Premotec synchronus motor control question

[Wolfsongaudio]

Hello,

I have a Wilson Benesch Circle 'table. It uses a Premotec 250 RPM 50 hz motor with a very simple control circuit. A/C to switch/fuse holder then to circuit that contains 3 parts. 2 film caps at 22nf ( .22uf ) and a single 1 M ohm resistor across of the caps. The caps are wired in series.

The issue is that every few times the power switch is engaged the motor will not start. It does get power as I can feel it vibrate. If I give the spindle a very gentle nudge the spindle gets going. It happens about 3 times out of 10.

I have replaced the motor, caps and resistor. I have bypassed the switch and fuse holder to no avail. Cleaned the switch and resoldered all connections. No joy.

I contacted WB and they are baffled. A trip back to the UK for service is not cheap and I just can not see how the circuit can have failed after 10+ years of perfect operation.

The issue happens with or without a load ( belt connected ) to the spindle.

Any ideas of experiences would be appreciated.

Mike

 

[Arch Stanton]

Sticking oil, I think.

 

[Wolfsongaudio]

Arch,

I have cleaned the bearing, replaced the oil in the 'tt bearing. Also the problem exists with the old and a brand new motor.

 

[Arch Stanton]

Sorry, you are right, I was distracted.

 

[volken]

Hello,

I have a Wilson Benesch Circle 'table. It uses a Premotec 250 RPM 50 hz motor with a very simple control circuit. A/C to switch/fuse holder then to circuit that contains 3 parts. 2 film caps at 22nf ( .22uf ) and a single 1 M ohm resistor across of the caps. The caps are wired in series.

The issue is that every few times the power switch is engaged the motor will not start. It does get power as I can feel it vibrate. If I give the spindle a very gentle nudge the spindle gets going. It happens about 3 times out of 10.

I have replaced the motor, caps and resistor. I have bypassed the switch and fuse holder to no avail. Cleaned the switch and resoldered all connections. No joy.

I contacted WB and they are baffled. A trip back to the UK for service is not cheap and I just can not see how the circuit can have failed after 10+ years of perfect operation.

The issue happens with or without a load ( belt connected ) to the spindle.

Any ideas of experiences would be appreciated.

Mike

Are you sure these caps are wired in series mosttimes you need about 0.22 uF on these 9904 PM motors .

 

[nigel pearson]

Measure the two coils for DC resistance. If vastly different that's likely to be the cause. Oil has been a problem with similar Airpax types. Usually a black gunge is seen in the bearings if the wrong oil used ( motorcar oil is not ideal, sewing machine oil will flush it out).

The motor can be opened if a coil is open circuit. If the coil can be resoldered to get the same readings +/- 10% the steel can be filed to remove the zinc coating at the riviting points ( 3 flat sections on the join ). Normal 60/40 solder will solder steel if the iron has enough mass and power ( > 40 watts ). Linn repaired some motors this way I seem to remember in the 1980's. 0.22 uF ( 220 nF ) or 0.15 uF USA 60 Hz is usual. Class X2 caps seem the best easy to get types, 275VAC is a typical rating often with a 630VDC in the UL regs if given. I doubt 630V peak will be seen.

 

[nigel pearson]

Some older versions used a 15K > 3 watt resistor from 230VAC Live/hot to red motor phase and a 0.22uF red to blue ( no problem if neurtal and live are exchanged, it's just a convention to have the resistor to hot ). The two spare wires often grey to neutral. If that works it is an answer. This assumes a 115 V motor. That really should work if nothing else does. 90VAC is what a typical early Linn used red to grey, sometimes 80VAC blue to grey. You might try 95 to 75 VAC to find your preference by using 15 K to 20 K I would guess. It is hard to say if your motor is exactly the same ( check makers PDF if you can ). Try 2 x 15K first to get an idea ( 30 K in series, might be 60VAC, the motor should stand 130 VAC for a brief test, doubtless the motor would be OK, the turntable might get hot ). A capacitor balast is prefered as it runs cold, it could be more trouble.

 

[volken]

Hello,

I have a Wilson Benesch Circle 'table. It uses a Premotec 250 RPM 50 hz motor with a very simple control circuit. A/C to switch/fuse holder then to circuit that contains 3 parts. 2 film caps at 22nf ( .22uf ) and a single 1 M ohm resistor across of the caps. The caps are wired in series.

The issue is that every few times the power switch is engaged the motor will not start. It does get power as I can feel it vibrate. If I give the spindle a very gentle nudge the spindle gets going. It happens about 3 times out of 10.

I have replaced the motor, caps and resistor. I have bypassed the switch and fuse holder to no avail. Cleaned the switch and resoldered all connections. No joy.

I contacted WB and they are baffled. A trip back to the UK for service is not cheap and I just can not see how the circuit can have failed after 10+ years of perfect operation.

The issue happens with or without a load ( belt connected ) to the spindle.

Any ideas of experiences would be appreciated.

