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LP12 motor. Lack of torque.

Discussion in 'd.i.y.' started by Chops54, Apr 22, 2015.

  1. Chops54

    Chops54 pfm Member

    My Linn Sondek motor struggles to get the platter moving and the pulley is easily stopped. It wouldn't pull a hen off it's nest tbh. It also makes a cogging noise when it stalls and often won't get going again and that's with the belt removed. The psu is a completely recapped Valhalla and appears in good order. Im bloody sure it worked ok last time out. I'm getting 80 volts blue to grey and 83 volts red to grey on the motor. I've tried another motor off my Pagoda Systemdek with the same result. It's like there's voltage but not much current. What should I be looking for? Any ideas anyone? The Valhalla is out on the bench running the Systemdek motor atm. I thought I'd leave it running for a few hours just in case those caps needed some bedding in?
  2. orangeart

    orangeart KJF Audio Ltd.

    I don't know much about these motors as I've always been a DC man (might revisit that) but there needs to be a phase shift between poles I think, not sure how this works in your PSU but if you've got a wrong value cap or wrong polarity that would throw things out of whack.
  3. Chops54

    Chops54 pfm Member

    Hi Stefan. You might be on to something.There is a voltage difference and the motor turns in the right direction so maybe the voltage difference needs to be bigger?
  4. stevec67

    stevec67 pfm Member

    If the phase difference were wrong it wouldn't run or would go backwards. The Valhalla has been refurbed so the caps will/should be the right value to make it go. Power output of a synchronous motor like this is dependent on supply voltage, 83V seems low. Geddons run at 110V.

    Edit - there is no voltage difference driving the motor, it's a function of the sine wave to the field coils being phase shifted relative to the rotor. This is 90° out and always ahead, so the rotor is always chasing the stator and the speed is governed by the mains frequency.

    Can you lay your hands on another PSU (or make a Basik, it's only 10 minutes and a tagstrip) and give that a whirl?
  5. Barrymagrec

    Barrymagrec pfm Member

    Measure the voltage between red and blue - I would assume it should be in the 80 volt area, other wise you would seem to have a phase problem in the Valhalla.
  6. misterc6

    misterc6 Wasted and wounded, it ain’t what the moon did

  7. Chops54

    Chops54 pfm Member

    Thanks lads, I think I might be on the right track.
    I read that thread last night Malcolm but a quick re read suggests around ten volts difference between read and blue phase so I've adjusted the trimpot to give 92 volts on the red phase which is slightly less and the motor is pulling like a train now. I'm going to build it back up and see how she goes.
    I have an idea that the chap I got the Valhalla from might have turned it down in an attempt to reduce the motor noise.
  8. Dowser

    Dowser Learning to bodge again..

    I think it was Mr Tibbs that went into quite some detail about adjusting the voltage as low as possible for your specific motor to improve sound quality from reduced motor vibration. Think the recomendation was to dismount motor and hold it in your hand while adjusting for lowest vibration but continued operation.

  9. Chops54

    Chops54 pfm Member

    I read that thread Richard and I realised after reading Stefan's post that maybe the motor didn't have enough voltage difference between the two phases. Sure enough now I have around ten volts difference between phases the motor is pulling well. The motor is on the bench and I can feel a slight vibration when I hold it. Turning the trim pot to reduce the voltage to the red motor wire does indeed decrease the vibration but at the expense of motor torque. It's too much of a compromise for me I'm afraid so I'm going to build the deck back up and I'll trim the voltage back just far enough that the motor is able to comfortably start the platter moving.
  10. andyr

    andyr Registered User

    From the experiments I did in 2014, with a new motor speed controller I had bought, Mr Tibbs only had half the picture ... and this sent him down the wrong rabbit hole. :p

    Yes, lowering the voltage decreases the vibration that the 110v Premotec motor has. So that is a good thing - and the Lingo uses this; it starts off at full voltage, to get the platter turning, and then (AIUI) reduces it to about 80v, for lower motor vibration.

    But, if you have a speed controller that enables you to adjust the phase between the blue & red motor wires to get minimum motor vibrations ... the way is then open to increase the voltage to the motor. This gives you increased drive (torque) and the music sounds better. :D The speed controller I am using enables me to send +30% voltage to the Premotec ... and this doesn't increase motor vibrations (that I can detect) because the phase difference between the motor pairs is optimised. But the sound is very much better than at the 'normal' voltage.


  11. Chops54

    Chops54 pfm Member

    Interesting Andy. That must be a custom made speed control?

