Sluggish synchronous motor
 

In a Tandberg 64 I'm working on, with a Papst synchronous motor, when the machine is cold (just switched on), the motor runs too slow, but over the course of a couple of minutes it slowly picks up speed and eventually runs normally after say five minutes. I've disassembled the motor as initially the oil had hardened and it barely turned at all, and re-oiled the bearings with thin watchmakers oil. Supply voltage is ok, actually a little high (specified at 245V but measures 265V), and the starting cap is also ok (measures 1.48µF, marked as 1.5µF). Even when warm, it appears weaker than I'd expect it to be but that may be my imagination.

Anyone recognize these symptoms? Could it be that the oil is too thick, just contributing enough friction to hold the motor back, or could there be a shorted winding?

Re: Sluggish synchronous motor
 

Have you checked the windings Ricard?

Colin.

Re: Sluggish synchronous motor
 

No. I don't think I have seen any specs for the DC resistance, although I could of course just compare the two windings and see that if they have the same resistance, if not I guess that would indicate shorted turns.

The fact that the motor does turn on its own indicates to me that the windings at least aren't open, then it wouldn't be able to start on its own although it might be able to maintain speed once started manually I would have thought.

Re: Sluggish synchronous motor
 

You probably wouldn't see shorted turns with a DC resistance check, because shorts are most likely between neighbouring turns, which are probably not far apart along the length of the winding.

Current demand would be up a lot.

Is the motor getting the correct voltage?

David

Re: Sluggish synchronous motor
 

This seems to be a problem for record player motors too - there are numerous threads about it here, many with no real conclusion as to the cause or cure.

Nick.

Re: Sluggish synchronous motor
 

And is it getting hot?

Re: Sluggish synchronous motor
 

A couple of points, Ricard.

Watch oil is no good for sintered bearings as it's meant to stay put without "creeping" whereas sintered bearings are like a sponge and the oil needs to soak into it; warmed 3-in-1 is as good as any.

That's unlikely to be your problem if the rotor spins freely.

Another thing about self-starting synchronous motors, Papst or others, is that they can lose magnetism which will either stall them under power or they'll not achieve synchronous speed.

Re: Sluggish synchronous motor
 

Through age, or something else, Mike?

Nick

Re: Sluggish synchronous motor
 

I don't think the loss of magnetism applies in this case. Clock motors, for example, are permanent magnet synchronous types in which the rotor can be demagnetised. However tape-recorder-style synchronous motors normally seem to be hysteresis types (or possibly salient-pole in the larger sizes) neither of which contain permanent magnets. Papst tape recorder motors came in both induction and hysteresis synchronous models with the appropriate rotor construction although the stators are basically the same.

These are PSC (permanent split capacitor) motors that often have identical main and auxiliary windings. If this is the case with yours, you may be able to detect shorted turns by interchanging the two (making the motor run backwards) and comparing the winding voltages and currents. However if there are enough shorted turns to seriously affect performance, the motor will probably get quite hot. A disconnected auxiliary winding or a faulty cap will affect or prevent both starting and running, since the aux winding is required at all times to provide a rotating flux component.

The AC voltage across the cap can be higher than the supply voltage and its performance is fairly critical. Have you checked it for breakdown under rated AC volts?

Lucien

PS I was always warned off using 3-in-1 for critical or long-life applications as it tends to form a gummy residue with time and oxidation.

Re: Sluggish synchronous motor
 

True, but what would you recommend instead for sintered bearings?

Nick.

Re: Sluggish synchronous motor
 

Proper sewing machine oil.

David

Re: Sluggish synchronous motor
 

First of all, I'm probably wrong about the 'watch oil' designation, the oil was labeled 'Kriechöl', which means 'creeping oil', and I seem to remember it being intended for watches, but I could be wrong there. It is fairly thin, thinner than sewing machine oil. I tend to use it for bearings with very little play, such as the case with this motor, or capstan bearings.

Winding resistance is, oddly enough, 450 ohms no matter which of the three motor connections I measure between. I was expecting two windings with a common connection, e.g. 450, 450 and 900 ohms or something like that. The resistance measurements indicate some sort of delta arrangement.

Motor supply voltage is correct, or a even bit high (245 V according to the schematics, but measures as 265 V).

I have not noticed if the motor gets especially hot, but have not thought to feel it after a while to get a feeling for the temperature. Certainly it does not get hot enough to emit any odd smell (such as was the case with another machine where the motor was wired for 110V but the mains was 230).

Running the motor without the cap just causes some light humming, but no movement, as would be expected.

I have not tested the cap though, I would have thought thought that if there was a leakage or breakdown problem it would get worse the longer the machine was on which is contrary to the observed behavior. But perhaps under the circumstances it is the most likely culprit, so I'll investigate it next. It is made by Jensen, and the smaller 0.1µF Jensen cap across the motor switch is very often so leaky in these machines that the motor tends to display noticeable torque even in stop mode. I've got a whole bunch of various caps so it shouldn't be hard to test a replacement.

The thing is, it is very noticeable that it starts slow and works its way up to maximum speed over the course of a couple of minutes. My instinctive conclusion was that it must be as the motor heats up, but it could be something in the capacitor as well.

