Ahh, I get it. At least a few variable speed drives are capable of operating at an output voltage that is higher than the input voltage. For some applications it might make sense to install a VSD simply for voltage control, leaving the frequency conversion aspect of the device as a nifty side effect. Note that _most_ VSD systems are not capable of _boosting_ the output voltage significantly above the input.
As the converters typically work with an intermediate DC circuit at (root)2*ACvoltage, it is not very difficult to realise. DC is then switched according to what the motor needs, voltage and frequency can (but not necessarily) be varied in wide ranges as long as the motor survives it. It is a price, space, and EMC discussion.
(edited for Germanism)
[This message has been edited by Wolfgang (edited 02-03-2006).]