Effect of the Phasing Cap at different TT speeds? - diyAudio
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Old 26th September 2013, 04:38 AM   #1
andyr is offline andyr  Australia
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Default Effect of the Phasing Cap at different TT speeds?

On a recent thread that I started, I became aware that, for a basic Linn AC motor power supply - which consists of a resistor to reduce the mains voltage which the motor sees plus an X2-rated mains cap to provide a phase shift - the value of the cap is dependent on the motor speed (ie. the Hz of the mains supply to the motor).

Mark Kelly kindly provided the maths which show that whereas the value (for 50Hz mains) is 220nF for 33rpm ... the value for 45rpm (which is 67.5Hz) should be 120nF.

So I am wondering what the effect is of having the wrong value of phasing cap for 45rpm? IOW, if you use 220nF instead of 120nF - what will the effect be, for 45rpm?

Would the effect be, by any chance, a lower torque - so the motor struggles to start up from scratch?

Why I ask this is because currently, I have a 220nF cap fitted. At 33rpm (with the power coming straight from the wall), I find the platter gets up to speed rapidly. But, when I select 45 on my speed converter (which requires the motor to have only a 120nF cap), I find the platter can only just start to rotate - so I have to start it off at 33 and then, when the platter is spinning, flick the speed switch to 45.

So I'm wondering whether, if there was the correct 120nF cap on the motor (for 45rpm), it would have higher torque and have no problem spinning up the platter from scratch? (Just like at 33rpm with the 220nF cap.)


Regards,

Andy
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Old 26th September 2013, 05:54 AM   #2
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Andy

Thanks for bringing this to the forum.

There are two things going on at once here. The first is as you describe, the incorrect cap value will change the phase angle away from 90 degrees and thus reduce the torque. It's not a simple task to calculate the exact phase angle change because it depends on the Q of the LC filter formed with the motor winding.

The second thing is that the voltage supplied to the motor is not the voltage "seen" by the motor windings. When the motor is running it acts as a generator, the voltage developed is called the back EMF. With a synch motor this is always in phase with the supply voltage so the net voltage across the windings is the difference between V supply and the back EMF. This difference in voltage drives current through the windings creating the magnetic field which drives the motor.

The back EMF developed is proportional to the run speed; IIRC with the Premotec motor it's about 60 V at 250RPM, so it will be about 80 V at 337.5 RPM. If you are supplying say 100 V, the difference is thus 40 V at 33 and only 20 at 45, so there is less available to drive the motor.

Net result: to get decent performance with variable frquency, you need to be able to vary the capacitance and the voltage with the frequency. In practice it's easier to eliminate the capacitance altogether using quadrature outputs.

If you are stuck with a phase faking cap, you can simply wire a 270 nF cap in series with the existing 220nF. Wire a switch in parallel with the added 270nF cap. When the switch is open (for 45) the two caps in series act like a 121 nF cap. When the switch is closed (for 33) the 270 nF is bypassed so you get 220 nF. Don't open or close the switch while the drive is running.

Last edited by Mark Kelly; 26th September 2013 at 06:00 AM.
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Old 26th September 2013, 08:26 AM   #3
andyr is offline andyr  Australia
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Quote:
Originally Posted by Mark Kelly View Post
Andy

Thanks for bringing this to the forum.

There are two things going on at once here. The first is as you describe, the incorrect cap value will change the phase angle away from 90 degrees and thus reduce the torque. It's not a simple task to calculate the exact phase angle change because it depends on the Q of the LC filter formed with the motor winding.

The second thing is that the voltage supplied to the motor is not the voltage "seen" by the motor windings. When the motor is running it acts as a generator, the voltage developed is called the back EMF. With a synch motor this is always in phase with the supply voltage so the net voltage across the windings is the difference between V supply and the back EMF. This difference in voltage drives current through the windings creating the magnetic field which drives the motor.

The back EMF developed is proportional to the run speed; IIRC with the Premotec motor it's about 60 V at 250RPM, so it will be about 80 V at 337.5 RPM. If you are supplying say 100 V, the difference is thus 40 V at 33 and only 20 at 45, so there is less available to drive the motor.

Net result: to get decent performance with variable frquency, you need to be able to vary the capacitance and the voltage with the frequency. In practice it's easier to eliminate the capacitance altogether using quadrature outputs.

If you are stuck with a phase faking cap, you can simply wire a 270 nF cap in series with the existing 220nF. Wire a switch in parallel with the added 270nF cap. When the switch is open (for 45) the two caps in series act like a 121 nF cap. When the switch is closed (for 33) the 270 nF is bypassed so you get 220 nF. Don't open or close the switch while the drive is running.
And thank you for the answers, Mark.

Given that I need to be able to vary the capacitance for the 2 speeds, I was actually thinking of permanently wiring a 120nF cap in place - and then having a switch to either bring in a 100nF in parallel ... or leave it out.

And sorry, your info about the back-EMF increasing with the higher rotation speed - so reducing the difference available to drive the motor, if the supply is constant - suggests that, ideally, I should supply more voltage to the motor at 45rpm. Am I correct here?

Sure, quadrature outputs from the speed converter would be nice ... but I'm stuck with what I've got.


Thanks,

Andy
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Old 26th September 2013, 08:50 AM   #4
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Andy

1. Yes that would work. My scheme is a little simpler.

2. Yes, if you increase the voltage by about half the speed difference you'll be close enough. 45 is 35% faster than 33 so about 20% voltage increase should work.
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Old 26th September 2013, 08:55 AM   #5
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Or you could just use the LP12 45 rpm pulley adaptor... ;-)
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Old 26th September 2013, 09:20 AM   #6
andyr is offline andyr  Australia
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Quote:
Originally Posted by sq225917 View Post
Or you could just use the LP12 45 rpm pulley adaptor... ;-)
It's actually now 2013, SQ - you appear to be locked into the 70s.


Regards,

Andy
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Old 26th September 2013, 09:25 AM   #7
andyr is offline andyr  Australia
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Quote:
Originally Posted by Mark Kelly View Post
Andy

2. Yes, if you increase the voltage by about half the speed difference you'll be close enough. 45 is 35% faster than 33 so about 20% voltage increase should work.
Thank you, Mark. I'll discuss with Peter whether your 12v / 2x120v output transformer will be sufficient to do this (given the actual PCB output voltage is 13v) ... or whether I need to go to a lower "primary".


Regards,

Andy
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Old 26th September 2013, 11:13 AM   #8
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Yes that transformer was designed to interface with an amp running +/- 30V rails and bootstraps on the drivers, so it swung 20V RMS per primary easily. The whole system was designed with generous overhead to keep distortion low.
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Old 26th September 2013, 11:18 AM   #9
andyr is offline andyr  Australia
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Quote:
Originally Posted by Mark Kelly View Post
Yes that transformer was designed to interface with an amp running +/- 30V rails and bootstraps on the drivers, so it swung 20V RMS per primary easily. The whole system was designed with generous overhead to keep distortion low.
GR8.

Andy
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Old 27th September 2013, 12:05 AM   #10
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Quote:
Originally Posted by andyr View Post
It's actually now 2013, SQ - you appear to be locked into the 70s.


Regards,

Andy
70's, as if ! I'll have you know I'm firmly in the 80's..
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