Mains-powered synchronous clock motor - Page 1
Forum section: electronics / projects

author: icon
author_rank: Regular Contributor
timestamp: July 14, 2012, 04:58:28 pm
content: HiI have an old Smiths electric clock that I'd like to convert to run off a low voltage AC supply. The motor consists of two sets of windings wrapped around a former. The inner windings are the 'primary', I presume - they were connected to the mains input, but there's a break somewhere. There were a hundred turns wrapped around the outside of that, that powered a 2.5V bulb for illumination. The former has a large crack in it, but it seems to fit very neatly into the motor shell even so.The former then fits inside this shell with interlocking teeth.I don't understand what makes it a 240V motor rather than a 2.4V or 24V motor. Is it the number of windings? Is there some significance to the 'secondary' winding from the motor's point of view, or is that just a convenient way of making 2.5V? There's what looks like a 'brush' that contacts the rotor.Any clues to the operation of this motor would be gratefully received.John

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 14, 2012, 08:41:57 pm
content: I think that the secondary winding is just what it looks like, a clever way to get 2.5 volts without using a transformer. I've seen the same motors in laundry machines controlling the timer, but then with only one winding. Why don't you just put a low voltage (3-5 volts) AC on it and see what happens? And the thing you call a brush, isn't that just something to prevent the motor running the wrong way around?

author: SeanB
author_rank: Super Contributor
timestamp: July 14, 2012, 09:04:22 pm
content: If you take an old microwave apart and look at the turntable motor there is a similar motor. It might be possible to fit the winding from that in place of the faulty one, if it is the right size. Just make sure it is a 240V motor and not a 36VAC motor as many have, otherwise you will need the dropper resistor as well.Otherwise it is easy enough to rewind this one, you just need a roll of 40SWG enamelled copper wire and a lot of patience in undoing the original coil and rewinding the same number of turns neatly on the core, keeping the same inside diameter including the crack, and varnishing it together again afterwards.

author: icon
author_rank: Regular Contributor
timestamp: July 14, 2012, 09:22:59 pm
content: Quote from: SeanB on July 14, 2012, 09:04:22 pmAh no - Iwantto change the voltage it operates at - I just don't know what to change. More turns? Fewer turns? How many?CheersJohn

author: Zero999
author_rank: Super Contributor
timestamp: July 15, 2012, 12:29:32 am
content: You need to use fewer turns of thicker wire, for example, if it's rated to 240VAC and you want to power it from 24VAC, re-wind with a tenth of the turns but ten times the crossectional area. There should be no need to count all of the turns which can be estimated quite accurately by measuring the wire thickness with a micrometer and the coil dimensions with vernier callipers.

author: Rerouter
author_rank: Super Contributor
timestamp: July 15, 2012, 12:47:54 am
content: the "crack" is an air gap, its a designed break,from the 6 protusions on the core, my first though would be that its a 3 phase syncronous motor :/ however i also feel i am very wrong,what i feel i could be more right on is that it acts similar to a stepper motor, like most automotive clocks, where your outer winding would actually be 3 offset winding wound in opposite directions, (for north and south for each AC cycle) to bring mains down to 10 RPM (6 poles) and once again i may be wrong here,and yes hero is correct, motors torque is a function of current per turn, so using an inverse ratio of wire size to turns, should allow you to scale to what ever voltage you please, being aware the wattege will remain the same, so for 24V in place of 240, your looking at 10 times the current,

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 15, 2012, 01:05:43 am
content: Wind a test coil of maybe 100 turns or so (using wire that uses up most of the space for the coil) and energize it using a transformer powered from a variac. (If you don't have a variac, you can make a track of 50 or 60Hz sine wave, play it through an audio amplifier, and adjust the volume to change the voltage.) Find the voltage at which the motor starts operating (cut in voltage) and the voltage where the current starts increasing sharply (saturation voltage). Pick a voltage between those two points and run the motor for a while, making sure it operates properly and doesn't overheat. Once you're happy with the results, you can find the V/turn value and the number of turns you really need for your application.

author: amyk
author_rank: Super Contributor
timestamp: July 15, 2012, 08:02:07 am
content: Here's a good resource on doing the rewind:http://sound.westhost.com/clocks/ocm.htmlYou'll need to change turns ratio between the primary and secondary if you're changing the voltage; a 240V primary and 100-turn secondary giving 1:100 voltage ratio means there were originally ~10,000 turns on the primary. If you change that to e.g. 24V on the primary, that means 1000 turns of wire with 10x the area on the primary, and the secondary remains the same at 100 turns, giving a 1:10 ratio instead of the original 1:100.Quote from: RerouterIt's a single-phase synchronous reluctance motor. Seems to be 24 poles, 50Hz, 250 RPM.

author: G7PSK
author_rank: Super Contributor
timestamp: July 15, 2012, 09:08:46 am
content: The brush as you called it is actually a pawl, this stops the motor starting in the wrong direction. When power is first switched on the motor bounced back and forth against the pawl until it got enough momentum to start, on really old smiths electric clocks you had to start the motor manually so the pawl was just there to stop you starting in reverse.It is possible that the low volt volt winding would do as the drive if you put an ac source on it thus saving having to rewind the the coil. The usual reason the motors burnt out on those clocks was the mechanism became stiff or jammed which reduced the back emf and lead to the overheating of the coils.Some of the clocks worked not directly from the mains but were driven by a master clock some where in the building these were often seen in schools and hospitals or large office buildings, this type were driven by a pulse from a mechanism that was controlled by a pendulum clock of high accuracy, The school I went to had such a system with the master clock in the heads office. It had two pendulums each filled with about half a liter of mercury in a glass cylinder the clocks were connected with a 3 wire system and you could have fun by taking the clock of the wall and reversing two of the wires.

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 15, 2012, 09:10:28 am
content: Quote from: Rerouter on July 15, 2012, 12:47:54 amIt looks like plastic, so I don't think it is an air gap...

author: G7PSK
author_rank: Super Contributor
timestamp: July 15, 2012, 01:05:59 pm
content: The gap looks like a crack to me, it is uneven on its side when looked at with magnification on the screen. I also cannot remember there being ant air gaps when I worked on this sort of clock in the 70's, I used to a horologist back then.

author: icon
author_rank: Regular Contributor
timestamp: July 15, 2012, 08:35:15 pm
content: The former is indeed made of plastic. And I'd say it was a crack - for some reason the former is very battered, despite being encased in steel. Go figure. There's an odd mixture of plastic, Bakelite and what I'd call 'paxolin' in the clock - must have been some crossover period.I'm in the process of cleaning the oil off before a trial reassemble. It's filthy.Thanks for the info so far.John

author: G7PSK
author_rank: Super Contributor
timestamp: July 15, 2012, 09:24:23 pm
content: Many of the old plastics de or re polymerized and shrank in doing so and therefore crack some times they even turn to powder or granules.