This really isn't a big mystery and can happen on filters or powerheads. It is a small point on the impeller that needs a check. Some background on how mag drive work will help to make sense. The current flowing through a coil around the impeller makes an Electromagnet. The impeller also is a magnet made up of alternating sections of positive and negative. So if the power comes up and a positive pole is attracted to the correct portion of the impeller, it starts spinning the correct way. If it happens that a negative comes up, the impeller portion will be pushed rather than pulled for that 1/60 second until the AC flow reverses. Looking at an impeller carefully , you may see that there is a small stop of some sort to stop the impeller from starting the wrong way if the power comes on reverse of what is needed. It is designed for the impeller to bang into the stop, pause and then as the power reverses, the impeller goes the right direction.
When your mag drive goes the wrong way, the little stop is not there. Often broken off. Some people get by with repeatedly plugging/ unplugging until they hit it at the right time.
I favor fixing the stop or replacing the motor.
I believe on the Koralia it is a small rubber tip stuck on a small nub of plastic on the cage? Been a while since I looked at one. Could be wrong about that.
Last edited by PlantedRich; 06-14-2014 at 02:00 AM.