Smiths sectric motor repair

[Clocksbotherme]

I have a Smiths sectric Bijou motor refusing to work. Had a broken coil which I rewound and stepped down to run on an ac/ dc transformer but still not working. All appears well except the rotor won’t turn - it starts to turn initially then stops and won’t budge. Any help out there - I don’t know where to go from here.
Thanks
D

[Techman]

Originally Posted by Clocksbotherme

I rewound and stepped down to run on an ac/ dc transformer but still not working.

It has to be AC only - no DC.

AC straight from a transformer - no diodes/rectifier. No regulators, just AC only.

[Robsradio]

I have done this on a few clocks but sometimes it takes a bit of fiddling with wire gauge and number of turns to get the right balance of required running torque vs current consumption and heat.
But please make sure it is AC output from the mains adapter.

[Nickthedentist]

For initial testing, run the motor itself horizontally, separate from the rest of the clock movement.

[Mike Phelan]

Does the rotor spin OK with no mains applied?

IMHO, much better with the correct wire gauge!

[Brigham]

Originally Posted by Mike Phelan

Does the rotor spin OK with no mains applied?

IMHO, much better with the correct wire gauge!

And safer!

[Robsradio]

Originally Posted by Brigham

And safer!

But how can you calculate the wire gauge required when re-winding a coil for low voltage operation? There is an element of trial and error, I certainly wouldn't be experimenting with wire gauge for mains operation!

[julie_m]

You'd have to wind on 100 turns, measure L and R, and then use the fact that inductance goes up with turns squared while resistance goes up with turns (actually a bit more, because the outer turns will be longer; but a lower bound on the figure is better than nothing) to get an impedance that would dissipate just a few watts at the intended operating voltage. Then measure again, and add or remove turns if required.

[TrevorG3VLF]

The original winding can be used to extract data for rewinding. You will need the same Ampere turns which may be influenced more by the resistance than the inductance.
Information to be collected should be:
Wire diameter, length, weight, resistance.
Number of turns.
Core diameter and length.

I have wound a solenoid for a slave clock (30000 turns) but this I think could be much more difficult

[Mike Phelan]

There's nothing critical about numbers of turne nor wire size!
I've used 44 SWG and as much wire on the spool as it will take.

As for a slave clock it's even less critical as it just has to pull the armature in every half-minute.

If the clock will run but not start, usual reasons are that the start rotor (spider) isn't central in the magnetised rotor, or the latter has become less magnetised over the years.

[Leon Crampin]

My approach is simply to neatly fill the bobbin with as much wire as it will take, using a convenient gauge for winding - say 30 SWG.

Then apply 50 Hz power via a Variac and find the lowest voltage at which the loaded motor will reliably start. Increase this by 30% to get an idea of the running voltage, and soak test the motor at this voltage. If the motor is no more than hand hot, this represents a reasonable fix.

If you fill the bobbin with copper, the motor cannot be less efficient than the original configuration.

Leon.

[Mike Phelan]

As Leon says.
I still say that there's no earthly reason not to use the correct wire size and run the clock on mains, though. Mine have been running for years like that.

[Hartley118]

The earthly reason for not rewinding with the original wire gauge may be the angelic care and patience required to handle 44 SWG or finer.

My own experience has been with getting vintage magnetic pickup heads rewound rather than clock motors, but I suspect that the fine coil winding skills and machinery of the mid 20th century may be hard to find in these days of lower impedances.

Martin

[Mike Phelan]

I just use one of our two members here to do the rewinding.

[Leon Crampin]

Winding a lower resistance coil favours reliability if a good transformer also is used, is safer in terms of risk of insulation breakdown, and removes the requirement for earthing - again, assuming a properly double insulated transformer.

Having struggled with 46 odd SWG wire in the past, a quick and robust low voltage rewind gets my vote.

Leon.

[Hartley118]

Originally Posted by Mike Phelan

I just use one of our two members here to do the rewinding.

So do I!

Martin

[Nickthedentist]

Hang on chaps, let's not argue about the pros and cons of re-winding for low voltage here. I think we've scared the original poster away!

As I hinted at in post 4, the first thing I would do is to run the motor on its own. If and only if it works like this, should you couple it up to the rest of the movement. If it doesn't work reliably alone, then we can start discussing the appropriate number of turns, voltage, current and so on.

A common problem with these Bijous stopping is wear to the rotor cock spring, which develops a small hole causing it to act as a brake (rather than a low-friction way of limiting axial movement). Sometimes clocks in this state will run horizontally, but stop soon after they're put upright. Repair is not difficult.

[Robsradio]

Just picked up a lovely octagonal Bakelite wall clock sporting the earlier ‘Smith Sectric’ branding on the dial and an early Bijou movement, which sadly when I got home and tested had an open circuit coil :-(
A dig through my ‘might come in handy’ boxes revealed a good motor coil, slightly later but fits and works
Just have to find some wall space for it!

[Brigham]

Is that the 'Oxford'?
I'm trying to learn the different model names, and Smiths catalogues are nowhere near as common as Belling ones.

[Robsradio]

Hi, I don’t know the model name, but here are a couple of pics... it even has the rare to find hanging bracket, (not in photo).
Happy New Year to all (in advance)
Rob