Excel stores time as fractions of a day, so 4:30 is stored as 0.19 days.
=CONVERT(A1,"hr","mn") converts 0.19 hrs to minutes, giving you 11.25.
You'd first have to convert 0.19 days to the correct number of hours.
Try:
=CONVERT(A1*24,"hr","mn")
or just =A1*24*60, formatted as a number.