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60 WPC Amplifier for DIY Turntable Motor Drive
60 WPC Amplifier for DIY Turntable Motor Drive
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Old 16th July 2018, 03:12 PM   #321
hirscwi is offline hirscwi  United States
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Join Date: Mar 2014
Location: Indianapolis, IN, USA
60 WPC Amplifier for DIY Turntable Motor Drive
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Originally Posted by hirscwi View Post
OK, It might be solved. I'm running 3 TDA7492 amps, each with one phase from the SG4 - to both channels. Each amp channel driving a secondary of the transformer and the primaries connected in parallel. I get a bit of clicking at startup but then it quiets down. I'm running some extended testing to see about heat and will (hopefully) report success.
Well, it was not solved. When I re-cased the thing, the clicking problem was back. I was getting distorted sine waves to each output. So here's what I tried:

I connected the secondaries in series and used one channel of each amp. The primaries remain connected in parallel. This eliminated the clicking sound but I was still getting a distorted sinewave when connected to the motor. I wonder if the load was too low impedance?

When I reduced the input from the SG4 to the amp boards, the distortion disappeared. I suppose I was overdriving the amps. However, the output to the motor is only about 40VAC (rms) under load. But the motor runs very stably and cool. No over heating in the transformers or amps either. So I went a step further and switched to the 45 RPM section of the pulley (Empire table with Papst motor) and reduced the frequency accordingly. It works even better. I get less noise from the motor (1300 RPM vs 1800) and no heat. I could put the primaries parallel also but I'm not sure I see a need to do so.

I've run some 1-2 hour tests and everything seems to be fine. Am I doing something that will have bad long-term effects? Or is it just the Papst motor is so oversized that this approach works.

BTW, using full 120VAC and the capacitor for phase splitting, the motor was drawing about 60 watts. A Lenco draws about 15-19 W and VPI about 5 W, I think. So maybe just over engineered?

I hope to mount a cart on the table today and the listening test will be the real crucial one.
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Old 18th July 2018, 04:20 AM   #322
hirscwi is offline hirscwi  United States
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60 WPC Amplifier for DIY Turntable Motor Drive
Yes, it's definitely working great. Sounds very nice - I've been listening to it for the past couple hours. Attached is a photo of the table and controller.


Now that's finished, I do have a question: What is the difference between the Papst motor and the Hurst motor used in the VPI tables? I think I understand that the Papst is a 3 phase synchronous - correct? So how is the Hurst different? I'd appreciate this information purely for my education, thanks.
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Old 18th July 2018, 11:09 AM   #323
Pyramid is offline Pyramid  United States
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The Papst motor is a 3 phase hysteresis type which is synchronous, but has some differences from a typical AC synch motor. It does not use a permanent magnet for the rotor, so it does not exhibit cogging. Because of this, it is smoother, but can fall out of synch at higher torque loads (an AC synch motor maintains speed until it stalls), is less efficient and has a lower power factor than a similarly sized AC synch motor. Both motors require higher voltage to maintain torque at higher speeds, the the Papst motor can start to "slip" similar to an AC induction motor at 45/78 RPM (drop below speed, but still spin), where an AC synch motor would stall.

IMHO, the Hurst motors are cheaply constructed, the bearings and rotor being the weak points, both inconsistent from one motor to the next. The rotors do not appear to be well balanced and coupled with a sloppy bearing (and only one bearing at that, on the top only), some motors exhibit unacceptable vibration as well as the expected cogging.

Nice looking table and good job of construction BTW.
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Old 18th July 2018, 01:59 PM   #324
hirscwi is offline hirscwi  United States
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Join Date: Mar 2014
Location: Indianapolis, IN, USA
60 WPC Amplifier for DIY Turntable Motor Drive
Quote:
Originally Posted by Pyramid View Post
The Papst motor is a 3 phase hysteresis type which is synchronous, but has some differences from a typical AC synch motor. It does not use a permanent magnet for the rotor, so it does not exhibit cogging. Because of this, it is smoother, but can fall out of synch at higher torque loads (an AC synch motor maintains speed until it stalls), is less efficient and has a lower power factor than a similarly sized AC synch motor. Both motors require higher voltage to maintain torque at higher speeds, the the Papst motor can start to "slip" similar to an AC induction motor at 45/78 RPM (drop below speed, but still spin), where an AC synch motor would stall.

IMHO, the Hurst motors are cheaply constructed, the bearings and rotor being the weak points, both inconsistent from one motor to the next. The rotors do not appear to be well balanced and coupled with a sloppy bearing (and only one bearing at that, on the top only), some motors exhibit unacceptable vibration as well as the expected cogging.

Nice looking table and good job of construction BTW.
Thank you for the information - it's most appreciated.
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