DC motor speed controller

[winggo]

I started out using an AC motor from Hurst. You are right, the big problem is getting the drive pulley the exact size, and I mean exact. The ratio of the drive pulley diameter to the platter diameter is so great that very small changes in diameter on the drve pulley makes a big difference on the other end.

My second attempt was with a DC motor/controller package from Oriental motor company. It was the smallest one they make and cost me $240 or so. The controller is way more complex than necessary and it took a while to get it configured right. It runs beautifully at a very constant speed but you can hear it run from across the room. Way too loud.

My next try will be with a Maxon 110184 and a small DC motor speed control I purchased on EBay. The controller has not arrived yet but I will let you know how it works out.

Good luck

George

[Bobken]

Hi,

I don't think there should be any problem with precise pulley sizes, if you use a DC motor. I would have thought with any DC motor it is essential to be able to vary the voltage to set the overall speed accurately, so the exact size of pulley is not very important here.

In fact this was one of the main reasons I finally tried out DC motors, after many different attempts with AC synchronous types where the nominal rotational speed is already fixed by the 'mains' frequency. In these AC cases, accuracy of the pulley size is vital, of course as you suggest, unless one does not care about the absolute pitch of any music being played.

When I was experimenting with AC motors, I used a nicely machined alloy pulley (made by Michell Eng.) This was attached to the motor shaft by a grub screw, but, because of the inevitable (although small) shaft/pulley clearance, when the fixing-screw was tightened, the pulley was pulled slightly off-centre. The run-out was only about a thousandth of an inch (or at most 2 thou.) which I could see under magnification, and could just about feel with a finger nail gently resting on the pulley's indented circumference as it rotated.

To correct this, I eventually burnished the pulley with a wooden coctail stick soaked in metal polish (Brasso) and fixed in a small hand vice, and when this was done until the 'bearing' surface of the pulley was running absolutely true and polished, the sound did improve noticeably.

Subsequently going over to using a DC motor, and with no temp. compensation or feedback, like yourself I was initially very concerned about longer term speed stability, especially following the enforced decision to abandon the feedback arrangement like your proposal.

However, this has remained rock solid for about two years now, much to my surprise and great relief. The speed was carefully adjusted initially when the motor was in use for a minute or so, it doesn't vary regardless of summer or winter nor however long the deck is in use, and I have never needed to re-adjust it since setting it up. Also, I can detect no pitch change due to stylus drag, even on heavily modulated passages, which was another initial concern of mine, and is common with many (most?) AC motors. I believe this must be due to the higher torque of the DC motor I used, and perhaps aided by the excellent Michell main bearing.

Merely removing a couple of thousandths of an inch off the diameter of the pulley with the AC motor (when I trued it up) did noticeably upset the pitch of music being played, and the reduction in speed was readily seen by the stand-alone tachometer/stroboscope I checked it out against. It could only have been running fractions of a % slower, but it was immediately audible to me.

Bearing in mind the inertia of a very heavy platter, and the drive in my case being merely through a circular-section neoprene band of only about 2mm cross sectional area, the effect of such tiny discrepancies like the pulley surface being slightly off-centre was surprising, but it clearly indicated just how critical this is and how important the surface finish and true-running abilities of the pulley are, for the best sonic results. It still amazes me that any vibrations or whatever can be transmitted via a thin rubber band to a substantial platter, to the extent which it does.

Incidentally, since having the use of a highly accurate tachometer, I have checked several other AC driven turntables, and not one was running at the correct speed, at any time, and there is very little one can do about this, from a practical point of view!

So, if you do change over to a good DC motor and a suitable variable voltage PS, and pay careful attention to the surface finish and concentricity of the pulley, I think you will find this to be far more important than worrying about any tachometer feedback arrangements, which may or may not be such a good thing, anyway.

[Vinyl-Addict]

Originally posted by Matt Rowland
In the meantime I am going to buy a better motor to experiment with, probably the Maxon 110189.

Cheers for now

Matt

PS anyone know where to get pulleys accurately machined?

I'm frantically looking to get a good motor myself. I spoke with a turntable manufacturer the other day who tried the Maxon 110189 and he said it was not good. Interestingly he uses the same DC drive system as Teres. He did offer to let me try out a Maxon 110945 but I think it's a brush motor.
I can machine pulleys in my shop. I posted another thread a few weeks ago for help on a motor build project. I'm willing to trade my machining capabilities for a good working motor, AC with power supply or DC. I just want a good, reliable motor.

[peranders]

Originally posted by argofanatic
Take a look at a circuit by Rod Elliot ...

