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Old 20th March 2016, 04:55 PM   #1
Pyramid is offline Pyramid  United States
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Default 3 Phase BLDC motor for turntable use?

I've been kicking around the idea of a 3 phase BLDC motor and controller for a couple of years now. I've experimented with a lot of the Hurst AC synch motors, but vibration was ALWAYS a problem. I found the Hurst motors to be very inconsistent in build quality. Problem with the BLDC approach, I could never find a motor that I liked or that was cost effective, until a recent trip to Japan when I found a fantastic 3 phase BLDC motor in the Akihabara electronics mart. Inspired by a couple of conversations with Mark Kelly on the subject, I finally decided to pull the trigger on the controller now that I found a suitable motor, mainly as a non-commercial project-- just to see what was possible.

The motor is 24V / 12W / 1500 RPM and has about 4x the torque rating of a 10W 600 RPM Hurst AC synch motor, although it is smaller diameter (45mm vs 59mm) and only slightly longer (60mm vs 40mm). When I spin the motor by hand, I feel no cogging at all, even though it has iron stators and a PM rotor. The motor uses precision ball bearings that are pressed onto both ends of the shaft so there is no end play at the pulley at all. I think this is one of the reasons the Hurst motors are so noisy; that and they only have one bearing at the top of the shaft, the magnet/rotor is left to fly around at the bottom with no support. I've built 2 phase controllers for the Hurst motors, but of the 6-8 motors I have, not all of them would work; some of them would start vibrating horribly as they got up to speed unless I damped the motion with my thumb. I contacted Hurst about this and they said that was typical and that I should attach a small collar to the shaft to control end-play like VPI does on some of their motors. Didn't seem like a very workable solution to me. The bearings in the BLDC motor initially gave me pause, until I fired it up and realized it was far quieter than any of the Hurst motors I have. The construction of the stators is similar to AC induction motors with all of the windings on one core assembly rather than on 2 bobbins like the AC synch motors (another source of vibration in the latter). The BLDC bearings are sealed and never need lubrication; and unlike friction bearings, they can handle much higher radial loads without having to replace them when they wear. The motor runs quieter than any of the Hurst motors I have and is comparable to the 2W Premotec motors for noise.

The controller is the same basic design as my other controllers (DDS based) except that it has 3 phases instead of one. I also needed to adjust the output voltage and phase based on the speed of the motor and the torque required. Having independent control of all three elements creates an extremely smooth rotation with much more torque than an AC synch motor. The other HUGE advantage is the range of speed control with this method: With the Hurst AC synch motor, I would be lucky to get 3:1 speed range, even when driving both phases with quadrature signals. The high end (90Hz max) would have almost no torque and the low end (30Hz) would cog excessively. With the BLDC motor, I can easily get 100:1 speed range running the motor from 15 RPM to 1500 RPM (<1RPM to ~83 RPM at the platter) and the torque is surprisingly high even at 78 RPM. I start the motor at ~5Hz (4.2 RPM at the platter) and can ramp up to 93.6Hz (78 RPM at the platter) in a matter of seconds with zero burn out on the belt and all 3 speeds (33/45/78) using the same pulley. The controller drives a 3 channel class D amp powered by a 24V 1A wall wart. The amp fits in the same sized housing as the controller (about the size of a pack of cigarettes) and never gets over 100 deg F.

Since I have complete control over the phase of the 3 windings, I can also control the direction of rotation in software. I can easily switch between CW rotation for belt drive or by putting a rubber grommet on the pulley and switching to CCW, use it as a direct rim drive. I had a local machine shop turn an aluminum housing from a piece of 3.5" solid aluminum bar; the standard SAMA housing on my Scout was too wide to allow the motor to get close enough for the pulley to make contact with the platter. I wasn't happy with the vinyl grommet so I stuck with belt drive for now, but I'm going to keep looking for a softer silicone washer to use for rim drive.

I did some listening tests with the new motor vs the Hurst AC synch motor on a VPI Scout table and there is a noticeable improvement. Dynamics definitely took a step or two in the right direction. Because the motor has virtually no vibration, the noise floor seems to have dropped as well and the back ground is eerily quiet. Detail and clarity seems to have improved, but only because they don't get buried in the noise as before. I don't think it change the sonic signature of the table, the detail was always there, I just couldn't hear it as well. I wished I had found this motor earlier, but now that I have it, I'll never go back to AC synch motors again.
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Old 20th March 2016, 05:27 PM   #2
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So basically an SL-1200 motor run a bit faster?
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Old 20th March 2016, 05:39 PM   #3
Pyramid is offline Pyramid  United States
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Apples and Oranges. The SL1200 is DD, this is for belt drive/rim drive tables.