Mike

View attachment 1685512.pdf

 

[nigel pearson]

Thanks for PDF. Clearly it is the one I know. With propper regard for safety could we have some AC measurements when it runs. Red grey and blue grey. 80 Vrms is about the minimum for it to start on load. Washing up gloves are what I use for safety, not ideal. Better than taking the maximum risk. Solder on the meter prods is safest.

 

[Wolfsongaudio]

Caps are in series. I have a photo of the arrangement but not sure how to post it.

The .pdf sure looks like the motor ( original and the replacement ).

If someone can assist with photo posting then that might help?

 

[hego]

This usually motors with permanent magnets ( Airpax, Premotec, Philpis,...and direct drives) are prone to have problems with the too hight torque and the cooging effect is the consequence.

For experience the easy solutión for avoid the cooging is use a low torque posible through low the voltage like do a Nigel´s Controller or similars. I use the old Norre Ttpsu with this mod.The torque must be low but enough for the a "round rotation".Round quiet and silence.My old P3 tuns well @ 170V AC ( Europe 240V AC).Now I can´t to mesure the vibrations.

http://www.lencoheaven.net/forum/index.php?topic=13981.0

http://www.norre.dk/TTPSU/TTPSU.html


And other origen of the problem is the DC voltage in special on toroid trafos (very easy to saturate) and " no round" motors. Is the Reign of the vibrations.A simple DC blocker is usefull and can help.

Mains DC and Transformers

A commercial solution.

DC Blocker Trap Filter – Populated PCB | ATL Hi-Fi Audio Company

Cheers

 

[kevinkr]

<snip>

If someone can assist with photo posting then that might help?

Click "Submit Reply." If you see a button for "Go Advanced" click it, otherwise you are already in advanced mode.

Under additional options you will see "Manage Attachments" click it, then in the section labelled "Upload files from your computer" you will see "Browse" click it and navigate to where your photo is. (Make sure it is not too large.)

Click "Upload" once you have added all of the pictures. Add your text if you have not done so already, and then post.

 

[Wolfsongaudio]

Photos of the caps and resistor in situ with incoming AC and the motor. Resistor is just visible under the left cap.

 

[Wolfsongaudio]

More info.

I replaced the entire power supply and motor, bypassed the switch and fuse holder. It works! That is until I install the brass pulley for the belt! As soon as the pulley is on the motor shaft it goes back to intermittent! Very confused!😕😕

 

[Simon1972]

if you want to quieten a premotec motor, run it on equal voltage, find the exact phase compensation cap for optimum phase for your motor by hand (kiterally) and lastly try running it with additional drag applied, either eddy current to the platter of viscous drag in the bearing.

The best measuring premotec setup I've had was with Meldano's controller and a high viscous drag LP12 bearing.

 

[Wolfsongaudio]

Update: Wilson Benesch replied several times and we have a solution. The 2 caps and one resistor network were designed for 230V operation. I removed one 22 uf cap and the 1 M ohm resistor and the motor works every time. The odd thing is that this 'table operated 100% flawlessly here in the US for the last 14+ years.

Thanks to all who replied.

 

[jan.didden]

Since it happens on the old and the new, maybe you are on an unusual low mains voltage?

Edit: just now read the above post. Seems thatw as the issue!

Jan

 

[volken]

Update: Wilson Benesch replied several times and we have a solution. The 2 caps and one resistor network were designed for 230V operation. I removed one 22 uf cap and the 1 M ohm resistor and the motor works every time. The odd thing is that this 'table operated 100% flawlessly here in the US for the last 14+ years.

Thanks to all who replied.

That was my answer in the first place :

Are you sure these caps are wired in series mosttimes you need about 0.22 uF on these 9904 PM motors .

 

[nigel pearson]

You might have a more forceful sound now. I remember my old French ERA mk6 turntable was said to work at 20 VAC when spinning, although was usually at about 90 VAC out of the 240 VAC we have in the UK even now. Linn used 66 VAC when the Lingo and started at 90 VAC ( mostly your motor is the same type ). The weird thing is that although the rotational speed is the same at lets say 60 to 100VAC with this type of motor the sound isn't. If it was just that less is best it would be simple vibration via the motor to the plinth. It isn't and started a whole debate. What seems best is to get the capacitor to match the motor and then fine adjust the voltage to your taste. That is the red and blue motor referenced to grey being equal is the ideal thing. From my memory the resultant voltage red to blue is 1.414 times higher when the ideal phase angle arrived at ( 90,90,127 ). I don't think I ever got it that good myself. The free lunch is more torque with less vibration. If wondering the motor is just two identical coils one on top of the other with what looks like triangular teeth between the coils ( like an altenator ). As far as I can see the teeth set the angle and vibration. Regardless of is sine, triangle or square wave the current waveform into the motor is a sine-triangle hybrid. The vibration is mostly controlled by phase angle and voltage. The motor seems to be at least a second order filter and gives identical current waveforms on sine and triangle waves ( about 10% THD even when sine output is 0.05% THD ). It is very easy to test this filter function with a frequency generator. At about 100 Hz the motor becomes reluctant to work and current wave is much smaller. 67.5 Hz is fine with about 100 VAC to offset the loss of torque.

Seeing as you got this far you might want to fine tune the sound.