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premotec motor run capacitor
premotec motor run capacitor
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Old 25th May 2019, 10:09 PM   #1
Matt Rowland is offline Matt Rowland  United Kingdom
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Default premotec motor run capacitor

Seeking help with choosing a run capacitor value for a 16Vac, 2VA, 2phase Premotec motor. The spec is for 8u2F at 50Hz for 300rpm. I need to run it at 250rpm and will be using a power frequency generator that can be tuned to the correct frequency for 331/3 platter speed (250rpm motor speed). How do I calculate the correct value of capacitor for 90deg phase shift, given that I don't know the exact frequency I need to run the motor at for the desired speed?
Cheers. Matt
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Old 26th May 2019, 02:14 AM   #2
PRR is offline PRR  United States
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If 300RPM needs 50Hz, then 250RPM needs 41.66666..Hz. Basic math.

I *speculate* that the new cap is very approximately 9.84uFd, but this does not account for motor inductance.

The lower frequency suggests a lower voltage to avoid over-heating from the increased current at the lower frequency. However this may only matter if the motor WORKS, like a power saw cutting a knot. Running light, as it should in a turntable, back-EMF is high and current should not be high. (Anyway a 2VA motor big enough to see probably has enough surface-area to stay cool.)
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Old 26th May 2019, 08:12 AM   #3
Matt Rowland is offline Matt Rowland  United Kingdom
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Thanks PRR. I can ramp up the voltage until it works reliably. I can use 9.84 as a starting point and adjust as necessary.
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Old 26th May 2019, 04:24 PM   #4
Pyramid is offline Pyramid  United States
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The Premotec motor is rated for 300RPM at 60Hz and 250RPM at 50Hz. No mfr would make a motor that requires 41.666Hz to run at a standard RPM rating, since these motors are designed to run from the frequency of the power available at the wall socket.

If Premotec made a motor to run at 300 RPM at 50Hz and 360RPM at 60Hz, it would be the first one I've ever heard of, and the motor would have 20 poles instead of 24.
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Old 26th May 2019, 07:05 PM   #5
Matt Rowland is offline Matt Rowland  United Kingdom
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I thought it was a Premotec, badged as a Project motor. Maybe they make their own. Anyway, if you look at the photo in the Ebay link, you'll see it runs at 300rpm at 50Hz and 360rpm at 60Hz. Pro-Ject 16v 50Hz Replacement Motor for Debut II, Xpression, RPM 4 etc. DECO | eBay
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Old 26th May 2019, 10:14 PM   #6
as_audio is offline as_audio
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In my experience a motor like this is weak at reduced frequency and may run hot.
Better idea is to buy a 50 Hz / 250 rpm motor.
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Old 27th May 2019, 07:27 AM   #7
Matt Rowland is offline Matt Rowland  United Kingdom
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Thanks for the feedbak as_audio. Because I already have the motor I try it anyway and see if I have the same result.
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