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Overload trip

8 watt 250 volt AC motor / gearbox.

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Stephen Spice08/02/2019 19:14:00
21 forum posts
52 photos

I have recently completed ( all but the extended dial ) John Wildings. A Small Weight Driven Tower Clock.

I opted for the automatic electric winding and was very pleased with the result

With a Mercury tilt switch the clock wound its self three times in 24 hours and only had to wind for less than 10 seconds at a time.

This worked very well for some months but unfortunately I had a chain climb on its self jamming the motor which was still trying to work and causing it to overheat.

This did not blow a fuse of course because it didn't short to earth. And if I hadn't been there I assume it might have resulted in a fire.

Unfortunately I am not able to replicate the problem and so cant make sure it couldn't happen again. Everything being absolutely free I cant see how it could possibly happen.

But as it did I just cant use it short of adding some sort of overload trip.

I know this can easily be done with much larger motors. But this being just an 8 watt motor with a 20 .1 gearbox. presents a problem.

This very nice motor / gearbox cost me £ 430. And enquiries with the manufacturers I could have it rewound with an overload in the wiring for another £ 600. I haven't won the Lottery recently so this is out of the question. And if there was another problem I wonder if this would mean a rewind every time.

If anyone has any ideas about this it would be much appreciated.

If I can't solve this problem I see no alterative but to alter this clock to manual winding which would a pity after putting so much work into the electric winding alone. And then I would have another problem with the required long weight drop.

Thanks. Stephen Spice.

Brian Sweeting08/02/2019 19:25:52
413 forum posts
1 photos

May I ask what the voltage of the motor is and is it ac or dc?

Michael Gilligan08/02/2019 19:34:19
15455 forum posts
668 photos


The thread subtitle declares:

"8 watt 250 volt AC motor / gearbox."


Brian Sweeting08/02/2019 19:36:56
413 forum posts
1 photos
Posted by Michael Gilligan on 08/02/2019 19:34:19:


The thread subtitle declares:

"8 watt 250 volt AC motor / gearbox."


Thank you MichaelG, I hadn't read or adsorbed that bit of the title, I'll sit in the corner quietly for a bit.

Jeff Dayman08/02/2019 19:38:56
1789 forum posts
45 photos

Hi Stephen, I think what you may need is a fuse or circuit breaker inline with your motor wiring that senses over current condition during motor stall, and shuts off. An automotive or marine type inline ATC fuse is likely the cheapest option. Probably can get them in UK at car parts stores, lots of available current ratings. Second link - You can buy circuit breakers that have a manual push button reset, in many current ratings. You choose one that breaks at say 500 mA or a bit more above the normal operating current of your motor. Electronics supply firms will be able to get these. Hope these ideas help.

Michael Gilligan08/02/2019 19:39:51
15455 forum posts
668 photos


Just thinking aloud ...

Might a reasonable alternative be to detect a 'stoppage' rather than an 'overload' ?

... It may be simpler to implement.


Brian Sweeting08/02/2019 19:41:55
413 forum posts
1 photos

Right, 8amps and 230vac would be a suitable size for a regular motor overload device however a temperature sensing motor protection thermistor might be a better solution.

This link is to an American source but gives a good insight into the device.


Michael Gilligan08/02/2019 19:43:25
15455 forum posts
668 photos
Posted by Brian Sweeting on 08/02/2019 19:36:56:

Thank you MichaelG, I hadn't read or adsorbed that bit of the title, I'll sit in the corner quietly for a bit.


It happens a lot round here, Brian

Do keep thinking whilst you are sitting quietly.


Ian Parkin08/02/2019 21:06:03
765 forum posts
185 photos

Remember that the current draw for this motor is only 33mA so even a fuse will struggle to protect that

I’d agree that some way of detecting a stoppage would be best

David George 108/02/2019 21:23:37
1189 forum posts
405 photos

Hi you could put a spring loaded disc on the motor shaft which disconnects power when overloaded.


SillyOldDuffer08/02/2019 21:38:14
5612 forum posts
1154 photos

I'm guessing from the high price that the motor and gearbox are proper clock items and it might be undesirable to mess with them by trying to place sensors etc. Possibly the protective measures would spoil the look of a fine clock.

