Sluggish synchronous motor

Is the magnetic field pulling the rotor into one of (the closed one) the bearings causing friction when powered up. Try it 'upside down' ?

[ricard]

I don't think so. The way the motor operates normally is that the rotor rests against a thrust bearing at the bottom of the motor, and the magnetic field is not powerful enough to lift it from the bearing (there is a bit of play). The grease in the thrust bearing had hardened and I replaced it with new fresh grease. Again, the motor turns freely when rotated by hand in this state.

When operated upside down, the rotor would seem to scrape against the end of the stator radial bearing. In fact there seemed to be some sort of a cork washer here (together with a metal one), or other material that had disintegrated as could be observed when the motor was opened.

But it's an interesting theory, I'll try it and see what happens. Just standing the machine up so the machine operates on its side would relieve the lower thrust bearing from its task.

Incidentally, on another model 6 I have the motor design indeed seems to be upside down, i.e. instead of the rotor resting on a thrust bearing, the stator is at the bottom and the motor rests on that.

[DOFFERY]

Ricard,
I cannot see the bearings being the problem, there is so little to them. One last measurement to make ,the inductance, same procedure as for resistance should again be equal any two wires, approx. 1.7 H.

Colin.

[Leon Crampin]

Take the motor apart and check that the self-aligning bushes are free to swivel into alignment. After checking for running clearance and lubricating the bushes with thin oil, assemble the motor and strike the lamination pack a few times with a soft mallet to align the bushes.

Leon.

[ricard]

I've had the problem you describe with the B&O motor in the simpler Tandbergs (models 8, 14 and 15). When reassembling those motors it's virtually impossible to get the bearings aligned, but tapping the motor after reassembly fixes the problem. However in that case it is apparent that something must be done as the motor doesn't turn freely at all until tapped.

In the case of the external-rotor Papst here there is only one radial bearing - could be comprised of two separate ones inside a long tube, I can't tell, but they don't seem to swivel. Indeed, after reassembly, the motor turns freely so I can't really imagine that it would need to be tapped in order to align anything.

Colin, regarding the inductance, I don't have an inductance meter, but could possibly connect a capacitor with known value in parallel, pulse it, and measure the resonant frequency with an oscilloscope in order to get the inductance.

[ricard]

I finally got around to measuring the inductance of the coils. Very crudely done, by connecting each coil in turn parallel to a 1µF capacitor, applying a 15V DC pulse, and measuring the frequency of the resulting ringing with the aid of an oscilloscope. The circuit is quite heavily damped so it was hard to get an accurate reading, but the period was approximately 10 ms which after doing the math comes out at 2.5H which at least is in the right ballpark as Colin's stated 1.7H above, and above all doing the measurement on all three terminals in succession gave the same result.

One thing I noticed though, that when applying 15V DC, the rotor would tend to try and rotate up to half a turn and stop there. That seemed quite far to me, although I don't really have any experience with this type of motor. I was expecting to feel some form of 'cogging in' when rotating the rotor by hand with the DC applied, but not the rotor going that far. Does that indicate that this motor type indeed does have permanent magnets in its rotor?

Tried operating the machine upside down to see if the motor went up to speed faster, but it was the same as right side up.

[Lucien Nunes]

Not permanent magnets in the conventional sense, but the motor works by pushing the material of the rotor around its hysteresis loop as it slips past the flux vector, leaving a net magnetisation along one axis of the rotor once it pulls-in to synchronism. This will remain after switch-off and tend to align with the stator flux when you apply DC. If you energise the stator sufficiently strongly, you might find that by rotating the rotor you can drag the axis around it. Because the hysteresis motor has a homogeneous rotor, the reluctance does not vary with angular position hence does not experience cogging.

Lucien

[ricard]

Did an experiment to boost the supply voltage in order to see if that would help. I was thinking of the Beocord 2000 I have which also has a Papst synchronous motor, where in play mode there is a resistor in series with the motor, whereas it gets the full supply voltage in the wind modes where more torque is needed.

Added a 45 AC source from a second transformer in series with the motor, which brought the supply voltage from 260V up to 305V. Interestingly enough, it still takes half a minute before it actually reaches its synchronous speed. I was thinking along the lines of the supply voltage for some reason originally being to low for it to run properly, but I'm now thinking that the added voltage causes the motor to warm up faster, hence it takes shorter time for it to start working properly.

