Why does one synchronous motor lock to same phase angle and one locks randomly

Thread Starter


Joined Nov 28, 2012
I have two single phase 120 volt synchronous motors, one locks to the same phase angle on every start and one locks randomly. I watched several videos and they all skip this feature on explaining how they work. Can anyone explain this. My only idea is the one that locks to the same phase angle only has 2 stator windings, but I don't know. Thank for any help.


Joined Sep 30, 2009
Synchronus motors can mean many different things. A link or picture of what you working with would be helpful. The small motors in an electric clock or timer are synchronous motors, is that what your talking about?


Joined Jul 18, 2013
This random start direction is common on the rotatory table motors in microwave ovens.
It was also common on the older wall clocks, they posses a mechanical trigger that would 'flick' the rotor if it ran in the wrong direction at power up.
It was not uncommon to see a clock running backwards after a power outage due to the trigger failing.
On my M.W. the T.T. runs alternately in each direction when started.


Joined Mar 14, 2008
It was not uncommon to see a clock running backwards after a power outage due to the trigger failing.
When I was a kid, we had a wall clock and bedroom clock that were not self starting, and both had a knob on the back connected to the mechanism to start the clock when the power was lost.
The clocks would run in either direction, depending on which direction the knob was spun.

schmitt trigger

Joined Jul 12, 2010
You mean a synchronous clock motor, like the Hammond or Telechron?

According to several websites dedicated to repair and restoration of old electric clocks, the immense majority of failures occur due to mechanical problems. Lubrication dried up, bushings have worn out, that kind of stuff.

The reason being that those motors have a very feeble torque, and any friction can stop them.