Versatile DC Motor PWM Speed Controller

Cheers

Something tells me to avoid switching circuits due to the very weak signals from the cartridge.

One other idea is to use a regular syncron motor and David White's crystal controlled AC source.
http://www.wnaudio.com/turntablepsu.html

[Matt Rowland]

Plenty more food for thought guys!
I've decided to try a maxon REMax29, 226756 (about £80!). I 'll try to use the original Linn pulley to start with, but it does mean the drive voltage for the motor will be down in the region of 1.5v for 33.33rpm.

Cheers

Matt

[harwoodspark]

Hi Matt, if your going to spend £80 on a dc motor why not just buy the correct dc motor from Origin Live? It comes complete with the correct pulley for your application you just specify.I've had the full OL kit fitted to my Linn for something like four years now and I've never had any problems.It certainly cured the Linn's overblown bass! Mine is the earlier design with the speed control panel mounted in the plinth and the dc supply sits on the floor.The rotary control is in the hole where the original Linn rocker switch used to be.I'm building up another turntable shortly, out of all sorts of spare bits I have but I shall be buying a new OL motor and building the control board myself.The dc supply can be bought from Maplins or ARD for less than a tenner!
Si.

[Pjotr]

Originally posted by Matt Rowland
Thanks for the suggestions. I'm trying to avoid the tacho route if possible. I understand the principle of using a thermistor but am unsure of the best way to incorporate it into the circuit or how to choose component values to make it operate correctly. I've attached the proposed circuit in hope of suggestions!

Cheers

Matt

Hi Matt,

The way of speed stabilisation as in that PDF has many drawbacks for a TT. 1st it makes the motor stiffer. The result is that irregularities in the commutation and magnetic circuit of the motor comes out harder as speed irregularities. 2nd it relies on a negative output resistance of the driving circuit to compensate for the DC resistance of the motor. Since the motor resistance is indeed temperature dependant you need to compensate for it. But how do you measure accurately the motor winding temperature, especially when it has an ironless armature?

IMHO the best way to drive a TT with a DC motor is just the opposite. Drive the motor with current so it becomes a torque motor. Now put just enough current in it to keep the platter at the correct speed. That guarantees minimum noise from the motor. For that you need a speed sensor, either a tacho or a pulse encoder,. to control the current and thus the speed. Such a feedback loop is a 1st order loop that has no overshoot or whatever.

Just my 2 cents, cheers

[harwoodspark]

Originally posted by Pjotr




IMHO the best way to drive a TT with a DC motor is just the opposite. Drive the motor with current so it becomes a torque motor. Now put just enough current in it to keep the platter at the correct speed. That guarantees minimum noise from the motor. For that you need a speed sensor, either a tacho or a pulse encoder,. to control the current and thus the speed. Such a feedback loop is a 1st order loop that has no overshoot or whatever.

Hi Pjotr, this is an interesting approach. Could you draw a block diagram, not a circuit diagram, so I can better understand the 1st order feedback loop.My technical abilities only reach so far, I'm an electrical engineer and my abilities are not as good on the electronics side!
Thanks, Si.

[Pjotr]

Hi Si,

Can draw a drawing but there are plenty drawings provided in books on control theory.

It is not that difficult. The current through the motor causes a torque inside the motor. That torque is acting on the moment of inertia of the rotating parts with rpm’s (or better Rad/s) as a result. Analogue to an electric circuit you can consider the torque as a current source, the moment of inertia as a capacitor and the rpm as a voltage across the capacitor. So this system behaves as a current source charging a capacitor with a voltage across the capacitor as a result. Such a system is a 1st order system, in fact it is just an integrator (when we ignore losses).

A normal motor driven by a voltage is more or less the same. The current is initially set by the voltage and DC resistance of the motor. When the motor starts turning it is also acting as a generator, generating a back EMF. When the speed is such that the back EMF is near equal to the driving voltage the speed will not go higher because the current then ceases. So there is a kind of speed stabilisation depending on de resistance of the motor. The lower the motor resistance the better it works but also the more the irregularities of the magnetic circuit and armature geometry comes out as speed irregularities.

When driving a motor with current the back EMF becomes inactive and there is no speed control that way, only torque control. So you need a speed sensor (which can be very linear and noise free) to control the current. Now you have also the possibility to tailor the behaviour between the sensor and the current drive with i.e. a PID control or to make the loop very slow.

Cheers

[harwoodspark]

Many thanks for that information Pjotr.The Origin Live speed control I have fitted to my Linn works by sensing the back emf of the motor. When I build my next turntable I will be using an OL motor but I'll try your alternative idea.It does seem the right way to go. Si.