (under loaded conditions) the transformer's resistive and reactive components cause the output voltage to drop below its no-load level. This is a logical assumption, but one that's not necessarily so. Depending on the power factor of the load, the output full-load voltage may actually be larger than the no-load voltage.

http://www.ecmweb.com/mag/electric_basics_transformer_voltag...

Many more examples:
http://www.google.es/search?hl=en&q="no-load voltage"

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Note added at 1 hr (2006-03-09 07:56:35 GMT)
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Another bilingual reference here:

Corriente de arranque Starting current
Tiempo para encendido Starting time
***Tensión en vacío No load voltage***
http://www.loyje-fydesa.com/balastoelectronico5133.htm