I'm wondering if I've reassembled the motor incorrectly, causing something not to align properly, but short of some part of the thrust bearing at the bottom of the machine falling out during disassembly I cannot really think of what. And if the thrust bearing were missing (basically just a steel ball) it would be a lot noisier.

Colin, did you have a spare motor on hand? I'm thinking of replacing it, as the rest of the electronics has responded well to capacitor replacement and alignment.

[DOFFERY]

Hi Ricard,
Yes I do have a motor for the 6, alas I will be away for a couple of days so unable to pack and forward to you. Let me have your address, we can arrange payment when I return.
How about £100.00 for a start plus p/p?

Colin

[ricard]

Got back at it tonight, I just can't leave this one be (it's both a strength and weakness of mine). After having verified that the additional 45 volts does not cause the motor to reach its rated speed I started thinking about other options. If it's not the supply voltage, perhaps it was something to do with the run cap. I'd previously tested replacing it with a couple of X2 caps with the same capacitance, with no noticable change, but what if the value were wrong for some reason? I remember discussing the run capacitor with a service man many years ago when I had another machine that ran too slowly, and remember him saying that the motor will run the best with the run cap having its optimum value, anything more or less will cause it to run slower. (This was regarding an asynchronous motor, but I'm imagining a similar reasoning would hold for synchronous ones).

So, time for some experimenting. I tried parallelling the existing 1.5µF with a couple of other values, 0.33µF, 0.47µF and 1.0µF. Somewhere around 0.47µF to 1.0µF there was a marked change in how fast the motor accelerated, and it seemed to reach its synchronous speed much easier and faster with the larger capacitance. Tried again replacing the original cap with a couple of others with the same total value, sure enough, same response as with the original cap. Back with the original, paralleled with 0.47µF, for a total of 2µF. Yep, much better again.

Also tried the machine standing up, to see if it made a difference whether the rotor was resting on the lower thrust bearing or not, which it didn't.

So the original cap seems ok, but it seems that the capacitance is too small. It might tie in with the apparently larger inductance I measured (see above), although that measurement was by no means accurate so might not have any bearing on the issue at all. I wonder if there could be a mistake during motor manufacture, or it is incorrectly labeled? Still, it seems odd that it would have passed quality control at the factory if it takes a while to 'warm up'. Then again one never knows, these machines could have been burned-in at an elevated temperature, followed by final testing and inspection, so the problem went unnoticed at the factory.

Right now the machine is cooling down as I want to see how it works when the machine is cold.

Colin, thanks for your speedy response, seems as if the motor in the machine will actually be ok in the end though, so I'll get back to you if its not satisfactory.

[DOFFERY]

Glad you appear to have resolved your problem Ricard, I was going to lower the price of the replacement motor to £99 . Trust you realise I was joking?

Colin.

[ricard]

I dismantled the motor again tonight, after having thought of something. I'd looked at the service manual last time I had it apart, but had failed to notice the lubrication instructions, which are on the last page. The thrust bearing should apparently be lubricated with SAE 80 hypoid oil, a.k.a. gearbox oil. Hypoid is used for lubrication where there is extreme pressure, e.g. between the teeth of the gears in a gear box. I wouldn't have thought that one would get the same force in a tape recorder motor, but nevertheless, having had a bottle around for quite some time it was easy to test.

Shame to say, what I had used to lubricate the bearing with originally was multi-purpose grease. I figured that the actual movement wouldn't be that great since the bearing basically consists of a steel ball resting in a plastic bearing, and that grease would stay in place better than a drop of oil. Nevertheless, sometimes when the motor was cold there was a slight 'swishing' sound which seemed to come from the motor.

Anyway, after re-lubrication and remounting it, it now seems like it reaches the right speed even when cold (still have the extra 0.47µF cap in the motor circuit as it seems to give extra torque for the motor which can't be a bad thing. BTW, in one of Tandberg's service notes, it lists the recommended value as 1.6µF, not the 1.5µF as fitted (and noted in the serivce manual)).

So ... mental note ... do read the service manual from start to finish first and heed the recommendations ...

Also ... don't tighten the mounting screws too hard. I snapped on of the 84 mm screws, of which I had no replacement on hand, so had to makeshift together a replacement using various other bits I had lying around, which took the better part of an hour just to find the things and figure out how to put them together.

