I replaced synchron motors in my motorised valves in my central heating system but they both failed in a matter of weeks.

Should I replace the whole valves or just the activators?

[scratches head and ponders whether it will rain tomorrow]
[smilez] uh, I need to do a search, brb.
Uh, is this a plumbing question? What are the valves for?
Synchron - Synchronous Motors (http://www.hansen-motor.com/synchindex.html)

Thanks doug238. The valves are zone valves in my central heating system

I suggested the moderator moves this question to Heating and air conditioning.

Does your valve have a disk you can turn by hand? If so, does it turn fairly easy? If so, I doubt changing valve and all will help. If it turns hard, you might look at disassembling it and cleaning it. Putting it back together again could require an unobtainable new gasket.

If I invested in a new motor, I would put an inline fuse on it sized to 125% of the amps the motor draws.

You might have the wrong motors. Honeywell zone valves uses a special motor setup. Check your voltage going to the motors. If to low or to high it could also cause a problem.

I am sure you did not just replace the motors for the heck of it so the reason for the original motors failure might still be present and causing trouble with the replacement motors.