On 09/07/15 12:15, robobass wrote:
Trying to think of another way to measure, I attached a small 220v axial fan to the inverter. The pitch of the fan was about a major third higher than when run on the mains, meaning about 25% higher.

Some types of of AC motor respond to voltage control of the speed, IIRC
shaded pole was one. I have a Comair Rotron axial fan and have used it
with an EBM Ziehl speed controller intended for fan speed control to
slow it down but it was run off 240V 50Hz mains, I presume if you can
slow them down by reducing the voltage then the opposite may also hold
if your inverter is seen to put out a higher voltage.