    The turntable is back together and whilst it's so much better the motor still groans slightly on start up. I'm going to up the voltage up a touch more and see where that takes me.
  12. Dowser

    Dowser Learning to bodge again..

    Understood Si.

    Interesting Andy - I assume this is similar to trimming the size of .2 (or .22? can't remember) uF phase capacitor. What speed controller do you have? :)

    Also, have you tried adding really thick oil to the bearing to introduce some drag? SQ does this with his Linn bearing a I think, finding it reduces effects of motor cogging.

    Richard (running Linn oil still, and a DIY Geddon...but which does at least have a start/run switch to reduce voltage once platter is turning :))
  13. Chops54

    Chops54 pfm Member

    Can you honestly hear a difference when you drop the voltage Richard?

    Like SQ I've always felt that a turntable should be driven. Maybe that's why I like idlers though they have their own issues.
  14. YNWOAN

    YNWOAN 100% Analogue

    I also use a high drag bearing in my deck and it does seem to improve speed linearity by increasing motor torque (both listening and measurement support this hypothesis).
  15. Dowser

    Dowser Learning to bodge again..

    What a very logical question...and one I can't answer, never listened. It always started the motor in the run position anyhow (current Geddon is one someone else built, so I don't even know how much it drops the voltage). Lenco is currently in the main system, I will give it a try when I put the Linn back (might even pull the geddon apart and measure the voltage first...just to create some expectation bias :)).

    Mark - I must admit thicker oil is on my list of things to try (especially now I have fitted a new motor), but I have a pre-Cirkus (circa 1977) bearing, so am a little concerned about different oils (I think Alan had a problem once, destroying a bearing because the oil soaked into plastic liner, causing it to expand).

    Sorry - going a bit off topic now!

  16. andyr

    andyr Registered User

    It's not really 'custom-made', Chops - though it certainly isn't a Linn one! :D It's a prototype of a speed controller that is shortly (so I'm told by the designer! ;) ) to be released in its Production form.


  17. Chops54

    Chops54 pfm Member

    Don't get too biased Richard, I want an honest answer ;)

    My thread drive turntable downstairs has an inverted grease bearing and a heavy platter, around 9kg. A 9 watt dc motor does a good job of getting it up to speed. I used to lube it with some fancy super slippery oil but since I moved on to grease, the deck seems to exhibit a more solid sound, like a good idler. I takes around fifteen minutes for the motor to warm up and the speed to stabilise but I can put up with that.

    Going back to the Linn, I've adjusted the trim pot on the Valhalla as low as I feel I can go but I noticed the belt was dropping and seemed to be juddering on start up. I don't know if it's the right thing to do but I've cleaned the belt and dusted it with talc. Start up is much smoother now without all the sub chassis jiggling about. I do have some final setting up to do so we'll see how it goes.
  18. andyr

    andyr Registered User

    That's what I thought, D ... but Steve Tuckett tells me that you only ever get 90 deg phase shift from a cap - whether its value is 0.2uF or 0.22uF. Whereas the optimum phase is a bit less than 90 deg (but of course you need a speed controller that allows you to adjust phase, to prove this - which the Lingo cannot do).

    The speed controller I have is the 'Number9', designed by Steve Tuckett.

    No I haven't ... yet! ;) I know Simon does this and he swears by it.


  19. sq225917

    sq225917 Bit of this, bit of that

    Chops. if the motor is struggling and easily stopped, then you've likely either lost one phase or the cap that provides the phase shift has died. Check your connections.

    The motor runs best with equal voltage on each phase and the correctly trimmed phase. You can drop voltage on one phase to reduce vibration but it's better to arrive at the same result with correctly trimmed phase.

    Check the resistance of your motor windings, they should be the same with the motor out of circuit. If it's dead I have an unused spare you can have for a steal.

    Might be time to look at a new speed controller. The LP12 motor price is £110 and you can build a new PSU for that much money.
  20. davidsrsb

    davidsrsb pfm Member

    Wrong understanding.
    The voltage across and current through an ideal capacitor are 90 degrees out, but this does not make your phases 90 degrees. Ideally the two phase should be the same voltage and the voltage between them 1.414 times higher. The two windings are meant to be the same in any of these Premotec or similar two phase motors. Any imbalance causes extra cogging.

    In practice, manufacturing differences between the windings means that smoothest running may require SLIGHT adjustment of the relative voltage and phase away from 90. It is not predictable which way this tweak will be from one motor to the next. There are digitally controlled psus that can do this, but then you need something like an acoustic guitar pickup and scope to measure the vibration and optimise.

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