One of my alternators referred to above is the same make/output as yours and as mentioned I didn't have an issue. I can't comment on the Mastervolt device though, but it can't put more current through the stationary alternators field than would happen without it fitted.
Thanks for the replies...
Indeed, an 'Alpha Pro' is Mastervolts offering of an Adverc, Stirling etc. It phases the amount of amps as well as monitoring the temperature of the batteries via sensors to ensure that batteries are fully charged and not just partially as in a car etc.
It's a complex but professional system with a 160ah Leece Neville alternator that charges the domestic bank (4 x 200ah gels) at 12v and the starters (2 x 110ah traction) at 24v via a Mastervolt DC-DC converter.
The previous owner as mentioned had to replace the alternator when the ignition had been left on beyond it's 'beep' beep' which occurs at 90-120 secs. SO I thought one of our electrical geniuses may have a solution which would in essence, switch the power off to the ignition. Very occasionally, the 1.7 ton Kelvin K3 stalls if the revs are reduced to a minimum in a hurry when in one of those 'oh shit' situations and to get down to the engine room in a hurry (or remembering to in such a situation) is not always practical.
Incidentally isn't the beep there to tell you you have left ignition on. Guess it may not be loud enough though.