component identification

Bit before the days of little inverter, Selenium rectifier package , like my pics

Difficult to test as so much droppage, usually replace with some Si diodes and sag resistors if appropriate.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Selenium stack bridge, by the looks of it. Equipment it's in ?

Arfa

Simple selenium bridge rectifier. Terminal marking shows what the function is (AC in, DC out) and the design is familiar to any old-timer... (Oh, crap, gave that one away, didn't I?)

On Thu, 26 Mar 2009 08:01:17 -0700, Smitty Two put finger to keyboard and composed:

ITT B30C400 BRIDGE RECTIFIER DIODE

VINTAGE ... 30V 400MA full wave selenium rectifier

- Franc Zabkar

Good old Selenium rectifier ! Not seen one of those in years.

On 3/26/2009 7:01 AM Smitty Two spake thus:

So this far down in the thread we know by now that it's a selenium bridge rectifier.

Question is, why can't he just replace it with a silicon bridge rectifier? Someone pointed out that Si has less voltage drop, but would that really be an issue? (Of course, it might help to know *something* about the equipment surrounding this component ...)

Thanks. Figured them damn numbers must be specs of a sort!

Ancient European slide projector with rotary carousel. Trying to help a friend get the thing going again, via email since he lives in another city.

Problem with it is that this motor:

turns very slowly.

It didn't make sense to me that a rectifier would be fed by a motor winding, but there you go. So now it seems the motor is 240, but there's a secondary winding in there putting out about 17, and he's getting about 12 VDC out of the bridge, so that part seems OK now.

Is there anything on that type (shaded pole?) of motor to fix, or is it an R & R job? (remove and replace)

you

Combined motor and "transformer" very common for "Dansette" type record players of the Selenium rectifier era. When higher power amps came in then they had to split apart.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Smitty Two Inscribed thus:

That is an AC shaded pole motor with a tapped winding feeding the rectifier. It also means that one side of the circuit is directly connected to the mains.

These commonly get glued up bearings. The cure is to strip it clean and lubricate the shaft on both sides. Use only a very fine oil ! Sewing machine oil is ideal. Reassemble. Job done.

The shaft should rotate freely without any binding. The only other possible problem would be shorted turns on the coil ! But if that was the case I would expect smoke !

This used to be very common practice back in the day. Record players often used a motor constructed in that way. FWIW, I can't remember ever having one of those motors, which were used in all sorts of things from tape recorders to fan heaters, that had any kind of electrical problem. It used to be very common for the oilite type bearings to completely dry out leaving a gummy residue, which then made the motor run slowly. Usually, when the motor runs, the rotor moves forward a couple of mm in its bearings. If it is not able to do this, that can cause it to drag on the 'brake' pad that's sometimes fitted in there. They don't have a lot of power even when working correctly. Also, the bearings are usually 'self-centering' as in they are fixed in a sort of ball mount. If the motor has been knocked or jarred, the bearing balls can be knocked out of line, and don't re-centre if it has gone gummy in there. The motor in the picture looks as though it may have simple fixed bearings, though.

Usually, these motors are very easy to service, being held together by two long screws which pass through one bearing plate, through the stator core, and into the other bearing plate, where the holes are threaded. In the case of this one, the rivets that they have used would have to be drilled out first, and then long screws with nuts substituted to hold it back together. Servicing usually involves cleaning the shaft and bearings with a solvent such as IPA, and soaking the bearings in a light machine oil for a while, before poking the rotor shaft through each in turn, and giving it a 'swing around' - a bit like working a joystick if you will - to make sure that the ball mounts are free to move. They should be tight enough to stay put wherever you leave them, but free enough to move fairly easily. The rotor and bearings are then assembled back onto the stator core, 'working' the ball mounts on the bearings as needed, until the rotor runs totally smoothly with the bearing plates refixed to the core by the screws. You should be able to easily spin the rotor with your fingers, and it should spin on for quite a while - several seconds at least.

When it's all back together, it doesn't hurt to add a little more machine oil to the bearings, particularly if there are felt 'resevoir' pads in them or by them. On some motors of this type that I have seen, there is actually a little oil hole in the bearing housing. Good luck to your friend. Hope he succeeds in rescuing it !

Arfa

Are shaded pole motors synchronous to the mains frequency unless overloaded ? I was always surprised how much axial movement there was of the rotor between the bearings, is this a necessary part of the design ? maybe to assist startup as relatively low torque even when rotating.

In the present case could low revs be a consequence of a fault in the electronics, via back emf? is the motor speed the same when the Se rectifiers are disconnected , not just the electronics disabled?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

N_Cook Inscribed thus:

Yes !

Generally it allows the rotor to align itself in the magnetic field.

Very unlikely ! Even if you reduce the voltage the speed tries to stay constant. Smoke is more likely particularly if stalled or shorted turns.

Thanks. My friend says the motor spins freely by hand, and since the secondary voltage seems correct, giving 12 VDC on the other side of the rectifier, I'm doubtful about shorted windings. But, he now says that the mechanism jammed for a few seconds (not longer) just prior to failure. That seems like it ought to be a good clue, but I haven't been able to fathom it yet.

Thanks for your usual comprehensive treatise! I forwarded your comments to my friend. I'm off for a weekend of work and play at "her" house in another city, so he's on his own for a bit now, and maybe by the time I get back he'll have it sorted out. The machine is critical to his business, not just a hobby, so he's anxious to have it operational.

In that case it is possible that there is some sizing in some other part of the mechanism.

It looks like a belt drive. If the motor still runs slow without the belt it could be that the bearings have worn oval allowing the rotor to catch the frame ! For that matter there could be debris in the gap that could cause the problem.

Pete I knew you were an OF because of all the knowledge you have to share. WW

All right then, here's what happened with this. (Recall that the motor is 240VAC and has a secondary winding to power some simple DC stuff through the bridge) My friend disconnected the AC supply to the rectifier, and presto, the motor returned to normal speed.

Thinking something downstream could be drawing too much current, he reconnected the rectifier supply wires and then disconnected the DC side of the rectifier. Motor slowed down. So he replaced the rectifier, and all is well.

But I'm still puzzled, and since I was never on-site I didn't do any of the tests myself. If the rectifier had some fault that was drawing too much current and it pulled the voltage down, how could he have measured

As it was a selenium stack, I would suspect that one arm was leaky. Enough to make it draw excess current, but not enough to represent a 'serious' failure that would load up the supply really hard - such as happens when one arm of a silicon bridge fails short circuit. With the other three arms functioning normally, the result may well have been a DC output sufficient for the rest of the circuitry to work.

Out of interest, did your friend replace with a silicon bridge, and re-measure the AC in / DC out voltages ? Looking again, assuming that a resevoir cap follows the rectifier, with 17v AC in, you would expect to see around 24v DC at the output, rather than the 12v that was apparently measured with the defective bridge in place.

Arfa