Re: Sluggish synchronous motor
 

It does sound like an ordinary 3-phase winding, IIRC Papst used both arrangements along with their dual-voltage and pole-changing 6-lead equivalents.

Regarding bearing oils, I prefer high-performance synthetics. Many precision motors going back as far as the '60s were lubricated during manufacture with PAO or diester-based oils, for example Studer specified a particular diester formulation for capstan motors, IIRC a Klüber product, for which I normally keep an equivalent by Anderol. I usually have to hand grades such as 423 (low viscosity - ISO 5) and 465 (ISO 68). These oils are suitable for sintered bearings running continuously at high temperatures without turning gummy, losing volatile fractions by evaporation or oxidation. Of the two base formulations, PAO has the better compatibility with rubbers, plastics and mineral oils, while diester typically has better viscosity stability. PAG formulations have poor compatibility with other oils so I avoid them.
Examples here:Anderol bearing oils

There are some good mineral oils too, including turbine oils, but you have to be on guard for what additives are in them (e.g. zinc) especially if you are using them via wick oilers where the additives can concentrate in the wick. Of course general-purpose household oils can have any amount of nasty gunk in them, they don't have to meet a rigorous specification just to stop door hinges squeaking!

Lucien

Re: Sluggish synchronous motor
 

The Tandberg service manual recommends Teresso 43 or 47 (from Esso); I don't know if those oils are still available, but if nothing else it might be possible to check their specification. From their usage as described in the service manual they didn't seem to specialized, as they were recommended for the reel table bearings as well.

Initially the oil in this motor had gummed up. What I'm not sure of is if that just affected the oil that on the bearing surfaces or whether the oil that has crept into the bearing also would have degenerated; in that case it would be necessary to somehow dissolve and drive the old oil out before re-lubrication. If not, it would not really even be necessary to re-lubricate as oil would tend to creep back out of the sintered bearing once the gummy residue had evaporated.

Incidentally, the oil (and other materials - lacquer?) composition in the motor seems to have a significant impact on the total smell of the recorder; this Tandberg smells more like a Beocord 2000 I have which also has a Papst motor, rather than the more typical Engel that Tandberg used at the time in the more upmarket machines. I've got another model 6 with an Engel motor, which I'm going to compare smell-wise at some point.

Re: Sluggish synchronous motor
 

Mainly age, Nick.

Thanks, Lucien, I've never serviced a Tandberg but it seems like it's the sort like a pre-Series 7 Ferrograph.
That sounds like it's just what you want, then. If it does creep it won't be any good for watches!

Re: Sluggish synchronous motor
 

Ricard,

Is your motor type HSZ 32 50- 4 - 400D? if so your measurements are correct. I presume you have checked for insulation?

If you get stuck I have many motors for various makes, including Papst types.

Colin.

Re: Sluggish synchronous motor
 

That's the exact type, yes. No, I haven't measured insulation (I suppose you mean winding to chassis resistance, preferably with a megger or other instrument capable of reading high resistances?)

I did try replacing the original 1.5µF motor capacitor with a couple of modern day polyester types, but it made no difference, it still starts slowly, with very little torque.

Thanks for the offer Colin, I'll think about it, question is if this particular machine is worth restoring if the motor needs to be replaced. There are other issues, and it's not in its original state either - for instance the remote control connector has been replaced with a DIN connector. (From a practical view a wise move though if you don't happen to have an original remote pedal for this machine.)

Re: Sluggish synchronous motor
 

Ran the motor for 15 or 20 minutes with no significant load (i.e. tape recorder in play mode but no tape). The motor does not heat up significantly, in fact it was barely luke warm to the touch, which was cooler than I expected.

Re: Sluggish synchronous motor
 

Just as a matter of interest Ricard, with the belt removed, does the motor run at full speed from the start? It's difficult to access, I realise, but if you feel with your fingers it is possible. You could also use a card about 70mm circumference with markings something like a strobe stuck on the top of the motor.

If the motor does run well from the start the problem could be due to other problems such as undue friction from the feed lower clutch half. Don't forget to check the flywheel brake lifts off (if fitted).

Colin.

Re: Sluggish synchronous motor
 

I removed the belt (no problem as I have the top cover removed at the moment anyway). Set machine to wind (i.e. the flywheel idler not engaged). Same problem. Starts slowly then gets up to speed after a minute or so.

Even used a black felt-tip pen to mark a cross on the pulley, and got out my trusty old stroboscope lamp (actually the salvaged power lamp from a Philips EL3547 :)) to verify it. Definitely the case. After a minute or so it locks onto synchronous speed, until then it's running slow.

No matter if cold or warm it spins freely by hand. There's definitely more friction in the rest of the drive than in the motor even when it's cold. When under power, it feels very week when trying to stop it by hand (haven't measured the torque though) when cold. I wonder if the bearing could have overheated when the original oil gummed up, causing the magnetic properties of some part of the motor to change. The axle didn't look glazed when I removed it so don't think that is what has happned though.

Also measured resistance from the windings to ground. Reads infinity on my digital multimeter, so at least there's not a short to ground anywhere.

'Tis a mystery. Sure, a replacement could be had, but I'm still very curious as to what's happened with this one...