What voltage will appear across the secondary terminals of an autotransformer if the common winding should open?

In an autotransformer, the primary and secondary are not fully isolated as they share part of the winding. When the common winding opens, the autotransformer effectively becomes non-functional in the traditional sense of transferring electrical energy from primary to secondary through a complete circuit. However, the voltage that appears across the secondary terminals is still directly related to the primary voltage.

Since the autotransformer is designed to step up or step down the voltage, without an open circuit, if we assume the common winding has opened, the remaining section of the winding could still experience the primary voltage. Therefore, the voltage across the secondary terminals would equal the primary voltage.

This is because, in this scenario, the secondary winding can still pick up the same voltage potential as the primary side, since it essentially remains connected to it until a complete disconnection occurs. Thus, it would reflect the primary voltage present in the system at that moment.