Prime motor construction

Prime motor construction

Postby Mr_Putty » Wed Sep 12, 2018 12:16 pm

A bit of history. I have determined that my Prime motor is adding noise to my TT via my belt. With a string belt that is tight the noise sounds like rumble. Same for the rubber belt. It is audible at normal listening levels. If I reduce the tension on the string belt the noise goes away! But, the belt is so loose I suspect slipping and speed variation. If I ignore the “rumble” there are sonic improvements that are desirable with a tight string belt. With the rubber belt the speed is really too fast, and always has been, on any of the pulley settings. With the string belt the speed is slow. I should add that lifting the motor off its base did not affect the rumble I want to eliminate. Now to my motor. When I got it there was enough vibration that I wondered if it was bad. After the recommended break-in it felt much better, and after about 40 hours it seemed fully seated and smooth. The pulley and brass collar are both tight. And the pulley looks true when spinning. SO, what to do? Is there a diagram of the motor showing it’s bearings? Are they user replaceable? I have seen there are precision ceramic bearings online. Would they be recommended? I know a speed controller would be useful, but the concern here is the unwanted noise. Are other Prime users having this problem or solved it? BRF, your comments please.
Thanks,
JTA
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Re: Prime motor construction

Postby 10ovr » Wed Sep 12, 2018 1:01 pm

Try putting a mouse pad under the motor see if it changes,,,
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Re: Prime motor construction

Postby Mr_Putty » Wed Sep 12, 2018 2:53 pm

10ovr wrote:Try putting a mouse pad under the motor see if it changes,,,

I stopped using a mouse pad a long time ago. Mine were unreliable, and I recently lifted the motor by hand while listening with no change in rumble. So it’s not coming from the motor through the TT platform and back to the stylus. I just switched back to the rubber belt to confirm and it’s still audible.
JTA
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Re: Prime motor construction

Postby spinblackcircle » Wed Sep 12, 2018 3:09 pm

There have been cases of motor magnets being damaged in shipping. While I could pickup the sligtest motor noise/vibration when I had a Classic. the motor on my Prime is the quietest VPI motor I've had.

If your table is under warranty, I would get your dealer involved. If it isn't, maybe a new motor will fix you up.

http://www.hurst-motors.com/papbdirectdrive.html#buy

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Re: Prime motor construction

Postby Mr_Putty » Wed Sep 12, 2018 3:34 pm

spinblackcircle wrote:There have been cases of motor magnets being damaged in shipping. While I could pickup the sligtest motor noise/vibration when I had a Classic. the motor on my Prime is the quietest VPI motor I've had.

If your table is under warranty, I would get your dealer involved. If it isn't, maybe a new motor will fix you up.

http://www.hurst-motors.com/papbdirectdrive.html#buy

3201-001

What’s odd is that I just did the motor lift test with rubber TT belt again, with the same record as last night and headphones to eliminate ambient noise, and now hear the rumble decrease when the motor is lifted. The motor also seems very smooth, but last night it had some roughness I could feel. So, I guess it’s possible it’s in the last stages of break-in. The variation in vibration is what has me wondering about the bearing condition and replacement with better ones. If the bearings are serviceable I’m willing to try cleaning them, which should only help with smoothness. I can do the string belt test later tonight for a second comparison.
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Re: Prime motor construction

Postby Brf » Wed Sep 12, 2018 3:39 pm

Unfortunately, the Hurst motors used by VPI suffer from manufacturing inconsistencies. I know that VPI tries their best to screen out the poor performers, but sometimes a perfectly good Hurst motor will start to vibrate and get noisy. This is because the Hurst motor is only supported from the top by a single friction bearing (sorry no roller balls used, therefore, nothing to upgrade) and the heavy magnet structure that hangs below the bearing can start to vibrate when out of balance. A better design would have both the top and bottom supported by a roller bearings.

As you have pointed out, there exist a brass collar just under the pulley. The brass-collar serves two purposes 1) it locks the shaft to aligns the rotor with the windings, and 2) provide a small flywheel affect to help filter and damp cogging. You can attempt to adjust the rotor/winding alignment by releasing the brass collar set screw and move the pulley shaft to fine tune the rotor/winding alignment. I’ve done this successfully to help reduce both noise and vibration.

Sometimes the noise and vibration will dissipate when you lift the motor up, but this could be a result of realigning he rotor/winding with a non-perpendicular gravitational force. Make sure that your motor is on a level surface.

Another alternative is to replace the Hurst motor with another motor or another motor design design like a BLDC motor.
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Re: Prime motor construction

Postby Mr_Putty » Sat Sep 15, 2018 2:19 am

Brf,
Thanks for the detailed comments. I found a bit less motor vibration after loosening the pulley set screws a bit. I may try a rebalance by rotating the pulley a bit. A superior motor replacement would be welcome but I have no knowledge of which specific one would be a good choice. And I’m only about nine months in using this TT. I made some changes to my Prime isolation blocks and really like what changed. I also removed the string belt and went back to two rubber belts. So far every record I have played sounds more musical. Maybe the motor was resonating a bit in addition to being rough, as it is also is not as noticeable. Only time will tell if the changes I made are stable. I have an idea about isolating the motor so that it does not rest on the TT base, but need to work out the details before proceeding. It would be suspended above the TT base.
Cheers,
Oktoberfest is here!,
JTA
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