Hurst Motors: 300 RPM vs 600 RPM--Upgrade or Myth?

Hello,
The BLDC motor starts with 5RPM to make life easy for the belt but other test howed that the Sota is a real winner. I will order one today if my friends will give me the info i need to order one for him too!
You can see some pics of my diy turntable earlier in this thread.
My platter is 14,17 inch (Sota will make a bigger pulley) weight 28,4 pounds
Total turntable including motor unit a little above 100 pounds.
The turntable base is supported by 3 Townshend seismic pods. For further dampening i made a lead shot filled rubber tube that will be put in a circle on the pom base underneath the platter. By moving/ shaping this tube i can restore level and equal load for the 3 springs which are located around the center of gravity.
Sota offered me to make a motor housing but i will need one higher than the standard one . I think the center of the pulley has to be at 5,5 inch height.
So i think i will ''design '' it self. There is a company close by that can machine pom material which has good properties and i can include some cavities to fill with lead shot like i did with my arm base.
I can make it circular, rectangular or even conical shaped. I dont have to much surface on the base to put a motor so the housing will have the same surface as a blu ray disc maximum. Would a conical shape with the smallest diameter in the upper part( like the white pom part) give an advantage?
I presume lead shot will give more effect in the lower part by giving a more stable '' set up ''
The motor unit could be attached to black pom base by means of a big nylon bolt and a threaded hole in the motor unit. Pom is like PTFE cannot be glued by normal folks. Can add a damping material between the pom parts like these
https://nl.mouser.com/datasheet/2/780/pro_1702-1669903.pdf
Or use 3 or 4 feet with the right load and no bolt?
Of course starting with the right motor and the right supply gives us a big advantage.
In the past the French audiophiles have done test with sand filled loudspeaker cabinets. One of the conclusions was not to use thick panels but to use thinner ones so they WOULD vibrate which would be absorbed by the sand. It could well be the same theory could be applied to the motor housing but one should know which would be the frequency range to be damped.
Greetings, eduard
 

Attachments

  • 20200612_192345.jpg
    20200612_192345.jpg
    382.7 KB · Views: 152
  • 20200604_221810.jpg
    20200604_221810.jpg
    890.5 KB · Views: 149
  • 20180712_233040.jpg
    20180712_233040.jpg
    735.1 KB · Views: 151
  • arm support pom en lood.jpg
    arm support pom en lood.jpg
    108.1 KB · Views: 158
Hurst Vibration

Hans-

The plots below were made with the same 5.5W 600 RPM motor in the same SAMA housing with the accelermeter located at the same point on the SS ring of the housing, with pulley attached but no belt (or platter). Adding the platter to the equation seems to reduce the vibration levels about 3dB.

The first plot is with the motor driven from single phase wall power with factory phase cap; the main response is at 120Hz (cogging) at ~-52dB.

The second plot is with the motor driven from a dual phase supply set to 90° phase offset and full voltage. The cogging response is down ~9dB.

The third plot is with dual phase drive, but the voltage reduced to 90VAC. The cogging response is about the same, but the motor started vibrating at ~40Hz at a level of -36dB. The response at 60Hz also came up dramatically at a level of ~-52dB. This got worse the lower the applied voltage was.

The motor has the brass collar on the shaft and in the past, I was able to reduce the random vibration that seems to occur with these motors, but it is mostly hit and miss. YMMV...


Thanks for some really nice measurements.
I replaced a broken Premotec in my RP3 with a Hurst Model 42mm (at 18% of the price of Premotec).

I measured the vibration form the Hurst motor by placing the the Ortofon HMC10 tip on the glass platter, with no belt and motor running, and measured the sound output at a very high volume than would hit 100dB SPL (@120Hz) with a calibrated mike. No increase from with the motor turned on.
I then repeated the test with the tip resting on a plastic block on the plinth, and got a faint output at 120Hz, about 27dB SPL. the volume was cranked up to the point of near acoustic feedback.

From this I'd say with the motor noise down 70dB I'm happy, and the motor noise is mainly conducted thru the drive belt, and not by conduction thru the plinth and platter.


I don't think a lower noise would make much of a difference.