What is the torque rating for the SL1200 motor?
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Old 20th March 2016, 08:34 PM   #4
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They are both BLDC so why are they not both apples?
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Old 20th March 2016, 09:13 PM   #5
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Forgive the truly ignorant question, but I'm having trouble putting DC and 3 phase together. Is the 'phase' component indicating how many controller-timed circuits provide DC the coils? Hence a motor whose rotor contains 3 sets of 2 opposed magnets would use one 'phase' pulse per matching coil per revolution.

Thanks for any illumination available.

skip
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Old 20th March 2016, 09:47 PM   #6
russc is offline russc  England
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Very interesting Pyramid. I love it when folk transfer technology from one field to another (and share it).
I considered BLDC for my turntable build but having no experience of BLDC motors I wrongly inferred that they were (only) high revving thus not suitable.
Skip Pack - like so many things, BLDC is a marketing term. It's a brushless (therefore half-right!) motor usually powered by a DC battery through a controller, the controller switches the phases while sensing the rotor position.
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Old 20th March 2016, 10:22 PM   #7
Pyramid is offline Pyramid  United States
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Russc and Skip Pack-

BLDC (aka EC motors) are an interesting type of hybrid. Normally, they are used in industrial applications and the controllers are DC switching (trapezoidal waveform) type. The DC voltage level determines the speed, and Hall Effect sensors signal the control circuits when to switch (or commutate) the drive signal, so they behave like conventional DC motors in that the voltage drives the speed and commutation is done electrically instead of mechanically. This type of drive circuit produces a lot of torque ripple (cogging). It also requires feedback from the motor to control the speed.

In these types of motors, there are 3 windings spaced 120 deg apart so they can be driven instead by AC sinewave signals with 120 deg phase shift between them and if done so, they behave like AC synchronous motors with the frequency determining the speed. If driven this way, the amplitude of the AC signal is still important and should closely approximate the DC level for the same speed. For lowest noise operation, the phase relationship of the 3 signals is also important. By controlling all three aspects (frequency, amplitude and phase angles) the motors can be controlled with extreme speed accuracy and very low noise. As far as I know, there are no other belt drive or rim drive tables using this type of motor. The advantages are: Wide speed range, high torque, low vibration and very good speed accuracy.

IME, this represents a drive system for BD or rim drive that is an order of magnitude better than using AC synch motors with regards to low vibration and higher torque while maintaining the advantage of frequency determined speed.

Billshurv- The SL1200 motor is a low torque BLDC motor driven as a DC motor; i.e. it uses a feedback mechanism for speed control. My implementation is a feedforward system for motor speed control.
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Old 21st March 2016, 12:29 AM   #8
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Quote:
Originally Posted by Pyramid View Post
Billshurv- The SL1200 motor is a low torque BLDC motor driven as a DC motor; i.e. it uses a feedback mechanism for speed control. My implementation is a feedforward system for motor speed control.
The motor in the 1200 is the same as the SP15 just at lower voltage. SP15 is 3kg.cm and would accelerate the platter to speed in less than a revolution. How much more torque do you need? the SP10 at 16kg.cm was barking and could get a 10kg platter to speed in under 1/4 revolution.

I am interested in what the advantages are in your drive system. Like most things in hifi there is so much **** spoken about motor drive nice to consider something not ruined by marketing to the true believers
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Old 21st March 2016, 07:20 AM   #9
ppap64 is offline ppap64  Canada
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Hi Pyramid
Have to contain my excitement about this, so will wait for you to share further developments on this, but am seriously hoping you make this available.
I have purchased a couple papst aussenlaufer motors with the hope that someone would offer a high quality 3-phase supply that I could use to complete my rim drive table. Unfortunately, since Mark Kelly ceased making such supplies, there doesn't appear to be any available solutions.
Now, maybe a smoother, quieter, torquier motor than the esteemed papst motor ??
Very cool !!
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Old 21st March 2016, 12:57 PM   #10
Pyramid is offline Pyramid  United States
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Quote:
Originally Posted by billshurv View Post
The motor in the 1200 is the same as the SP15 just at lower voltage. SP15 is 3kg.cm and would accelerate the platter to speed in less than a revolution. How much more torque do you need? the SP10 at 16kg.cm was barking and could get a 10kg platter to speed in under 1/4 revolution.
I see. I have no intention of turning this into a "BD vs DD" thread. Surely, there must be plenty of other threads where these arguments have been made already?

My O/P was in regards to replacing a Hurst AC synch motor with a BLDC motor. My turntable is belt drive and I have no desire to convert it to DD, so I am stuck with a belt drive or rim drive system. Changing motors has made a definite improvement.

Quote:
I am interested in what the advantages are in your drive system.
As I stated in my previous post:

IME, this represents a drive system for BD or rim drive that is an order of magnitude better than using AC synch motors with regards to low vibration and higher torque while maintaining the advantage of frequency determined speed.

I would add: There is zero burn out on the belt, even when starting in 78 RPM mode with a heavy platter; mine is only 8lb, but it would just as easily start a 20lb aluminum platter without the familiar "chirp" on the belt (using a 600 RPM pulley or 300 RPM pulley).
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