I've gotten curious about the accuracy or not of these. I am speaking of the motor-driven timers with a 24-hour dial on the front.
Obviously not something someone would use where precision is required so this is purely an academic question.
I was under the impression that these were driven by tiny sync motors and therefore as accurate as the power line frequency just like a traditional electric wall clock. Not easy to set precisely but should repeat nearly to the second day after day. Or so I thought.
Recently, using a timer on Christmas lights, I noticed variation as much as a minute. But that was a newer timer and I don't know what kind of motor it has. I know on the older ones, the motor rotor could randomly start in either direction but there was device in gear train that would kick it to reverse if it happened to start by running in reverse. Doesn't that sound like a sync motor?
So for a few days I've been running a little experiment. I have a number of timers of various vintages. I ran each to where it just kicked on then unplugged it. I put the group on a couple of outlet strips and powered them at the same time, each hooked to an indicator lamp. At the expected time each night I put a smartphone running an NTP-connected clock app nearby and video the scene with another phone so as to produce a record of when each turned on. I was expecting them to switch on a few seconds before the designated time (based on how many seconds elapsed during the setup adjustment between the timer triggering and my unplugging it) but the variation has been much greater, nearly a minute early on one but generally all over the map and different each night and not in unison as though the power line frequency* had varied. Maybe this is from mechanical variations from the switch part of the timer snapping over. Or maybe they are not sync motors after all.
- I think the utilities used to make up for lost or gained time keeping it
variation I see though it could be some of it.
Again, there is no practical point to this; a digital timer can be used where precision is needed. I am just curious.