Another way of approaching the problem might be to connect the clock to power via a cheap mains timer that strictly rations how long the clock is given power. Given that the clock only winds for 10 seconds 3 times per day, you could set the mains timer so that power was only applied to the winder, say, 4 times a day in 1 minute bursts. Then, even if the motor jammed, it wouldn't matter much because the power would be automatically disconnected after 1 minute allowing it to cool for 6 hours. ( I suspect an 8W motor could be stalled for 60 seconds without overheating.) The timer is an easy no mods fix plugged unobtrusively into the mains socket.

Looking at the various models sold by Screwfix it looks as if the mechanical models switch on for a minimum period of about 5 minutes (which might still be OK) but some at least of the electronic ones can do 1 minute.


Martin Cargill08/02/2019 22:01:56
129 forum posts

Have a look on Ebay there are loads of thermostatic switches available in a range of temperature settings. One of these fastened to the motor and wired into its power supply will be able to cut the power if the motor overheats. Wired in this manner it will reset and try again once the motor cools down but it will cycle like this without letting the motor reach any serious temperatures.


Emgee08/02/2019 22:17:19
1445 forum posts
212 photos

Hi Stephen, depending of course on the power required and rpm for the drive to the winding gear have you considered using a small DC motor with gearbox attached as the power source, you would of course have to provide a control for the motor, these are widely available with speed control and low cost, the existing 240/250v ac mains supply could be used to power a suitable ac transformer and rectifier circuit for the low DC voltage required for the new motor.
A picture or 2 would help with judging physical size limitations.


Stuart Smith 508/02/2019 22:19:38
87 forum posts
25 photos

This might be suitable:

John Haine08/02/2019 22:30:25
3007 forum posts
160 photos

It would help if we knew more about the motor/gearbox. 8W is a very low power motor - is this one of those little synchronous motor/gearbox combinations? If it overheats when jammed that suggests that the stall current is quite a lot larger than the running current, though probably not nearly enough to blow a fuse. A possible solution would be a current sensor and a bit of circuitry to switch off the power and probably sound an alarm if the current rises above a threshold. Possibly a clip-on current sensor could be persuaded to work, or a proper current transformer. A quick search on RS indicates that most off the shelf protection relays can't really go down to the current you have.

not done it yet09/02/2019 05:08:57
4484 forum posts
16 photos

Check its stall current and fit a slow-blow fuse? Should start even with a mometary power surge, but blow after a few seconds of stall.

Doesn’t make the problem go away, mind, but no point in having even a one amp fuse for a motor that size. It will draw rather more than 8W when stalled. A timing circuit to ensure continued operation would be good, too. It needs protection against burning out the motor winding\

Speedy Builder509/02/2019 09:42:02
1981 forum posts
139 photos

Over temp fuse attached to the frame of the motor ?? Most bread makers, microwaves and toasters seem to have them somewhere inside - ebay.

Andy Carruthers09/02/2019 10:18:20
274 forum posts
23 photos

Just before go and sit in the corner... would a shear-pin work?

SillyOldDuffer09/02/2019 10:34:57
5612 forum posts
1154 photos
Posted by not done it yet on 09/02/2019 05:08:57:

Check its stall current and fit a slow-blow fuse? ...

Probably not necessary to be that clever. An 240V 8W motor will draw 33mA at full load. We don't know how hard the motor works when winding the clock, but it might be a significant proportion of 8W if it lifts a heavy clock weight in 10 seconds. Even so, the fuse only has to cope with a short burst of current. 32mA is a standard size instrument fuse and I'd use the ordinary fast blow version.  It shouldn't blow when presented with 33mA for only 10 seconds and it's more likely to blow quickly if the motor is genuinely in trouble.

A suitable holder and box would also be necessary - this type of fuse won't clip into a mains plug!

When looking at the fuses listed by RS Components I noticed they're using a new-to-me suffix to encode the fuse type. In case the scheme is unfamiliar to anyone else:

FF - very fast blow
F - fast blow (the common type)
M - medium blow
T - slow blow
TT - very slow blow



Edited By SillyOldDuffer on 09/02/2019 10:37:30

Roger Hart09/02/2019 10:35:52
116 forum posts
27 photos

You might like to try the small synchronous motors used in central heating diverter valves. They are designed to operate stalled (and get a bit hot!) without burning out. In the stalled mode there is usually a resistor and a diode in series with the motor. They are internally geared down a lot so operate at a civilised speed.

These motors are pretty cheap in the usual place and come with a small pinion and have a useful torque.

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