Colin, honestly I didn't think you were joking. For some types of equipment, such as tape recorders, the prices can be all over the place, anything from £10 for a Tandberg 6 to £ 400 for a Teac X-10 in good condition. Also, I'd just checked up a replacement flex cable for my video camera, the actual part cost the equivalent of about £22, but the shipping would have been another £20, which seemed overly excessive given that it hardly weighs anything. So I was already in the "in the real world people actually have to pay huge amounts" mindset when I read your mail. For me, these old machines are a fairly low budget hobby in general, although certain machines I'm willing to spend quite a lot on (the Tandberg 6 not being one of them though).

[Radio Wrangler]

"Hypoid" is the name of a tooth shape used in right-angle gearing, and hypoid oils are the specially formulated lubricants for these tooth-shapes where the pressures are significantly greater than in ordinary parallel gearing, whether straight-cut or helical. Some manufacturers specified hypoid in car gearboxes to simplify servicing, but the drive for greater economy stopped that.

Hypoid oils impose a lot of drag, Ricard, though not as much as grease, so are you sure it's Hypoid 80 and not just a plain 80 grade?. I'm very surprised at them being needed in a tape recorder. They are also a bit smelly.

I'm glad to see this come to a successful conclusion, it had been mystifying me.

£100 for a motor is close enough to the sort of silliness going around that it wasn't certain that it was a wind-up, there was just enough doubt.

Cheers
David

[julie_m]

Well, surely that's the best kind of wind-up, when it's just about within the bounds of plausibility?

Of course, if it's actual wind-up tape recorder motors you're after, you want a Butoba, not a Tandberg.

[ricard]

David, the service manual specifically says "SAE 80 hypoid oil". I was also surprised. It does have a distinctive smell, but it is not that strong, and smells more just like "metal" for want of a better description.

[Leon Crampin]

Hypoid gears are designed to operate with their shaft centre lines offset. This means that there is a sliding component as well as the rolling motion present in geared drives. The sliding motion places great demands on the oil film strength and hypoid oils contain additives of chlorine and sulphur compounds to achieve this film strength - hence the distinctive smell.

The downside of these oils is that the additives are corrosive to non-ferrous metals, especially brass, copper and bronze when they are hot.

As our friend Ricard points out, there's no substitute for reading the manual. Putting hypoid oil in a gearbox containing bearings with brass cages or bronze selector forks spells disaster.

Leon.

So that's why an old motorcycle I had specified in the handbook SAE90 'plain' oil, you live and learn.

[DOFFERY]

Ricard,
In the original series 6, the run cap was 1.1mfd, no mention oil/bearings The 62/4 was a 1.5mfd cap plus/minus 20%. The 6X was again originally 1.5mfd, later changed to 1.6mfd.
As for oil, correct as you mentioned but what "is a fraction of a drop" as stated in the manual, beats me.

However it would seem you have cracked it, your original run cap. must be rather on the low side.

[DOFFERY]

Ricard,
From the Papst service data.

Colin.

[ricard]

Originally Posted by DOFFERY

Ricard,
In the original series 6, the run cap was 1.1mfd, no mention oil/bearings The 62/4 was a 1.5mfd cap plus/minus 20%. The 6X was again originally 1.5mfd, later changed to 1.6mfd.
As for oil, correct as you mentioned but what "is a fraction of a drop" as stated in the manual, beats me.

Yes, I thought that was strange too, especially given the fact that when oiling the ordinary bearings, you actually oil the top one, then the oil is supposed to reach the bottom bearing via a wick; it would seem that 'a fraction of a drop' would not go far in reaching the lower bearing.

However it would seem you have cracked it, your original run cap. must be rather on the low side.

Actually, when I switched it on yesterday, it did run slowly for a minute or so, which wasn't as good as directly after I'd oiled it, but still better than with the grease. It gets up to speed so quickly now though that one wouldn't think it a problem if I hadn't got a stroboscope on it or actually measured the tape speed.

As for the run cap, I've both measured it to be reasonably close to 1.5µF, and also tried replacing it with a couple of modern capacitors adding up to 1.47µF with the same performance, so it does seem that the required value at least in this case is higher.

I was also in contact with another collector who has heard of people adding between 0.5µF and 1µF to the existing capacitor to get the motor speed and torque right.